Motors

The Heart of Your Workshop Presentation for the SCWG 19 November 2013 Roy Mitton, FEC, P.Eng. Sr Member IEEE

If the Heart Stops

So does productivity!

Discussion Topics Brief History What is a Motor? Types of Motors Typical uses of each Analysis if won’t run Maintenance

Ancient History

More Recent History

(Very Brief)

* 1824 – a French physicist formulated the existence of rotating magnetic fields * 1861 – James Maxwell derived his famous four equations that described how electric and magnetic fields are generated and altered by each other * 1879 – first primitive induction motor * 1888 – Telsa patented a working motor * 1892 – Westinghouse developed the first practical induction motor * 1896 – Westinghouse and General Electric signed a crosslicensing agreement for the squirrel-cage rotor

What is a Motor? * Device that converts electrical energy to mechanical energy * A Motor is the opposite of a generator!

Generator Motor or Both?

What are the Principle Parts of a Motor? Other Components Not all motors have these

Basic All motors have these

* Stator * Rotor * Magnetic Field * Permanent magnet * Field Winding * Electromagnet

* Armature Winding * Frame

* * * * * * *

Brushes Slip Rings Commutator Exciter Starting Capacitor Running Capacitor Bearings

Stator

Rotor

Complete Motor

A Little (very little!) bit of theory * Voltages are generated in windings by rotating the winding mechanically through a magnetic field * Or, by mechanically rotating a magnetic field past a winding * Interaction between an electric motor’s magnetic field and winding currents will generate force within the motor.

A motor can be: * Powered by direct current (DC) sources * * * *

Batteries Wind generator Photovoltaic or Solar Cells Rectifiers

* Powered by alternating current (AC) sources * Power grid * Inverters * generators

Simple DC Motor

Simple AC Motor

Basic Types of Motors * DC (direct current) Brushed Brushless * AC/DC Universal * AC (alternating current) Synchronous Induction

What are the Differences And Why Different Types?

Design of Motor * Field Winding on Rotor or Stator? * This makes the magnetic field

* Armature Winding on Rotor or Stator * This is the coil – electric field

* Life expected or needed * Cost

Use of Motor * Type of power supply * Start-up Requirements * What is the initial load?

* Running Requirements * How will the load vary?

* Environment * Need to be fully enclosed such as a submersible?

Brushed DC Motors Advantages

Disadvantages

• Simple • Brush speed maintenance control • Medium life • Highest • Costly brushes torque at and low speed commutator

Typical Applications

• Steel mills • Paper making machines • Treadmill exercisers • Electric trains (traction motors) • Toys • Consumer appliances

Brushless DC Motors

Advantages

• Long lifespan • Low maintenance • High efficiency

Disadvantages

• High cost • Requires computer control

Typical Applications

• Hard disk drives • CD/DVD players

Universal AC/DC Motors Advantages

• High starting torque • High speed • Easy speed control • Compact • Lightweight

Disadvantages

Typical Applications

• Brush • Handheld power maintenance tools • Shorter lifespan • Blenders • Noisy • Vacuum cleaners • Only small ones are economical

Universal AC/DC Motors Field and Rotor Series Circuit

Synchronous AC Motor Wound Rotor Advantages

• Efficient • Low power factor

Disadvantages

• Costly

Typical Applications

• Industrial • Electric utility

• pf correction • Pumped storage

Hysteresis Advantages

Disadvantages

• Accurate • Very low speed control efficiency • Low noise • expensive

Typical Applications

• Clocks, timers • Sound recording equipment

Induction AC Motor

* Electric current in the rotor needed to produce torque is induced by electromagnetic induction from the magnetic field of the stator winding * No additional current or energy is needed for the transfer of energy from stator to rotor * Essentially it is a transformer in motion

Induction Motor Types * Three-phase * Wound Rotor * Requires slip rings * Enables external resistor or contactors for control * Becoming less common due to computer controls

* Squirrel Cage Rotor * Most common – 90% of all 3-phase AC induction motors

* Single-Phase * Squirrel Cage Rotor

Induction Motors

The Worlds Workhorses

Advantages

Low cost Robust Reliable Broad range of sizes High starting torque Can be variable speed or variable torque • Low maintenance • Direct AC connection • • • • • •

Disadvantages

• High starting current • Relatively low efficiency

Typical Applications

• Pumps, fans, blowers, compressors • Stationary power tools • Appliances

Single-Phase Induction Motor

* Probably more in use today than all the other types put together * Least expensive * Lowest maintenance

* Only one stator winding (the main winding) * Single phase power supply * Rotor is a squirrel cage

However, one main problem ---

It is not Self Starting

* With single phase supply, the main winding carries AC and produces a pulsating magnetic field * The rotor is energized by induction but because of the pulsating field, no torque is generated * The rotor will vibrate but not rotate * Therefore some starting mechanism is required to give a kick start

Single Phase Induction Motor Starting Without Start Mechanism

Split Phase Starting

Single Phase Induction Motor Starting – Most Common

Capacitor Start Capacitor Start Capacitor Run

Torque-Speed Curves for Various Starting Circuits

That’s Enough Theory What if a Heart Stops?

Need to Diagnose

Where do you Start? What type of motor? What Happened? What is your confidence to investigate and/or resolve? What is your time worth?

What Type of Motor?

Small hand tools

Probably a Universal AC/DC * Brushes wear out * Wont start or intermittent starting * Excessive sparking * Buy new brushes and replace

* Faulty trigger/switch * If you can’t test/replace yourself - toss it

* Smoke and/or excessive heat * Burned out winding - toss it

Electricity or Smoke?

What Type of Motor?

Small stationary machines * If it has brushes * Brushes wear out * Wont start or intermittent starting * Excessive sparking

* Buy new brushes and replace

* If no brushes, probably an AC Induction < one HP * Some are standard NEMA sizes and replacement is probably available but likely expensive from manufacturer * If import machine, probably non standard odd-ball * Replace tool may be cheapest option

What Type of Motor?

Large Stationary Machines

* Single phase, AC Induction, usually less than 5 HP * Most common failures: * Starting and/or run capacitors * Easy to replace

* The centrifugal switch and/or switch contacts * A little more involved

* Winding * Shop repair or replace motor

Induction Motor

Won’t start at all and makes no noise * Suspect supply problems * Circuit breaker that supplies the motor * Wiring – loose or broken * Start switch

* Test these with a multimeter * Measure voltage * Test continuity

* If all above OK, then probably a failed (open) winding * Replace motor

Induction Motor

Tries to Start and then stops

* The stopping is probably a tripped circuit breaker * What caused it to trip? * A breaker trips on high current * Higher inrush and/or shorted winding. Lets look at a breaker curve:

* Something jamming the motor * Remove the problem! * Starting and/or run capacitors * Easy to replace * The centrifugal switch and/or switch contacts * A little more involved

Capacitor Start and Run

Thermal-Magnetic Time/Current Characteristic Curve

Time

Multiples of Rated Current

Single Phase Induction Motor

Starting Capacitor

Single Phase Induction Motor

Centrifugal Switch

Single Phase Induction Motor

Centrifugal Switch

Induction Motor Stops unexpectedly while running

* Probable overload * Feeding material too fast? * Overheated * If motor has a “red” overload button, wait 5-10 minutes and push to reset

* If trips again, investigate further. Do not keep resetting and trying again!

Induction Motor Any other symptoms – e.g. smell, smoke?

* Obviously there is heat * Sawdust somewhere it shouldn’t be? * Loose connection and sparking

* If lots of smoke, most likely, a “cooked” winding from a short circuit

Tough Luck!

Maintenance

* This is integral with safety * * * *

Sharp tools Proper alignment Clean shop Clean Machines

* With motors, not a lot you can do * Replace brushes if excessive sparking or ozone smell * Vacuum and/or blow out dust

Dust in the Motor

Dust in the Motor Open up and Clean

References * Electric Machinery, A.E. Fitzgerald & Charles Kingsley, Jr. Second Edition * AC Induction Motor Fundamentals, Microchip Technology Inc., AN887, Rakesh Parekh * Wikipedia.org, Various articles: Wound Rotor Motor, Electric Motor, Universal Motor, Brushed DC Electric Motor, Induction Motor, AC Motor, Alternator, Linear Motor * Electronics.howstuffworks.com, How does a brushless electric motor work? * Iiutaimottola.com/Tools/Motor.htm, What to do When the Motor in Your Woodworking Power Tool Fails

Thanks for Listening

And Keep your Heart Pumping! Questions?

Cheers!

And Every Day is a Good Day