Electrical Systems, Manutronics
Technician Certificate in Manutronics Automation Dublin Institute of Technology Bolton Street Dublin 1
Manutronics Electrical Systems First Year Course Version 2
Lecturer: John McGrory
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Electrical Systems, Manutronics Table of Contents TABLE OF CONTENTS .................................................................................................................. 2 OBJECTIVES OF THIS COURSE: ................................................................................................. 6 INTRODUCTION.......................................................................................................................... 6 COMPLETION TIME .................................................................................................................... 6 DESIGNED FOR .......................................................................................................................... 6 PRIOR KNOWLEDGE.................................................................................................................. 6 COURSE AIMS ............................................................................................................................ 6 CONTENT .................................................................................................................................... 7 CHAPTER 1 INTRODUCTION........................................................................................................ 8 CHAPTER 2, SAFETY .................................................................................................................... 9 A FEW QUICK POINTERS: ................................................................................................................. 9 CHAPTER 3, UNITS, GREEK/CIRCUIT SYMBOLS AND DIMENSIONAL ANALYSIS.............. 10 UNITS .......................................................................................................................................... 10 GREEK SYMBOLS.......................................................................................................................... 11 CIRCUIT DIAGRAM SYMBOLS .......................................................................................................... 12 DIMENSIONAL ANALYSIS ........................................................................................................ 13 TEST YOURSELF ........................................................................................................................... 17 SECTION 4, VOLTAGE (ELECTRICAL PRESSURE) ................................................................. 18 AC AND DC ................................................................................................................................... 18 MAINS VOLTAGE SUPPLIES ............................................................................................................ 18 LOW VOLTAGE SUPPLIES ............................................................................................................... 18 HIGH VOLTAGE SUPPLIES .............................................................................................................. 18 HOW VOLTAGE IS MEASURED ......................................................................................................... 19 THE FORCE BEHIND ELECTRICITY ................................................................................................... 19 TEST YOURSELF ........................................................................................................................... 20 SECTION 5, ELECTRICAL CURRENT ........................................................................................ 21 HOW TO MEASURE CURRENT ......................................................................................................... 21 W HAT IS AN ELECTRIC CURRENT? .................................................................................................. 22 RECAPPING IN BRIEF: .................................................................................................................... 25 CONDUCTORS AND INSULATORS .................................................................................................... 26 TEST YOURSELF ........................................................................................................................... 27 SECTION 6, RESISTANCE, CONTROLLING CURRENT ........................................................... 28 RESISTANCE IN ELECTRIC CIRCUITS ............................................................................................... 28 RESISTORS .................................................................................................................................. 29 HEAT AND LIGHT ........................................................................................................................... 29 TEMPERATURE COEFFICIENT OF RESISTANCE ................................................................................. 30 OHM SWEET OHM!......................................................................................................................... 31 SYMBOLS AND UNITS OF RESISTANCE ............................................................................................ 31 RESISTANCE VALUE DETERMINATION ............................................................................................ 32 Modern System for Resistance Quantifying ........................................................................... 32 Numbering by colours............................................................................................................. 32 CONNECTION OF RESISTORS (SERIES OR IN PARALLEL) .................................................................. 34 Series...................................................................................................................................... 34 APPLICATIONS .............................................................................................................................. 35 VOLTAGE DIVISION ........................................................................................................................ 35 VOLTAGE DIVIDERS, W HAT MIGHT GO WRONG? ............................................................................. 37
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Electrical Systems, Manutronics FAULT FINDING PROCESS FOR VOLTAGE DIVIDERS: ....................................................................... 38 VARIABLE RESISTORS ................................................................................................................... 38 PARALLEL .................................................................................................................................... 39 W HY PARALLEL?........................................................................................................................... 39 W HAT ABOUT THE OHMS?.............................................................................................................. 39 EXAMPLE 1................................................................................................................................... 40 EXAMPLE 2................................................................................................................................... 41 QUICK REVISION AND REMINDER ................................................................................................... 42 SECTION 7, OHMS LAW .............................................................................................................. 43 W HERE WOULD YOU USE OHM'S LAW? .......................................................................................... 45 EXAMPLE OF INDIRECT CURRENT MEASUREMENT ............................................................................ 46 REVISION ..................................................................................................................................... 47 SECTION 8, ELECTRICAL POWER............................................................................................. 48 W HAT'S W ATT! ............................................................................................................................. 50 POWER AND CURRENT CONSUMPTION ............................................................................................ 50 COMBINING OHMS LAW AND THE POWER TRIANGLE. ...................................................................... 53 THE POWER TRIANGLE .................................................................................................................. 55 POWER RATING ............................................................................................................................ 56 RESISTOR RATINGS....................................................................................................................... 57 CALCULATING THE POWER RATING OR A RESISTOR ......................................................................... 57 SECTION 9, NETWORK THEOREMS.......................................................................................... 58 INTRODUCTION ............................................................................................................................. 58 SINGLE AND DOUBLE SUBSCRIPTS................................................................................................. 58 NETWORK THEOREMS ................................................................................................................... 59 SUPERPOSITION THEOREM ............................................................................................................ 59 Example .................................................................................................................................. 59 KIRCHHOFF’S LAWS (1860)........................................................................................................... 62 Example 1 ............................................................................................................................... 63 THEVENIN’S THEOREM .................................................................................................................. 64 Example 1 ............................................................................................................................... 65 Example 2 ............................................................................................................................... 66 CHAPTER 10, STATIC CHARGES............................................................................................... 67 Attracting dust and dirt............................................................................................................ 69 From a spark to a fire.............................................................................................................. 69 Chip damage .......................................................................................................................... 70 CHAPTER 11, ELECTROMAGNETISM ....................................................................................... 72 USING ELECTROMAGNETISM .......................................................................................................... 72 PERMANENT MAGNETS ................................................................................................................. 72 Attraction and Repulsion of Magnetic Poles........................................................................... 74 Altering a Magnetic Field ........................................................................................................ 75 Magnetic Flux ......................................................................................................................... 75 How Materials Become Magnetised ....................................................................................... 76 An Application......................................................................................................................... 76 ELECTROMAGNETISM .................................................................................................................... 78 Right-Hand Rule ..................................................................................................................... 79 Electromagnetic Properties..................................................................................................... 80 EXAMPLE ..................................................................................................................................... 83 THE ELECTROMAGNET .................................................................................................................. 83 The Solenoid........................................................................................................................... 84 The Relay................................................................................................................................ 85 The Speaker ........................................................................................................................... 86
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Electrical Systems, Manutronics MAGNETIC LEAKAGE AND FRINGING ............................................................................................... 87 ELECTROMAGNETIC INDUCTION ..................................................................................................... 88 Relative Motion ....................................................................................................................... 88 Polarity of the Induced Voltage............................................................................................... 88 Induced Current ...................................................................................................................... 89 Forces on a Current-Carrying Conductor in a Magnetic Field................................................ 89 FARADAY'S LAW ........................................................................................................................... 90 LEFT HAND RULE.......................................................................................................................... 91 APPLICATIONS OF ELECTROMAGNETIC INDUCTION .......................................................................... 92 Automotive Crankshaft Position Sensor ................................................................................. 92 DC Generator ......................................................................................................................... 93 CHAPTER 12, MEASURING INSTRUMENTS ............................................................................. 97 ANALOG METER MOVEMENTS ....................................................................................................... 97 THE AMMETER.............................................................................................................................. 99 Multiple-Range Ammeters ...................................................................................................... 99 Effect of the Ammeter on a Circuit........................................................................................ 100 THE VOLTMETER ........................................................................................................................ 101 Multiple-Range Voltmeters ................................................................................................... 102 Loading Effect of a Voltmeter ............................................................................................... 103 THE OHMMETER ......................................................................................................................... 104 Zero Adjustment ................................................................................................................... 104 Multiple-Range Ohmmeter.................................................................................................... 104 THE W ATTMETER........................................................................................................................ 106 Multimeters ........................................................................................................................... 107 USE OF VOLTMETER AND AMMETER............................................................................................. 108 QUICK SUMMARY ........................................................................................................................ 109 CHAPTER 13, ALTERNATING CURRENT AC AND VOLTAGE .............................................. 110 INTRODUCTION ........................................................................................................................... 110 CHAPTER 14, SINE WAVE CHARACTERISTICS..................................................................... 112 THE SINE W AVE ......................................................................................................................... 112 THE POLARITY OF A SINE W AVE .................................................................................................. 112 THE PERIOD OF A SINE W AVE ..................................................................................................... 113 THE FREQUENCY OF A SINE W AVE .............................................................................................. 114 RELATIONSHIP OF FREQUENCY AND PERIOD ................................................................................ 114 VOLTAGE AND CURRENT OF A SINE WAVE..................................................................................... 115 INSTANTANEOUS VALUE .............................................................................................................. 115 PEAK VALUE............................................................................................................................... 115 PEAK-TO-PEAK VALUE ................................................................................................................ 116 AVERAGE VALUE ........................................................................................................................ 117 CHAPTER 15, INDUCTION FOR AC SYSTEMS ....................................................................... 118 INDUCTANCE .............................................................................................................................. 118 SELF-INDUCTANCE ...................................................................................................................... 118 CHAPTER 16, CAPACITORS..................................................................................................... 122 TYPES OF CAPACITOR ................................................................................................................. 122 CAPACITOR COLOUR CODES ........................................................................................................ 126 CHARGING AND DISCHARGING CAPACITOR................................................................................... 128 CHARGING AND DISCHARGING CAPACITOR.................................................................................... 128 CHARGING AND DISCHARGING PROCESSES ................................................................................. 129 CAPACITANCE ............................................................................................................................ 130 CAPACITORS IN PARALLEL........................................................................................................... 130 CAPACITORS IN SERIES............................................................................................................... 130
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Electrical Systems, Manutronics TIME CONSTANT OF RC CIRCUIT.................................................................................................. 132 CAPACITORS IN USE ................................................................................................................... 132 Suppression circuits.............................................................................................................. 132 Preventing interference......................................................................................................... 132 SAFETY NOTE ............................................................................................................................. 133 CHAPTER 17, TRANSFORMERS .............................................................................................. 134 STEPPING DOWN ........................................................................................................................ 135 REMEMBER MAGNETISM? ............................................................................................................ 135 HOW DOES IT WORK?.................................................................................................................. 136 THE 'TURNS RATIO'...................................................................................................................... 137 CHAPTER 18, AC-POWER, TRUE OR APPARENT ................................................................. 139 THE APPARENT POWER ............................................................................................................... 141 THE TRUE POWER ...................................................................................................................... 141
CHAPTER 19, NATIONAL RULES FOR ELECTRICAL INSTALLATIONS .............................. 145 SCOPE ..................................................................................................................................... 145 OBJECT ................................................................................................................................... 146 FUNDAMENTAL PRINCIPLES FOR SAFETY OF ELECTRICAL INSTALLATIONS.............. 147 Protection against direct contact .......................................................................................... 147 Protection against indirect contact........................................................................................ 147 Protection against thermal effects in normal service............................................................ 147 Protection against over current............................................................................................. 147 Protection against fault currents ........................................................................................... 148 Isolation and switching.......................................................................................................... 148 Protection against environmental conditions (external influences) ...................................... 148 Verification and certification.................................................................................................. 148 DEFINITIONS........................................................................................................................... 149 IEC CLASSIFICATION SYSTEM FOR ENCLOSURES .......................................................... 153 525 VOLTAGE DROP IN CONSUMERS' INSTALLATIONS............................................ 154 CURRENT-CARRYING CAPACITIES OF CONDUCTORS .................................................... 155 STANDARD TYPES OF WIRES AND CABLES NORMALLY USED. ..................................... 156 CENELEC CABLE CODING SYSTEM .................................................................................... 156 INTRODUCTION TO PROTECTION OF CIRCUITS ......................................................................... 159 PROTECTION AGAINST OVERLOAD CURRENT ................................................................. 159 PROTECTION AGAINST SHORT-CIRCUIT CURRENT......................................................... 159 FAULT-LOOP IMPEDANCE ............................................................................................................. 160 INSULATION RESISTANCE ............................................................................................................. 161 HEALTH & SAFETY ...................................................................................................................... 162 TEST YOURSELF RESULTS ..................................................................................................... 163 CHAPTER 1, TEST YOURSELF RESULTS. ...................................................................................... 163 CHAPTER 10, TEST YOURSELF RESULTS. .................................................................................... 164 CHAPTER 15, TEST YOURSELF RESULTS. .................................................................................... 165 REFERENCES............................................................................................................................. 167
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Electrical Systems, Manutronics Objectives of this course: INTRODUCTION With the growth of electronic/electro-mechanical devices in the control of industrial equipment it has become clear there is an increasing need for all personnel involved with the installation and maintenance to develop electrical skills. This course introduces basic electrical skills, terminology and concepts in an easy to follow methods. COMPLETION TIME Chapters 1 to 2 Chapters 3 to 8 Chapters 9 to 16 Chapter 17 to 18 Chapter 19
3 Hours 20 Hours 20 Hours 4 Hours 4 Hours
DESIGNED FOR Students who wish to retrain in electrical skills, including electrical terminology, passive components and their electrical circuit function. PRIOR KNOWLEDGE The course is from the beginning and does not need any special instruction except standard mathematics and obvious comprehension. A general engineering background would be an advantage. COURSE AIMS At the end of this year each student should be able to: x x x x x x x x x x x x
Recognise and apply the correct terms used to describe and measure an electrical power supply. Measure the voltage, current, or resistance, within a simple electrical circuit, using multimeter. Predict the results of wiring resistors in series and parallel, in simple circuit. Understand the relationship between power, current and potential difference. Describe typical common industrial examples of current and power ratings. Identify methods of preventing damage to electronic components caused by static electricity. Construct and use an electromagnet. Use a multimeter to measure ac voltage and current. Compare the shapes of various waveforms and construct a simple sine wave. Correctly identify and use capacitors in an electrical circuit. Use capacitors in circuits to provide smoothing, arc suppression and time delays. Use a transformer to raise and lower voltages.
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Electrical Systems, Manutronics CONTENT x x x x x x x x x x x x x
Use of multimeters Resistors in series and parallel Measurement of voltage, current and resistance Ohms Law Power rating in dc circuits Colour coding of resistors Measurement of ac and dc voltage, current, resistance and capacitance Static electricity and its prevention Electromagnetism Capacitor colour codes Waveforms Smoothing, are suppression and time delays Transformers Simple ac and dc circuitry
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Electrical Systems, Manutronics Chapter 1
Introduction
Electricity is one of the most widely forms of energy we know. It is invisible, it’s dangerous, but it makes things work. It provides heat for our kettles, light for our homes & offices and power for our industries. Without electricity our world would be a much different place. You have probably heard of Amps, Volts, Watts, Resistance and other electrical terms. As you work through this course you will come to understand what they all mean, how they relate to one another and how to apply them for the purposes of engineering. In fact you might already know more about electricity that your realise. Perhaps you have studied something about electricity at school, from your parents or friends, but have forgotten all the theory and formulas. The idea of this course is to take you from the very beginning and work through the concepts and applications. Just as an additional note it is worthwhile remembering that as electricity is invisible and we need to use tools to observe what is happening. A plumber knows when there is a problem by seeing the water leak out of pipes. Electricity is not the same thus we use the tools to investigate what is happening.
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Electrical Systems, Manutronics Chapter 2, Safety When working with electricity or any equipment, safety should be your No. 1 concern. The following is not a comprehensive list for safety and should only be used as a quick pointer guide. A few quick pointers: x Do not remove the cover of any equipment, if it’s connected to the mains or a battery without isolating the course of supply before hand. x
Before working on any equipment make sure that it is isolated and not just in an idle state. In many circuits it is possible for it to look safe, but they can still be lethal.
x
To isolate a circuit it is not enough to push in an emergency stop, you must always remove the fuses, lock the supply in the off position with a pad lock and put a visible label on the lock. Do not leave the fuses or keys at the circuit keep them in your pocket.
x
Tell somebody what you are doing and how it will affect outside elements such as fuses etc.
x
If in doubt do not touch, stay away and ask for help.
x
Circuits that use capacitors should be de-energised to avoid the capacitor discharging while maintenance is been completed on the circuit.
x
Even low voltage circuits can be dangerous if they involve capacitors.
x
If anybody gets an electrical shock do not touch them. Isolate the source of supply and call help. If the voltages are very small (240V) use an insulator such as a wooden brush or branch of a tree to move the isolated source from the person. But only if the voltages are small.
x
Always where possible move the danger from the subject and not the subject from the danger.
x
Never touch or operate a circuit with wet hands.
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Electrical Systems, Manutronics Chapter 3, Units, Greek/Circuit Symbols and Dimensional Analysis Units The majority of mistakes in engineering develop from the incorrect use of multiples and their related symbols. The following is an extract from the log tables. Make sure you know and use them correctly. You might mistake 10kV for 10Volts, the latter being quite low voltage. But in fact 10kV it is 10,000Volts which is absolutely deadly, so beware. All the calculations we perform require that you know the multiples and there symbols and use them correctly or your answers will be wrong.
By convention we do not express numbers to more that two decimal places. Use engineering notation to correctly note the numbers.
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Electrical Systems, Manutronics Greek Symbols
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Electrical Systems, Manutronics Circuit diagram symbols Here are all the circuit diagram symbols you should need for this year. For the full complement of symbols refer to BS3939. A copy of it is in the library. Negative polarity Positive polarity Junction of conductors Switch Battery Fixed resistor
Variable resistor
Signal lamp Fuse
Capacitor
NPN Transistor
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Electrical Systems, Manutronics DIMENSIONAL ANALYSIS When doing physics problems, you'll often be required to determine the numerical value and the units of a variable in an equation. The numerical value usually isn't too difficult to get, but for a novice, the same can't be said for the units. This self-instruction unit deals with dimensional analysis, which is a useful method for determining the units of a variable in an equation. Another use of dimensional analysis is in checking the correctness of an equation which you have derived after some algebraic manipulation. Even a minor error in algebra can be detected because it will often result in an equation which is dimensionally incorrect. The following lists the objectives of this unit. Read these objectives carefully. (1) (2)
Given the definition of a physical quantity, or an equation involving a physical quantity, you will be able to determine the dimensions and SI units of the quantity. Given an equation, you will be able to determine if the equation is dimensionally correct or incorrect.
Most physical quantities can be expressed in terms of combinations of five basic dimensions. These are mass (M), length (L), time (T), electrical current (I), and temperature, represented by the Greek letter theta (M). These five dimensions have been chosen as being basic because they are easy to measure in experiments. Dimensions aren't the same as units. For example, the physical quantity, speed, may be measured in units of metres per second, miles per hour etc.; but regardless of the units used, speed is always a length divided a time, so we say that the dimensions of speed are length divided by time, or simply L/T. Similarly, the dimensions of area are L2 since area can always be calculated as a length times a length. For example, although the area of a circle is conventionally written as Hr2, we could write it as p r (which is a length) × r (another length). Now that you can determine the dimensions of physical quantities, it'll be useful to write the SI units for the quantities. SI stands for International System (Système Internationale). The SI unit for mass is the kilogram, for length the metre, for time the second, for current the ampere, and for temperature the kelvin. Notice that kelvin is abbreviated as just K. The degree symbol, °, and the word "degree" are not used with kelvin. As a quick example, let's look at speed, which has dimensions of length divided by time or L/T. Its SI units are then metres divided by seconds, represented as m/s or m·s-1.
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Electrical Systems, Manutronics Some combinations of SI units are given special names. For example, the unit of energy, kg·m2/s, is given the special name joule, which is abbreviated as J. Study the information presented below. a) b) c) d) e)
energy force frequency power charge
joule (J) newton (N) hertz (Hz) watt (W) coulomb (C)
kg·m2/s2 kg·m/s2 (cycles)·s-1 J/s = kg·m2/s3 A·s
You might be wondering when to write joule, J, or kg·m2/s2 as the energy unit. The SI convention is that if there is no number in front of the unit, then the unit is written as a full word. For example, you would write "energy is expressed in joules," with "joules" written out in full, since there is no number associated with it in the sentence. If there is a number, then "J" ( or less commonly, "kg·m2/s2") is used. Thus, you would write "the energy is 6.4 J". Some quantities have no dimensions. For example, the sine of an angle is defined as the ratio of the lengths of two particular sides of a triangle. Thus, the dimensions of the sine are L/L, or 1. Therefore, the sine function is said to be "dimensionless". There are many other examples of "dimensionless" quantities listed in the following table. (a) all trigonometric functions (b) exponential functions (c) logarithms (d) angles (but notice the discussion in the next paragraph) (e) quantities which are simply counted, such as the number of people in the room (f) plain old numbers (like 2, p, etc.) Notice that some quantities which are "dimensionless" have units. For example, angles can be measured in units of radians or degrees, but angles are "dimensionless". Another familiar example is a frequency unit, (cycles) per second. The second, of course, is a time unit but the cycles are "dimensionless". That's the reason for cycles being written in parentheses above. Take a few moments and learn the "dimensionless" quantities above. We've now covered all the basics so let's get into some fine points. There are two special cases of quantities which are "dimensionless". First, the argument of a trigonometric function, and second, the exponent in any exponential function. The argument of a trig function is an angle, of course, so it's "dimensionless"; and an exponent of an exponential function is the same thing as a logarithm so
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Electrical Systems, Manutronics it's "dimensionless". These facts often are useful in helping to determine the dimensions of a quantity. For example, if we're given that y = ekt, where t is the time, we can state that k must have dimensions of time-1 in order that the exponent be "dimensionless". A common notation, which means "the dimensions of a quantity", is simply the quantity written inside square brackets [ ]; thus, [area] = L2. In an algebraic expression, all terms which are added or subtracted must have the same dimensions. This implies that each term on the left-hand side of an equation must have the same dimensions as each term on the right-hand side. For example, in the equation a = bc + (1/2)xy, "a" must have the same dimensions as the product "bc", and the product "(1/2)xy" must also have the same dimensions as "a" or "bc". Remember that the "1/2" in "(1/2)xy" is just a plain old number and so it has no dimensions. An equation in which each term has the same dimensions is said to be dimensionally correct. All equations used in any science should be dimensionally correct. The only time you'll encounter one which isn't is if there is an error in the equation. So dimensional analysis is a valuable tool in helping you to detect an equation in which you made an error in algebra, for example. Let's do a sample question. This is a typical question which is fairly difficult. We're given an equation (shown on the left) involving force, radius, length, speed, and distance, and are asked for the dimensions and SI units of eta, (K), which is a viscosity. First, we rearrange the equation to solve for K and then convert it to an equation involving dimensions. Note that the negative sign is interpreted as "-1". Okay, you finish it off. When you've worked out the answer, move on to see if you're right or not.
where F is force r is radius is length v is speed R is distance What are the dimensions and SI unit of K (viscosity)?
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Electrical Systems, Manutronics Well, the correct answer for the dimensions is M·L-1·T-1. The corresponding SI unit is kg·m-1·s-1. If you didn't get this answer, try the question again. You've probably just made a simple error with your exponents.
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Electrical Systems, Manutronics Test yourself Test 1 See if you can determine the dimensions of the following quantities: 1. volume 2. acceleration (velocity/time) 3. density (mass/volume) 4. force (mass × acceleration) 5. charge (current × time) Test 2 Now find the dimensions of these: 1. pressure (force/area) 2. (volume)2 3. electric field (force/charge) 4. work (in 1-D, force × distance) 5. energy (e.g., gravitational potential energy = mgh = mass × gravitational acceleration × height) 6. square root of area Test 3 You have already determined the dimensions of each of the following quantities. Can you determine the corresponding SI units? 1. volume 2. density 3. pressure 4. energy Test 4 For each case below, write the corresponding special name of the SI unit. 1. energy 2. power 3. kg·m/s2 4. charge 5. force 6. kg·m2/s2 7. kg·m2/s3 8. A·s 9. (cycles)·s-1 10. J/s 11. frequency
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Electrical Systems, Manutronics Section 4, Voltage (Electrical Pressure) Everything that works by electricity needs a power supply of some kind. Different kinds of equipment need different types of power supply. A calculator, for example, may run off a 1.5 volt “AA” battery. A car's electrical system usually needs a 12 volt supply using a lead/acid battery. Household appliances such as a TV or kettle need a 240 volt supply. The unit of measurement for electrical pressure - the volt - is named after the Italian scientist Alessandro Volta who made the first battery about 200 years ago. Ac and dc There are two types of electrical power supply ac and dc. These letters stand for alternating current and direct current. Alternating current is the type of electricity supplied to your factory or home by the electricity boards. Direct current is the type of electricity you get from a battery. Some equipment is designed to work with ac supplies while other with dc supplies. Mains voltage supplies The mains electricity which is supplied to your home is 240 volts, 50Hz ac. In a factory there may also be a 415 volts, 50Hz and occasionally higher ac supplies. Both these high voltages are very dangerous. If you were to come into contact with the mains supply you would receive an electric shock which could be severe enough to kill you. Low voltage supplies Because the mains voltage could have a fatal affect if touched by accident low voltage supplies were introduced. These low voltage supplies drop the voltage and current down to levels making them much safer in the event of an accident. Most control panels in Europe use 110Vac, 50Hz for the control side with the capability of switching any voltage through the use of relays. The voltages we use in the labs are usually 6, 12, 24 and 36 Volts with voltages getting higher for the machines which need more power. They have a drawback however: the actual potential to produce work is reduced if using low voltage supplies. High voltage supplies High voltage dc supplies are used to power 'third rail' electric trains and the kind of motors you find in steel and paper mills. High voltage dc supplies can be very dangerous and should be treated with great care. Like ac mains supplies, they can cause a shock that could kill you. They also have a greater burning affect when they spark for an equivalent ac voltage.
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Electrical Systems, Manutronics How voltage is measured The abbreviation for volts is V, but as well as volts you may sometimes need to measure smaller voltage levels, millivolts (symbol mV), which are thousandths of a volt, and even in microvolts (symbol µV), which are millionths of a volt. (The sign µ is the Creek letter mu). Very high voltages are usually measured in kilovolts (symbol kV). One kilovolt is a thousand volts. The cables carried by electricity pylons, for example, operate at 132 kV, or even 275 kV or 400 kV. High Kilovolts (kV), 1kV = 1000volts, i.e. 1E+03volts Volts (V) Millivolts (mV), 1000mV = 1volts , i.e. 1E-03volts microvolts (V), 1000,000 V = 1volts, , i.e. 1E-06volts Low
The force behind electricity We've talked long enough about voltage now - and you've measured it yourself without actually defining what it is. In fact, voltage is the force behind electricity. It's sometimes called electrical pressure', or, more scientifically, electromotive force, 'emf'. Voltage alone doesn't explain what electricity is, though it helps explain how electric current moves. The next section will look at electric current itself. One of the best ways to think about voltage is by comparing it with the water pressure in a plumbing system. A 240 V supply is like the pressure of water that comes to a tap from a storage tank high up in the loft. A 6V supply, on the other hand, is like the water pressure in a tap only a few feet below the supply tank. The pressure which drives the water is due to the difference in levels of the tank and the tap. The actual levels can be called potentials. The difference in levels, which is what actually does all the work, could be called a potential difference. In fact, this phrase potential difference, 'pd' for short, is sometimes used to refer to voltages. This similarity is not completely true for electricity as electricity is not affected by gravity in the same way water is.
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Electrical Systems, Manutronics Test Yourself Try tick the correct answers from the lists given below. From question 6 onwards fill in your answer. 1.
The electrical force of a power supply is measured in? a. Kilojoules ___ b. Volts ___ c. Ohms ___ d. emf ___
2.
What voltages would you expect to power your electrical kettle at home? a. 24V ___ b. 240V ___ c. 2400V ___ d. 24kV ___
3.
How many µV are in 1V? a. 1000 b. 1000,000 c. 0.0000 d. 0.001
___ ___ ___ ___
High Voltage is dangerous? a. True b. False
___ ___
4.
5.
Low Voltage has the potential to provide more power than high voltage? a. True ___ b. False ___
6.
What do the letters emf and pd stand for? ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________
7.
What are the units called volts really measuring? ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________
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Electrical Systems, Manutronics Section 5, Electrical Current In the section 3, Voltage you looked at the voltage that pushes electricity into a piece of equipment. Now you will move on to what it is that is actually used by a machine in order to make it work. This is the electric current. Current flow in an electrical circuit is equivalent to the 'flow' in a water system. In a domestic heating system, for example, when the pump is switched on and applies pressure to the system water flows around the system (or circuit) and through the radiators. Similarly, if there is a potential difference (voltage) across a closed electrical circuit, current will flow. However, we must be careful in pursuing this analogy with water. Water will flow as long as the level of supply is higher than the level where it's going. An electric current however, unlike water, will only flow if it can return to its source. The route it takes is known as a circuit. As soon as you break the circuit - by switching it off, for instance, or by cutting a wire, then the current flow stops. An electrician does a great deal of testing. As well as testing for voltage, he sometimes wants to know whether current is flowing in a circuit, and if so, how much. How to measure current Current is measured in Amperes, named after a French physicist from the 19th century. Amperes are usually known simply as amps, but you'll find that you sometimes need smaller units for measuring current. To an electronics engineer, 1 amp may be quite a large current. A small pocket calculator, for example, might take only 0.0000933 A. As we stated in chapter 2, units, we never write more that two decimal places without using engineering notation. You will find that microamps (symbol - µA) are used to measure very small amounts of current. One microamp is equal to one millionth of an amp, so that calculator would be rated at 93.3µA. (To get µA from Amps move the decimal place six places to the right.) Another unit of measurement for very small currents is the milliamp (mA), which is equivalent to one thousandth of an amp. This is the level of current you'll find in the signals going around inside televisions and radios. The 93.3 µA calculator, in fact, would probably be spoken of as 0.093 mA. (If you haven't had occasion to convert units like this and need some help, have a word with your lecturer before you go much further.) An electrical engineer working with heavy equipment might need to do calculations in thousands of amps. An eight carriage 'third rail' electric train, for example, takes about 2000 A. One thousand amps is equal to one kiloamp, (kA), so this current could also be written as 2 kA.
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Electrical Systems, Manutronics
Around the home you'll find that a television takes just under 0.5 A, but an electric kettle takes perhaps 10 A. Both of course operating at 240V, 50Hz ac. You may be wondering why it is important to know how much current a piece of equipment is taking. One answer is that voltage and current combine to tell us how much power a component is taking (Power = Voltage X Current). Don't worry for the moment how voltage and current combine - you'll be learning more about this later. For now it is enough to know that if you can measure the voltage across a component and the current flowing through it, you can tell how much power is being used. Another reason is current flow generates heat and so equipment is designed to dissipate the heat from a safe (rated) level of current. If the rated level of current is exceeded due to a fault or the application of a larger voltage than intended then the equipment will overheat and damage may result. That's why we put fuses in circuits, to protect against overcurrent. What is an electric current? We know from section 3 that a power supply produces a force in a certain direction. If a force is pushing in a particular direction, there must be something that it is pushing and so there must be something in a working electric circuit that is moving. In fact, early studies of electrical effects did show that there was something moving in the connecting wires, although nobody could explain exactly what it was. We know now what actually was moving were tiny particles of matter called electrons. Every material no matter if it is wood, metal, plastic Glass or ceramic is composed of millions atoms. The atom has a center known as a Nucleus which is composed of +Ve protons and neutrons. Each Atom includes a number of electrons (negatively charged -) and they are far too small to be seen, even by the most powerful microscope. Electrons whizz around the centre of their atoms, like planets orbiting round the sun.
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Electron Negative Charge
+
-
Nucleus of +Ve protons and neurons Orbit of electron
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Electrical Systems, Manutronics At this point it is worth noting that each atom is held to its adjacent one by attraction forces, see the diagram below. Perhaps in some cases the atom shares electrons with its neighbour. In solids such as wood, glass and copper the bonds between then is very strong. If the bond can stretch then the material is malleable (mouldable) such as copper or aluminium. At room temperature copper and aluminium can be hammered into patterns and extruded into cables. Glass on the other hand at room temperature would shatter and thus is a brittle bond between atoms. Liquids have a much weaker bond between allowing them to flow and gases have one of the weakest bond attractions. This however has not anything to do with the conduction or insulation of materials.
-
- +
+ -
-
-
-
-
-
- +
+ -
-
-
-
- +
+
-
-
-
-
-
-
-
-
- +
+
-
-
-
-
-
Attraction forces Mainly the Electron Attraction Force as shown in the diagram below dominates the conduction or insulation of a material. If this attraction force is very high then the material will not conduct electrically. The flow of electricity requires that electrons move but if they are bonded too tightly to the nucleus of the atom then the electron cannot move. In most materials the electrons are so tightly bound to Electron Negative Charge
Electron Attraction Force
+ -
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Nucleus of +Ve protons and neurons Orbit of electron
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-
Electrical Systems, Manutronics the atoms that they cannot move away to form a current. These materials are called insulators or non-conductors. In some materials, however, called conductors, the electrons are not so tightly linked to their atoms and they can be dislodged from their orbits by the force from an electric power supply. In nature there are two types of charges. One is Positive charge and the other is Negative charge. These charges are governed by two rules, which state that like charges repel one another and unlike charges attract.
Unlike charges Attract
Like charges Repel +
+
-
-
-
+
When an electric circuit is connected up to a power supply - some of the electrons, which are said to carry a negative electric charge, are free to move and they then flow towards the positive terminal of the power supply. The electrons are so small they can travel through the tiny gaps between different atoms. In the conduction of electricity a positive charge is introduced to the circuit as shown below. The nearest atom to the positive supply releases an atom and suddenly changes from being neutral to being positive. What happens then?
Electon moves
-
- +
+ -
-
-
-
-
-
- +
+ -
-
-
-
-
+ -
The electron in the neighbouring atom is then attracted to the new positive charge and then moves. The neighbour now becomes positive and the process repeats itself. You are now probably thinking about the direction of the flow. We know now that electrons flow from -Ve to +Ve terminals or points. However, in
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Electrical Systems, Manutronics the 19th century when current flow was first discovered scientists didn't know anything about electrons (we still can’t see them). So, they assumed that current flowed from +Ve to -Ve. That convention remains today. When engineers talk about current flow (mostly in electrical systems) they mean from +Ve and -Ve. When they talk about electron flow (mostly in electronics) they mean from -Ve to positive. The difference is just an historical accident. Recapping in brief: x An electric current is a flow of electrons x A conductor is a material in which electrons can move freely x An insulator is a material in which electrons are tightly bound to their parent atoms Understanding how much current a piece of equipment takes is very important from the point of view of safety. This is because as current flows along a wire or through a component, it generates heat. The more current that flows, the greater the number of electrons are passing along the circuit. The heat produced is rather like the heat produced by friction. It is as if the electrons are all jostling with one another and rubbing against the atoms as they go. When too much current is taken through wiring, the heat produced can be enough to melt the plastic insulating the cable and, possibly, start a fire in the surroundings. Since it is the number of electrons that flow around a circuit that is responsible for what's called the electric current, you may be wondering how many electrons are needed to flow to make one amp of current. Because electrons are so small it takes a huge number of them to produce any noticeable effect. In fact, for every amp of current there are 6.26 x 108 electrons passing a point every second. With numbers that big, no wonder we use a measurement like amps, rather than electrons per second!
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Electrical Systems, Manutronics Conductors and Insulators There are hundreds of conducting and insulating materials. Some examples you may find in electric circuits are listed below. Don't attempt to memorise the list. Keep it handy for quick reference and refer to it when you need the information. Only a small sample of materials is listed here. There are actually hundreds of different materials in use, far too many to include here. All metals are conductors and most (but not all) non-metallic solids are insulators. A notable exception is carbon, which is a non-metal and yet is a good conductor. COPPER The most widely used material for electrical wires BRASS Used for rigid parts in switches, contacts and terminal blocks NICHROME A stainless steel alloy used for electric fire elements CARBON Not a metal, but nevertheless a conductor; used to make resistors WATER conducts electricity and that's why it's so important to keep it away from any electrical equipment. This also explains why there are special regulations controlling what kind of switches you're allowed to in the bathroom, and why cars have trouble starting on damp mornings. GLASS and CERAMIC Rigid and cover wires and cables. very strong in compression; used for overhead line insulators PLASTICS are some of the most widely used insulators. PVC is flexible and fairly tough so it's used to cover wires and cables. Resin bonded glass fibre is used to make printed circuit boards like the one used in laboratory. Polythene is probably the best insulator of all but in its pure form is rather soft. It can be used to cover cables.
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Electrical Systems, Manutronics Test Yourself Try tick the correct answers from the lists given below. From question 6 onwards fill in your answer. 1.
The path around which electricity flows is called a ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________
2.
The plastic which covers the copper wires used in connecting up circuits is called ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________
3.
Electrons in a __________________ will be dislodged from their atoms when a voltage is applied.
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Electrical Systems, Manutronics Section 6, Resistance, Controlling Current If an electric current is like a stream of water then the path it flows along - the circuit - is like the river bed. Sometimes there are obstacles in the way that reduce the flow of water. It's just the same with electric circuits. At first glance, electrical resistance seems like a problem. It reduces the amount of current that can get through. But it can also be used to our advantage. You can use it to control a current. A river bed is never absolutely smooth. Even one without big stones or boulders resists while it guides the flow of water. In the same way, everything in an electric circuit resists the flow of current. Even the wires the current goes along provide some resistance, however small. The better the conductor, the lower the resistance. The resistance in insulators is very high: virtually no current flows through a good insulator. Resistance in electric circuits The connecting wires of an electric circuit should have a very low resistance. The current can get easily to wherever it's supposed to be going. This is why the wires are usually made of copper. Copper wire has a very low resistance and is a good conductor. In some circuits you might want some resistance. You may want to reduce the electric current. If so, you could reduce the current flow by putting some resistance in its path. Suppose you want to use a lamp in a piece of equipment that uses a 240V power supply. If the 240V is too high for the lamp it will push too much current through it and the lamp may burn out. The answer could be to put in some kind of resistance in the circuit to limit the current through the lamp.
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Electrical Systems, Manutronics Resistors There are specially made components called resistors that are designed to do nothing but resist the current flow. There are resistors in radios, TV’s, washing machines, cars and computers. They're also in a wide range of industrial control equipment. They are used in lots of different machines where you want to control the flow of the current. In a radio, for example, when you turn the volume knob, you are varying a resistance in the audio signal circuit. Without resistance there you couldn't control the volume to an "easy" listening level. Modern resistors are normally made of metal oxide which has better life, stability and temperature characteristics than the older type made of carbon. If we want to carry heavier currents these smaller resistors are not suitable as they would burn out so we have to use wire wound types. These also have very good stability but need space for their size and also for cooling. Heat and light However, when an electric current meets with resistance in a circuit, it generates heat just by working its way through the conductor. That's what heats the element of an electric fire. There's so much current the wire actually gets red hot. Filament lamps use a thin tungsten wire or filament. When a current is forced through this filament by a voltage the heat produced is enough to make the filament white hot. Such heat produced is useful if you need it. But it can be a problem if you don't. You can't stop resistance causing heat any more than you can stop a car's brakes from getting hot. Overheating is a major cause of electrical breakdowns, and in bad cases can be a fire risk. Overheating happens when components are faulty or circuits are badly designed. High powered electronic equipment is sometimes fitted with fans to keep it cool. Whenever you install any electrical equipment make sure that all its ventilation holes are clear so that any excess heat can escape.
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Electrical Systems, Manutronics Temperature coefficient of resistance The resistance of all pure metals increases with increase of temperature, whereas the resistance of carbon, electrolytes and insulating materials decreases with increase of temperature. Certain alloys, such as manganin show practically no change of resistance for a considerable variation of temperature. For a moderate range of temperature, such as 100°C, the change of resistance is usually proportional to the change of temperature; and the ratio of the change of resistance per degree change of temperature to the resistance at some definite temperature, adopted as standard, is termed the temperature coefficient of resistance and is represented by the Greek letter 9. The variation of resistance of copper for a range over which copper conductors are usually operated is represented by the graph below. If this graph is extended backwards, the point of intersection with the horizontal axis is found to be -234.5°C. Hence, for a copper conductor having a resistance of 1 1 at O°C, the change of resistance for 1°C change of temperature is (1 /234.5) 1, namely 0.004264 1,
D0
0.004264[: / o C ] 1:
0.004264 / o C
The maximum power which can be dissipated as heat in an electrical circuit is limited by the maximum permissible temperature, and the latter depends upon the nature of the insulating material employed. Materials such as paper and cotton become brittle if their temperature is allowed to exceed about 100°C; whereas materials such as mica and glass can withstand a much higher temperature without any injurious effect on their insulating and mechanical properties. When an electrical device is loaded (e.g. when supplying electrical power in the case of a generator, mechanical power in the case of a motor or acting as an amplifier in the case of a transistor), the temperature rise of the device is largely due to the I2R loss in the conductors; and the greater the load, the greater is the loss and therefore the higher the temperature rise. The full load or rated output of the device is the maximum output obtainable under specified conditions, e.g. for a specified temperature rise after the device has been loaded continuously for a period of minutes or hours.
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Electrical Systems, Manutronics Ohm sweet ohm! If the resistance of a circuit is high then you need a high voltage if you want to push a large current around the circuit. If the voltage drops but the resistance remains the same then less current will flow. So voltage, current and resistance in a circuit are all related to each other. The man who discovered the relationship between these ideas was Georg Ohm, a German physicist working in the 1820s. It perhaps isn't surprising then, that the unit for measuring resistance, the ohm, is named after him. This relationship is now known as Ohm's Law, and we will be studying it in Section 7. For the moment, all you need to know is that the greater the resistance of a circuit, the more it reduces the current flow. You will also have to know how to measure resistance with your multimeter. This is an important skill which is often needed. There is only one way to learn it, and that's by doing it. Before you do that, though, let's look at the symbols used for ohms and one way to check the resistance of a resistor. Symbols and Units of resistance The resistance of a component or a part of a circuit is measured in ohms. The scientific symbol for this is 1 (which is the Greek letter omega - our letter O). As well as measuring resistance in ohms, it is often useful to work in higher values such as kilohms (k1: one thousand ohms) and megohms (M1: one million ohms).
High Kilohms (k1), 1k1 = 10001, i.e. 1E+031 Ohm (1) Milliohms (m1), 1000m1 = 11 , i.e. 1E-031 Low
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Electrical Systems, Manutronics Here are the traditional scientific symbols: 1 k1 = 1 M1 =
1,000 1 1,000 k1
=
1,000,000 1
Resistance Value Determination Modern System for Resistance Quantifying Most electronic service technicians use a different, more modern, system of symbols for resistance. It also uses megohms, kilohms and ohms, but there is no need for the Creek omega. Here it is: 11 = 1R0 1k1 = 1K0 1M1 = 1M0 In fact, when technicians talk about resistance values you'll hear them talk of ohms, 'K's and megs. In the latest MkS system, if there are decimal points in the number then these are replaced with the appropriate letters. The letters do not then come at the end of the numbers. For example: 1.51 is written 1R5 2.7k1 is written 2K7 4.7M1 is written 4M7. Numbering by colours Although all parts of an electrical circuit have a certain amount of resistance to current, one of the times when you're most likely to be thinking about the resistance of something is when you're working with resistors. If the resistor is broken it is not possible for an instrument to measure its value so we need some way of identifying what resistance the component should be. Resistors are the small, often tiny, components which help control a circuit by resisting current. As they are so small, it can be difficult to print their resistance value on them although technology has changed to allow this take place. However colour coding is still widely used and covered here for completion sake.
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Electrical Systems, Manutronics Each resistor has four or five coloured bands around it. You read the code from left to right. Make sure the band with the smallest gap between it and the next band is on the left. If a resistor has four bands, the first band on the left-hand side is one of 10 possible colours. Look at the table below to see what these stand for. In the second band you'll also see any one of ten possible colours. Together, the first two bands will give you a two-figure number. If the first, say, is yellow, that means 4. If the second is violet, that means 7. So a resistor starting yellow violet means you start with the number 47.
Third
First
Fourth
Second
Resistor
The third band is known as the multiplier. You use the number from the third band to multiply the figure you worked out from the first two bands. In the example here, suppose the third band was red. That would mean multiplying 47 by 100 so the value of the resistor would be 47OO1, or 4K7. Resistors aren't usually manufactured so that their resistance value is known exactly. Often it doesn't matter if they're a bit out one way or the other. This is why there is a fourth band of colours. It is the tolerance band. It tells you how accurate the resistance value is. For example, suppose our 4K7 resistor had red as its fourth band. It could be 2% out either way. Thus its true value would be guaranteed to be somewhere between 4K606 and 4K794. If the resistor has five bands, the first three indicate a value which is multiplied by the fourth band's value. The fifth band indicates the tolerance.
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Electrical Systems, Manutronics Why you need tolerance Designers are careful to specify tolerance values for their circuits. They do it deliberately - to make sure the circuit works efficiently and safely. If a resistor is replaced by one having a worse (higher %) tolerance, then there is no guarantee that the circuit will work properly. So you need to know the design tolerance when selecting resistors to repair damaged circuits. Of course, there is no need to actually calculate the limits. All you have to do is match the colour bands on the new resistor with those on the old one. Still, you might need to check the value of a resistor to see if it is within its correct tolerance limits. Resistors can change value as they get older. Connection of resistors (Series or in Parallel) Resistors can be connected in two ways. Either in series or parallel or combinations of both. When resistors are connected together, we can calculate the equivalent resistance which we can used in ohms law. Equivalent resistance is like replacing a network of resistors with one resistor of a similar value. Series Events that happen one after another are said to happen in series. If resistors are linked as shown in the diagram below they are said to be connected in series. This means that electricity need to enter the first one then exit it into the second one etc.
R1
R2
R3
To calculate the total resistance we simply add all the resistance values up. Total Resistance = R1 R 2 R3 ....Rn
where Rn is the last resistor in series.
Taking a few quick examples, Example 1 R1 = 2001, R2=1001,
R3=1501
The total would be =4501
Example 2 R1 = 200k1, R2=9001,
R3=20k1
The total would be =220,9001 (Don’t forget about the k in the value)
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Electrical Systems, Manutronics Let's carry on to look at applications of series resistors. Circuit diagram symbols for this course are shown in section 2. Applications Suppose you needed a 3K0 resistor and the storeman said 'I've only got 1K0 and 2K0 in stock'. You could say 'OK, I'll take one of each', knowing that 1K0 plus 2K0 makes 3K0. This is one sort of situation where knowing about resistors in series might be helpful. You don't always have to get an exact sum. Remember that resistors have tolerances; if you are working to 10% tolerance you have some margin to play with. Voltage division There is one use for resistors in series which you will often meet. Two resistors in series can be used to fix the potential of a point in a circuit at a level which is lower than that of the supply voltage. This is a R1 V1 technique that is widely used in transistor circuits in radios, TV’s and industrial E control machinery. A pair of resistors connected in this way is often called a R2 V2 voltage divider or a potential divider. The diagram shows it as a theoretical circuit. We know that current, voltage and resistance have a proportional relationship. You will not be at all surprised when I say that this can be extended to embrace series resistors. The supply voltage E is divided up between R1 and R2 in proportion to their values. The resistor with the highest ohms value gets the biggest share of the volts. The resistor with the lowest ohms value has the smaller (remaining) voltage across it. We can write this as a formula which you can use to work out how voltage is divided. Suppose the supply voltage is E and the potential difference across the resistor R1 is V1 then: R1 V1 E u R1 R2 In other words, to work out the voltage across a resistor if it is one of two resistors connected in series you should: A.
Multiply the supply voltage by the resistance in ohms of that resistor
B.
Add the resistance values of the two resistors
C.
Divide the result of A. by the result of B.
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Electrical Systems, Manutronics An alternative way of looking at the problem is this: If we can calculate the total E resistance and we know the Voltage (E) then we divide where RT is the Total RT resistance. This will give us the current. Knowing the current we can then calculate the voltage by multiplying each resistance by the current. Voltage Division Example In the example below suppose we were trying to find the voltage across the resistor labelled R1. The steps you should follow would be.
9V R1
1R0
3V
9V
6V R2
2R0
6V 0V
A.
The supply voltage Vs = 9V
B.
Total resistance RT = R1 + R2 = 1 + 2 = 31
C.
Current
D.
Voltage across R1
I
Vs RT
9 3
3amps
= VR1 = I R1 = 3x1=3 volts
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Electrical Systems, Manutronics Voltage Dividers, What might go wrong? Let's now think about the kind of faults which could develop in this circuit. Suppose you measured the voltage across R2 and the value actually was 9 volts. What might be wrong? Before reading on, you may want to think about this for a short while and look at the circuit diagram on the previous page again. If the whole of the supply voltage is across R2 it means that the potential divider isn't working the way it should be. Perhaps R1 has a 'short' so that it is zero ohms, or perhaps R2 is very much higher in value than R1. Let's think about some other kinds of faults. Suppose instead that the reading across R2 was (a) 0 V
(b) 7.65V
(c) 4.5 V
(d) 1.5 V.
What might be wrong in each case? Some suggestions are: (a)
A break in the circuit of R1, or a very high value for R1, or a 'short' across R2 or even not switched on!
(b)
7.65 V, that should be the voltage across R1 so it looks like the resistors are the wrong way round that is, 12K for R1 and 68K for R2
(c)
Could be a wrong value resistor here - the two resistors may be the same value. You would need to check.
(d)
Probably nothing wrong! The value could be this much out due to resistor tolerances.
Note that there can sometimes be more than one possible cause for a wrong value. In a situation like case (a) for example, the voltage check should be followed by a visual check for breaks in the circuit. If you could see nothing wrong, you would then probably check the values of the resistors. Remember, you will need to switch off the supply to the circuit when measuring resistance. You may also have to disconnect one end of the resistor you are testing from the circuit. This is because other resistors and components are connected to that resistor when in circuit and will affect the reading unless the resistor under test is disconnected from the circuit.
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Electrical Systems, Manutronics Fault Finding Process for Voltage Dividers: (a)
Think carefully about what might be wrong.
(b)
You need to be able to look at the circuit at R1, R2 - work out the approximate voltages you would expect. If when you test the circuit you don't obtain these approximate values, then you have to stop and think WHY.
(c)
Use Ohm's law where necessary to help you confirm any voltage checks you make.
(d)
Check the circuit visually.
(e)
Check the component values.
Variable resistors Using resistors as voltage dividers is a useful way of changing the voltage across part of a circuit. Sometimes, however, you need to be able to vary the voltage continuously rather than just drop the supply voltage from one level to another. This is where a variable resistor comes in handy. Because they vary the potential difference across part of a circuit, some variable resistors are also known as potentiometers, or 'pots'. It's a pot that varies the volume on your radio. It's a pot that varies the brightness on your TV. They are very useful devices. Here's a circuit diagram of a pot.
Compare it with the diagram of a voltage divider. You'll see it's similar in many ways. But a pot uses only one resistor, the value of which is divided by a movable contact. The more of the resistor you use, the greater the voltage drop. The less of the resistor you use, the smaller the voltage drop.
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Electrical Systems, Manutronics Parallel Events that happen at the same time as said to be in parallel. If resistors are where all the wires to the left are connected together and all the wires to the right are connected together then the circuit is said to be connected up in parallel. This means that a proportion of the electricity goes through each resistor at the same time. The amount of electricity passing each resistor is related to its resistance. Electricity takes the path of least resistance.
R1
R2
R3
Parallel Why parallel? There are many good reasons for using parallel circuits. One of the more important is the case of power supply. When electricity is supplied from the mains it is usual for all equipment to be connected in parallel to the supply. This is essential if all the items connected to the supply are to receive the full supply voltage. Of course, this only works if the mains can stand up under the load. Every item connected is a 'load' on the supply, and as the load increases the supply voltage tends to drop a little (although supplies can be "regulated" if necessary). One advantage of wiring pieces of equipment in parallel is that each can be switched off independently from all the rest. What about the ohms? When you had several resistors in series you could find the total ohms by adding up their values. There is a way to find the total value of a number of resistors connected in parallel, but it's not quite so easy as for series circuits. You can see that in a circuit where the Components are in parallel, the total current drawn will be the sum of all the currents in each branch. The more branches the more current. The combined resistance must be lower than the resistance in any one of the branches.
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Electrical Systems, Manutronics The formula for calculating the combined value of any number of resistors in parallel is: 1 RT
1 1 1 1 1 ...... R1 R2 R3 R4 Rn
One of the biggest mistakes in calculating the resistance in parallel is failure to invert the answer. The answer above will be the inverse of RT not RT itself Example 1
3R0 IT
I1
I2
3R0
IT
I3 3R0 Parallel 1 RT
1 1 1 3: 3: 3:
1 RT
0.33 0.33 0.33 0.99 1
?
1 RT
1 RT
1:
You will also note that the current total is equal to the sum of the currents in each branch. IT I1 I 2 I 3 If you connected a number of equal resistors in parallel then the combined value can be found by dividing the individual resistance values by the number of resistors. For example, if you connect three 3K3 resistors in parallel the combined value is:
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Electrical Systems, Manutronics 3K 3 1K1 3 And three 1K5 resistors: 1K 5 3
500 R
And two 4K8 resistors: 4K 8 2
2K 4
Look at the worked example below. Example 2
2R0 IT
I1
I2
4R0
IT
I3 6R0 Parallel 1 RT
1 1 1 2: 4: 6:
1 RT
0.5 0.25 0.16 0.91:
1 0.91 RT 1.1: RT You can see that when a high value resistor is connected in parallel with one of a lower value the result will be hardly any different from the lower value (although it is always less than the lower value). Why not try some values of your own? ?
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Electrical Systems, Manutronics Quick Revision and reminder Series Connection
R1
R2
R3
The total resistance will always be larger than the largest individual resistance. Parallel Connection
R1
R2
R3
Parallel The total resistance will always be less than the lowest individual resistance.
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Electrical Systems, Manutronics Section 7, Ohms Law By now you'll probably have noticed something about volts, amps and ohms. They are all related to each other. In fact they are all connected in such a way that if you known any two you can work out what the other one must be.
V I
R
It was Georg Ohm, a German scientist in the nineteenth century, who discovered this relationship. He set it out in a statement which is now known as Ohm's Law. It's a very useful law that will help you whenever you're working with electricity. The actual equation for ohms law is written but can be simplified as shown above using the triangle.
V=I x R V stands for Voltage in Volts. I stands for Current in Amps. R stands for Resistance in Ohms
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Electrical Systems, Manutronics For instance if you have values for V and R and want to calculate out the Current the you simply to the following
V
V = ---R
I
R
If you wish to calculate R and have values for V and I then you do the following.
V I
V = ---I
R
Lastly if you wish to calculate V and have values for I and R then you do the following: =IxR
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Electrical Systems, Manutronics Just to get you started use the following table and fill in the blanks. You are given two components in the ohms law equation. Calculate the third yourself. Number 1. 2. 3. 4. 5. 6. 7. 8. 9.
Volts (V) 220 400 110 24
Resistance(1) 2.2K
Current (I) 7A
32 48 33K 900 400
270K 1M
0.7 3 0.23mA 88 0.17mA
Where would you use Ohm's Law? In practice you can do a great deal of fault finding and other maintenance work without using an Ohm's Law calculation. In many situations you can use Ohm's Law to come to a rough idea of whether your meter reading of current, voltage or resistance is correct or not. If it's approximately correct you'll be able to go on and do another measurement on another part of the circuit. If you suspect circuit errors you'll be able to check again and inspect that part of the circuit in greater depth. Circuit designers use Ohm's Law quite a lot, but for most other work it is enough to remember that if you double the volts then you will double the current. If you double the ohms then you half the current. There is one situation where a knowledge of how to calculate using Ohm's Law is very valuable: when you need to measure the current flow in a circuit where the components are all connected to a printed circuit wiring board (PCB). We know that a circuit has to be broken in order to measure current. To do this on a PCB you would have to remove a component or cut one of the tracks (the fine copper connecting strips). Neither of these would be easy, and cutting a track will damage the board. In fact, on a modern multilayer PCB it could prove impossible to do, because many of the tracks are sandwiched between the layers of the board. Similarly in electrical equipment and circuits it is often not practicable to break into circuits to measure current. The trick is to locate a resistor through which the current flows, and measure the voltage across it.
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Electrical Systems, Manutronics The current can then be calculated using the voltage measured, the value of the resistor (from its colour code) and putting these into ohm's law to calculate I
V=I x R Example of indirect current measurement Suppose a reading was taken of the voltage across a resistor in a circuit and the result was 1.26V. The resistor was colour coded red, violet, red, with gold tolerance band. What was the current? The answer is worked out by Ohm's Law, but first note that the nominal resistor value is 27001, i.e. 2K7 or 2.7k1. Using Ohm's Law:
V
V = ---R
I I
1.26 2700
R
0.000466 A
Or I 0.4666mA 0.47 mA The calculator I used to do the division gave me a long string of 6s; but as the resistor has a tolerance of 5% the answer can really be found only to the same accuracy (±5%). We might as well correct the result to two figures; this gives I = 0.47 mA. In fact, for many practical situations an answer of 'it's about half a milliamp' would be close enough. You could probably get this close to the answer without even needing a calculator! J.McGrory, DIT Bolton Street. Version 2.00
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Revision Ohm's Law states that the current in a circuit is directly proportional to the applied voltage. This means, for example, that by doubling the voltage we produce twice the current; halving the voltage halves the current. The current I (amperes), the voltage V (volts) and the resistance R (ohms) are related as follows:
V
V = ---R
I
R
V I
V = ---I
R =IxR
V I
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Electrical Systems, Manutronics Section 8, Electrical Power Up until now you've found out a lot about electricity without talking about what it actually does. The work electricity is used to do whether it's driving motors, turning on lamps or cooking the dinner - has to have a way of being measured or rated. You've seen electricity in action, so now's the time to consider the work it does. Electricity is a form of energy or power. The words 'electrical power' are often used to mean the electricity supply. In fact, in its strict sense, electrical power means the rate at which electricity does its work. As it is consumed, electrical energy is converted into other kinds of energy, such as heat light or movement. Electrical power is measured in watts. If you use a kettle with a high wattage it will boil a set amount of water more quickly than a low wattage kettle. Something you probably also know from around the home is that a 100 watt bulb is brighter than a 60 watt bulb i.e. it dissipates more energy. As an introduction to the units used to measure electrical power, we will take a step back in time. In the eighteenth century just about everybody relied upon horses for the heavy jobs. They were used for pulling carts, hauling farm machinery, and even as personal transport. There were a few steam engines, but they were very inefficient and slow, and used mainly for pumping water. Then along came James Watt, who invented a completely new type of steam engine. It was smaller, lighter, more efficient and more powerful. It could drive heavy industrial machines, and more importantly - even vehicles. Brilliant though it was, James Watt had to advertise in order to sell his machines. He hit upon the idea of comparing his engines' power with the old familiar horse. The term 'horsepower' was born. One horsepower was defined by Watt as the rate at which one horse can work.
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Electrical Systems, Manutronics All work is measured in engineering terms as a force exerted (for instance by the horse straining against its load) multiplied by the distance moved (by the cart or wagon behind it). Power is the rate of doing work. In electrical terms, power is the rate at which an electrical device converts the electrical energy to some other form of energy, such as heat or mechanical movement. Electric Power (dc circuits) You can work out the power in dc circuits by simply multiplying the total circuit current squared by the circuit resistance:
P = I2 x R This equation can also be remembered using the power triangle which is similar to the one used for ohms law.
P I2
R
P stands for Power I stands for Current in Amps R stands for resistance in Ohms Another way if you wish to know the power in one component or element of the circuit, the same formula applies: you multiply the current passing through that component by the voltage across it. This formula is accurate when applied to dc circuits; it can only be used as a rough calculation in ac circuits.
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Electrical Systems, Manutronics What's Watt! It's a long way from horses, steam engines and James Watt to electrical power. Well, so you might think. But Mr Watt gave his name to the unit of electrical power - the watt (symbol W). One watt is 1/746 of one horsepower! In other words, 746 W = 1 HP. That doesn't represent much power. As we've seen before with other units, we add prefix letters. So we get two units which may be familiar to you. High Kilowatt (kw), 1kw = 1000watt, i.e. 1E+03watt Watt (w) Milliwatt (mw), 1000mw = 1watt , i.e. 1E-03watt microwatt (w), 1000,000 w = 1watt, , i.e. 1E-06watt Low
The first is the kilowatt. The prefix kilo means one thousand, so a kilowatt is 1000 watts. The second is megawatt. Mega means a million, so a megawatt is one million watts, or a thousand kilowatts. This is the scale used to describe the amount of power generated at a power station. The kilowatt is doubtless more familiar. You've probably used an electric bar fire. Each bar element is usually 1 kW. That means it converts electrical energy to heat at a rate of 1000 W. A typical kettle is 2.4 kW, while a pocket calculator may rate at only 0.00028 W. Power and current consumption You may wonder why it is useful to know how to work out power consumption. Well, suppose you have a 13 A socket and you would like to know how many 1 kW bars of an electric fire you can safely plug into the socket. A 13 A socket is one through which you can only draw 13 A safely. Though the formula watts = volts x amps is only accurate strictly for dc supplies, you can use it as a safe enough approximation of the examples of domestic equipment given here. The second example of a situation in which it's useful to know about power ratings and how they relate to current and voltage concerns a TV. Suppose you need to replace a fuse in your tv's plug but you don't known what fuse to use. You look on the back of the set and you see that it is rated at 120 W. What size fuse? These and many similar questions can be quickly answered now that you know the relationship between watts, volts and amps. I have already confirmed that the formula is something like Ohm's Law, only now we have:
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P I2
R
instead of
V I
R
However working out the different components is the same. If you have any two items you can workout what the third one can be.
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Electrical Systems, Manutronics If you wish to work out what the Power is you simply cover the power component and the calculation needed is obvious.
I2 = ---R
P I2
R
If you wish to work out the resistance you simply cover the resistance components and the calculation appears.
P I2
P = ---I2
R
Lastly if you wish to calculate the Current Squared you simply cover over the I2 component and the calculation appears. If you only need the Current value then you square route the answer. It’s that easy.
P
P = ---R
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Electrical Systems, Manutronics Combining Ohms Law and the Power Triangle. By combining Ohms Law and the Power Triangle we have one of the most powerful calculations in Electrical Engineering at our disposal. By a simple manipulation we can develop a calculation which has Voltage Power and Current. Thus we don’t need to calculate the resistance value at all. Or if we know the resistance we can work out the items without knowing the current. The Power Triangle
P = I2 x R Ohms Law
V=I x R Firstly we will work out the calculation for Power using Voltage and Resistance. If we take Ohms Law we can work out a calculation which gives us a formula with V and R to give us a current.
V
V = ---R
I
R
We can now replace I2 in the power triangle with the above calculation with V and R.
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Electrical Systems, Manutronics The calculation can be reduced to
V2 P = --R Secondly we will work out the calculation for Power using Voltage and Current. It is very similar to the Resistance one so be careful. If we take Ohms Law we can work out a calculation which gives us a formula with V and I to give us resistance.
V I
V = ---I
R
We can now replace R in the power triangle with the above calculation with V and I.
P
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= I2 x
V -I
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Electrical Systems, Manutronics Cleaning up the above calculation we get the following
P =I xV These two calculations can be worked out from Ohms Law and the Power Triangle and are very powerful tools for us to have. The power triangle As before, you cover up whatever value you need to calculate and the triangle leaves you with the formula you need. Now let's go through the example of the fire and the TV. To know the current that can be safely taken through the fuse, cover up the current symbol, I. The formula you need is.
P =I xV Changing this for I we get:
P -V
I=
and using the triangle shows that you've got to divide P by V. The mains supply is usually rated at 240 V, so for a one 1 kW bar fire: I
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1000 240
4.17 A
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Electrical Systems, Manutronics How many times will 4.17 go into 13? The answer is three at the most since you can't have only part of a bar. So now you know why you can only run a three bar fire from a 13 A plug. What about the second, TV example? You can do that in much the same way: I
120 240
0.5 A
This is a nice exact answer for once, but let's think a moment before we rush off to try to find a 0.5 A fuse. Fuses are in the circuit to provide protection. If the current drawn from the supply is higher than it should be, it means something is wrong. Before the current gets dangerously high, the fuse should 'blow' and cut off the supply. However, it isn't necessary to have a fuse in the wall plug which is matched exactly to the current drawn by the device. If we did put in a 0.5 A fuse in our 120 watt TV set, it would almost certainly 'blow' in a short time, even if there was nothing wrong with the set. The reason is simply that the fuse would be running too close to its rated value. Also, it isn't either practical or necessary to have a large range of fuses for 13 A wall plugs: the values usually available are 2 A, 3 A, 5 A and 13 A. So, for our TV set (and most TV sets in fact) we would fit a 3 A fuse. Use the guide below to help you select the right fuse; it should be made to the appropriate European Standard. A guide to fuses for 240 V 1 13 A plugs: 2A: up to 450 watts 3A: up to 700 watts 5A: up to 1000 watts 13A: up to 3000 watts Power rating Power ratings are used in several ways. The power rating of an electric motor tells you how much power it can (safely) develop. If you make the motor do more work than this it will probably try its best, but will overheat (just as you would if you over-exerted yourself), and may burn out. On equipment such as TV sets there is a rating which tells you how much power it takes from the supply. Loudspeakers are rated by the maximum amount of power they can take from an audio amplifier.
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Electrical Systems, Manutronics Resistor ratings The power rating of a resistor refers to the amount of heat it can safely dissipate (when mounted in the normal way). This is also known as a resistor's power dissipation. If a resistor is required to dissipate more power that its rating then it will overheat and may burn out. Its life can be shortened and its value may change if it is run above its rating for any length of time. Many of the resistors used in electronics are very small, rated at 0.3 W or even less. There are some places where a higher rating is used. You should NEVER replace a resistor by one of a lower rating. Using a smaller rating leads to overheating, a short life and, in the worst cases, could lead to fire. Calculating the power rating or a resistor You won't often need to calculate the power rating of a resistor. It usually happens only if you have to make a replacement in equipment for which there are no service charts, or if a particular resistor is always giving problems and is overheating.
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Electrical Systems, Manutronics Section 9, Network Theorems Introduction Not all circuits are as simple as the parallel circuit with a battery shown. Some circuits are very complex networks and would some method of methodically (Orderly or Systematically) investigating the circuit it may not be possible to fix or improve it. A network consists of a number of branches or circuit elements, considered as a unit, and is said to be passive if it contains no source of e.m.f. The equivalent resistance between any two terminals of a passive network is the ratio of the p.d. across the two terminals to the current flowing into (or out of) the network. When a network contains a source of e.m.f., it is said to be active. We shall now consider the principal theorems that have been developed for solving problems on electrical networks.
100:
400:
300:
+
-
Single and Double Subscripts The use of single subscripts is used to simply identify each component, say resistors or capacitors or current or voltage so we are able label them. For example I1, I2, I3, I4 or R1 or R2 etc. Double subscript notation however is used to avoid ambiguity in the direction of current, e.m.f or potential difference. In the diagram below S represents a d.c. source, the e.m.f. of which is acting from D towards A and is therefore designated EDA. The current in conductor AB flows from A to B and is designated IAB. For a simple circuit as shown it is obvious that: IAB
=
IBC
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=
ICD
=
IDA
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Electrical Systems, Manutronics Network theorems There are quite a few network theorems available to the designer but most are specific for a particular purpose and not useful as a generic solution model. The three network theorems I have chosen are Superposition theorem, Kirchhoff’s Laws (1860), Thevenin’s Theorem. Superposition theorem definition In a linear network containing more than one source of e.m.f., the resultant current in any branch is the algebraic sum of the currents that would be produced by each e.m.f., acting alone, all the other sources of e.m.f. being replaced meanwhile by their respective internal resistances. 100:
14k:
200: +
300:
600: +
Superposition theorem In a linear network containing more than one source of e.m.f., the resultant current in any branch is the algebraic sum of the currents that would be produced by each e.m.f., acting alone, all the other sources of e.m.f. being replaced meanwhile by their respective internal resistances. Let us consider the application of this theorem to the solution of the following problem. Example Question Battery A in the diagram below has an e.m.f. of 6.0V and an internal resistance of 2.01. The corresponding values for battery B are 4.0V and 3.01 respectively. The two batteries are connected in parallel across a 101 resistor R. Calculate the current in each branch of the network.
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Electrical Systems, Manutronics
Answer
Fig (a) represents the network with battery A only. Equivalent resistance of R and R2 in parallel is R x R2 ----------R + R2
=
I1
=
Hence
I2
=
and
I3
=
10 x 3 ---------10 + 3
=
6 --------------2 + 2.31
2.311
=
1.392 A
3 1.392 x ----------10 + 3
=
0.321A
1.392 – 0.321
=
1.071A
Fig (b) represents the network with battery B only. Equivalent resistance of R and R1 in parallel is R x R2 ----------R + R2
=
I4
=
2 x 10 ---------2 + 10 4 --------------3 + 1.667
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=
1.6671
=
0.856 A
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Hence
I5
=
2 0.856 x ----------2 + 10
=
0.143A
and
I3
=
0.856 – 0.143
=
0.713A
Superimposing the results for (a) on those for (b) we get the following: Resultant current for A = I1 – I6 = 1.392 – 0.713
=
0.679 A
Resultant current for B = I4 – I3 = 0.856 – 1.071
=
-0.215 A
i.e. the battery B is being charged at 0.215A Resultant current through R = I2 – I5 = 0.321 + 0.143 = 0.464A
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Electrical Systems, Manutronics Kirchhoff’s Laws (1860) First Law:
The total current flowing towards a node (a junction of two or more branches) is equal to the total current flowing away from that node, i.e. the algebraic sum of the current flowing towards a node is zero. Thus at node C in the diagram below.
I1 + I2 = I3
or
In general
. I=0
Where
I1 +I2 – I3 = 0
. represents the algebraic sum.
Second Law In a closed circuit, the algebraic sum of the products of the current and the resistance if each part of the circuit is equal to the resultant e.m.f. in the circuit. Thus for a closed circuit involving E1, E2, R1 and R2 in the above diagram we get. E1 - E2 = I1R1 – I2 R2 And for the mesh involving E2, R2 and R. E2 = I1R1 – I3 R In general
.E = .IR
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Electrical Systems, Manutronics Example 1 Using Kirchhoff’s Laws calculate the current in each branch of the network shown below.
Answer Applying Kirchhoff’s Laws to the circuit formed by A and R we have 6 = 2 I1 + 10 (I1 + I2) = 12 I1 + 10 I2
…….(a)
Similarly for the closed circuit formed by A and B 6 – 4 = 2 = 2 I1 – 3 I2
…….(b)
By multiplying (b) x 6 and subtracting from (a) we get: -6 = 28 I2 I2 = -0.215A Substituting for I2 in (b) we get: I1 = 1 – 1.5 x 0.2143 = 0.679A and
I3 = 0.678 – 0.214 = 0.464A
Which is the same as using the other example we have covered. Proving that it works.
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Electrical Systems, Manutronics Thevenin’s Theorem The current through a resistor R connected across any two points A and B of an active network (i.e. a network containing one or more sources of e.m.f.) is obtained by dividing the p.d. between A and B, with R disconnected, by (R + r), where r is the resistance of the network measured between points A and B with R disconnected and the sources of e.mf. replaced by their internal resistances. An alternative way of stating Thevenin's Theorem is as follows: An active network having two terminals A and B can be replaced by a constant-voltage source having an e.mf. E and an internal resistance r. The value of E is equal to the open circuit p.d. between A and B, and r is the resistance of the network measured between A and B with the load disconnected and the sources of e.mf. replaced by their internal resistances. Suppose A and B in fig.(a) to be the two terminals of a network consisting of resistors having resistances R2 and R3 and a battery having an e.m.f. E, and an internal resistance R,. It is required to determine the current through a load of resistance R connected across AB. With the load disconnected as in fig. b), Current through R3 = E1 / (R1 + R3) p.d across R3 = (E1 R3 ) / (R1 + R3) Since there is no current through R2 p.d across AB =
V
=
(E1 R3 ) / (R1 + R3)
Fig. (c) shows the network with the load disconnected and the battery replaced by its internal resistance R1. Resistance of network A and B = r = R2 + (R1R2 / (R1 + R2)) Thevenin's Theorem merely states that the active network enclosed by the dotted line in fig. (a) can be replaced by the very simple circuit enclosed by the dotted line in fig. (d) and consisting of a source having an e.m.f E equal to the opencircuit potential difference V between A and B, and an internal resistance r, where V and r have the values determined above. Hence, E Current through R = I = -----------r + R.
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Electrical Systems, Manutronics Thevenin's Theorem sometimes referred to as Helmholtz's Theorem is an application of the Superposition Theorem. Thus, if a source having an e.m.f. E equal to the open-circuit p.d. between A and B in fig. (b) were inserted in the circuit between R and terminal A in fig. (a), the positive terminal of the source being connected to A, no current would flow through R. Hence, this source could be regarded as circulating through R a current superimposed upon but opposite in direction to the current through R due to E1 alone. Since the resultant current is zero, it follows that a source of e.m.f. E connected in series with R and the equivalent resistance r of the network, as in fig. (d), would circulate a current I having the same value as that through R in fig. (a); but in order that the direction of the current through R may be from A towards B, the polarity of the source must be as shown in fig. (d). Example 1 C and D in the diagram below which is similar to our previous example represents the two terminals of an active network. Calculate the current through R. Answer With R disconnected as in fig (b) 6-4 I1 = -------------= 0.4A 2+3 and i.e.
p.d across CD = E1 – I1 R1 V = 6 – (0.4 x 2)
= 5.2V
When the e.m.f.’s are removed as in fig (c) Total resistance between C and D = r
=
2x3 ---------2+3
1.21
Hence the network AB in fig (a) can be replaced by a single source having and e.m.f. of 5.2V and an internal resistance of 1.21 as in fig (d) consequently. 5.2 I = ----------= 0.464A 1.2 + 10 namely the same value as obtained in the previous example.
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Electrical Systems, Manutronics Example 2 The resistances of the various arms of an unbalanced Wheatstone bridge are given in the diagram below. The battery has an e.m.f. of 2.0V and a negligible internal resistance. Determine the value and direction of the current in the galvanometer circuit BD using Kirchhoff’s laws. Answer Let I1, I2 and I3 be the currents in arms AB, AD and BD respectively as shown in the above diagram. Then by Kirchhoff’s first law Current in BC = I1 – I3 Current in DC = I2 – I3 Applying Kirchhoff’s second law to mesh formed by ABC and the battery, we get 2 = 10 I1 + 30 (I1 – I3) = 40 I1 – 30 I3
…..(a)
Similarly for mesh ABDA 0 = 10 I1 + 40 I3 – 20 I2
….(b)
and for mesh BDCB 0 = 40 I3 + 15(I2 + I3) – 30(I1 – I3 ) = -30 I1 + 15 I2 + 85 I3
….(c)
multiplying (b) by 3 and (c) by 4 and adding the two expressions thus obtained we have. 0 = -90 I1 + 460 I3 I1 = 5.111 I3 Substituting for I1 in (a) we have I3 = 0.0115A = 11.5mA Since the value of I3 is positive the direction of I3 is that assumed in the diagram namely from B to D.
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Electrical Systems, Manutronics Chapter 10, Static Charges Static electricity, called just 'static' for short, is the result of electric charges standing still. This is very different from an electric current which is a moving flow of charges. You may remember from a previous chapter that electrons flow to provide current. However it was possible to have a charged atom which is either Positive, Negative or Neutral. If there is a significant amount of the positive charged (or Negative) which is not moving then the objects said to have a static charge. When touched or brought near a grounded object then the charge flows causing sparks or current to flow.
Have you ever pulled off your jumper quickly and heard a crackling sound? The chances are that it contained a good proportion of artificial fibres. If you were in the dark you may have seen some tiny blue sparks. Something else you might have tried is rubbing a plastic comb against your coat sleeve and using it to pick up small bits of paper. You will be trying this in your practical work. Both nylon sparks and the ability of a comb to pick up small pieces of paper are examples of the effects of static electricity. The most dramatic and potentially damaging example of static is lightning. A flash of lightning happens when a cloud that has built up a static charge discharges it through anything that conducts electricity - including moist air - down to earth. When you see lightning, it's the result of an electrostatic discharge. But how did the clouds get charged up in the first place? The answer is in the same way as your plastic comb - by the movement of charged particles.
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Electrical Systems, Manutronics For a quick brief experiment that we don’t need to write up or go to the Laboratory all you'll need are the things that you probably already have around the house. Electrostatic charges are often produced by friction and this is the first thing you can demonstrate. You will need some small scraps of paper, about the size of confetti. You will also need a dry, clean plastic object. A comb is ideal. Rub the comb briskly against your coat sleeve. You should find that the comb 'charges up' and can be used to attract the small pieces of paper. If you have a party balloon handy you could also try it with this. Rub the balloon against your coat or shirt-front and you can make it stick to a wall. You have been demonstrating electrostatic charging by friction and electrostatic attraction. You can show electrostatic attraction if you have a colour TV. Just put the backs of your knuckles near to the screen when it is on and you will feel the attraction on the fine hairs. Most of the dust that collects on TV screens is drawn there by electrostatic attraction. Electrostatic attraction is caused by the same sort of voltage that drives an electric current since it is this sort of voltage which is inside your TV. Now, if you can get hold of a roll of insulation tape of the sort often used by electricians you can try a different sort of test. Go into a dark room and quickly pull some tape from the roll. If you look at the place where the tape parts from the reel, you should see a little line of blue sparks. The blue sparks are signs of electrostatic discharge. They may not look very impressive but even a small spark could be disastrous if there were petrol vapour or explosives around! Safely note: If you are working in an industrial plant where the materials handled present a risk of explosion or fire and which is classified as a hazardous area you must ensure you are aware of and follow all the rules and procedures covering any work you do, electrical or otherwise, in that area. Note : high risk areas for static sparking hazards are dusty atmospheres or where flammable gases or vapours could occur.
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Electrical Systems, Manutronics Here is one last test which I cannot guarantee will work. If you have a nylon or artificial fibre-based carpet and it is a dry day, try walking briskly around for a bit, then touch some earthed metal, such as a copper pipe in your plumbing system. You may be able to charge yourself up enough to feel a small shock. The last test works best on a very dry frosty day, and seldom works on a damp or humid day. The reason is that any moisture in the air tends to discharge the static as quickly as it forms, so on a damp day you wouldn't charge up enough to get a shock you could feel. To make static charges you need good insulation. Charge can only build up on a conductor if it is well insulated from earth. Most artificial materials like nylon and polythene are very good insulators and so are easily charged up. This is why cling-film used in packaging etc, clings so well. Now you know from your practical work that electrostatic charges can be produced by friction. This works best with good insulators such as most artificial fibres like nylon. An insulator is a material that completely resists the flow of current, so the charge can't 'leak' away easily. You should also have seen a charge produced by the rapid separation of one insulator from another. There are, in fact, many ways in which electrostatic charges can be made. Attracting dust and dirt In industry, static may cause practical problems because electrostatic charges can attract dust and dirt. This is usually no more than a nuisance and not often a serious problem. Static attraction is used to advantage by car manufacturers who give car bodies an electrostatic charge before starting to spray paint them. This attracts the paint to the car body and prevents a lot of waste. From a spark to a fire Electrostatic discharges are another matter. Even a tiny spark can cause disaster if highly flammable materials are around. In the petrochemical, biscuit manufacturing, flour-milling and paper-making industries, precautions against static electricity are vital to keep down the risks of fire or explosion.
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Electrical Systems, Manutronics Chip damage Semiconductors and integrated circuits (ICs), or 'chips' as they are called, used in electronic circuits, are easily damaged by static discharges. Even the charge picked up on your body just by walking around can damage some types of integrated circuit. So, if you work with transistors or chips, don't wear nylon. In fact, it is a good idea to touch some earthed metal to discharge yourself before picking up any of these items. It is also a good idea to avoid touching the connection pins or wires. C-MOS chips are particularly prone to static damage. In factories where a lot of these chips are handled, the floors are often carpeted with a conducting rubber mat underneath which is a metal earthing grid. The workers wear natural fibres in their uniforms and sometimes the workers are connected to earth via an earth lead and a wriststrap. Actually, for safety reasons, a resistor of at least 1 megaohm is connected between the earth lead and true earth limiting current flow.
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Electrical Systems, Manutronics Test yourself 1.
Friction is one cause of electrostatic charge. True / False
2.
You can charge yourself just by walking across a dry carpet. True / False
3.
Electrostatic sparks are harmless. True / False
4.
You are more liable to get electrostatically charged on a foggy day True / False
5.
You should never touch earthed metal when working with IC’s True / False
6.
Electrostatic attraction is useful when spray painting cars. True / False
7.
You should avoid wearing nylon underwear when working on electronic equipment. True / False
8.
Conductors cannot become electrostatically charged. True / False
9.
The purpose of wearing an earthed wristband is to discharge any electrostatic charge safely when working on sensitive circuits. True / False
10.
Lightning is a natural electrostatic discharge. True / False
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Electrical Systems, Manutronics Chapter 11, Electromagnetism I imagine that you must at some time have picked up pins with a magnet. You probably used a permanent magnet. I wonder if you thought how useful it would be if you could turn the magnet's effect on and off, just when you wanted. In this section we'll look at how electromagnetism works and some of its many applications.
N
S
Using electromagnetism Can we put electromagnetism to any practical use? Industrial electro-magnets are used to lift scrap iron such as car bodies. Another use of electromagnetism is an electromagnet which can be used as the heart of an electromagnetically operated switch, or relay. The use of an electromagnet in a relay means that remote or automatic control of a switch by an electric current becomes possible. Permanent Magnets If you wish to investigate the main properties of magnets you will need the small bar magnet and plotting compass, a sheet of plain paper and a pencil. The first property of magnets that most people notice is that they attract certain metallic objects. This is objects that contain ferrite, i.e. ferrous metals. Check that this is the case using pins, bag buckles and then test various other metallic objects you can see around you. You should find that only objects made from iron or steel are attracted. These are called ferromagnetic materials. Now test if the strength of the attractive force is the same for all parts of the magnet. You should find that the places where the attraction is strongest is at each end of the magnet. Indeed you should have found that there's no attraction at all in the centre of the magnet. The areas where the attractive force is strongest are called the poles of the magnet. North and South. To show how a magnet is influenced by the polarity of the earth we'll use the compass. If you place the compass on a flat surface well away from your bar magnet or any other ferromagnetic materials, you will find that it always comes to rest in a North-South direction after being disturbed. The end of the compass needle pointing to north is called North Seeking, more commonly the North Pole.
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N Now place the compass on a flat surface and slowly bring the end of the bar magnet marked North Pole close to W E the north pole of the compass. Now repeat the action with the other pole of N S the bar magnet. You should have found that when you brought the magnet's North Pole close, the north-seeking end of the compass N needle swung away from it. S Similarly, bringing the south pole of the bar magnet close attracts the north W E seeking end of the compass needle. So, the north pole of the magnet repels S the north seeking end of the compass needle. This happens because the north seeking end of the needle is magnetised as a north pole. The earth has a magnetic south pole at its geographical north pole! Did you notice the compass needle was repelled when the pole of the bar magnet was some distance from the compass? The area around a magnet where a magnetic effect can be detected is called a Magnetic Field. We can describe a magnetic field as having both magnitude and direction and you can draw a simple diagram to show these. Take a sheet of plain paper and place it on a table well away from any other magnets or ferromagnetic materials. Using the compass, arrange the paper with its longer side pointing in a north south direction. The bar magnet should then be placed in the centre parallel to the long side of the paper with its South pole to the north as in the diagram below.
N
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Electrical Systems, Manutronics Place the plotting compass near the corner of the North pole of the magnet as shown and pencil in a dot at each end of the needle. Then move the compass away from the magnet so that it's pointing at the dot you have just made and again mark a dot at the further end. Continue with this process until the row of dots returns to the other pole of the magnet or goes off the edge of the paper. Now join the dots together and the diagram above should appear. It is important to note that this profile is not only 2D but is in fact 3D and the resolution of the actual magnetic field lines is much much less that the crude ones shown. Attraction and Repulsion of Magnetic Poles When unlike poles of two permanent magnets are placed close together, an attractive force is produced by the magnetic fields, as indicated below. When two like poles are brought close together, they repel each other, as shown below.
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Electrical Systems, Manutronics Altering a Magnetic Field When a nonmagnetic material, such as paper, glass, wood, or plastic, is placed in a magnetic field, the lines of force are unaltered, as shown in (a) below. However, when a magnetic material such as iron is placed in the magnetic field, the lines of force tend to change course and pass through the iron rather than through the surrounding air. They do so because the iron provides a magnetic path that is more easily established than that of air. Figure (b) illustrates this principle.
Magnetic Flux The group of force lines going from the north pole to the south pole of a magnet is, called the magnetic flux, symbolised by I (the lower-case Greek letter phi). The number of lines of force in a magnetic field determines the value of the flux. The more lines of force, the greater the flux and the stronger the magnetic field. The unit of magnetic flux is the weber (Wb). One weber equals 108 lines. In most practical situations, the weber is a very large unit; thus, the microweber (Pwb) is more common. One microweber equals 100 lines of magnetic flux. Magnetic Flux Density The flux density is the amount of flux per unit area in the magnetic field. Its symbol is B, and its unit is the tesla (T). One tesla equals one weber per square meter (Wb/m2). The following formula expresses the flux density: I B A Where I is the flux and A is the cross-sectional area of the magnetic field.
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Electrical Systems, Manutronics How Materials Become Magnetised Ferromagnetic materials such as iron, nickel, and cobalt become magnetised when placed in the magnetic field of a magnet. We have all seen a permanent magnet pick up paper clips, nails, iron filings, and so on. In these cases, the object becomes magnetised (that is, it actually becomes a magnet itself) under the influence of the permanent magnetic field and becomes attracted to the magnet. When removed from the magnetic field, the object tends to lose its magnetism. Ferromagnetic materials have minute magnetic domains created within their atomic structure. These domains can be viewed as very small bar magnets with north and south poles. When the material is not exposed to an external magnetic field, the magnetic domains are randomly oriented, as shown in Fig (a). When the material is placed in a magnetic field, the domains align themselves as shown in Part (b). Thus, the object itself effectively becomes a magnet.
An Application Permanent magnets have almost endless applications, one of which is presented here as an illustration. The figure below shows a typical magnetically operated, normally closed (NC) switch. When the magnet is near the switch mechanism, the metallic arm is held in its NC position. When the magnet is moved away, the spring pulls the arm up, breaking the contact as shown in the first figure. Switches of this type are commonly used in perimeter alarm systems detect entry into a building through windows or doors and on safety cages. As the last figure shows, several openings can be protected by magnetic switches wired to a common transmitter. When any one of the switches opens, the transmitter is activated and sends a signal to a central receiver and alarm unit.
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Magnetic and switch set
Operation of a magnetic Switch
Connection of a typical perimeter alarm system
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Electrical Systems, Manutronics Electromagnetism We have covered how permanent magnets are made and some of their application. But imagine been able to turn on and off a magnetic affect just by pressing a button! Current produces a magnetic field, called an electromagnetic field, around a conductor, as illustrated below. The invisible lines of force of the magnetic field form a concentric circular pattern around the conductor and are continuos along its length. Although the magnetic field cannot be seen, it is capable of producing visible effects. For example, if a current-carrying wire is inserted through a sheet of paper in a perpendicular direction, iron filings placed on the surface of the paper arrange themselves along the magnetic lines of force in concentric rings, as illustrated in below (a). The field is stronger closer to the conductor and become; weaker with increasing distance from the conductor.
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Electrical Systems, Manutronics Direction of the Lines of Force The direction of the lines of force surrounding the conductor are indicated in the diagram below. They are circular (not concentric) around the cross section of the conductor perpendicular to the flow of the current. Right-Hand Rule An aid to remembering the direction of the lines of force is illustrated below. Imagine that you are grasping the conductor with your right hand, with your thumb pointing in the direction of current. Your fingers indicate the direction of the magnetic lines of force.
Illustration of Right-hand rule.
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Electrical Systems, Manutronics Electromagnetic Properties Several important properties relating to electromagnetic fields are now presented. Magnetomotive force; magnetic field strength In an electric circuit, the current is due to the existence of an electromotive force. By analogy, we may say that in a magnetic circuit the magnetic flux is due to the existence of a magnetomotive force (m.m.f.) caused by a current flowing through one or more turns. The value of the m.m.f. is proportional to the current and to the number of turns, and is descriptively expressed in ampere turns; but for the purpose of dimensional analysis, it is expressed in amperes, since the number of turns is dimensionless. Hence the unit of magnetomotive force is the ampere. If a current of I amperes flows through a coil of N turns, as shown below, the magnetomotive force is the total current linked with the magnetic circuit, namely IN amperes. If the magnetic circuit is homogeneous and of uniform cross sectional area, the magnetomotive force per metre length of the magnetic circuit is termed the magnetic field strength and is represented by the symbol H. Thus, if the mean length of the magnetic circuit of the setup below is L metres, H
=
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IN ---L
amperes per metre
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Electrical Systems, Manutronics Reluctance Reluctance (R) is the opposition to the establishment of a magnetic field in an electromagnetic circuit. It is the ratio of the mmf required to establish a given flux to the amount of flux, and its units are ampere-turns per weber. The formula for reluctance is Fm R = -----I This equation is sometimes known as "Ohm's law for magnetic circuits," because the reluctance is analogous to the resistance in electrical circuits. Permeability The ease with which a magnetic field can be established in a given material is measured by the permeability of that material. The higher the permeability, the more easily a magnetic field can be established. The symbol of permeability is P, and the formula is as follows: L = -----P RA where R is the reluctance in ampere-turns per weber, L is the length of the material in meters, and A is the cross-sectional area in square meters. The permeability of free space Po is numerically equal to 4S x 10 - 7 Wb/At.m. The absolute permeability of other materials is related to the permeability of free space by the relative permeability, i.e. P
=
PoPr
For air and other non-magnetic materials, the absolute permeability is the same constant. For magnetic materials, absolute permeability is not a fixed constant but varies nonlinearly with the flux density. The nonlinear variation of permeability is conveniently displayed as a functional plot of magnetic flux density, B, against magnetic intensity, H. The plot shown below illustrates a number of B-H curves for some common materials. From knowing that B = PH it is apparent that the absolute permeability is given by the slope of a tangent to the B-H curve at any particular value. B
=
I ---A
Where I is the flux and A is the cross-sectional area of the magnetic field. H
IN amperes per metre L
Where I is the current, N is the number of turns and L is the length of the flux path.
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Electrical Systems, Manutronics It is apparent that for an applied magnetic intensity, the magnetic flux developed in a coil with a ferrous core is many times greater than that through a similar coil with an air core. In most practical systems therefore, a ferrous core is normally used since it greatly facilitates the establishment of a magnetic flux.
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Example There are two amperes of current through a wire with 5 turns. (a) What is the mmf? (b) What is the reluctance of the circuit if the flux is 250 PWb? Solution (a)
(b)
N = 5 and
I = 2A
Fm =
NI
= (5)(2 A) = 10 At
R
=
Fm -----I
=
10 At -----------250PWb
= 0.04 x 106 At/Wb
The Electromagnet An electromagnet is based on the properties that you have just learned. A basic electromagnet is simply a coil of wire wound around a core material that can be easily magnetised. The shape of the electromagnet can be designed for various applications. For example, the diagram below shows a U-shaped magnetic core. When the coil of wire is connected to a battery and current flows, as shown in Part (a), a magnetic field is established as indicated. If the current is reversed, as shown in Part (b), the direction of the magnetic field is also reversed. The closer the north and south poles are brought together, the smaller the air gap between them becomes, and the easier it becomes to establish a magnetic field, because the reluctance is lessened.
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A good example of one application of an electromagnet is the process of recording on magnetic tape. In this situation, the recording head is an electromagnet with a narrow air gap, as shown below. Current sets up a magnetic field across the air gap, and as the recording head passes over the magnetic tape, the tape is permanently magnetised. In digital recording, for example, the tape is magnetised in one direction for a binary 1 and in the other direction for a binary 0, as illustrated in the figure. This magnetisation in different directions is accomplished by reversing the coil current in the recording head.
Electromagnetic Devices The recording head is one example of an electromagnetic device. Several others are now presented. The Solenoid Generally, the solenoid is a type of electromagnet that has a movable iron core whose movement depends on both an electromagnetic field and a mechanical spring force. The basic structure is shown below. When current flows through the coil, the electromagnetic field magnetises the core so that, effectively, there are two magnetic fields. The repulsive force of like poles and the attractive force of unlike poles cause the core to move outward against the spring tension, as shown in (b). When the coil current stops, the core is pulled back in by the spring. This mechanical movement in a solenoid is used for many applications, such as opening and closing valves, automobile door locks, and so on.
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Electrical Systems, Manutronics The Relay Relays differ from solenoids in that the electromagnetic action is used to open or close electrical contacts rather than to provide mechanical movement. The diagram below shows the basic operation of a relay with one normally open (NO) contact.
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Electrical Systems, Manutronics The Speaker Permanent-magnet speakers are commonly used in stereos, radios, and TV, and their operation is based on the principle of electromagnetism. A typical speaker is constructed with a permanent magnet and an electromagnet, as shown in Figure (a). The cone of the speaker consists of a paper-like diaphragm to which is attached a hollow cylinder with a coil around it, forming an electromagnet. One of the poles of the permanent magnet is positioned within the cylindrical coil. When current flows through the coil in one direction, the interaction of the permanent magnetic field with the electromagnetic field causes the cylinder to move to the right, as indicated in Figure (b). Current through the coil in the other direction causes the cylinder to move to the left, as shown in Part (c). The movement of the coil cylinder causes the flexible diaphragm also to move in or out, depending on the direction of the coil current. The amount of coil current determines the intensity of the magnetic field, which controls the amount that the diaphragm moves. As shown in the Figure below, when an audio signal (voice or music) is applied to the coil, the current varies in both direction and amount. In response, the diaphragm will vibrate in and out by varying amounts and at varying rates. Vibration in the diaphragm causes the air that is in contact with it to vibrate in the same manner. These air vibrations move through the air as sound waves.
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Electrical Systems, Manutronics Magnetic leakage and fringing Suppose dd in the sketch to the right represents a metal ring symmetrically situated relative to the air gap in the steel ring, and suppose the magnetising winding to be concentrated over a short length of the core. As far as ring dd is concerned, the flux passing through it can be regarded as the useful flux and that which returns by such paths as a, b and c is leakage flux. The useful flux passing across the gap tends to bulge outwards as shown roughly to the right, thereby increasing the effective area of the gap and reducing the flux density in the gap. This effect is referred to as fringing; and the longer the air gap, the greater is the fringing. The distinction between useful and leakage fluxes may be more obvious if we consider an electrical machine. For instance the sketch below shows two poles of a six-pole machine. The armature slots have, for simplicity, been omitted. Some of the dotted lines do not enter the armature core and thus do not assist in generating an e.m.f. in the armature winding; consequently, they represent leakage flux. On the other hand, some of the flux passes between the pole tips and the armature core, as shown below, and is referred to as fringing flux. Since this fringing flux is cut by the armature conductors, it forms part of the useful flux. Thus it is seen that the effect of leakage flux is to increase the total flux through the exciting winding, total flux through exciting winding leakage factor = --------------------------------------------useful flux The value of the leakage factor for electrical machines is usually about 1.15-1.25.
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Electrical Systems, Manutronics Electromagnetic Induction When a conductor is moved through a magnetic field, a voltage is produced across the conductor. This principle is known as electromagnetic induction, and the resulting voltage is an induced voltage. The principle of electromagnetic induction is widely applied in electrical circuits. The operation of electrical motors and generators is also based on this principle. Relative Motion When a wire is moved across a magnetic field, there is a relative motion between the wire and the magnetic field. Likewise, when a magnetic field is moved past a stationary wire, there is also relative motion. In either case, there is an induced voltage in the wire as a result of this motion, as the Figure below indicates.
The amount of the induced voltage depends on the rate at which the wire and the magnetic field move with respect to each other: The faster the relative speed, the greater the induced voltage. Polarity of the Induced Voltage If the conductor in the Figure above is moved first one way and then another in the magnetic field, a reversal of the polarity of the induced voltage will be observed. As the wire is moved downward, a voltage is induced with the polarity indicated in Figure (a). As the wire is moved upward, the polarity is as indicated in Part (b) of the figure. The lower-case v stands for instantaneous voltage.
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Electrical Systems, Manutronics Induced Current When a load resistor is connected to the wire in the above Figure, the voltage induced by the relative motion in the magnetic field will cause a current in the load, as shown in diagram below. This current is called the induced current. The principle of producing a voltage and a current in a load by moving a conductor across a magnetic field is the basis for electrical generators. The concept of a conductor existing in a moving magnetic field is fundamental to inductance in an electrical circuit. The lower-case i stands for instantaneous current.
Forces on a Current-Carrying Conductor in a Magnetic Field Figure (a) shows current outward through a wire in a magnetic field. The electromagnetic field set up by the current interacts with the permanent magnetic field; as a result, the permanent lines of force above the wire tend to be deflected down under the wire, because they are opposite in direction to the electromagnetic lines of force. Therefore, the flux density above is reduced, and the magnetic field is weakened. The flux density below the conductor is increased, and the magnetic field is strengthened. An upward force on the conductor results, and the conductor tends to move toward the weaker magnetic field. Figure (b) shows the current flowing inward, resulting in a force on the conductor in the downward direction. This principle is the basis for electrical motors.
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Electrical Systems, Manutronics Faraday's Law Michael Faraday discovered the principle of electromagnetic induction in 1831. He found that moving a magnet through a coil of wire induced a voltage across the coil, and that when a complete path was provided, the induced voltage caused an induced current, as you have seen. Faraday's observations are as follows: 1.
The amount of voltage induced in a coil is directly proportional to the rate of change of the magnetic field with respect to the coil.
2.
The amount of voltage induced in a coil is directly proportional to the number of turns of wire in the coil.
3.
The voltage induced across a coil equals the number of turns in the coil times the rate of change of the magnetic flux.
Part 1 of Faraday's law is demonstrated in the Figure below, where a bar magnet is moved through a coil, thus creating a changing magnetic field. In Part (a) of the figure, the magnet is moved at a certain rate, and a certain induced voltage is produced as indicated. In Part (b), the magnet is moved at a faster rate through the coil, creating a greater induced voltage.
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Electrical Systems, Manutronics Part 2 of Faraday's law is demonstrated in the Figure below. In Part (a), the magnet is moved through the coil and a voltage is induced as shown. In Part (b), the magnet is moved at the same speed through a coil that has a greater number of turns. The greater number of turns creates a greater induced voltage.
Left Hand Rule By observation of the experiments, it can be noted that the mechanical force exerted by the conductor always acts in a direction perpendicular to the plane of the conductor and the magnetic field direction. The direction is given by the left hand rule illustrated below. The rule can be summarised as follows: 1. Hold the thumb, first finger and second finger of the left hand in the manner indicated, whereby they are mutually at right angles. 2. Point the First finger in the Field direction. 3. Point the second finger in the Current direction. 4. The thumb then indicates the direction of the Mechanical force exerted by the conductor. By trying this with your left hand, you can readily demonstrate that if either the current or the direction of the field is reversed then the direction of the force is also reversed. Also you can demonstrate that, if both current and field are reversed, the direction of the force remains unchanged.
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Electrical Systems, Manutronics Applications of Electromagnetic Induction In this section, two interesting applications of electromagnetic induction are discussed: an automotive crankshaft position sensor, and a dc generator. Although there are many varied applications, these two are representative. Automotive Crankshaft Position Sensor An interesting automotive application is a type of engine sensor that detects the crankshaft position directly using electromagnetic induction. The electronic engine controller in many automobiles uses the position of the crankshaft to set ignition timing and, sometimes, to adjust the fuel control system. The diagram below shows the basic concept. A steel disk is attached to the engine's crankshaft by an extension rod; the protruding tabs on the disk represent specific crankshaft positions. As the disk rotates with the crankshaft, the tabs periodically pass through the air gap of the permanent magnet. Since steel has a much lower reluctance than does air (a magnetic field can be established in steel much more easily than in air), the magnetic flux suddenly increases as a tab comes into the air gap, causing a voltage to be induced across the coil. This process is illustrated in the Figure below. The electronic engine control circuit uses the induced voltage as an indicator of the crankshaft position.
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Electrical Systems, Manutronics DC Generator The sketch below shows a simplified DC generator consisting of a single loop of wire in a permanent magnetic field. Notice that each end of the loop is connected to a split-ring arrangement. This conductive metal ring is called a commutator. As the loop is rotated in the magnetic field, the split commutator ring also rotates. Each half of the split ring rubs against the fixed contacts, called brushes, and connects the loop to an external circuit.
As the loop rotates through the magnetic field, it "cuts" through the flux lines at varying angles, as illustrated in the diagram to the right. At position A in its rotation, the loop of wire is effectively moving parallel with the magnetic field. Therefore, at this instant, the rate at which it is cutting through the magnetic flux lines is zero. As the loop moves from position A to position B, it cuts through the flux lines at an increasing rate. At position B, it is moving effectively perpendicular to the magnetic field and thus is cutting through a maximum number of lines. As the loop rotates from position B to position C, the rate at which it cuts the flux lines decreases to minimum (zero) at C. From position C to position D, the rate at which the loop cuts the flux lines increases to a maximum at D and then back to a minimum again at A. When a wire moves through a magnetic field, a voltage is induced, and by Faraday's law, the amount of induced voltage is proportional to the number of loops (turns) in the wire and the rate at which it is moving with respect to the magnetic field. Now you know that the angle at which the wire moves with respect to the magnetic flux lines determines the amount of induced voltage, because the rate at which the wire cuts through the flux lines depends on the angle of motion.
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Electrical Systems, Manutronics The sketches below illustrate how a voltage is induced in the external circuit as the single loop rotates in the magnetic field. Assume that the loop is in its instantaneous horizontal position, so the induced voltage is zero. As the loop continues in its rotation, the induced voltage builds up to a maximum at position B, as shown in Part (a) of the figure. Then, as the loop continues from B to C, the voltage decreases to zero at C, as shown in Part (b). During the second half of the revolution, shown in Parts (c) and (d), the brushes switch to opposite commutator sections, so the polarity of the voltage remains the same across the output. Thus, as the loop rotates from position C to position D and then back to A, the voltage increases from zero at C to a maximum at D and back to zero at A.
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The plot below shows how the induced voltage varies as the loop goes through several rotations (three in this case). This voltage is a dc voltage because its polarities do not change. However, the voltage is pulsating between zero and its maximum value. When more loops are added, the voltages induced across each loop are combined across the output. Since the voltages are offset from each other, they do not reach their maximum or zero values at the same time. A smoother de voltage results, as shown in lower plot. The variations can be further smoothed out by filters to achieve a nearly constant dc voltage.
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Electrical Systems, Manutronics Test Yourself Explain the difference between magnetism and electromagnetism? What happens to the magnetic field in an electromagnet when the current through the coil is reversed? What is the induced voltage across a stationary conductor in a stationary magnetic field? When the speed at which a conductor is moved through a magnetic field is increased, does the induced voltage increase, decrease, or remain the same? When there is current through a conductor in a magnetic field, what happens?
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Electrical Systems, Manutronics Chapter 12, Measuring Instruments Analog Meter Movements The d'Arsonval movement is the most common type used in analog multimeters, In this type of meter movement, the pointer is deflected in proportion to the amount of current through a coil. The figure (a) shows a basic d'Arsonval meter movement. It consists of a coil of wire wound on a bearing-mounted assembly that is placed between the poles of a permanent magnet. A pointer is attached to the moving assembly. With no current through the coil, a spring mechanism keeps the pointer at its left most (zero) position. When current flows through the coil, electromagnetic forces act on the coil, causing a rotation to the right. The amount of rotation depends on the amount of current. Figure (b) shows a construction view of the parts of a typical movement.
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Electrical Systems, Manutronics The figure below illustrates how the interaction of magnetic fields produces rotation of the coil assembly. The current flows inward at the "cross" and outward at the "dot" in the single winding shown. The inward current produces a counter clockwise electromagnetic field that reinforces the permanent magnetic field below it. The result is an upward force on the left side of the coil as shown. A downward force is developed on the right side of the coil, where the current is outward. These forces produce a clockwise rotation of the coil assembly.
In addition to the d’Arsonval movement there are two other types of movements that are in use for analog meters. The Iron Vane and Electrodynamometer are another two versions available. With the electronic age most of these movements are being replaced with a circuit were the display is digital. This makes for easier reading, better accuracy and multi-functional apparatus. It is now not necessary to multiply out the values in our heads as the electronic circuit completes that for us. The electronic circuit will be covered later in your electronics course. However for the explanation of the operation of the Ammeter, Voltmeter and Ohms meter we shall use the analog layout because if we add the electronics in at this stage it may cause confusion and take away from the principles we are trying to get across.
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Electrical Systems, Manutronics The Ammeter A typical d’Arsonval movement might have a current sensitivity of 1mA and a resistance of 50:. In order to measure more than 1 mA, additional circuitry must be used with the basic meter movement. The figure (a) below shows a simple ammeter with a shunt (parallel) resistor (RSH) across the movement. The purpose of the shunt resistor is to bypass current in excess of 1 mA around the meter movement. For example, let us assume that this meter must measure currents of up to 10 mA. Thus, for full-scale deflection, the movement must carry 1 mA, and the shunt resistor must carry 9 mA, as indicated in figure (b).
Multiple-Range Ammeters The simple ammeter in the figure above has only one range. As you saw, it can measure currents from 0 to 10mA and no higher. Most practical ammeters have several ranges. Each range in a multiple-range ammeter has its own shunt resistance which is selected with a multipleposition switch. For example the figure to the right shows a four-range ammeter with a 0.1-mA (100-PA) meter movement. When the switch is in the 0.1-mA position, all of the current being measured flows through the coil. In the other positions, some of the current flows through the coil, but most of it flows through the shunt resistor. In the 1-mA position, 100PA (0.1mA) flows through the coil and 0.9mA through the shunt resistor for full-scale deflection.
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Electrical Systems, Manutronics In the 10-mA position, 100PA flows through the coil and 9.9mA through the shunt resistor for full-scale deflection. In the 100-mA position, 100PA flows through the coil and 99.9mA through the shunt resistor for full-scale deflection. Of course, for less than full-scale deflection, the current values are less. Effect of the Ammeter on a Circuit As you know from earlier coverage, an ammeter is connected in series to measure the current in a circuit. Ideally, the meter should not alter the current that it is intended to measure. In practice, however, the meter unavoidably has some effect on the circuit, because its internal resistance is connected in series with the circuit resistance. However, in most cases, the meter's internal resistance is so small compared to the circuit resistance that it can be neglected. For example, if the meter has a 50-: movement and a 100-PA full-scale current, the voltage dropped across the coil is VCOIL = ICOIL RCOIL = (100 PA)(50:) = 5 mV The shunt resistance for the 10-mA range, for example, is 5 mV VCOIL RSH = ------- = --------= 0.505 : ISH 9.9 mA As you can see, the total resistance of the ammeter on the 10-mA range is the coil resistance in parallel with the shunt resistance: RCOIL II RSH = 50 : II 0.505 : = 0.5 :
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Electrical Systems, Manutronics The Voltmeter The voltmeter utilises the same type of movement as the ammeter. Different external circuitry is added so that the movement will function to measure voltage in a circuit. As you have seen, the voltage drop across the meter coil is dependent on the current and the coil resistance. For example, a 50-PA, 1000-: movement has a full-scale voltage drop of (50PA)(1000:) = 50mV. To use the meter to indicate voltages greater than 50mV, we must add a series resistance to drop any additional voltage beyond that which the movement requires for full-scale deflection. This resistance is called the multiplier resistance and is designated Rm. A basic voltmeter is shown below fig(a) with a single multiplier resistor for one range. For this meter to measure 1V full scale, the multiplier resistor (Rm) must drop 0.95V because the coil drops only 50mV (0.95V + 50mV = 1V), as shown in fig (b).
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Electrical Systems, Manutronics Multiple-Range Voltmeters The simple voltmeter shown above has only one range. It can measure voltages from 0 to 1V and no higher. Most practical voltmeters have several ranges, each of which has its own multiplier resistor which can be selected by a switch. For example the system below shows a four-range voltmeter with a 50-PA, 1000: meter movement. When the switch is in the 50-mV position, all of the voltage is dropped across the coil. In the other positions, some of the total voltage is dropped across the coil, but most is dropped across the multiplier resistor. In the 1-V position, 50mV is dropped across the coil and 0.95V across Rm, for full scale deflection. In the 10-V position, 50mV is dropped across the coil and 9.95 V across RM1 + RM2 for full-scale deflection. In the 100-V position, 50 mV is dropped across the coil and 99.95 V across RM1 + R M2 + R M3 for full-scale deflection. Of course for less than full scale deflection the voltage values are less.
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Electrical Systems, Manutronics Loading Effect of a Voltmeter As we have seen from earlier coverage, a voltmeter is always connected in parallel with the circuit component across which the voltage is to be measured. Thus it is much easier to measure voltage than current, because you must break a circuit to insert an ammeter in series. You simply connect a voltmeter across the circuit without disrupting the circuit or breaking a connection. Since some current is required through the voltmeter to operate the movement, the voltmeter has some effect on the circuit to which it is connected. This effect is called loading. However, as long as the meter resistance is much greater than the resistance of the circuit across which it is connected, the loading effect is negligible. This characteristic is necessary because we do not want the measuring instrument to change the voltage that it is measuring. The meter movement just discussed has a full-scale coil current of 50-PA. This current flows through the multiplier resistors since they are in series with the coil. For example, on the 10-V range, the total multiplier resistance is Rm
=
9.95 V --------50 PA
= 199 k:
The internal resistance of the voltmeter is the multiplier resistance in series with the coil resistance: Rm + Rcoil = 199 k: + 1 k: = 200 k:
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Electrical Systems, Manutronics The Ohmmeter The meter movement used for the ammeter and the voltmeter can also be adapted for use in an ohmmeter. The ohmmeter is used to measure resistance values. A basic one-range ohmmeter is shown below. It contains a battery and a variable resistor in series with the movement. To measure resistance, the leads are connected across the external resistor to be measured, as shown in (b). This connection completes the circuit, allowing the internal battery to produce current through the movement coil, causing a deflection of the pointer proportional to the value of the external resistance being measured. Zero Adjustment When the ohmmeter leads are open, the pointer is at full left scale, indicating infinite (-) resistance (open circuit). When the leads are shorted, the pointer is at full right scale, indicating zero resistance. The purpose of the variable resistor is to adjust the current so that the pointer is at exactly zero when the leads are shorted. It is used to compensate for changes in the internal battery voltage due to ageing.
Multiple-Range Ohmmeter An ohmmeter usually has several ranges. These typically are labelled Rx1, Rx10, Rx100, Rx1k, Rx10k, Rx100k, and Rx1M, although some ohmmeters may not have all of the ranges mentioned. These range settings are interpreted differently from those of the ammeter or voltmeter: The reading on the ohmmeter scale is multiplied by the factor indicated by the range setting. For example, if the pointer is at 20 on the scale and the range switch is set at RX100, the actual resistance measurement is 20x100, or 2k:.
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Electrical Systems, Manutronics When the meter's leads are open, there is no coil current and the pointer is at infinity. In order to deflect the pointer toward the zero end of the scale when a resistance is measured, coil current must flow, and the smaller the measured resistance, the greater the coil current must be. Shunt (parallel) resistors are used to provide multiple ranges so that the meter can measure resistance values from very small to very large. For each range, a different value of shunt resistance is switched in. The shunt resistance increases for the higher ohm ranges and is always equal to the centre scale reading on any range. An example of a multiple-range ohmmeter circuit is shown below.
Remember, an ohmmeter must not be connected to a circuit when the circuit's power is on. Always turn the power off before connecting the meter.
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Electrical Systems, Manutronics The Wattmeter The wattmeter is an instrument for measuring electrical power. In construction and appearance it resembles a moving-coil voltmeter or ammeter, but it has no permanent magnet. Instead it has two fixed coils, FF as shown below, which set up the magnetic field in which the suspended coil, M, moves. When the instrument is in use, the coils FF are connected in series with the device X whose power consumption is to be measured. The magnetic field B, set up by FF, is then proportional to the current I drawn by X: BvI
The moving coil M is connected across the device X. In series with M is a high resistance R, similar to the multiplier of a voltmeter. M is indeed often called the volt-coil. The current I' through the volt-coil is small compared with the main current I, and is proportional to the potential difference V across the device X: I' v V The torque acting on the moving-coil is proportional to the current through it, and to the magnetic field in which it is placed: T v BI' Therefore
T v IV
So the torque on the coil is proportional to the product of the current through the device X, and the voltage across it. The torque is therefore proportional to the power consumed by X, and the power can be measured by the deflection of the coil. The diagram shows that, because the volt-coil draws current, the current through the fixed coils is a little greater than the current through X. As a rule, the error arising from this is negligible; if not, it can be allowed for as when a voltmeter and ammeter are used separately.
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Electrical Systems, Manutronics Multimeters A multimeter is an instrument which is adapted for measuring both current A.C/D.C, voltage A.C/D.C and Ohms. The Ohms part is removed for clarity and not shown below. It has a shunt R and a series of voltage multipliers R' as shown below. The shunt is connected permanently across the coil and the resistances in R’ are adjusted to give the desired full scale voltages with the shunt in position. A switch or a plug enables the various full scale values of current or voltage to be chosen, but the user does the mental arithmetic (Not the case with the digital units). The instrument shown in the figure is reading 1.7 volts; if it were on the 10-volt range, it would be reading 6.4. The terminals of a meter, multimeter or otherwise, are usually marked + and -; the pointer is deflected to the right when current passes through the meter from + to -.
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Electrical Systems, Manutronics Use of Voltmeter and Ammeter A moving-coil voltmeter is a current-operated instrument. It can be used to measure potential differences if we assume that the current which it draws is always proportional to the potential difference applied to it as the current varies. Since its action depends on Ohm's law, a moving-coil voltmeter cannot be used in any experiment to demonstrate that law. We use moving-coil voltmeters as they are more sensitive and more accurate than other forms of voltmeters. The current which they take does, however, sometimes complicate their use. To see how it may do so, let us suppose that we wish to measure a resistance R of about 100:. As shown in Figure (i), we connect it in series with a cell, a milliammeter, and a variable resistance; across it we place the voltmeter. We adjust the current until the voltmeter reads, say, V1 = 1 V; let us suppose that the milliammeter then reads I = 12 mA. The value of the resistance then appears to be R
=
V1 ---I
=
83: Approx
=
1 ------------12 x 10-3
=
103 ----12
But the milliammeter reading includes the current drawn by the voltmeter. If that is 2 mA, then the current through the resistor, I', is only 10 mA and its resistance is actually R
=
V1 ---I’
=
100:.
=
1 ------------10 x 10-3
=
1 ----10-2
The current drawn by the voltmeter has made the resistance appear 17% lower than its true value.
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Electrical Systems, Manutronics To try and avoid this error, we might connect the voltmeter as shown in (ii): across both the resistor and the milliammeter. But its reading would then include the potential difference across the milliammeter. Let us suppose that this is 0.05V when the current through the milliammeter is 10 mA. Then the potential difference V' across the resistor would be 1 V, and the voltmeter would read 1.05V. The resistance would appear to be
R
1.05 ------------10 x 10-3
= =
=
1.05 ----10-2
105:
Thus the voltage drop across the milliammeter would make the resistance appear 5% higher than its true value. To reduce errors, in low-resistance circuits the voltmeter should be connected as in Figure (i), so that its reading does not include the voltage drop across the ammeter. But in high-resistance circuits the voltmeter should be connected as in Figure (ii) so that the ammeter does not carry its current. As we see later, using a potentiometer to measure p.d. in Figure (i) is equivalent to using a voltmeter of infinitely-high resistance. In this case no current is drawn by the potentiometer, so this increases the accuracy of measuring R.
Quick Summary An Ideal Voltmeter has an infinitely high resistance. An Ideal Ammeter has an infinitely low resistance.
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Electrical Systems, Manutronics Chapter 13, Alternating Current AC and Voltage Introduction In recent lectures we have come across generating a DC supply by having a coil connected to a split ring communicator moving in a magnetic field. This principle gave us a current and voltage always in the positive direction deviating from 0 to full current/voltage and back to 0. It never went into the negative region. However constructing this equipment and generating large quantities of supply is difficult thus an alternative needs to be considered. If we consider say, instead of splitting each side of the coil we purely change the system to have two solid slip rings rather than one split ring we can be confident the brushed are always in contact with the ring and are not always jumping the gap.
The system below shows the generating of the voltages and current as the coil moves in the magnetic field when using slip rings. This sketch clearly shows the generation of a wave with every revolution of the coil in the magnetic field. This wave formation is called a sine wave and is the basis for AC electrical generation.
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Electrical Systems, Manutronics Chapter 14, Sine wave characteristics As you can see from the generating of electricity a generator is used. Both a DC and AC generator were discussed in previous sections. The cycle of current (Voltage) produced by the AC generator produces a varying current which actually matches the Sine wave we have used in maths in the past. This is no coincidence as they are both circular and both are dependent on where they are positioned to the start position. The Sine Wave The sine wave is one very common type of alternating current (ac) and alternating voltage. It is also referred to as a sinusoidal wave or, simply, sinusoid. The electrical service provided by the power companies is in the form of sinusoidal voltage and current. This is why we will investigate its properties in depth. The sketch below shows the general shape of a sine wave, which can be either current or voltage. Notice how the voltage (or current) varies with time. Starting at zero, it increases to a positive maximum (peak), returns to zero, and then increases to a negative maximum (peak) before returning again to zero.
The Polarity of a Sine Wave As we have seen, a sine wave changes polarity at its zero value; that is, it alternates between positive and negative values. When a sine wave voltage is applied to a resistive circuit an alternating sine wave current results. When the voltage changes polarity, the current correspondingly changes direction.
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The Period of a Sine Wave As you have seen, a sine wave varies with time in a definable manner. Time is designated by t. The time required for a sine wave to complete one full cycle is called the period (T), as indicated below. Typically, a sine wave continues to repeat itself in identical cycles, as shown in Part (b). Since all cycles of a repetitive sine wave are the same, the period is always a fixed value for a given sine wave.
The period of a sine wave does not necessarily have to be measured between the zero crossings at the beginning and end of a cycle. It can be measured from any point in a given cycle to the corresponding point in the next cycle.
Method 1:
The period can be measured from one zero crossing to the corresponding zero crossing in the next cycle.
Method 2:
The period can be measured from the positive peak in one cycle to the positive peak in the next cycle.
Method 3:
The period can be measured from the negative peak in one cycle to the negative peak in the next cycle.
These measurements are indicated above, where two cycles of the sine wave are shown. Keep in mind that you obtain the same value for the period no matter which of these points on the waveform you use.
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Electrical Systems, Manutronics The Frequency of a Sine Wave Frequency is the number of cycles that a sine wave completes in one second. The more cycles completed in one second, the higher the frequency. The sketch below shows two sine waves. The sine wave in Part (a) completes two full cycles in one second. The one in Part (b) completes four cycles in one second. Therefore, the sine wave in Part (b) has twice the frequency of the one in Part(a).
The Unit of Frequency is measured in hertz’s, abbreviated Hz. One hertz is equivalent to one cycle per second; 60Hz is 60 cycles per second; and so on. The symbol for frequency is f Relationship of Frequency and Period The relationship between frequency and period is very important. The formulas for this relationship are as follows: f
=
1 ---T
T
=
1 ---f
There is a reciprocal relationship between f and T. Knowing one, you can calculate the other. This relationship is logical because a sine wave with a longer period goes through fewer cycles in one second than one with a shorter period.
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Electrical Systems, Manutronics Voltage and Current of a sine wave There are several ways to measure the value of a sine wave in terms of its voltage or its current magnitude. These are instantaneous, peak, peak-to-peak, rms, and average values. Instantaneous Value The sketch below illustrates that at any point in time on a sine wave, the voltage (or current) has an instantaneous value. This instantaneous value is different at different points along the curve. Instantaneous values are positive during the positive alternation and negative during the negative alternation. For example, in the sketch below the instantaneous voltage is 3.1V at 1 Es, 7.07V at 2.5Es, 10V at 5Es, 0V at 10Es, -3.1V at 11Es, and so on. Instantaneous values of voltage and current are symbolised by lower-case v and i, respectively.
Peak Value The peak value of a sine wave is the value of voltage (or current) at the positive or the negative maximum (peaks) with respect to zero. Since the peaks are equal in magnitude, a sine wave is characterised by a single peak value, as is illustrated below. For a given sine wave, the peak value is constant and is represented by Vp or Ip. In this case, the peak value is 8 V.
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Electrical Systems, Manutronics Peak-to-Peak Value The peak-to-peak value of a sine wave, as illustrated below, is the voltage (or current) from the positive peak to the negative peak. Of course, it is always twice the peak value as expressed in the following equations: Vpp
=
2 Vp
Ipp
=
2 Ip
Where peak-to-peak values are represented by the symbols Vpp or Ipp. In the sketch below the peak-to-peak value is 16 V.
rms Value The term rms stands for root mean square. It refers to the mathematical process by which this value is derived. The rms value is also referred to as the effective value. Most AC voltmeters display rms voltage. The 220 volts at your wall outlet is an rms value. The rms value of a sine wave is actually a measure of the heating effect of the sine wave. For example, when a resistor is connected across an AC (sine wave) voltage source, as shown in Figure (a), a certain amount of heat is generated by the power in the resistor. Part (b) shows the same resistor connected across a DC voltage source. The value of the AC voltage can be adjusted so that the resistor gives off the same amount of heat as it does when connected to the DC source. The rms value of a sine wave is equal to the DC voltage that produces the same amount of heat as the sinusoidal voltage. The peak value of a sine wave can be converted to the corresponding rms value using the following relationships for either voltage or current: Vrms Irms
= SQR(0.5) Vp = SQR(0.5) Ip
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= =
0.707 Vp 0.707 Ip
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Using these formulas, we can also determine the peak value knowing the rms value as follows: Vp
=
(1/0.707) Vrms
Vp
=
SQR(2) Vrms =
1.414 Vrms
=
SQR(2) Irms =
1.414 Irms
Similarly, Ip
To find the peak-to-peak value, simply double the peak value: Vpp
=
2.828 Vrms
Ipp
=
2.828 Irms
And
Average Value The average value of a sine wave taken over one complete cycle is always zero, because the positive values (above the zero crossing) offset the negative values (below the zero crossing). To be useful for comparison purposes, the average value of a sine wave is defined over a half-cycle rather than over a full cycle. The average value is the total area under the half-cycle curve divided by the distance of the curve along the horizontal axis. The result is expressed in terms of the peak value as follows for both voltage and current: Vavg
=
(2/H) Vp
=
0.637 Vp
Iavg
=
(2/H) Ip
=
0.637 Ip
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Electrical Systems, Manutronics Chapter 15, Induction for AC Systems When we were looking at the electromagnetic affect the technical term for this is call induction. We induced the magnetic affect by enhancing the small magnetic field generated by the current in the conductor coil. Inductance When a current flows through a coil it sets up a magnetic field around the coil. Magnets themselves have magnetic fields around them. However if a magnetic bar moves within the coil and the magnetic field crosses over the coil then a current is induced in the coil circuit which can be measured. This is the case if either the coil or the magnet moves but once the movement does not stop.
Self-inductance You saw that if you pass a current through a coil of wire then you make an electromagnet. Also, if you move a magnet into a coil of wire you will induce a voltage in the wire. You also saw that when a magnet is spun very quickly inside a coil, two pulses of voltage are produced in every revolution. These two pulses have opposite polarity. This is, in fact, an alternating voltage. If you switch on a current in a coil then this is just like pushing a magnet into the coil and it will give you an induced voltage. This effect is called 'self-inductance' because the voltage is induced by the magnetic field produced by the coil itself, not by any other magnet. Coils designed to produce a significant self-induced voltage are called inductors, although electricians often call them chokes. An inductor is nothing more than a coil of wire. The coil is often wound on an iron core which concentrates the strength of the magnetic field and so increases the inductive effect. You may have taken the back off a radio at some time and seen a coil of wire wound over a black rod. That was an inductor and the rod was the magnetic core. It was part of a tuned circuit that you need to select the radio stations. There is no self-induced voltage in an inductor while the current is steady. Induced voltages only happen when the value of the current changes causing the magnetic field to expand or contract.
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Electrical Systems, Manutronics In a dc circuit, such as in the power supply units of radios, TVs, video recorders, computers etc., the current only changes when you switch on or off. When you switch on, inductance causes a slight delay in the build up of the current. This doesn't usually matter too much except in very high speed circuits. But when you switch off it is a different story. The induced voltage will try to keep the current going and can reach very high values in its attempt to do so. This is why you will often see a spark if you watch the contacts of a switch as you switch off. You will have gathered from what I have said that induced voltages always try to keep the current steady, they do not like change. In fact an inductor acts in an electric circuit to keep the current steady rather like a flywheel acts in a machine to keep its speed steady. In ac circuits the current keeps changing, it doesn't just switch on and off, if actually keeps on reversing its direction. An inductor, like the flywheel paddle, tries its best to smooth out the current to a steady one-way flow, but it can't succeed, as the only way to smooth out ac completely is to stop it altogether. What actually happens is that inductance reduces the flow of ac current considerably. In fact, if we want to restrict the flow of an ac current it is more efficient to use an inductor than a resistor. Sometimes in electronic circuits both ac and dc flow together in the same circuit. In these conditions the inductor has a selective effect, it tends to let the dc pass freely while it tries to stop just the ac part of the current.
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Electrical Systems, Manutronics Test yourself Circle the appropriate answer A current which flows first one way then the other is called: (a) (b) (c) (d)
commuting alternating reversible bipolar
The number of complete cycles which occur every second is called the: (a) (b) (c) (d)
repetition rate waveform vibration period frequency
The frequency of the ac supplied by the national grid is: (a) (b) (a) (b)
50 Hz 100 Hz 5 kHz 50 MHz
A big advantage of using ac is that: (a) (b) (c) (d)
an alternating current gives better heating than dc it allows electricity to be generated on a large scale the low pitched hum of a transformer at 50 Hz is very soothing electronic circuits can only work with ac
No, many electronic circuits use dc, for example, transistor radios use batteries which supply dc. An inductor: (a) (b) (c)
allows an alternating current to flow but stops a dc one lets both ac and dc currents flow equally well (c) allows a dc current to flow freely but tries to stop an ac one completely blocks the flow of any current
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Electrical Systems, Manutronics Chapter 16, Capacitors Capacitors are important components in the electronics and telecommunications industries. They are essential, for example, in radio and television receivers and in transmitter circuits. We shall describe how charges and energy are stored in capacitors, the series and parallel circuit arrangements of capacitors and the charge and discharge of a capacitor through a resistor, which occurs in many practical circuits. A capacitor is a device for storing charge. The earliest capacitor was invented (almost accidentally) by van Musschenbroek of Leyden, in about 1746, and became known as a Leyden jar. One form of it is shown in diagram below (i), J is a glass jar, FF are tin-foil coatings over the lower parts of its walls, and T is a knob connected to the inner coating. Modern forms of capacitor are shown at (ii) and (iv) in the figure. Essentially all capacitors consist of two metal plates separated by an insulator.
Types of capacitor The insulator is called the dielectric; in some capacitors it is polystyrene, oil or air. Item (iii) in the above diagram shows the circuit symbol for such a capacitor; T, T are terminals joined to the plates.
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Electrical Systems, Manutronics Take a practical example to help explain the capacitors operation. The following circuits contain a lamp capacitor but two different types of electricity supply.
Battery Lamp Capacitor Mains Lamp Capacitor In our practical work a lamp dimmed in the ac circuits because the capacitor reduced the amount of current flowing in the circuit. In fact the capacitor acted like a resistor. In the dc circuit the lamp did not light. In this case the capacitor blocked the current flow. When you apply a dc voltage across a capacitor, current flows, charging up the capacitor until the charge on the capacitor is equal to the applied voltage. At this point no further current will flow. A higher value capacitor will take longer to charge up than a small one because it has more capacitance. It's a bit like filling a bucket with water from a tap. A small bucket will take less time to fill than a large bucket. In a similar way a small capacitor will charge up more quickly than a large one. In ac circuits the capacitor doesn't block the current flow. It simply charges and discharges alternately - filling up and emptying the bucket.
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A Simple Transistor Circuit The diagram above shows a simple transistor circuit. This circuit is typical of parts of circuits in electronic motor speed control, alarm circuits and transistor radios. You will notice that it has two capacitors. In this circuit the transistor is only required to pass ac signals. The capacitors are used to block any dc current which might affect the ac signal. Capacitors are often connected in circuits to block dc current. There are many shapes, sizes and types of capacitor, but they all basically do the same job. The quality, value and voltage rating of capacitors depend upon their size and upon the type of insulating material used. The chart on the next page describes the most common capacitor types. The value of capacitance is measured in farads (symbol F). Most capacitors have a capacitance value equal to just a few millionths of a farad or even smaller:
High Farad (F) microfarads (F), 1000,000 F = 1Farad, , i.e. 1E-06Farads nanofarads (nf), 1,000,000,000nF = 1Farad , i.e. 1E-09Farads picofarads (pF), 1pF = 1000,000,000,000pF, i.e. 1E-12Farads Low
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Electrical Systems, Manutronics The chart shows that the electrolytic capacitor has the largest value of capacitance. To obtain this high value, electrolytic capacitors are made in a special way, using electrolysis. This is similar to the processes used in electroplating. The construction of the electrolytic capacitor means that in most cases it must only be connected one way round. We say it is polarised and connecting it round the wrong way will destroy it. To avoid this, electrolytic capacitors have either the positive or the negative terminal marked. Because of their construction, most electrolytic capacitors are not suitable for use with ac current, though there are a few special types that are. On most electrolytic capacitors you will coloured blue and purple with arrows on them marking the negative terminals. Or in some cases the short leg is a way of identifying the negative end Capacitor colour codes Many capacitors have their value directly marked on the case, but often, especially with the very small types, using a colour code similar to that used by the resistors marks their capacitance value. (a) (b) (c) (d)
Each colour represents a number. The number represented by a colour on a capacitor is the same as the number represented by the same colour on a resistor. The first two colour bands represent a number with a third colour band to represent a multiplier. The fourth colour band represents tolerance.
If you're not familiar with resistors then it may seem a bit strange, but don't worry, it will soon become clear, but don’t miss any chapters in the notes. The table on the next page shows the capacitor colour code. The first two bands give a two figure number. For example, if the first band is yellow, this means 4. If the second band is violet, this means 7. So a capacitor starting with yellow-violet gives a number 47. Now comes the bit that makes it different from resistor colour codes. The number 47 is the value of the capacitor in microfarads. The third colour band is the multiplier which gives the position of the decimal point. For example, if the third band were coloured orange, the number 47 would have to be multiplied by 0.001 or one thousandth 1/1000. This would mean that the value of the capacitor was 47pF x 1/1000 which is 0.047EF. Capacitors aren't usually manufactured so that their capacitance value is known exactly. It doesn't matter if the value is a bit out one way or another. The fourth or tolerance band tells you how far out from its marked value the capacitor could be. Tolerance is marked as a percentage value (usually 10% or 20%). A capacitor with a 10% tolerance will have a capacitance value which will not vary by more than 10% of its marked value. There's no need to work out these tolerance values, but it is important to make sure when you are fitting or replacing capacitors that the tolerance of the
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Electrical Systems, Manutronics capacitor is the same or lower than that necessary. You can get the tolerance value from the component being replaced or from the circuit specification. Take a look at the colour-coded capacitor in the picture. You will see that it has five colour bands. Up till now I have only mentioned four colours: the fifth colour gives the working voltage of the capacitor. The working voltage is an important piece of information on a capacitor. Some capacitors are colour coded. and others have their value marked directly on them along with the working voltage. If you put a capacitor with too small a working voltage in a circuit then it will fail (I've seen some types explode!). Note: The capacitor colour code that I'm using here is one of the most widely used from RS Components. Unfortunately, there is no set standard for capacitor colour codes and there may be some minor differences between different manufacturers. These differences are usually in the application of the multiplier band.
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Electrical Systems, Manutronics Charging and Discharging Capacitor (i) shows a circuit which may be used to study the action of a capacitor. C is a large capacitor such as 500 microfarad (see later), R is a large resistor such as 100 kilohms (105 1), A is a current meter reading 100-0-100 microamperes (100EA), K is a two-way key and D is a 6 V d.c. supply.
Charging and discharging capacitor When the battery is connected to C by contact at X, the current I in the meter A is seen to be initially about 60 EA. Then as shown in graph (ii) above, it slowly decreases to zero. Thus current flows to C for a short time when the battery is connected to it, even though the capacitor plates are separated by an insulator. We can disconnect the battery from C by opening X. If contact with Y is now made, so that in effect the plates of C are joined together through R and A, the current in the meter is observed to be about 60 EA initially in the opposite direction to before and then slowly decreases to zero as shown. This flow of current shows that C stored charge when it was connected to the battery originally. Generally, a capacitor is charged when a battery or p.d. is connected to it. When the plates of the capacitor are joined together, the capacitor becomes discharged. Large values of C and R in the circuit above help to slow the current flow, so that we can see the charging and discharging which occurs, as explained more fully later. We can also show that a charged capacitor has stored energy by connecting the terminals by a piece of wire. A spark, a form of light and heat, passes just as the wire makes contact.
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Electrical Systems, Manutronics Charging and Discharging Processes When we connect a capacitor to a battery, electrons flow from the negative terminal of the battery on to the plate A of the capacitor connected to it. At the same rate, electrons flow from the other plate B of the capacitor towards the positive terminal of the battery. Equal positive and negative charges thus appear on the plates, and oppose the flow of electrons which causes them. As the charges accumulate, the potential difference between the plates increases, and the charging current falls to zero when the potential difference becomes equal to the battery voltage Vo. The charges on the plates B and A are now + Q and - Q, and the capacitor is said to gave stored a charge Q in amount. When the battery is disconnected and the plates are joined together by a wire, electrons flow back from plate A to plate B until the positive charge on B is completely neutralised. A current thus flows for a time in the wire, and at the end of the time the charges on the plates become zero. So the capacitor is discharged. Note that a charge Q flows from one plate to the other during the discharge.
A capacitor charging (resistance is showing because some is always present, even if only that of the connecting wires)
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Electrical Systems, Manutronics Capacitance The ratio of the charge on either plate to the potential difference between the plates is called the capacitance. Since Q V, then Q /V is a constant for the capacitor. Q Q C = ---Q = CV V = --V C
Capacitors in Parallel The circuit to the right shows three capacitors connected in a simple parallel arrangement. The total charge flowing in the circuit is Q = Q1 + Q 2 + Q 3 As Q= CV We can the write Q = (C1V + C2V + C3V) Q = (C1 + C2 + C3) We can now replace the capacitors in parallel with one equivalent capacitor. Q = Ce V,
where Ce is the total equivalent capacitance.
Capacitors in Series In a similar fashion the circuit to the right shows three resistors connected in a simple series circuit. The total charge flowing in the circuit is Q = C1V1 + C2V2 + C3V3 And V = V1 + V2 + V3 V=
Q --- + C1
Q --- + C2
Q --C3
1 1 1 V = [ --- + --- + --- ]Q C1 C2 C3
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Therefore the capacitors in series can be replaced by one equivalent capacitor. Q V = ---Ce 1 1 --- = --- + Ce C1
1 --- + C1
1 ---C2
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Electrical Systems, Manutronics Time constant of RC circuit. As we have seen when a capacitor charges or discharges through a resistance a certain time is required for the capacitor to charge fully or discharge fully. The voltage across a capacitor cannot change instantaneously, because a finite time is required to move charge from one point to another. The rate at which the capacitor charges or discharges is determined by the time constant of the circuit. The time constant of a series RC circuit is a time interval that equals the product of the resistance and the capacitance. The time constant is symbolised by W (Greek letter tau), and the formula is as follows:
W = RC Capacitors in Use You've now learnt how to recognise different capacitors and how they work in dc and ac circuits. In this section I'll be describing how capacitors are used in several important industrial circuits. A voltage across a capacitor causes current to flow into it. In other words, the capacitor becomes charged. When the voltage level is reduced, the capacitor will put current (electrons) back into the circuit. The action of a capacitor is similar to the springs in a car which absorb the bumps in the road and try to level out the ride (smoothing). Very large voltage variations, such as those producing sparks or arcs, can be reduced by capacitors. This is called arc suppression. Suppression circuits Another typical application of capacitors is in suppression circuits, such as across the points in the distributor of a car. The traditional name for these capacitors was 'condenser' and it is a term still often used. The capacitor is sometimes said to 'condense' the spark energy. It actually absorbs the spark energy - the spark charges up the capacitor. The capacitor then discharges into the ignition circuit, until the next high voltage spark is produced. Preventing interference Since capacitors absorb the peaks or 'spikes' in electrical supplies, they are often useful for preventing radio interference carried by the supply. Capacitors can be connected across car dynamos or alternators and across the coil for radio interference suppression, at source. A fluorescent light has a capacitor connected across its starter circuit to stop it producing interference when it switches on or off (see below). You may have noticed that capacitors are mainly used for radio interference suppression in ac circuits and arc suppression in dc circuits. But if you think about it, in both cases they're doing the same job. Radio interference is caused by arcs sending out signals at radio frequency.
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Electrical Systems, Manutronics In the case of radio interference the capacitor is carrying out secondary
suppression (after the interference has been radiated by a spark or similar). In the case of arc suppression the capacitor carries out primary suppression (before the spark can be generated and radiation of energy occurs). Safety note The voltage stored in large capacitors can be very high. A good quality capacitor can hold its charge for days, even months. You can get a nasty shock working on equipment with such capacitors, even with the supply switched off. To prevent people getting an electric shock from the terminals of a charged capacitor, a resistor is often connected across its terminals to discharge it. This type of resistor is called a bleed resistor. But don't worry, the capacitors and voltages we use in the laboratory are not large enough to give you a considerable electric shock.
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Electrical Systems, Manutronics Chapter 17, Transformers Alternating current is a major source of power in this country. One of the reasons for this is that voltage levels can be conveniently raised or lowered, to required values, using a transformer. In the national grid system shown in the diagram on the next page, you can see that on the route from the power station to your house the electricity is 'transformed' many times. The power station alternator produces electricity at about 22 000 V (22.OkV). This is then fed into a transformer and increased or stepped up to either 110 kV, 220 kV or 400 kV for transmission along overhead wires. The high voltage is essential to reduce the power lost in forcing the electricity along miles of cable. The big advantage of using ac on the grid, rather than dc, is that transformers can be used to step voltage levels up or down. Stepping up voltage for distribution systems reduces current flow, reducing power losses. As the current reaches the town the voltage is reduced or stepped down to 33 kV or 11 kV for local distribution.
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Electrical Systems, Manutronics Getting even closer to home, at your local substation another transformer further reduces the voltage to 400 V and 240 V. These voltages are present in the cables that run up your street, but only the 240 V connections are made to your house. The 400 V connections are reserved for factories and heavy industry.
Stepping down Small users too, need to raise and lower voltages and they use a transformer for the job. All of the distribution stages shown in the diagram contain at least one transformer. As you have probably realised, most electronic equipment requires only small voltages. Often voltages of 1.5 V to 24 V are sufficient. So, much of the transformer's work on this equipment is to 'step down' the voltage from 240 V to these extra low voltages. As we have seen, transformers change voltage levels in a circuit, and there are two basic types: the step up transformer and the step down transformer. But how do they work? Remember magnetism? In a previous section we showed that when an electric current passes through a wire, it creates a magnetic field? Do you also remember that the strength of the magnetic field can be increased by winding the wire into a coil? Winding the coil on an iron or suitable metal core concentrates the magnetic field down that core. If you can't quite remember these things, take another look before carrying on.
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Electrical Systems, Manutronics How does it work? Take a look at this drawing of a transformer.
You will notice that there are two coils of wire wound on two limbs of the same piece of metal or core. The coil on the left, known as the primary winding, is connected to an ac supply. A transformer will not work on dc because only a varying magnetic field can induce voltages in the secondary coil. Also, a coil has a much lower resistance to dc and sufficient current may flow and burn out the primary winding. The coil on the right is the secondary winding. Because the current is ac it flows first in one direction through the coil and then reverses and flows in the other direction. This means that the magnetic field produced also reverses, first clockwise around the core and then anticlockwise. So, the magnetic field produced by the primary winding links with the secondary winding as it builds up around the core, see diagram.
What happens next is very important. Because the strength of the field around the secondary winding is changing, an emf or voltage is induced into the secondary winding.
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Electrical Systems, Manutronics The 'turns ratio' In the transformer in diagram, the voltage induced in the secondary winding will be the same as that applied to the primary, because they have the same number of turns. Because the ratio of the number of primary turns (Np = 4) to the number of secondary turns (Ns = 4) is Np
4 4
Ns Or 1:1
then the ratio of primary volt (Vp = 240 V) to secondary volts (Vs = 240 V) is also 1:1. In other words, the ratio of primary volts to secondary volts must be the same: Np
Vp
Ns
Vs
i.e.
4 4
240 240
What would happen if we doubled the number of secondary turns, from 4 to 8? N p Vp Ns
Vs
Vs
?
Vs
Vp u
Ns Np
240 u
8 4
480V So, as you would expect, if the ratio of Np:Ns is 1:2 or in this case 240:480. In other words, this transformer has stepped up the voltage form 240 V to 480 V, simply by putting twice as many turns on the secondary as there are on the primary, see diagram.
When the secondary winding has fewer turns than the primary winding, the voltage steps down. For example, if the ratio of Np:Ns is 2:11, then the ratio of Vp:Vs will also be 2:1 .
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You may recognise the circuit symbol for a transformer which is shown on the previous page. Notice that the symbol may give you an idea of the 'turns ratio' by the number of windings drawn. Many transformers make good use of the ability to tap off a particular voltage by reducing the number of coils on one winding relative to the other. The electricity supply industry uses these tapping points to raise and lower the voltage by small amounts to compensate for voltage drops in long cables.
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Electrical Systems, Manutronics Chapter 18, AC-Power, True or Apparent We mentioned earlier that the simple equation for power in dc circuits, P = VI, does not hold good in ac circuits. This is a slightly "heavier" chapter than some of the previous ones so take it slowly, ensure you understand each "bit" as you move along. Let me give you an important rule to start off with: In an AC circuit, power is equal to the product of V x I (power = VI) only when V or I are in phase. This only occurs in a purely resistive component see diagram (a). Remember when you looked at inductance earlier in this course, you found it resisted current. This causes the current waveform to lag 90 behind the voltage waveform EL across the inductance. See diagram (b) In the capacitor circuits, where we used the water bucket analogy, the voltage across the capacitor took time to charge up to maximum value. Here the capacitor voltage EC lags the current by 90. See diagram (c).
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Electrical Systems, Manutronics A second important power rule in ac circuits is: Reactive components (inductances and capacitances) are "wattless" they do not dissipate power - only the resistive component does that. A circuit that had inductance, resistance and capacitance would have voltage and current waveforms out of phase, as you can see in the diagram above. We need to use something simpler than a mass of waveforms to help us calculate power in such an ac circuit. Let's "freeze" the waveforms at an instant in time and use a pictorial way to show the amplitude of voltages and the current and the phase angles between them. This pictorial representation is called a vector diagram.
The vector diagram above shows the current I as common to each component R, L and C. All the voltages are then shown in relationship to I. The resistive voltage ER is in phase with I. EL leads 1 by 90 [as shown in the waveform diagram (b)] and E lags I by 90' [as c shown in diagram (c)]. What we need to know now before we can calculate power is what the combined affect of voltages ER, EL and EC is i.e. one voltage resulting from combining all three. We'll call that voltage Et (E total), the resultant of ER, EL and EC. Now I want to show on the vector diagram how Et is arrived at from the other voltages. If I do that from the three voltages ER, EL and EC things get a little complicated so let me use a simplified circuit with only resistance and inductance.
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Electrical Systems, Manutronics Now we have reduced things to a single voltage Et and current I let's look at ac power. If you remember we said that reactive components L and C do not dissipate power, the only component that does is the resistance. So we can say: True (resistive) power = ER x I (watts) But if we were to measure total circuit voltage and current with a multimeter we would measure Et and I and the apparent power would be P = Et x I i.e. Apparent Power = Et x I (watts) So, from the information available about a circuit in terms of voltage and current there are two types of power. One that "looks" as if it's the power being consumed: the Apparent Power and one that is the real, actual power being consumed: the True Power Don't worry about your domestic electricity bills by the way, the wattmeter on your supply records the True Power used. Now there is a relationship between True Power and Apparent power that allows us to calculate power in any ac circuit with any mix of R, L or C. This is the: Power Factor =
True Power ----------------------Apparent Power
We can put in the values we know for True and Apparent power in terms of Et, ER and I:
Power Factor
ER I Et I
The I’s cancel out and we have Power Factor
ER Et
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If you look at the last figure you can see ER and Et and in between them phase angle >. Now the mathematical bit that can't be avoided. If you've done any trigonometry at school etc. you should be able to see that: E Power Factor =
ER also = cosine > Et
= COS > (for short) If you've not done any trigonometry don't worry about it. It's unlikely you will ever have to use it. Just remember where cos > in ac power calculations has come from if you see an ac power equation anywhere. So, to finally arrive at a statement covering ac power in any circuit from: True _ Power Apparent _ Power
CosI
True Power = Apparent Power x cos > and as Apparent Power = Et X I True Power = Et X I X cos > = Et I cos > (watts) You can see now that the equation for ac power is somewhat different from the simple dc power equation and this is because of the reactive (L and C) components in ac circuits and the phase shifts they cause between voltages and current. Note: the unit of power is the same for ac or dc - watts.
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Electrical Systems, Manutronics Well, finally to demonstrate the practical effects of True and Apparent Power let's look at an example: On a piece of industrial equipment we have a low voltage electric heater. It's circuit voltage and current are Et 10 volts and I = 2 amps. It has a resistance of 4 ohms and an inductance (or inductive reactance) of 3 ohms. The power factor, cos >, works out at 0.8. So the circuit looks like:
From our previous work: True Power = Et I cos> Substituting the values we know for Et and I and cos> we get: True Power = 10 X 2 X 0.8 = 16 watts This is what a wattmeter would measure if connected to the heater supply. But what if we measured E and I with a multimeter and did the basic (but not ac accurate) power calculation? We would get: Apparent Power = Et I =10 x 2 =20 watts So, the True Power and the power we want to pay for is 16 watts. The Apparent Power is higher and is 20 watts.
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Electrical Systems, Manutronics So you can see, in many instances, using the basic power equation, power = VI is satisfactory as long as you remember it is not an accurate equation for ac power and will result in a higher figure than the actual TRUE POWER. Incidentally, the electricity companies, when supplying electricity to an industrial site, have to provide cables and switch gear rated high enough to cater for any load and any mix of inductance, from motors, light fittings etc, and any capacitance in the works system. So the gear they install is rated more highly than would be required for purely resistive loads. The equipment costs more, so the electricity company charges the customer more for providing the supply. However if the company can improve the power factor that their load shows to the electrical supply, and make it look more resistive, nearer True power, then the electricity company will reduce the cost of supply to the customer. As most factory electrical loads are inductive, due to motor loads (lots of windings) and light fittings, a capacitive effect needs to be included in the system which will cancel out the inductive effect i.e. You can see that EL and EC act in opposite directions. If they were equal they would cancel out and there would only be ER and I, in phase, a purely resistive load. So, on your works site you may see, in switch rooms or near switch gear, large metal boxes, often tubular, standing some feet high. These can be large condensers (or capacitors) connected to the works power supplies to cancel out or reduce the effect of the inductive component of the load, to improve power factor, and bring down costs. Well that's as far as we want to go on ac power in this module. I'm sorry it got slightly mathematical in some places, that's difficult to avoid. But don't worry about the maths, in most jobs you'll never need them. What you need to remember is that there is True Power and Apparent Power in ac circuits and: True Power = Et I cos> or VI 1 cos> Have a mind's eye view of the vector diagram and how it is linked to the waveform diagram and the phase shifts between voltage and current caused by inductance and capacitance in the circuit. Again, don't strive to remember every detail, you can refer back to this workbook any time you need to.
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Electrical Systems, Manutronics Chapter 19, NATIONAL RULES FOR ELECTRICAL INSTALLATIONS SCOPE 11.1 These Rules apply to electrical installations such as those of.. x residential premises x commercial premises x public premises x industrial premises x agricultural and horticultural premises x prefabricated buildings x caravans and caravan sites and similar sites x construction sites, exhibitions, fairs and other temporary installations (future) x docks and marinas (future) 11.2 These Rules apply to the following: x circuits supplied at nominal voltages up to & including 1000Vac or 1500V dc x circuits, other than the internal wiring of apparatus, operating at voltages exceeding 1000V and derived from an installation having a voltage not exceeding 1000V a.c., e.g., discharge lighting, electrostatic precipitators x any wiring not specifically covered by the specifications for equipment x fixed wiring for telecommunication, signalling, control and the like (excluding internal wiring of apparatus) 11.3 These rules do not apply to: x electrical traction equipment x electrical equipment of automobiles x electrical equipment on board ships x electrical equipment in aircraft x installations in mines and quarries x radio interference suppression equipment, except in so far as it affects the safety of the installation -systems for distribution of energy to the public, or power generation and transmission for such systems x lightning protection of buildings Note: Atmospheric phenomena are covered in so far as effects on the electrical installations are concerned (e.g., with respect to selection of lightning arresters). 11.4 These Rules apply to electrical equipment only as far as its selection and application in the installation is concerned. This includes prefabricated assemblies of electrical equipment.
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Electrical Systems, Manutronics OBJECT 12.1 These Rules lay down the requirements for the design and erection of electrical installations so as to ensure safety and proper functioning for the use intended, 12.2 Chapter 13 of these Rules states the fundamental principles. It does not include detailed technical requirements which may be subject to modifications on account of technical developments. 12.3 Part 3 and subsequent parts of these Rules deal with technical requirements, the observance of which is intended to ensure that the electrical installations conform to the fundamental principles of Chapter 13.
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Electrical Systems, Manutronics FUNDAMENTAL PRINCIPLES FOR SAFETY OF ELECTRICAL INSTALLATIONS General These Rules are intended to ensure the safety of persons, livestock and property against dangers and damage that may arise in the reasonable use of electrical installations. In electrical installations, the major risks are shock currents and excessive temperatures likely to cause fires, bums and other injurious effects. Risk of physical injury can be caused by electrically driven mechanical equipment. Protection against direct contact Persons and livestock shall be protected against dangers that may arise from contact with live parts of the installation. This protection can be achieved by one of the following methods: x preventing a current from passing through the body of any person or any livestock x limiting the current that can pass through a body to a value lower than the shock current Protection against indirect contact Persons and livestock shall be protected against dangers that may arise from contact with exposed conductive parts. This protection can be achieved by one of the following methods: x preventing a fault current from passing through the body of any person or any livestock x limiting the fault current that can pass through a body to a value lower than the shock current x automatic disconnection of the supply on the occurrence of a fault likely to cause a current to flow through a body in contact with exposed conductive parts, where the value of that current is equal to or greater than the shock current. Protection against thermal effects in normal service The electrical installation shall be so arranged that there is no risk of ignition of flammable materials due to high temperatures or electric arcs. There shall be no risk of bums to persons or livestock during normal operation of electrical equipment. Protection against over current Persons and livestock shall be protected against injury, and property shall be protected against damage due to excessive temperatures or electromechanical stresses caused by any over currents likely to arise in live conductors.
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Electrical Systems, Manutronics This protection can be achieved by one of the following methods: x automatic disconnection if an over current attains a dangerous value taking into account its duration x limiting the maximum over current to a safe value and duration. Protection against fault currents Conductors, other than live conductors, and any other parts intended to carry a fault current shall be capable of carrying that current without attaining excessive temperatures. Note 1:
Particular attention should be given to earth fault currents
Note 2:
For live conductors, compliance with the previous clause assures their protection against fault currents, including over currents caused by faults.
Isolation and switching Effective means shall be provided in the appropriate locations for the isolation of the supply from every installation, circuit and item of equipment in order to prevent or remove danger. Protection against environmental conditions (external influences) Equipment shall be suitable for the expected adverse conditions of its environment or alternatively it shall be provided with the necessary supplementary protection. Equipment in areas of fire risk shall be so constructed and erected and where necessary provided with supplementary protection so as to prevent danger. Verification and certification After completion every installation shall be subjected to a process of inspection and testing in order to verify that it complies so far as is reasonably practicable with the requirements of these Rules. An approved type of certificate shall be provided stating that the installation complies with these Rules.
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Electrical Systems, Manutronics DEFINITIONS The following definitions apply for the purposes of these Rules. For the definitions of other terms, reference should be made to IEC Publication 50 "International Electrotechnical Vocabulary" and IEC Publications dealing with the particular subjects concerned. The definitions are given in alphabetical order. Accessory: A device, other than current-using equipment, associated with such equipment or with the wiring of an installation. Ambient Temperature: The temperature of the air or other medium where the equipment is to be used. Appliance: Any device that utilises electricity for a particular purpose, excluding a luminaire or an independent motor. Arm's Reach: A zone extending from any point on a surface where persons usually stand or move about, to the limits which a person can reach with the hand in any direction without assistance. Authorised Person: A person who is competent for the particular purposes of these Rules in relation to which the expression is used and who is also either the occupier, or a contractor who is for the time being under contract with the occupier, or a person employed, appointed or selected by the occupier or such contractor, to carry out work or duties incidental to the generation, transformation, conversion, switching, controlling, regulating, storage, transmission, distribution or use of electrical energy. Barrier: A part providing protection against direct contact from any usual direction of access. Basic Safety Insulation: The insulation applied to live parts for basic protection against electrical shock. Note 1: Basic safety insulation can serve also as the insulation necessary for operation of the equipment Note 2: Basic safety insulation will normally need supplementary measures in order to complete the protection against electrical shock. Bonding: See Equipotential Bonding. Breaking Capacity: A value of current that a protective device is capable of breaking at a specified voltage and under prescribed conditions of use and operation.
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Electrical Systems, Manutronics Building Void: A space within the structure or components of a building, which may be accessible at certain points. Bunched Cables: Two or more cables contained within a single conduit, duct, ducting or trunking or, if not enclosed, not separated from each other. Cable Channel: An enclosure for cables consisting of a horizontal duct with removable covers that permit inspection of the cables. It may form part of the building structure. Cable Coupler: A means enabling the connection, at will, of two flexible cables. It consists of a connector and a plug. Cable Ducting: A duct exclusively for cable. Cable Tray: A cable support consisting of a continuous base with raised edges and no covering. Note: A cable tray may be perforated or non-perforated. Cable Trunking: See Trunking. Cable Trench: A temporary opening in the ground to permit the burial of a cable. Cable Tunnel: An enclosure or corridor containing appropriate supporting structures and/or enclosures for cables, and having dimensions allowing persons to pass freely throughout the entire length. Caravans and Caravan Parks: See Section 708, Clause 708-2 Circuit: Part of an electrical installation supplied from the same origin and protected against over currents by the same protective device. Circuit Breaker: A mechanical device capable of making, carrying and breaking currents under normal circuit conditions and also capable of making, carrying for a specified time, and breaking currents under specified abnormal circuit conditions such as those of short circuit. Class I Equipment: Equipment having basic insulation throughout, and depending on the earthing of exposed conductive parts for protection against indirect contact in the event of failure of the basic insulation. Class II Equipment: Equipment having double insulation or reinforced insulation, or a combination of these throughout, and whose intermediate parts are protected by supplementary insulation so that there is no risk of indirect contact in the event of failure of basic insulation. Note: Class II equipment is marked with the symbol
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Class III Equipment: Equipment that will not give rise to electric shock because it is designed for supply from SELV, (Safety Extra Low Voltage 50Vac or 120Vdc) and in which voltages higher than extra-low voltage are not generated. Note: This equipment is normally not earthed, but in special cases Class III equipment may have earthing facilities in which event it would be supplied by PELV (q.v.) Conductive Part: A part capable of conducting current although it may not necessarily be used for carrying service current. A system of tubing intended for enclosing cables and wires in order Conduit: to protect them from mechanical damage, and which allows them to be drawn-in and withdrawn. Contactor (mechanical): A mechanical device having only one position of rest, operated electromagnetically and capable of making, carrying, and breaking currents under normal circuit conditions, including overload operations. Note:
A contactor is usually intended to operate frequently.
Conventional Operating Current (of a protective device): A specified value of the current which causes the protective device to operate within a specified time, designated conventional time. Conventional Touch Voltage Limit (symbol UL): The maximum value of the touch voltage permissible to be maintained indefinitely in specified conditions of external influences. Current-Carrying Capacity (of a conductor): The maximum current that can be carried continuously by a conductor under specified conditions without its steadystate temperatures exceeding a specified value. Current-Using Equipment: Equipment intended to convert electrical energy into another form of energy, for example, light. heat or motive power. Danger to health, or danger to life or limb from shock, bum, or injury Danger: from mechanical movement of electrically driven equipment to persons (and livestock where present), or from fire, attendant upon the use of electrical energy. Design Current (of a circuit): The current intended to be carried by a circuit in normal service. Direct Contact: Contact of persons or livestock with live parts.
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Electrical Systems, Manutronics A mechanical switching device that, in the open position, Disconnector: complies with the requirements specified for the isolating function. Distribution Board: An assembly of protective devices, including two or more fuses or circuit breakers, arranged for the distribution of electrical energy to final circuits or to other distribution boards. Distribution Circuit (of buildings): A circuit supplying a distribution board, an item of switch gear, or an item of control gear. Double Insulation: Insulation comprising both basic safety insulation and supplementary insulation. Note: See also Class II equipment. AND THE LIST GOES ON………………….
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Electrical Systems, Manutronics IEC CLASSIFICATION SYSTEM FOR ENCLOSURES IEC classification system for enclosures intended to prevent contact with live or moving parts inside the enclosure, to prevent the ingress of solid foreign bodies including dust, and to prevent the ingress of moisture. This classification system - the "IP" system - is taken from IEC publication 529. The first digit indicates the degree of protection against contact by persons with parts inside the enclosure as well as that of the protection against ingress of solid foreign bodies. The second digit indicates the degree of protection against ingress of moisture.
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Electrical Systems, Manutronics 525 VOLTAGE DROP IN CONSUMERS' INSTALLATIONS 525.1 Under normal service conditions the voltage at the terminals of any powerusing equipment shall be not less than the lower limit specified in the relevant standard, or in the absence of such a standard, the lower limit specified by the manufacturer. 525.2 The cross-sectional area of every current-carrying conductor shall be such that the drop in voltage between the main supply point and any point in the installation is not more than the following when the conductors are carrying normal full-load current: a) In installations rated not greater than 80A, and parts of larger installations rated not greater than 80A: 4% of nominal voltage. This requirement does not apply to extra-low voltage circuits. b) In other installations the voltage drop within the installation shall not exceed a value appropriate to the safe functioning of the associated equipment in normal service. Note: A greater voltage drop is admissible for: x x
motors during starting periods, and other equipment with high inrush current.
provided in both cases it is ensured that the voltage variations remain within the limits specified for the equipment concerned. 525.3 The following temporary conditions are excluded from the requirements of this section: x x
voltage transients, and voltage variations due to abnormal operation.
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Electrical Systems, Manutronics CURRENT-CARRYING CAPACITIES OF CONDUCTORS Introduction The Rules in this section are intended to ensure a satisfactory operating life for cables and insulated conductors having regard to the conductor temperatures and ambient temperatures. This section applies to the following types of conductors: x insulated conductors and unarmoured cables x armoured multi-core cables containing all the conductors of a single-phase circuit or of a three phase circuit x cables with metallic screen or sheath x flexible cables x mineral insulated cables Other considerations affecting the choice of size of conductor are dealt with in other sections, for example, protection against electric shock, protection against over currents, voltage drop limits, and motor starting currents. General The steady-state temperature of a cable or insulated conductor carrying current in normal operation shall not exceed the values given in Table 52B. The corresponding maximum permissible value of current shall be selected in accordance with Sub-clause 523.1.2 or determined in accordance with Subclause 523.1.3. TABLE 52B MAXIMUM STEADY-STATE TEMPERATURE OF CABLES AND INSULATED CONDUCTORS
Note: 1 For mineral insulated cables, higher continuous operating temperatures are permissible depending upon the temperature rating of the cable, its terminations, the environmental conditions and other external influences; the manufacturer's instructions will normally contain such information. Note: 2 It is recommended that the temperatures of bare conductors should not, in general, exceed 90'C in normal operating conditions.
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Electrical Systems, Manutronics STANDARD TYPES OF WIRES AND CABLES NORMALLY USED. Table 52A in Chapter 52, "Acceptable Types of Wiring System", groups wires and cables under four broad headings, (apart from bare conductors). This annex lists the normal types of cable used in Ireland, and gives the numbers of the appropriate national standards - IS, BS, and VDE. The international CENELEC code is also given where it applies. Cables not listed are not excluded provided: x they have an equivalent level of safety and performance, or x they comply with CENELEC harmonised standards. CENELEC CABLE CODING SYSTEM The international CENELEC Code for power cable types is as follows. First Digit: (Status)
H: A:
Harmonised Cable Type Approved National Type
Second/Third Digit: 03: 05: (Voltage level) 07:
300/30OV 300/500Y 450/75OV
Fourth Digit: (core insulation)
V: R: S:
PVC Rubber, including butadiene Silicone rubber
Fifth Digit: (Sheathing)
V: R: N: J:
PVC Rubber, including butadiene PCR (Polychloroprene rubber) Glass fibre braiding No sheath
Sixth Digit:
U: R: K:
Solid Stranded Fine Stranded
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Electrical Systems, Manutronics INTRODUCTION to Protection of Circuits Every circuit, except where specifically exempted, is required to be protected against over current. Over current may be either overload or short-circuit current, and the respective requirements are given separately. In certain cases overload protection or short-circuit protection may not be necessary (see Section 473). General-purpose fuses and miniature circuit breakers (MCBS) usually provide complete over current protection. Requirements for the application of this measure are given in Section 473. PROTECTION AGAINST OVERLOAD CURRENT General A protective device shall be provided to interrupt overload currents flowing in the circuit conductors before such currents could cause a temperature rise detrimental to insulation, joints, terminations or surroundings of the conductors. PROTECTION AGAINST SHORT-CIRCUIT CURRENT Note: These Rules take into account only cases of short circuit anticipated between conductors belonging to the same circuit. General Protective devices shall be provided to interrupt any short-circuit current flowing in the conductors before such a current could cause danger due to thermal and mechanical effects produced in conductors and connections. Determination of prospective short-circuit current. The prospective short-circuit current at every relevant point of the installation shall be determined either by calculation or by measurement. Where appropriate the value of short circuit current at the main supply point shall be obtained from the supply authority. Characteristics of short-circuit protective devices. Each short-circuit protective device shall meet the conditions specified in Subclauses 434.3.1 and 434.3.2. The breaking capacity shall be not less than the prospective short-circuit current at the place of installation, except where the following condition applies: A lower breaking capacity is permitted if another protective device having the necessary breaking capacity is installed on the supply side. In that case, the characteristics of the devices shall be co-ordinated so that the energy letthrough by these two devices does not exceed that which can be withstood without damage by the device on the load side and the conductors protected by these devices.
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Electrical Systems, Manutronics Note: In certain cases other characteristics may need to be taken into account, such as dynamic stresses and arcing energy, for the device on the load side. Details of the characteristics needing co-ordination should be obtained from the manufacturers of the devices concerned. Fault-loop impedance The over current protective device for each circuit shall be selected with respect to the fault loop impedance of that circuit so that if a fault of negligible impedance occurs between a phase conductor and a protective conductor or an exposed conductive part, automatic disconnection of the supply will take place within the time specified. The following condition fulfils this requirement: ZL x I a < Uo where ZL
is the impedance 0 of the fault loop comprising the source, the live conductor up to the point of the fault and the protective conductor between the point of the fault and the source, and
Ia
is the current A ensuring the automatic operation of the disconnecting protective device in accordance with Sub-Clause 413.1.6.5 or Sub-Clause 413.1.6.6, and
Uo
is the nominal voltage V to earth a.c. rms
For final circuits that supply: x -socket-outlets having rated currents not exceeding 63 A, or x -hand-held Class 1 equipment, or x -portable equipment intended for manual movement during use the disconnecting time shall not exceed the values given in Table 41 A.
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Electrical Systems, Manutronics Insulation resistance The insulation resistance of conductors, with all switches closed and with all fuses in place shall be measured before the installation is made live. x x x
between all live conductors connected together and the protective conductor, and between all phase conductors connected together and the neutral conductor, and between each phase conductor and all the other live conductors connected together.
Note 1: For this test items of fixed equipment and electric lamps should be removed or disconnected. Where it is impractical to remove lamps or disconnect equipment, the switches controlling them may be left open. Note 2: Items of equipment that may be damaged by insulation resistancetesting must be disconnected before the test commences. Such items of equipment include semiconductors, diodes, etc. Where electronic devices cannot be easily disconnected, the phase and neutral conductors must be connected together at the device, to enable the test to earth to be made. Note 3: Large installations may be divided into groups of not less than fifty points each and each group tested separately. The insulation resistance, measured in Table 61A, is satisfactory if each circuit, without the appliances, has an insulation resistance not less than the appropriate value given in Table 61A. Measurements shall be carried out with direct current. The testing apparatus shall be capable of supplying the test voltage specified in Table 61 A when loaded with 1mA. The insulation resistance of items of fixed equipment shall be measured separately: x x
-between electrical circuits and exposed conductive parts, and -between electrical circuits and the protective conductor.
In the case of the protective measure "protection by electrical separation", (Clause 413.5) the separation of the live parts from those of other circuits and from earth shall be verified by a measurement of the insulation resistance. The resistance values obtained shall be in accordance with Table 6 1 A, with the appliances, as far as possible, connected. Note: In the case of IT systems, the circuit rated voltage is that between phases.
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Electrical Systems, Manutronics Health & Safety List five kinds of dangers to persons (from electricity or electrical machinery) that must be guarded against and briefly explain each one. (i) (ii) (iii) (iv) (v)
Death or injury by electrocution, that is, by actually coming in contact with voltages above 20 or 30Volts. Generally speaking the higher the voltage the more dangerous it is. Being burned by electrical arcs and sparks. These are frequently caused by short circuits when wires being handled live. Injury to eyes either from the radiation of an intense arc near the eye or from molten metal or such being thrown into the eye from the nearby explosive force of a short circuit or similar trouble. Falling or a similar blow (from a ladder for instance) caused by electrical shock or other electrical trouble. This may involve two or more persons working together. Injury from electrically driven or controlled machinery for example from a motor being inadvertently started.
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Electrical Systems, Manutronics Test Yourself Results Chapter 1, Test yourself Results. Answer to Test 1 You are correct if you wrote down: 1. Volume 2. Acceleration (velocity/time) 3. Density (mass/volume) 4. Force (mass × acceleration) 5. Charge (current × time)
L3 L/T2 M/L3 M·L/T2 I·T
Answer to Test 2 Now find the dimensions of these: 1. Pressure (force/area) 2. (volume)2 3. Electric field (force/charge) 4. Work (in 1-D, force × distance) Energy (e.g., gravitational potential energy = mgh = mass 5. × gravitational acceleration × height) 6. Square root of area
M·L-1·T-2 L6 M·L·I-1·T-3 M·L2/T2 M·L2/T2 L
NOTE: Dimensions of work = Dimensions of energy
Answer to Test 3 You are doing very well if your answers are: 1. Volume m3 2. Density kg/m3 3. Pressure kg·m-1 s-2 4. Energy kg·m2/s2 Answer to Test 4 The correct responses are: 1. Energy joule (J) 2. Power watt (W) 3. Kg·m/s2 newton (N) 4. Charge coulomb (C) 5. Force newton (N) 2 2 6. Kg·m /s joule (J)
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7. 8. 9. 10. 11.
Kg·m2/s3 A·s (cycles)·s-1 J/s Frequency
watt (W) coulomb (C) hertz (Hz) watt (W) hertz (Hz)
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Electrical Systems, Manutronics Chapter 10, Test yourself Results. 1.
True. Friction is a major cause of electrostatic charge.
2.
True. Just walking across a carpet especially if it is dry and contains synthetic fibres, can charge you up.
3.
False. Electrostatic sparks can cause fire or explosion in certain environments.
4.
False. A foggy day usually means damp air. The moisture in the air would tend to discharge electrostatic charges.
5.
False. It's usually a good idea to touch earthed metal to discharge yourself, but make very sure that there is no risk of touching any live metal at the same time because this would give you a dangerous shock.
6.
True. The electrostatic attractions helps to give a more even coverage and to reduce waste of paint.
7.
True. Nylon (or any other artificial fibre) tends to build up electrostatic charge.
8.
False. Actually if metal is insulated from earth then it can build up a big charge just as you can yourself, after all your body is an electrical conductor, which is why electric shocks happen if you're careless (i.e. current passes through your body). It is good wiring practice to 'bond' isolated bits of metal to earth using a earthwire.
9.
True. Earthed wristbands are used where very static sensitive components are being fitted into circuit boards.
10
True. Lightning is a natural form of electrostatic discharge.
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Electrical Systems, Manutronics Chapter 15, Test yourself Results. A current which flows first one way then the other is called: (b)
alternating
The number of complete cycles which occur every second is called the: a) repetition rate No, this could mean per hour, or per minute. (b) waveform No, this refers to shape rather than frequency. (c)
vibration period
No, this refers to the time for one cycle, not how many there are in a second. (d)
frequency
The frequency of the ac supplied by the national grid is: (a) 50 Hz A big advantage of using ac is that: (a)
an alternating current gives better heating than dc
Either, ac or dc could be used equally well for heating. (b) it allows electricity to be generated on a large scale (CORRECT ANSWER) (c)
the low pitched hum of a transformer at 50 Hz is very soothing
The hum is irrelevant and in any case is usually considered to be a nuisance. (d)
electronic circuits can only work with ac
No, many electronic circuits use dc, for example, transistor radios use batteries which supply dc.
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An inductor: (a)
allows an alternating current to flow but stops a dc one
No, this describes the action of a capacitor. (b)
lets both ac and dc currents flow equally well
No, this describes a resistor. (c)
allows a dc current to flow freely but tries to stop an ac one
Yes, this is the correct answer. (d)
completely blocks the flow of any current No this is incorrect.
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Electrical Systems, Manutronics References I wish to acknowledge the following books were used during the construction of these lecture notes. Any material referred therein is the ownership and copyright of the author or publisher. Fraser C, “Integrated Electrical and Electronic Engineers for Mechanical Engineers”, published by McGraw-Hill Book Company. Hughes, McKenzie I, “Hughes Electrical Technology”, published Longman Scientific and Technical. Floyd T, “Electrical Circuit Fundamentails” published by Merrill Publishing Company. Crompton AJ, “ Basic Electromagnetism and its application”, published by Van Nostrad Reinhold Ltd.
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