Gustafson, Robert J., and Mark T. Morgan. 2004. Electric Motors. Chapter 8 in Fundamentals of Electricity for Agriculture, 4rd edition, 205-248. St. Joseph, Michigan: ASAE. © American Society of Agricultural Engineers.

CHAPTER 8

ELECTRIC MOTORS 8.1 ADVANTAGES OF ELECTRIC MOTORS One of the principal advantages of electrical energy is the ease by which it can be converted to mechanical energy. Over 60% of the electrical energy generated in the U.S. is used by electric motors, according to the Department of Energy. The electric motor is an efficient means of converting electrical energy into mechanical energy. As shown below, efficiency of an electric motor surpasses that of both gasoline and diesel engines. Approximate Efficiency Electric Motor 50-99% Gasoline Engine 25% Diesel Engine 40% Electric motors have many advantages over energy, including: • Low initial cost • • Relatively inexpensive to operate • • Easy to start • Capable of starting a reasonable load • • • Can be automatically and remotely controlled • • Capable of withstanding temporary • overloads

other means of producing mechanical Long life, many motors are designed for 35,000 hours of operation Compact Simple to operate Low noise level No exhaust fumes Minimum of safety hazards

206

CHAPTER 8 ELECTRIC MOTORS

To make use of these advantages, we need to understand the basic principles of how an electric motor converts electrical energy to mechanical energy, the characteristics of various types of motors, how some of the characteristics are measured, what characteristics can be determined by nameplate data, and how motors are controlled and protected. These topics will be addressed in the following sections. For discussion, electric motors are often classified in several ways. One classification is by the type of electrical service required; for example, single-phase alternating current, three-phase alternating current, or direct current. Other classification systems are based on such items as type of starting mechanism, rotor style, frame or enclosure style, application and power output.

8.2 AC MOTOR PRINCIPLES The vast majority of electrical motors used in homes and on farms are alternating current motors. To understand the principles of operation of a simple ac motor, a brief review of three basic electrical principles is in order. They are: properties of electromagnets, electromagnetic induction, and alternating current. An electromagnet can be produced by winding insulated wire around a soft iron core. When current passes through the coil of wire, a magnetic field is produced with a north (N) pole at one end of the iron core and a south (S) at the other. The orientation of the N and S poles is dependent on the direction of current flow and changes each time the current changes direction. It is important to remember that the electromagnet produces a magnetic field only when current is flowing in the coil. Induction is the phenomenon by which a current is induced in a conductor as it passes through a magnetic field or as the field varies around the conductor. As discussed in Chapter 4, the direction of current flow depends on the direction of the wire movement and the orientation of the magnetic field. The magnitude of the induced voltage is controlled by (a) the strength of the magnetic field, (b) the rate at which the flux lines of the magnetic field are being cut by the conductor, and (c) the number of conductors cutting across the magnetic field. Current which periodically changes its direction of flow is alternating current. Current in the U. S. is generally at 60 Hz, or cycles per second, meaning the current changes direction of flow 120 times each second. Combining the principles reviewed, operation of an inductive-type electric motor can be shown. An electric motor is designed with a stationary part called a stator and a rotating part called a rotor. In some texts, the rotor is referred to as an armature. The stationary section, stator, contains pairs of slotted cores made up of thin sections of soft iron. The cores are wound with insulated copper wire to form one or more pairs of definite magnetic poles (Fig. 8.1). The stator windings are connected to an ac source to form electromagnets. One common type of rotor, the squirrel cage rotor, derives its name from its resemblance to an exercise cage for pet squirrels (Fig. 8.2). For a squirrel cage rotor, a cylinder made up of thin sections of a special soft steel has slots cut in the surface. Bare copper, brass or aluminum bars are mounted in the slots. The bars are short circuited at each end by rings but there are no electrical connections to this type of rotor.

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

207

POLE

POWER SOURCE WINDING FIG. 8.1 SCHEMATIC OF A TWO-POLE STATOR The rotor must be carefully balanced on a central shaft. The shaft extends beyond its support bearings at one or both ends to provide for pulleys or other drive mechanisms. Another type of rotor, the wound rotor, will be discussed later. Assume a simplified rotor is inserted into a stator in the position shown in Fig. 8.3. If the poles of the electromagnet (stator) are as shown, the north pole will induce a north pole in the upper portion of the rotor. Likewise the south pole of the stator will induce a south pole in the lower portion of the rotor. Because like poles tend to repel each other, the rotor will rotate clockwise. If the polarity of the magnets are maintained, when the rotor arrives at the horizontal position (Fig. 8.4), the unlike poles will tend to attract, drawing the rotor further around. If, as the rotor again approaches a vertical position (180 degrees rotation from the start) the polarities of the stator poles are reversed, the rotor will continue to be rotated in the same direction.

SHAFT

COPPER BARS END RING FIG. 8.2 SQUIRREL CAGE ROTOR

208

CHAPTER 8 ELECTRIC MOTORS

N

N

S

S

FIG. 8.3 SIMPLE AC MOTOR, POSITION 1 N

S

N

S

FIG. 8.4 SIMPLE AC MOTOR, POSITON 2 If the stator is connected to an ac source, the polarity of the electromagnetic poles will continue to alternate. As the rotor continues to spin, theoretically it will adjust itself to the frequency of the source. For a 60 Hz source this would mean a rotational speed of 60 revolutions per second or 3600 revolutions per minute for the simple twopole motor. This rotational speed, equal to the speed of the rotating magnetic field in the stator, is called the synchronous speed. As more sets of poles are added to the stator, the rotor does not travel as far to reach the next pole; therefore the speed of the motor is reduced. The synchronous speed of a motor can be expressed as a function of the number of poles and the frequency of the source as

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

209

Frequency of Source 60 s Revolutions per Minute = (Number of Poles/2) 1 min More simply: 120 × Frequency RPM = Number of Poles In practice, however, the actual rotating speed is less than the theoretical speed (synchronous speed) due to slip. Slip occurs due to the fact that the rotor bars must be cutting across the stator’s lines of magnetic flux in order to induce a rotor voltage. This fact means that the rotor must rotate slower than the theoretical speed. Usually under no load, a motor runs 4 to 5% slower than the theoretical speed. In summary, in order to create torque in an induction-type motor, there must be slip (rotor bars cutting lines of magnetic flux). This means the actual speed of an induction-type motor will always be less than the synchronous speed. The type of motor selected largely depends on the starting requirements of the equipment to be driven, the load during operation and the types of power sources available. Selection of motors will be discussed more fully in section 8.5. The following section will briefly discuss the design and operating characteristics of various types of single-phase motors. Table 8.1 summarizes some of the important characteristics of each type of single-phase motor.

8.3 SINGLE-PHASE MOTORS A common type of motor used in the home, on the farm, and in light industry is the single-phase, alternating current motor. Many single-phase motors are designated as small or fractional-horsepower (less than 1 hp at 1700-1800 rpm) but can be as large as 10 hp or more where three-phase power is not available. A problem arises with single-phase motors in that they are not inherently self-starting. If the rotor of the simple motor described earlier were to stop with the rotor in the alignment shown in Fig. 8.5, there would be no force to start the rotor turning since the magnetic poles of the stator N

N

S

S

FIG. 8.5 NON-START POINT FOR SIMPLE SINGLE-PHASE MOTOR

Starting Current High; five to seven times full-load current. Medium, three to six times fullload current.

Medium, three to five times fullload current. Low, two to four times full-load current.

Load-Starting Ability Easy Starting loads. Develops 150 % of fullload torque. Hard starting loads. Develops 350 to 400 % of full-load torque.

Hard starting loads. Develops 350 to 450 % of full-load Easy starting loads. Develop 150 percent of full-load torque

Type/ Power Range Split-Phase 35 to 370 W 1/20 to 1/2 hp Capacitorstart 100 W to 7.5 kW 1/8 to 10 hp Two-value capacitor 15 to 150 kW 2 to 20 hp Permanentsplit capacitor 35 to 750 W 1/20 to 1 hp

Inexpensive, simple Construction. Has no starting winding switch.

Simple construction, long service, w/ min. maintenance. Requires more space to due to larger capacitor.

Simple construction, long service. Good general-purpose motor suitable for most jobs. Nearly constant speed with varying load.

Inexpensive, simple construction. Small for a given motor power. Nearly constant speed with a varying load.

Characteristics

Yes

Yes

Yes

Yes

Electrically Reversible

Typical Uses

Fans and blowers.

Conveyors, barn cleaners, elevators, silo unloaders.

Compressors, grain augers, conveyors, pumps. Specifically designed capacitor motors are suitable for silo unloaders and other augers.

Fans, centrifugal pumps; loads that increase as speed increases.

TABLE 8.1 Types of Single Phase Motors and Their Characteristics

* Reversible by brush ring change. Source: Soderholm and Puckett (1974).

Soft-start 7.5 to 560 kW 10 to 75 hp

Low, 1.5 to 2 times fullload current

High

Hard starting loads. Develops 350 to 450 % of full-load torque.

Universal or series 5 W to 1.5 kW 1/150 to 2 hp

Easy starting loads

Low, two to four times full-load current.

Very hard starting loads. Develops 350 to 450 % of fullload torque

Wound-rotor (Repulsion) 125 W to 7.5 kW 1/6 to 10 hp

Synchronous Very small, or > 200 hp

Medium

Easy starting loads.

Shaded pole 3 to 370 W 1/250 to 1/2 hp

Excellent for large loads requiring low starting torque

Constant speed.

High speed, small size for a given hp. Typ. directly connected to load. Speed changes with load variations.

Larger than equiv. size splitphase or capacitor motor. Running current varies only slightly with load.

Inexpensive, moderate efficiency, for light duty.

Yes

Yes

No*

No

Crop driers, forage blowers, irrigation pumps, agitators.

Clocks and timers. Large compressors

Portable tools, kitchen appliances.

Conveyors, dray burr mills, deep-well pumps, hoists, silo unloaders, bucket elevators.

Small blowers, fans, small appliances.

212

CHAPTER 8 ELECTRIC MOTORS

are directly in line with the induced poles in the rotor. Since all real rotors are round and symmetrical, this condition would occur each time a single-phase motor is started. Therefore, all single-phase motors require some type of starting mechanism. The starting torque available and starting current requirements will vary with the type of mechanism used. Often single-phase motors are classed by their type of starting mechanism. Starting mechanisms will be discussed for each type of single-phase motor. For any motor, during the starting period a current of a magnitude 2 to 7 times larger than the full-load current is required. The magnitude of the current surge will depend on the motor type and design as well as the load to be started.

8.3.1 SPLIT-PHASE MOTORS Split-phase (SP) induction motors are inexpensive and widely used for fractional (less than one) horsepower applications. The mechanism used to start a split-phase motor is a second stator winding, called a starting or auxiliary winding, connected in parallel with the main, or running, stator winding. Split-phase motors are sometimes referred to as resistance-start motors because the auxiliary winding is made of smaller wire and with fewer turns than the main windings. Due to the higher resistance of the smaller wire and lower inductance of the fewer turns, the current and magnetic field reach a maximum in the auxiliary windings before the main windings. This “phase shift” between the main winding and starting winding along with offsetting the windings by 90° (as shown in Fig. 8.6) creates a rotating magnetic field subsequently starting the motor rotating. Since the rotor cannot line up with both sets of windings simultaneously, a starting torque is always available at start-up. The action of the two sets of windings to split the single-phase current into two phases yields the name split-phase motor. The direction of rotation of this type of motor can be changed by reversing the line connections to the auxiliary windings.

RUNNING WINDING

STARTING WINDING

FIG. 8.6 SPLIT-PHASE MOTOR WINDING CONFIGURATION

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

MAIN WINDINGS

ROTOR

213

CENTRIFUGAL SWITCH

AUXILIARY WINDINGS FIG. 8.7 SPLIT-PHASE MOTOR EQUIVALENT CIRCUIT SCHEMATIC (SP) Once the motor reaches approximately 75% of full speed, the auxiliary windings are deactivated, usually by a centrifugal switch (Fig. 8.7). A centrifugal switch opens or closes due to centrifugal forces above a specific rotating speed. The auxiliary windings of this type of motor are not designed to operate for extended periods of time because their higher resistance (denoted by the R in series with auxiliary windings in Fig. 8.7) creates higher losses and heating. If the motor does not come up to speed, or for some other reason the auxiliary windings are not deactivated, heat build-up will likely damage or “burn-out” the auxiliary windings. The smaller size of the auxiliary windings has the advantage of a small space requirement. However, the small wire and small phase shift (20°-30°) limits the starting current and starting torque of the motor. Split-phase motors are only suitable for handling easy starting loads such as ventilation fans. They are rarely used for motors larger than one-half horsepower because of their relatively high starting current. Generally split-phase motors are limited to low starting torque applications where low cost is more important than high starting currents.

8.3.2 CAPACITOR MOTORS Simple capacitor-start induction-run (CS-IR) motors are nearly the same as split-phase motors, except that a capacitor is connected in series with the auxiliary windings (Fig. 8.8). The capacitor creates a larger phase shift between the starting and running winding’s currents. The capacitor improves starting characteristics because (a) increasing the split in the single-phase currents creates a wider time interval between the two magnetic field peaks increasing starting torque and (b) it allows use of more copper in the auxiliary windings and thus reduces starting current requirements. Like the split-phase motor, a centrifugal switch disconnects the capacitor and starting winding at 75% of rated speed. The simple capacitor-start motor shown in Fig. 8.8 has approximately twice the starting torque and about one-third less starting current than a split-phase motor.

214

CHAPTER 8 ELECTRIC MOTORS

CAPACITOR

MAIN WINDINGS

CENTRIFUGAL SWITCH

ROTOR

AUXILIARY WINDINGS FIG. 8.8 CAPACITOR-START MOTOR SCHEMATIC (CS-IR) Two other types of capacitor motors are available. Both types differ from the standard capacitor-start motor because their auxiliary winding remains in the circuit at all times. This implies the wire used for the auxiliary winding must be able to withstand the heat build-up due to the continuous current flow. Two-value capacitor motors (CS-CR, capacitor-start, capacitor-run) motors are similar to CS-IR motors for starting (Fig. 8.9). However, during running, a smaller, continuous duty capacitor remains in series with the auxiliary windings. This capacitor gives greater efficiency by lowering the line current required by the motor. In this motor, a centrifugal switch simply disconnects the starting capacitor at 75% of rated speed and leaves the auxiliary windings and run capacitor connected. CS-CR motors have slightly higher starting and running torque than CS-IR motors and can therefore handle more difficult starting loads. Starting current requirements for the two types are about the same.

CAPACITORS

MAIN WINDINGS

ROTOR

CENTRIFUGAL SWITCH

AUXILIARY WINDINGS FIG. 8.9 TWO-VALUE CAPACITOR MOTOR SCHEMATIC (CS-CR)

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

MAIN WINDINGS

215

ROTOR

AUXILIARY WINDINGS FIG. 8.10 PERMANENT-SPLIT CAPACITOR MOTOR (PSC) Permanent-split capacitor (PSC) motors are similar to CS-CR motors except the same value of capacitance is used for both starting and running (Fig. 8.10). This has the advantage of eliminating a centrifugal switch. However, since the capacitor is not the ideal value for either starting or running but a compromise between the two, the starting torque for these motors is much lower than for other capacitor motors. The PSC motor is more sensitive to voltage variation than other squirrel cage induction motors (Fig. 8.11). This can be an advantage, because speed can be controlled by varying the voltage, or a disadvantage, since the motor will slow down with increases in voltage drop of the feed wires. The simple method of speed control makes PSC motors suitable for shaft-mounted fans and blowers. For ratings of one-third horsepower and above, most capacitor-start motors are manufactured for dual-voltage operation. For example, the main windings of a 115/230 V motor are in two sections that are connected in parallel for 115 V operation and in series for 230 V operation (Fig. 8.12). Many manufacturers provide a wiring diagram to clarify which connections should be made for high and low voltage operation.

Torque Reducedvoltage torque vs. speed curves

Operating points

Fan or pump load curve

Speed FIG. 8.11 SPEED CONTROL OF A PSC MOTOR

216

CHAPTER 8 ELECTRIC MOTORS

FIG. 8.12 DUAL-VOLTAGE MOTOR CONNECTIONS Capacitor motors are generally also electrically reversible by reversing the auxiliary winding leads if they are accessible outside of the motor. PSC motors can actually be reversed while they are running by switching the capacitor between the two identical starting and running windings. The capacitor-type motors with a centrifugal switch must slow down enough for the switch to close before they can be reversed. It is also important to point out that capacitors designed for motor starting and motor running are not interchangeable, even though both are about the same physical size. Motor starting capacitors may have 20 times the capacitance of those used during motor running. Motor starting capacitors are also usually electrolytic-type capacitors. These capacitors are not designed for continuous use while a motor is running.

8.3.3 WOUND-ROTOR MOTORS Wound rotor motors get their name from the fact that their rotors are made up of wire windings connected to a commutator ring and brushes much like a generator armature (Fig. 8.13). The commutator ring allows for external connection to specific windings in the rotor via the brushes. Depending on design, the brushes are connected to external resistances during starting or short-circuited. The brushes connect only selected windings on the rotor. These selected windings can be shifted with respect to the stator windings. The stator winding current induces a current in the shifted rotor windings. This produces a magnetic field in the rotor that is offset from the stator poles. The fields from the two currents oppose each other and thus produce a torque. These motors have excellent starting torque and low starting currents. Therefore, they are commonly used for frequent starting and stopping of heavy inertial loads. However, these motors are more expensive than split-phase or capacitor motors and also require more maintenance because of brush and commutator wear. Two types of wound-rotor motors used for agricultural applications are discussed in more detail in the following paragraphs.

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

217

FIG. 8.13 REPULSION-START MOTOR (RS-IR) Repulsion-start induction-run (RS-IR) motors start as repulsion motors but switch to operate as induction motors. At a predetermined speed, the commutator is lifted to eliminate wear and then all of the rotor windings are short-circuited via a centrifugal switch to give the equivalent of a squirrel cage winding. The repulsion-start induction-run motors are the most common type of wound-rotor motors. Repulsion (R) motor is a term often used for all wound-rotor motors. However, a true repulsion motor is a type in which the brushes short-circuit only selected windings on the commutator in such a manner that the magnetic field axis of the rotor is shifted from the magnetic axis of the stator all of the time (Fig. 8.14). Unlike the RS-IR motor, a true repulsion motor starts and runs based on repulsion. The commutator is not removed and the rotor bars are not short-circuited. The speed of this type of motor is controlled by the load and position of the commutator brushes. This type of motor is sometimes referred to as a variable-speed motor. It is also reversible by changing the angle of the commutator with respect to the stator poles.

N

N S

S FIG 8.14 REPULSION MOTOR WITH COMMUTATOR POSITIONED FOR CLOCKWISE ROTATION

218

CHAPTER 8 ELECTRIC MOTORS

COPPER LOOP

FIG. 8.15 SHADED-POLE MOTOR

8.3.4 SHADED-POLE MOTORS Shaded-pole motors are simply constructed, low cost motors for loads with low starting torque requirements (Fig. 8.15). Instead of an auxiliary winding, shaded pole motors have a continuous solid copper loop around a small portion of each pole. The shading loop delays the build up of a magnetic field in that portion of the pole and gives the motor some starting torque. Use of this type of motor is limited to light loads such as small room fans and small mechanical devices due to low efficiency, very low starting torque, and poor power factor.

8.3.5 UNIVERSAL OR SERIES MOTOR The universal or series motor (UNIV) (Fig. 18.16) gets its designation from having the stator and rotor in series. This type of motor will operate on either ac or dc power sources. It is usually used as a special purpose motor. Often it is built into portable equipment such as drills, grinders, sanders, vacuum cleaners, and food mixers, although very large series motors are used in locomotives to produce high torque at low speed to start a heavy train and less torque at high speed just to keep the train moving. The advantages of this type of motor include high power-to-size ratio and rapid acceleration. This type of motor does not operate at a fixed speed, but rather runs as fast as the load allows. A good example of this characteristic is an electric drill where the motor slows as the load increases. Although the drill runs very fast with no load, the friction of the bearings and motor limit the speed to a safe level.

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

219

FIG. 8.16 UNIVERSAL OR SERIES MOTOR

8.3.6 SYNCHRONOUS MOTORS Synchronous motors are constant-speed motors. In fact their most important characteristic is that their output speed is very exact. Their use in agriculture is limited to clocks and timers. However, in other industries three-phase synchronous motors are more widely used. Most synchronous motors used in industry are very large horsepower and relatively low speed. For example, a synchronous motor rated at 750 hp and 360 RPM would be lower in cost to install than a squirrel cage induction motor. These large synchronous motors start like three-phase induction motors. Then, when they approach their synchronous speed, a dc voltage is applied to the armature (rotor) to create poles of fixed polarity. These poles will be attracted to the rotating magnetic poles of the stator and “lock” into the synchronous speed of the stator field. Synchronous motor speed can be controlled by both design and the ac line frequency.

8.4 THREE-PHASE MOTORS Three-phase motors (usually referred to as polyphase motors) are the most common in industry. They are available in the induction, synchronous, and wound-rotor type of designs similar in principle to their single-phase counterparts. However, a three-phase motor has a set of stator windings for each of the phases and no starting windings. Three-phase motor windings may either be wye or delta connected (Fig. 8.17). For balanced phase voltages, both types are similar in performance. Having three-phase ac power permits a simple, low-cost design. With the three windings 120° apart, there are no positions where torque is not produced to turn the rotor, as there are for single-phase motors. Therefore, three-phase motors do not require a starting mechanism; they are inherently self-starting. This eliminates many expense and maintenance problems associated with single-phase motors.

220

CHAPTER 8 ELECTRIC MOTORS

FIG. 8.17 THREE-PHASE MOTORS WITH WYE AND DELTA STATOR WINDINGS Three-phase motors are common from one-half horsepower and up. Those that produce more than 1 hp are typically referred to as medium or integral horsepower motors. Starting torque is generally high with low-to-moderate starting currents, four to six times full-load current. Some typical uses of three-phase motors include crop dryer fans and irrigation pumps. Use of three-phase motors is often limited by the availability of three-phase power. More discussion of three-phase power and phase converters was given in Chapter 4. Three-phase induction motors are available in four main designs (A, B, C, and D) as specified by NEMA (National Electrical Manufacturers Association) standards. Differences in the designs of the squirrel cage create very different performance characteristics for each type of motor. NEMA design B is the most commonly used threephase general purpose motor. Table 8.2 summarizes some of the characteristics of NEMA design three-phase induction motors. Although wound-rotor and synchronous motors were described in Section 8.3, there are also three-phase versions of both types. Three-phase wound-rotor motors have been commonly used for variable speed operations or those that require unusual amounts of ruggedness. The wound-rotor induction motor is used where sudden stoppage of the rotor is possible due to a jam. Rock crushers and automobile crushers are examples. One important application of three-phase synchronous motors is to increase the power factor of a three-phase power system. An interesting fact of the synchronous motor’s design is that it can have either a leading or lagging power factor. This leading power factor can act like a capacitor bank to improve the entire system’s power factor. The direction of rotation of a three-phase motor can be reversed by switching the connections of any two of the three power lines. Switching two of the power lines reverses the direction of the rotating magnetic field in the stator. Controlled switching of two-phase lines while the motor is running is sometimes used to reverse or dynamically brake an ac motor. This is referred to as plug reversing or plug braking.

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

221

TABLE 8.2 Characteristics of Three-Phase Induction Motor Designs Starting Starting Torque (% Current (% NEMA of rated of full-load Design torque) current) A Average; High; typically 150 to 200% 800% or greater

Characteristics General purpose, more overload capacity & higher efficiency than design B. Requires reduced-voltage starting above 7.5 hp.

Applications Same as design B where more chance for overload is possible

B

Average; Typically 150%

Medium; 450 to 800%

General purpose, most commonly used. For easy starting loads. Fullload slip typically < 3%.

Fans, blowers, rotary pumps, machine tools

C

High; 200 to 250%

Medium; 450 to 800%

For hard-to-start loads. Dual rotor bar design.

High inertia loads, conveyors, reciprocating pumps, compressors, pulverizers

D

Very High; > 275%

Medium; 450 to 800%

Maximum torque produced at starting. Lower efficiency and higher slip (5 to 13%) than other designs.

Very high inertia loads, punch presses, elevators, cranes, hoists

8.5 MOTOR TERMINOLOGY AND SELECTION Successful motor selection entails choosing a motor that will meet load requirements without exceeding the motor’s temperature and torque limitations within the physical environment of operation. The most common motor failure is due to overheating of the windings. Overheating breaks down the thin varnish-like insulation on the windings causing shorting of the windings and motor failure. Overheating can be the result of either excessive current flow or inadequate ventilation. Accumulation of dust and dirt on and in the motor can be a severe problem when it prevents proper motor cooling. The first step in motor selection is to determine the load characteristics, such as power or torque requirements, speed, and duty cycle. Starting and running torques must both be considered. Starting, or locked-rotor, torque requirements vary by type of load from a small percentage of full load, as for fans, to several times full load,

222

CHAPTER 8 ELECTRIC MOTORS

as for conveyors and silo unloaders. At all times, from starting to full speed, the torque supplied by the motor must be more than that required by the load. The greater the excess torque, the more rapid the acceleration. By knowing the motor performance characteristics, a motor that has enough torque to start the load, accelerate to full speed, and handle the maximum overload can be selected. Basic motor performance is described by a speed vs. torque curve. Fig. 8.18 shows a speed-torque curve for a general purpose squirrel cage induction motor. The figure shows how torque varies as speed increases from zero to maximum speed. Several locations on the speed vs. torque curve have been given special names because of their significance in matching motor and load characteristics. They are: Locked-Rotor Torque—motor torque at zero speed or the maximum torque available to start the load. Pull-Up Torque—lowest value of torque produced by the motor between zero and full-load. This may be less than the locked-rotor torque for some motors. Full-Load Torque—torque necessary to produce the motor's rated horsepower at rated speed. Breakdown Torque—maximum torque a motor can carry without an abrupt drop in speed that may make the motor stall or be inoperative. Acceleration Torque—torque available for acceleration. This is not a specific point on the curve, but is the difference between motor torque produced and torque required by the load during acceleration. Typical speed versus torque curves for NEMA type A, B, C, and D, three-phase motors are shown in Fig. 8.19.

FIG. 8.18 SPEED VS. TORQUE FOR A GENERAL PURPOSE SQUIRREL CAGE MOTOR

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

223

300

Design D

TORQUE (% Full-load torque)

250

200

Design C

150

Design A or B

100

50

0 0

20

40

60

80

100

SPEED ( % Synchronous speed)

FIG. 8.19 GENERAL SPEED VERSUS TORQUE FOR THREE-PHASE MOTORS Once the required motor torque characteristics have been met, several other factors about the motor design need to be considered. They include starting current requirements, temperature rating, duty cycle, enclosure type, and service factor. The motor nameplate carries a good deal of the essential information about the motor. A typical nameplate is shown in Fig. 8.20.

FIG. 8.20 TYPICAL ELECTRIC MOTOR NAMEPLATE

224

CHAPTER 8 ELECTRIC MOTORS

The following list gives a brief summary of the items generally found on the nameplate of a motor. Some items are discussed in more detail following the list. Name of the Manufacturer Frame Designation—the NEMA designation of frame size. Power or Horsepower—full-load wattage or horsepower rating for output power. Motor Code—letter designating starting current requirement. Cycles or Hertz—frequency of the source to be used. Phase—number of phases of the source (single-phase, three-phase). Revolutions per Minute—rated speed of the motor at full-load. Voltage—voltage or voltages at which the motor is designed to operate. Thermal Protection—indicates if built-in overload protection is provided. Amperes—rated current at full-load. Ambient Temperature or Temperature Rise—maximum environmental temperature at which the motor should operate, or temperature rise of the motor above ambient at full-load. Time Rating—duty rating, continuous or intermittent. Service Factor—the amount of over load the motor can tolerate continuously at rated voltage and frequency. Insulation Class—a designation of winding insulation generally used only for rewinding. Identification of Bearings—type of bearings sleeve or ball. Power Factor—power factor at full-load appears on some recently manufactured motors. Efficiency—NEMA nominal efficiency of the motor. Standard size frames and shaft heights have been established by NEMA for integral horsepower motors. Standardization allows interchangeability between motors from different manufacturers. A NEMA frame designation appears on the motor nameplate. Shaft height in inches for integral horsepower motors may be obtained by dividing the first two numbers of the frame size by four. Shaft height in inches for fractional horsepower motors may be obtained by dividing the frame size by 16. Both the mounting method/hole pattern and the shaft height are important when selecting a replacement motor to match an existing installation. In cases where the maximum starting current that the motor draws may strain the power system, the designation of the starting current (locked-rotor current) for the motor is helpful. A motor code, designated by a letter on the nameplate, indicates the starting current required. Table 8.3 shows some of the common letter designations. The higher the locked-rotor kilowatt-ampere rating the higher the starting current surge will be. Motors with very high starting current may not be permitted to start on full voltage due to the branch circuit design or power company regulations. In this case, reduced-voltage starting as described in Section 8.7.3 may be required.

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

225

TABLE 8.3 Motor Code Letters, Applied to Motor Starting on Full Voltage Code Letter D E F G H J K L M N P V

EXAMPLE 8.1

Locked-Rotor kVA per hp 4.0 to 4.5 4.5 to 5.0 5.0 to 5.6 5.6 to 6.3 6.3 to 7.1 7.1 to 8.0 8.0 to 9.0 9.0 to 10.0 10.0 to 11.2 11.2 to 12.5 12.5 to 14.0 22.4 and up

Locked-Rotor kVA per kW 5.4 to 6.0 6.0 to 6.7 6.7 to 7.5 7.5 to 8.4 8.4 to 9.5 9.5 to 10.7 10.7 to 12.1 12.1 to 13.4 13.4 to 15.0 15.0 to 16.8 16.8 to 18.8 .

LOCKED-ROTOR CURRENT CALCULATION

Calculate the approximate locked-rotor current for a 1/2 hp 240 V motor with an H motor code. Solution From Table 8.3, an H motor code implies 6.3 to 7.1 kVA/hp 1 VA 6,300 hp × 1/2 hp × 240 V = 13.1 A VA 1 7,100 hp × 1/2 hp × 240 V = 14.8 A Locked-rotor current would be in the range of 13.1 to 14.8 A. The service factor is a multiplier indicating the maximum continuous load that the motor can safely handle. For example, a service factor of 1.25 on a 10 hp motor indicates that it could provide 12.5 hp on a continuous basis. Care must be taken to ensure that other design limits like ambient temperature are not exceeded when operating under these conditions. Both bearing and winding insulation life are reduced as the operating temperature of the motor increases. Nameplate data on temperature can be in one of two forms.

226

CHAPTER 8 ELECTRIC MOTORS

TABLE 8.4 Common Insulation Classes and Their Ratings Insulation Class A B F H

Temperature Rating 105°C 130°C 155°C 180°C

Sometimes it will state temperature rise or degree C rise. Temperature rise indicates the change in temperature inside the motor when operating at full load. If the ambient temperature of the air around the motor is 40°C and the temperature rise of the motor is 50°C, points on the motor frame will reach 90°C under full load. The second approach is to list the ambient temperature rating for the motor and its insulation class. Nearly all electric motors are designed for a maximum ambient temperature rating of 40°C (104°F). This implies that the motor should not be operated in an environment above this temperature without special precautions for cooling the motor. The insulation is usually standardized to have one of four temperature ratings. The temperature capability of each insulation class is defined as being the maximum temperature at which the insulation can be operated to yield an average life of 20,000 hours. The ratings for common insulation classes are shown in Table 8.4. A common rule of thumb is that a 10°C increase of winding temperature will result in a 50% reduction in the expected motor lifetime (Fig. 8.21). This increase in winding temperature can be due to either high ambient temperatures or overloading the motor in normal ambient conditions. Manufacturers often classify motors as continuous duty or intermittent duty. Motor duty cycle, or time, refers to how frequently the motor is started and for how long it will run each time it is started. Continuous duty is defined as the type of service in which the motor is operated at or near full load for more than 60 minutes at a time. This would be the common situation for many loads. Intermittent duty is the type of service in which the load is only on for 10, 20, or 30 minutes at a time with a rest or cooling period between operations. Some examples of this type of load, which may be serviced by an intermittent duty motor, include refrigerators and domestic water pumps. Most motors are designed for continuous duty. The reason for making intermittent duty motors is a matter of cost. Heat dissipation is not as critical on an intermittent duty motor; therefore, some components can be constructed less expensively. Intermittent duty motors are not generally recommended for agricultural or commercial applications. There are two main types of bearings used in motors, sleeve and ball bearings (Fig. 8.22). The choice of which type to use depends mainly on the method and frequency of lubrication and mounting orientation. The sleeve bearing consists of a brass or bronze collar in which the shaft rotates. Sleeve bearings generally require more frequent lubrication than ball bearings and are not well adapted to mounting positions where the motor shaft is not nearly horizontal. Ball bearings consist of steel balls that

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

227

100000

Class A

B

F

H 20,000 hr average life

Average life in hours

10000

1000

100

10

1 100 120 140 160 180 200 220 240 260 280 300

Hottest temperature °C FIG. 8.21 AVERAGE INSULATION LIFE VS. TEMPERATURE roll in a special cage around the shaft. Ball bearings are used on larger motors and whenever end thrust is present on the motor shaft due to a belt, chain, or gear. Ball bearings have less friction and require less frequent lubrication. However, ball bearings are noisier and more expensive than sleeve bearings.

BRASS SLEEVE

FIG. 8.22 SLEEVE AND BALL BEARINGS

228

CHAPTER 8 ELECTRIC MOTORS

Nominal efficiency is another parameter found on most three-phase motor nameplates. Nominal efficiency refers to the average efficiency of a large population of motors of the same design. Since variations in materials and manufacturing processes result in motor-to-motor efficiency variations, the full-load efficiency for a population of motors of a given design is not a single value but rather a range. NEMA nominal efficiency ratings and the minimum guaranteed efficiency for any motor given such a rating are given in Table 8.4. The full-load efficiency of a motor, when operating at rated voltage and frequency, should not be less than the minimum efficiency. Modifications in design to produce higher efficiency motors means a higher production cost. Energy cost savings over time from improved efficiency need to be balanced against higher initial costs in order to select an appropriate motor. Minimum efficiency is the value that the manufacturer guarantees all motors of that rating will meet or exceed. TABLE 8.4 NEMA Efficiency Levels for Three-Phase Medium Motors with Continuous Ratings Nominal Efficiency 99.0 98.9 98.8 98.7 98.6 98.5 98.4 98.2 98.0 97.8 97.6 97.4 97.1 96.8 96.5 96.2 95.8 95.4 95.0 94.5 94.1 93.6 93.0 92.4 91.7 91.0

Minimum Efficiency 98.8 98.7 98.6 98.5 98.4 98.2 98.0 97.8 97.6 97.4 97.1 96.8 96.5 96.2 95.8 95.4 95.0 94.5 94.1 93.6 93.0 92.4 91.7 91.0 90.2 89.5

Nominal Efficiency 90.2 89.5 88.5 87.5 86.5 85.5 84.0 82.5 81.5 80.0 78.5 77.0 75.5 74.0 72.0 70.0 68.0 66.0 64.0 62.0 59.5 57.5 55.0 52.5 50.5

Minimum Efficiency 88.5 87.5 86.5 85.5 84.0 82.5 81.5 80.0 78.5 77.0 75.5 74.0 72.0 70.0 68.0 66.0 64.0 62.0 59.5 57.5 55.0 52.5 50.5 48.0 46.0

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

229

TABLE 8.5 Full-Load Efficiencies of Energy Efficient Motors (from NEMA Standard MG 1-2002 Table 51) 2 POLE hp

4 POLE

6 POLE

8 POLE

Nom. Eff. Min. Eff. Nom. Eff. Min. Eff. Nom. Eff. Min. Eff. Nom. Eff. Min. Eff.

OPEN MOTORS 1 1.5 2 3 5 7.5 10 15 20 25 30 40 50 60 75 100 125 150 200 250 300 350 400 450 500

82.5 84.0 84.0 85.5 87.5 88.5 89.5 90.2 91.0 91.0 91.7 92.4 93.0 93.0 93.0 93.6 93.6 94.5 94.5 95.0 95.0 95.4 95.8 95.8

80.0 81.5 81.5 82.5 85.5 86.5 87.5 88.5 89.5 89.5 90.2 91.0 91.7 91.7 91.7 92.4 92.4 93.6 93.6 94.1 94.1 94.5 95.0 95.0

82.5 84.0 84.0 86.5 87.5 88.5 89.5 91.0 91.0 91.7 92.4 93.0 93.0 93.6 94.1 94.1 94.5 95.0 95.0 95.4 95.4 95.4 95.4 95.8 95.8

1 1.5 2 3 5 7.5 10 15 20 25 30 40 50 60 75 100 125 150 200 250 300 350 400 450 500

75.5 82.5 84.0 85.5 87.5 88.5 89.5 90.2 90.2 91.0 91.0 91.7 92.4 93.0 93.0 93.6 94.5 94.5 95.0 95.4 95.4 95.4 95.4 95.4 95.4

72.0 80.0 81.5 82.5 85.5 86.5 87.5 88.5 88.5 89.5 89.5 90.2 91.0 91.7 91.7 92.4 93.6 93.6 94.1 94.5 94.5 94.5 94.5 94.5 94.5

82.5 84.0 84.0 87.5 87.5 89.5 89.5 91.0 91.0 92.4 92.4 93.0 93.0 93.6 94.1 94.5 94.5 95.0 95.0 95.0 95.4 95.4 95.4 95.4 95.8

80.0 81.5 81.5 84.0 85.5 86.5 87.5 89.5 89.5 90.2 91.0 91.7 91.7 92.4 93.0 93.0 93.6 94.1 94.1 94.3 94.5 94.5 94.5 95.0 95.0

80.0 84.0 85.5 86.5 87.5 88.5 90.2 90.2 91.0 91.7 92.4 93.0 93.0 93.6 93.6 94.1 94.1 94.5 94.5 95.4 95.4 95.4 -

77.0 81.5 82.5 84.0 85.5 86.5 88.5 88.5 89.5 90.2 91.0 91.7 91.7 92.4 92.4 93.0 93.0 93.6 93.6 94.5 94.5 94.5 -

74.0 75.5 85.5 86.5 87.5 88.5 89.5 89.5 90.2 90.2 91.0 91.0 91.7 92.4 93.6 93.6 93.6 93.6 93.6 94.5 -

70.0 72.0 82.5 84.0 85.5 86.5 87.5 87.5 88.5 88.5 89.5 89.5 90.2 91.0 92.4 92.4 92.4 92.4 92.4 93.6 -

77.0 82.5 84.0 85.5 85.5 87.5 87.5 88.5 88.5 90.2 90.2 91.7 91.7 92.4 92.4 93.0 93.0 94.1 94.1 94.1 94.1 94.1 -

74.0 77.0 82.5 84.0 85.5 85.5 88.5 88.5 89.5 89.5 91.0 91.0 91.7 91.7 93.0 93.0 93.6 9.36 94.1 94.5 -

70.0 74.0 80.0 81.5 82.5 82.5 86.5 87.5 87.5 87.5 89.5 89.5 90.2 90.2 91.7 91.7 92.4 92.4 93.0 93.6 -

ENCLOSED MOTORS 80.0 81.5 81.5 85.5 85.5 87.5 87.5 89.5 89.5 91.0 91.0 91.7 91.7 92.4 93.0 93.6 93.6 94.1 94.1 94.1 94.5 94.5 94.5 94.5 95.0

80.0 85.5 86.5 87.5 87.5 89.5 89.5 90.2 90.2 91.7 91.7 93.0 93.0 93.6 93.6 94.1 94.1 95.0 95.0 95.0 95.0 95.0 -

230

CHAPTER 8 ELECTRIC MOTORS

Energy efficient motors first appeared on the market around 1975 as a result of the recent energy crisis. These motors provide higher efficiency through higher cost designs and materials. This obviously results in a more expensive motor. However, even small improvements in the efficiency can result in significant reductions in operating costs over the life of a motor. Table 8.5 below defines efficiencies of energy efficient polyphase induction motors as defined by NEMA standards. Due to the National Energy Policy Act (NEPACT) of 1992, most applications have required energy-efficient motors since late 1997. In 2001, the U.S. electric motor industry agreed to the definition of premium efficient motors and included them in the NEMA industry standard. The NEMA PremiumTM efficiency motors are expected to help assist industrial motor users and utilities to optimize motor systems efficiency. Table 8.6 defines that the full-load efficiencies for NEMA PremiumTM efficiency electric motors rated less than 600 V. If a motor is provided with thermal protection that meets the definitions provided by NEMA standards, the words “Thermally Protected” will be listed on the nameplate. Motors rated above 1 hp and marked with the words “OVER TEMP. PROT. -.” followed by the numeral 1, 2, or 3 are provided with winding overtemperature protection devices that do not meet the definition of “Thermally Protected.” The numeral indicates the type of winding overtemperature protection provided as follows: Type 1 – Winding Running and Locked-Rotor Overtemperature Protection Type 2 – Winding Running Overtemperature Protection Type 3 – Winding Overtemperature Protection, Nonspecific Type The winding running overtemperature protection (Type 2) is required to maintain the winding temperatures under running load conditions 5°C less than the insulation class rating + standard ambient (40°C). Some motors may also be marked with a standard code, A for automatic reset or M for manual reset. With automatic reset, the thermal protection will automatically reset itself after a cool down period. Depending on how the motor is controlled, this may or may not automatically restart the motor. With manual reset, the operator will have to manually push a button to reset the thermal protection after a cooling down period. Different applications will require different types of thermal protection. Electric motors may need to operate under adverse environmental conditions. Dust, dirt, or moisture may be present for many applications. Therefore, selection of the proper type of enclosure is important for protection of the motor and its safe operation. Common motor enclosures used in agricultural and industrial applications are open, drip-proof, splash-proof, totally enclosed, and explosion-proof types (Fig. 8.23). An open motor is one that has ventilation openings that permit the passage of air over and around the windings. A drip-proof enclosure is an open type that protects a motor from liquids and solid particles falling zero to 15° downward from vertical. Outside air is pulled through openings in the end bell or shield of the motor (Fig. 8.23). These motors may be used outside, but only in dust-free areas, and should be protected from the weather. TABLE 8.6 (facing page). Full-Load Efficiencies for NEMA PremiumTM Efficiency Motors Rated 600 Volts or Less (Random Wound) (from NEMA Standard MG 1-2002 Table 52)

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

2 POLE HP

4 POLE

231

6 POLE

Nom. Eff. Min. Eff. Nom. Eff. Min. Eff. Nom. Eff. Min. Eff.

OPEN MOTORS 1 1.5 2 3 5 7.5 10 15 20 25 30 40 50 60 75 100 125 150 200 250 300 350 400 450 500

77.0 84.0 85.5 85.5 86.5 88.5 89.5 90.2 91.0 91.7 91.7 92.4 93.0 93.6 93.6 93.6 94.1 94.1 95.0 95.0 95.4 95.4 95.8 95.8 95.8

74.0 81.5 82.5 82.5 84.0 86.5 87.5 88.5 89.5 90.2 90.2 91.0 91.7 92.4 92.4 92.4 93.0 93.0 94.1 94.1 94.5 94.5 95.0 95.0 95.0

1 1.5 2 3 5 7.5 10 15 20 25 30 40 50 60 75 100 125 150 200 250 300 350 400 450 500

77.0 84.0 85.5 86.5 88.5 89.5 90.2 91.0 91.0 91.7 91.7 92.4 93.0 93.6 93.6 94.1 95.0 95.0 95.4 95.8 95.8 95.8 95.8 95.8 95.8

74.0 81.5 82.5 84.0 86.5 87.5 88.5 89.5 89.5 90.2 90.2 91.0 91.7 92.4 92.4 93.0 94.1 94.1 94.5 95.0 95.0 95.0 95.0 95.0 95.0

85.5 86.5 86.5 89.5 89.5 91.0 91.7 93.0 93.0 93.6 94.1 94.1 94.5 95.0 95.0 95.4 95.4 95.8 95.8 95.8 95.8 95.8 95.8 96.2 96.2

82.5 84.0 84.0 84.0 84.0 89.5 90.2 91.7 91.7 92.4 93.0 93.0 93.6 94.1 94.1 94.5 94.5 95.0 95.0 95.0 95.0 95.0 95.0 95.4 95.4

82.5 86.5 87.5 88.5 89.5 90.2 91.7 91.7 92.4 93.0 93.6 94.1 94.1 94.5 94.5 95.0 95.0 95.4 95.4 95.4 95.4 95.4 95.8 96.2 96.2

80.0 81.5 81.5 86.5 87.5 88.5 90.2 90.2 91.0 91.7 92.4 93.0 93.0 93.6 93.6 94.1 94.5 94.5 94.5 94.5 94.5 94.5 95.0 95.4 95.4

82.5 87.5 88.5 89.5 89.5 91.0 91.0 91.7 91.7 93.0 93.0 94.1 94.1 94.5 94.5 95.0 95.0 95.8 95.8 95.8 95.8 95.8 95.8 95.8 95.8

80.0 85.5 86.5 87.5 87.5 89.5 89.5 90.2 90.2 91.7 91.7 93.0 93.0 93.6 93.6 94.1 94.1 95.0 95.0 95.0 95.0 95.0 95.0 95.0 95.0

ENCLOSED MOTORS 85.5 86.5 86.5 89.5 89.5 91.7 91.7 92.4 93.0 93.6 93.6 94.1 94.5 95.0 95.4 95.4 95.4 95.8 96.2 96.2 96.2 96.2 96.2 96.2 96.2

82.5 84.0 84.0 87.5 87.5 90.2 90.2 91.0 91.7 92.4 92.4 93.0 93.6 94.1 94.5 94.5 94.5 95.0 95.4 95.4 95.4 95.4 95.4 95.4 95.4

232

CHAPTER 8 ELECTRIC MOTORS

FIG. 8.23 THREE MOTOR ENCLOSURE TYPES A splash-proof enclosure has openings so constructed that drops of liquid or solid particles can only enter the motor from an angle more than 100° downward from the vertical (Fig. 8.23). These motors are also suitable for outdoor use in dust-free areas or indoors in areas that are washed periodically. Totally enclosed motors have an enclosure that prevents the free exchange of air between the inside and outside of the case but is not completely airtight (Fig. 8.23). Many totally enclosed motors have a fan on one end of the motor that blows air over the outside of the case. This is called a totally enclosed fan cooled (TEFC) motor. This type of motor is recommended for dusty or wet areas. Totally enclosed motors cost 20% to 40% more than drip-proof motors, but may reduce repair bills in the long run. An explosion-proof motor is a type of totally enclosed motor. This kind of motor must be used in areas where hazardous vapors or dust conditions could cause an explosion or fire. Both the motor enclosure and the wiring box must be explosion-proof.

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

233

8.6 MEASUREMENT OF MOTOR CHARACTERISITICS By measuring the mechanical output of a motor and the electrical input to the motor under different load conditions, data can be developed to show the characteristics of the motor. The mechanical output can be measured either with a dynamometer or Prony brake test apparatus. A voltmeter, an ammeter, and a wattmeter are necessary to measure electrical input. A schematic of a simple Prony brake test apparatus is shown in Fig. 8.24. The Prony brake uses a frictional load against a drum driven by the motor shaft. With this apparatus, the torque created by the frictional force against the drum can be measured and controlled. From the Prony brake test, torque and horsepower can be calculated. Torque is the length of the lever arm L times the force F. Power or work per unit time can be calculated from the torque and revolutionary speed as Work Force × Distance Power = Time = = F × 2πL × N Time In units of horsepower, output power is expressed as 2πFLN Output Power = 33000 hp where F = force in pounds, lb L = lever arm in feet, ft N = rotational speed of shaft in revolutions per minute, RPM 1 hp recalling 33,000 = ft-lb/min

FIG. 8.24 SIMPLE PRONY BRAKE APPARATUS

234

CHAPTER 8 ELECTRIC MOTORS

FIG. 8.25 ELECTRICAL INSTRUMENTATION FOR MEASUREMENT OF POWER INPUT TO A SINGLE-PHASE MOTOR

In units of watts, output power can be expressed as Output Power = 2πFLN watts where F = force in Newtons, N L = lever arm in meters, m N = rotational speed in cycles per second, Hz If the instrumentation as shown in Fig. 8.25 is used in conjunction with the Prony brake, factors such as efficiency and power factor can also be calculated at any load condition. True input power is measured by the wattmeter. Motor efficiency is the ratio of output power as measured by the Prony brake to the input power as measured by the wattmeter. Power Out % Motor Efficiency = Power In × 100% Apparent power can be calculated from the product of current and voltage measurements. Therefore, power factor can be determined from the ratio of true to apparent power, or Wattmeter Reading True Power Power Factor = Apparent Power = Volts × Amperes

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

EXAMPLE 8.2

235

MOTOR CHARACTERISTIC CALCULATIONS

Calculate the horsepower output, efficiency, and power factor from the following data. Electric meter readings Im = 11.4 A Prony brake data F = 2 lb

Vm = 110 V

P = 960 W

L = 1 ft

N = 1,720 RPM

Solution 2π × 2 × 1 × 1720 2πFLN hp = 0.66 hp Power = 33000 hp = 33000 Output 746 W/hp % Efficiency = Input × 100% = 0.66 hp × 960 W × 100% = 51% Power Factor =

960 W Watts = = 0.76 Volts × Amperes 110 V × 11.4 A

If efficiency and power factor are calculated over a range of motor loads, plots of efficiency and power factor can be developed. Fig. 8.26 shows such a plot for a threephase motor. The horizontal axis is often scaled as a percent of the full-load rating of the motor.

FIG. 8.26 TYPICAL THREE-PHASE MOTOR EFFICIENCY AND POWER FACTOR

236

CHAPTER 8 ELECTRIC MOTORS

8.7 MOTOR PROTECTION AND CONTROL No matter what type of electric motor is selected, controls to start and stop the motor and some type of overload protection are needed. Since overload protection is often built into the control system, these two topics are discussed together. Electric motors present some special challenges from the standpoint of overcurrent protection. Electric motors will “try” to provide the power required by the load, even if it results in excessive currents and self-destruction due to excessive winding temperatures. When excessive currents persist for sufficient time to cause damage and overheating of the motor, the motor is said to be overloaded. However, currents of up to six times the running current occur normally at starting and must be allowed. Control contacts and wires must be capable of carrying these larger currents without causing damage or excessive voltage drop. Therefore, overload protection must be specifically matched to the motor. There are two main types of motor protective equipment available: fuses and thermal-overload devices. As will be discussed in the next section, only fuses are commonly included with manually operated switches. However, thermal-overload devices are used on both manual and electromagnetic controllers. Time-delay fuses afford both short-circuit and overload protection. As described in Section 4.9, time-delay fuses can tolerate an overload for a brief period, thereby allowing for starting current surges. Either plug or cartridge-type fuses can be used for motor protection. Thermal-overload devices with either bimetallic elements (much like circuit breakers) or eutectic elements (with action much like normal time-delay fuses) are both available. A thermal-overload switch may be built into the motor itself or provided with the motor controller. The overload device generally opens the supply lines directly on a fractional horsepower motor. With larger motors a relay system with overload contacts must be used to disconnect the higher currents. Thermal-overload devices built into motors or controllers can be either manualreset or automatic-reset. Manual-reset means a button must be pressed to reset the tripped mechanism. This type is generally recommended for general purpose motors because the condition causing the overload can be corrected before the motor restarts. Unexpected start-ups, which might present safety hazards, are avoided. Automatic reset mechanisms automatically attempt to restart the motor after the thermal device cools. A means of disconnecting the motor from the electrical supply is required for all motor circuits. Either a manual or automatic means of opening and closing a circuit to an electric motor can also be used as a controller. Types of controllers will vary by both size of motor and type of application. For example, motors of 1/3 hp or less can be controlled directly by plugging and unplugging at a receptacle. Larger motors may also be plugged in, but must also have one of the types of controllers described in the following sections.

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

8.7.1 MANUAL MOTOR CONTROLLERS

237

Manual switches are most often used to control small motors of 1/2 hp or less. These switches are low cost and can be purchased with built-in overload protection. Control switches for electric motors must be able to withstand the high starting current and arcing that occurs when the circuit is opened due to the highly inductive nature of the motor. Quick-make, quick-break switches equipped with arc quenchers are used. This type of switch is rated by horsepower and voltage. Regular toggle-type switches as used for light switches should not be used to control motors. They can withstand the starting surges, but are not equipped with arc quenchers and therefore usually burn out quickly. For manual control, a fusible knife switch may be used. Time-delay fuses can usually be sized to avoid blowing during starting and still provide overload protection. The knife-switch must be rated at or above the horsepower of the motor. In many applications an inverse-time-delay circuit breaker is permitted as a motor controller. The time-delay action allows for motor starting currents. Breakers are generally tested at six times the breaker rating to permit motor starting. Care must be taken when sizing the breakers to protect the motor and still not trip due to starting inrush current. Motor current for any manual switch should not exceed 80% of the motor full-load rating. For example, a 20 A switch should not be used for motors with full-load current over 16 A. However, a 20 A switch could be used for a 1 hp, 115 V (16 A) motor. Some snap action switches, like the one shown in Fig. 8.27, are designed for motors and provide overload protection. They are preferred over ordinary switches and are useable up to 2 hp.

FIG. 8.27 SNAP-ACTION MOTOR CONTROL SWITCH

238

CHAPTER 8 ELECTRIC MOTORS

MOTOR HEATER OVERLOAD CONTACT (NC) STOP

START

COIL AND CONTACTS OF ELECTROMAGNETIC RELAY

FIG. 8.28 SINGLE-PHASE MOTOR STARTER CIRCUIT

8.7.2 MAGNETIC MOTOR STARTERS Magnetic motor starters are widely used for controlling motor loads. This type of starter should be used in all motors larger than 2 hp and is an essential element in automatic control systems. The difference between a manual switch and a magnetic switch is in how the motor is started. Instead of a manual switch to open and close the circuit directly, the magnetic switch works with manual or automatic control of a magnetically controlled switch (relay). The operation of the magnetic controller will be described in more detail in a Chapter 9. Most magnetic starters also have built-in overload protection. For example, in the unit shown in Fig. 8.28, as current passes through the motor circuit, it passes through a heater. Heat is given off, but under normal conditions is not enough to cause the bimetallic overload switch to open. However, under an overload situation the excess heat will cause the overload switch to open. When the overload contacts are opened, the coil loses power and the contact points open, stopping current flow to the motor. To restart the motor, the overload control is reset, and the start button pressed again.

8.7.3 REDUCED-VOLTAGE AC MOTOR STARTERS As ac induction motors become larger and larger, their high starting currents begin to have detrimental effects on the power system. By reducing the starting current surge, higher horsepower motors can be used without adversely affecting other loads on the line due to voltage drop as the motor starts. In order to avoid this effect, there are several systems for reduced-voltage starting, including resistance starting, reactor starting, autotransformer starting, wye-delta starting, part-winding starting, and electronic soft starting. Most of these systems attempt to reduce the current draw by slowly stepping up the voltage supplied to the motor during starting. Perhaps the most sophisticated is the electronic soft starting, which uses solid-state electronics, such as thyristors or silicon-controlled rectifiers (SCRs), to control the ramp up to full voltage. For example, in many cases single-phase motors over 5 or 7.5 hp will need to use an electronic soft starter. Soft starting a motor limits the starting current to as low as 1.5 to 2 times the full-load current. However, the reduced starting current also reduces the available starting torque to 50% to 90% of full-load torque. Therefore, this type of

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

239

motor starter is matched to easy starting loads such as drier fans, forage blowers, and irrigation pumps.

8.7.4 VARIABLE SPEED DRIVE CONTROLLERS The induction motor is basically a constant speed device that operates within a few percent (typically within 3% to 5%) of the synchronous speed of the electrical supply. There are several methods of controlling the speed of motors. Some of these include: • Changing the number of poles in the motor • Changing the power supply voltage • Changing the resistance of wound-rotor motors • Changing the frequency of the power supply. The first method, changing the number of poles, is an option in some motors. By using several sets of windings, several speeds can be achieved. The maximum number of speeds available by changing the number of poles is usually limited to four. Twospeed motors are very common. The second option of changing the supply voltage was discussed briefly for singlephase PSC motors (section 8.3.2). The slip of the motor increases proportionally to the decrease in voltage squared. However, this simultaneously decreases breakdown torque and can cause the motor to stall. This method of speed control is not recommended. Slip can also be varied by changing the amount of resistance in the rotor for wound-rotor motors. This method of speed control uses external resistors in series with the rotor circuit to limit the rotor current. This method will cause the speed to vary with the load and also is inefficient due to the power lost by the resistors. The final option of changing the frequency of the power source is very feasible and economical due to the recent development of solid-state power switching devices. The basic concept is to convert the 60 Hz ac to dc and then create the desired frequency AC voltage (Fig. 8.29). With a variable-frequency drive (VFD), a 50% decrease in the applied frequency should decrease the motor speed by about 50%. The amount of slip and the breakdown torque are virtually unaffected. This means that the torque vs. speed curve is basically shifted to the left (Fig. 8.30). Likewise, if the frequency is increased by 50% over the motors rated frequency, the torque vs. speed curve will shift to the right. If the motor is used to drive a fan or pump, represented by the dashed load line in Fig. 8.30, then the motor will operate at the intersection of the motor curve and the load curve.

60 Hz AC SUPPLY AC to DC rectifier circuit

DC

DC to AC inverter circuit

Variablefrequency AC

FIG 8.29 BLOCK DIAGRAM FOR VARIABLE-FREQUENCY MOTOR DRIVE

240

CHAPTER 8 ELECTRIC MOTORS

Torque

30 Hz

60 Hz 90 Hz Fan or pump load curve Speed

FIG. 8.30 VARIABLE-FREQUENCY CONTROL OF MOTOR SPEED Applications of VFDs can save power. Recall that horsepower output is related to both speed and torque by the equation 2πFLN Power output = 33000 hp Consider an example of operating the load at a slower speed to reduce air or water flow through a system. This operating condition also reduces the torque required by the load and the motor outputs less power. Compare this to using a dampener or valve to restrict the flow of air or water and requiring the motor to run at the rated motor speed and forcing the fluid through the restriction. This method wastes power. When operating the motor at a condition higher than rated speed, both speed and required torque may increase. This can easily overload the motor by exceeding its horsepower rating. Therefore, if it is desired to operate a load such as a fan or pump above 60 Hz, then the motor must be sized carefully to handle the load.

8.8 MOTOR BRANCH CIRCUITS Motor branch circuits require special procedures for selecting the short circuit and ground-fault protection, conductor size, motor control circuits, controllers, disconnect, and overload protection. Fig. 8.31 illustrates the required components for all motor branch circuits. Article 430 of the NEC code is specific to motors, motor circuits, and controllers. This section will briefly cover several of these special procedures for motors operating under 600 V ac.

8.8.1 DISCONNECTING MEANS NEC article 430 section IX describes the requirements for the disconnecting means for all motor branch circuits. According to the code, each motor and controller must have a disconnecting means located “in sight from” the motor or controller locations, respectively. “In sight from” means visible and within 15 m. But, if the controller’s disconnect is also located in sight from the motor and driven machinery location, it can also serve as the motor disconnect.

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

241

Disconnect Ground-fault & short-circuit protection

Branch circuit conductors

Controller Overload protection devices M FIG. 8.31 MOTOR BRANCH CIRCUIT COMPONENTS Each disconnect must open at least all ungrounded conductors, plainly indicate whether in the open or closed position, and be readily accessible. According to the NEC code, the disconnecting means must be capable of being locked in the “off” position. The equipment used for the disconnecting means can include any of the listed items: • Motor circuit switch, properly sized in hp rating • Molded case circuit breaker • Molded case switch • Instantaneous trip circuit breaker, as part of a listed combination controller • Self-protected combination controller • Manual motor controller, marked as “suitable for motor disconnect.” For stationary motors of 1/8 hp or less, the branch-circuit overcurrent device is permitted to serve as the disconnecting means. For stationary motors of 2 hp or less, a general use switch (rated > 2 times full-load motor current) or a general purpose snap switch (ac rated > 1.25 times full-load motor current) or a manual motor controller (described above) can be used as the disconnecting means. In addition, a horsepowerrated plug and receptacle can serve as the disconnecting means for most motors. In some cases, a switch or circuit breaker can serve as both the controller and disconnecting means, or several motors can be served by one disconnecting means. The disconnecting means for motor circuits rated 600 V nominal or less shall have an ampere rating of at least 115% of the full-load current rating of the motor (see NEC code section 430 for further details).

8.8.2 SHORT-CIRCUIT AND GROUND-FAULT PROTECTION Short-circuit and ground-fault protection devices are required to protect the motor branch-circuit conductors, the motor control apparatus and the motors against overcurrent due to shorts and grounds. You should recall the difference between a short circuit

242

CHAPTER 8 ELECTRIC MOTORS

and a ground fault as described in Chapter 4. Briefly, a short circuit is a direct connection of very low resistance between two conductors resulting in very high fault currents. A ground fault is a connection between an ungrounded conductor and the ground, which may vary in resistance. Typically, dual-element fuses are used for short-circuit and ground-fault protection on motor branch circuits because they can be sized closer to the motor’s full-load amperes providing better protection. However, non-time-delay fuses and inverse time circuit breakers, along with other “listed” devices, can be used. “Listed” devices are those that are tested and approved by an independent testing laboratory. Based on the specifications of the NEC, a dual-element time-delay fuse can be rated up to 175% of the full-load current of single-phase or three-phase motors, except for wound-rotor motors which have a limit of 150%. But, if the motor cannot be started without blowing the fuse, a maximum of 225% of full-load current is allowed. For non-time-delay fuses, the rating can be up to 300% (or 400% if less than 600 amperes and unable to start under 300%) of the full-load current to allow for starting currents. These fuses have to be rated much higher than time-delay fuses in order to withstand the inrush of starting current. Likewise, inverse time circuit breakers can be used if sized up to 250% of rated full-load current. In all cases, the full-load current rating of the motor must be taken from Appendix Tables A.3 or A.4 unless the motor has a current rating larger indicated on the nameplate.

EXAMPLE 8.3

BRANCH CIRCUIT SHORT CIRCUIT SIZING 1

Motor Starter & Thermal overloads

M Determine the maximum size time-delay fuse permitted for the 10 hp 230 V singlephase squirrel cage induction motor in the figure.

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

243

Solution From Table A.3, the full-load amps for the motor are 50 A. The multiplier for a dual-element fuse is 175%, and 1.75 × 50 = 87.5 A. Standard fuse sizes are 80, 90, 100, etc. Since the next higher standard size fuse is allowed, select the 90 A fuse.

EXAMPLE 8.4

BRANCH CIRCUIT SHORT-CIRCUIT SIZING 2

What maximum size non-time-delay fuse is permitted for the motor in the above example? Solution The multiplier for a non-time-delay fuse is 300%, and 3.00 × 50 = 150 A. Standard size fuses in this range are 100, 125, 150, 175, 200, etc. Since this matches a standard size, select the 150 A fuse. If the motor cannot be started using this fuse, the size can be increased to a maximum of 400%. Since 4.00 × 50 = 200 A, and this is also a standard size, select the 200 A fuse.

8.8.3 WIRE SIZING Motors perform best at rated voltages, therefore wires must be sized to avoid excessive voltage drop. Branch circuit conductors to individual motors should be selected to carry 125% of full-load current of the motor with 2% or less voltage drop. The 125% factor allows for a certain degree of overload. Notice that motor branch circuit conductors do not have to be sized 150% to 300% to match the short-circuit and ground-fault protection ratings. This is because the required overload protection will adequately protect the circuit conductors. When the conductors supply more than one motor on a branch circuit, the wire is sized for a current value of 125% of the largest motor current plus 100% of the additional motor currents. Appendix Tables A.3 and A.4 give full-load currents for singleand three-phase motors. Current values from those tables should be used in the wire selection process unless the motor nameplate current is known and larger. If the nameplate current is larger, it should be used. Wire size required to meet the voltage drop and current carrying limitations is determined in the same manner as described for feeder wires in Section 5.4 of this text. The following two examples will demonstrate the process.

244

CHAPTER 8 ELECTRIC MOTORS

EXAMPLE 8.5

BRANCH CIRCUIT WIRE SIZING FOR MOTORS 1

What size of copper conductor would be required for a 1/2 hp, single-phase motor located 20 m from the service entrance, if (a) The motor is wired for 120 V? (b) The motor is wired for 240 V? Solution From Table A.3, full-load current is 9.8 A at 120 V and 4.9 A at 240 V. (a) Allowable Voltage Drop = 2% × 120 V = 2.4 V E 2.4 V Allowable Resistance = I = = 0.20 ohm 1.25 × 9.8 A 0.2 ohm R/1000 m = 40 m × 1000 = 5 ohm/1000 m From Table A.2, No. 10 needed. Check allowable ampacity, Table A.5 – okay. (b) Allowable Voltage Drop = 2 % × 240 V = 4.8 V E 4.8 V Allowable Resistance = I = = 0.78 ohm 1.25 × 4.9 A R/1000 m =

0.78 ohm 40 m × 1000 = 19.5 ohm/1000 m

From Table A.2, No. 16 needed. Check allowable ampacity, Table A.5 – No. 14 or No. 12 copper could be used for the branch circuit, but No. 16 may not be used for branch circuit wiring.

EXAMPLE 8.6

BRANCH CIRCUIT WIRE SIZING FOR MOTORS 2

Calculate the copper branch circuit conductor size needed for serving two motors, 3/4 hp and 1/2 hp, on the same 240 V branch circuit, if the motors are located 15 m from the service entrance. Solution From Table A.3, Appendix A

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

245

3

1 /4 hp — 6.9 A full-load /2 hp — 4.9 A full-load Total Current for calculations = 1.25 × 6.9 A + 4.9 A = 13.5 A

Allowable Voltage Drop = 2% × 240 V = 4.8 V E 4.8 V Allowable Resistance = I = 13.5 A = 0.35 ohm R/1000 m =

0.35 ohm 30 m × 1000 = 11.8 ohm/1000 m

From Table A.2, need No. 14 for allowable resistance, and No. 14 is okay for 13.5 A load with any insulation suitable to the environment (Table A.5).

8.8.4 CONTROLLER

The controller is any switch or device that is normally used to start and stop a motor by making and breaking the motor circuit current. The actual device that can serve as the motor controller varies by motor size and application. Stationary motors < 1/8 hp for continuous running devices like clocks can use the branch-circuit protective device as the controller, whereas portable motors of < 1/3 hp can use the attachment plug and receptacle. Regardless of the device used, it must be capable of interrupting the locked-rotor current of the motor. The horsepower rating of the controller must not be lower than the horsepower rating of the motor and in some circumstances must be up to 1.4 times the motor rating. Unless a number of motors drive several parts of a single machine, or a group of motors are protected by the same overcurrent device, each individual motor requires its own controller. The controllers’ enclosure must be suitable for the environment to which it will be exposed.

8.8.5 OVERLOAD PROTECTION For any control device used, the heater/overload protection device is generally a removable item that should be selected based on the nameplate current rating of the motor to be controlled. Overload devices come in a wide range of rated tripping currents. Since the needs of a particular motor may not exactly match a standard, higher ratings will be necessary. Table 8.7 gives the recommended and maximum ratings for overload protection by percentage of full-load current rating. TABLE 8.7 Overload Protection Rating as Percent of Nameplate Full-load Current Motor Motors with service factor of 1.15 or greater Motors with a marked temperature rise not over 40°C All other motors

Recommended Maximum 125% 140% 125% 140% 115% 130%

246

CHAPTER 8 ELECTRIC MOTORS

8.8.6 GROUNDING For safety purposes the frame of each motor should be connected to the grounding system, equipment grounded. If an electrical fault develops in the motor, grounding will prevent hazardous voltages between the motor frame and the earth. By supplying a low resistance path for current, current due to the fault will not cause a hazard to people and should trip the overcurrent protection. Motor grounding is described further by the NEC article 430, section XII. Where the frame of a motor is not grounded, it should be permanently and effectively insulated from the ground.

8.8.7 SUMMARY The following measures must be provided for in the wiring system for a motor based on NEC article 430: • A means of disconnecting the motor from the electrical supply; • Branch-circuit short-circuit and ground-fault protection based on the full-load currents in Tables A.3 and A.4 to protect the conductors, motor and controller of the motor circuit from destructive fault currents; • Branch circuit conductors of appropriate size to carry 125 % of motor full-load amperes from Tables A.3 or A.4 and avoid excessive voltage drop; • A controller to start and stop the motor; • Motor overload protection based on the nameplate current to prevent overloading the motor and protect the conductors from overload currents under running conditions; and • Grounding of the motor frame for safety.

EXERCISES 1.

What is the theoretical speed of a motor on a 60 Hz source if it has (a) 2 poles? (b) 4 poles? (c) 6 poles?

2.

What is the theoretical speed of a 120 V, 6-pole motor on a 50 Hz source?

3.

An electric motor draws 2238 W and 12 A when operating on a 240 V 60 Hz source. Determine the (a) Power factor of the motor. (b) Apparent power input to motor. (c) True power input to motor. (d) Horsepower output of the motor if it is 75% efficient.

4.

The following is found on the nameplate of a motor: 3 hp 60 Hz 1 phase 120/240 V 34/17 A 1,740 RPM

FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE

247

(a) If the power factor of the motor is 0.80, what is the power input to the motor at full load? (b) What is the efficiency of the motor at full load? (c) How much does it cost to operate the motor at full load for 100 hours at $0.06 per kWh? 5.

(a) How many foot-pounds of work must be done in filling a 30,000 gallon tank with water from a well in which the water level is 159 ft below the tank? (1 gal = 8 1/3 lb). (b) What horsepower is needed to fill the tank in two hours assuming 100% efficiency? (c) If the motor and pump combination is 45% efficient, what size of motor is needed?

6.

What size of copper wire cable (THW insulation) is needed for the following motor if located 30 m from the service entrance? 3 hp 240 V single-phase

7.

What size of wire is needed for a 5 hp single-phase motor located 12 m from the source? Assume aluminum, UF insulation, and cable underground.

8.

What size copper wire would be needed for a branch circuit to a 0.75 hp 240 V motor located 75 feet from the service entrance?

9.

What size of copper wire would be needed for a branch circuit to a 7.5 hp 240 V motor on a vacuum pump located 25 ft from the service entrance?

10. What size overcurrent protection would be needed for the branch circuit in problem 9 if (a) Time-delay fuses are used? (b) Non-time-delay fuses are used? (c) Circuit breakers are used? 11. Given the following table of results from a motor dynamometer test of a singlephase motor, calculate horsepower, power factor, and efficiency for each load level. Then plot, as a function of horsepower output, the following: (a) Efficiency (b) Power Factor (c) Current (d) Motor speed Assume electrical instrumentation as shown in Fig. 8.25 and that the Prony brake used has a lever arm of 0.25 ft.

248

CHAPTER 8 ELECTRIC MOTORS

Meter Readings Motor Speed (RPM) 1,770 1,760 1,750 1,740 1,730 1,720 1,710 1,670

Force (lb) 0 1 2 4 6 8 10 11

Ammeter Voltmeter Wattmeter (A) (V) (W) 8.2 115 170 8.4 115 380 8.5 115 390 8.7 115 520 9.8 115 750 10.4 115 860 11.2 115 980 13.8 115 1,270

12. Given the following table of results from a motor dynamometer test of a singlephase motor, calculate output power in watts, power factor, and efficiency for each load level. Then plot, as a function of output power, the following: (a) Efficiency (b) Power Factor (c) Current (d) Motor speed Assume electrical instrumentation as shown in Fig. 8.25 and that the Prony brake used has a lever arm of 4.3 cm (0.043 m). Meter Readings Motor Speed (Hz) 30.0 29.7 29.5 29.3 29.1 28.8 28.6 28.3 27.9

Force (N) 0.0 5.8 12.0 17.8 24.0 29.8 36.0 41.8 48.0

Ammeter Voltmeter Wattmeter (A) (V) (W) 4.7 123 140 4.8 122 180 4.85 121 230 4.9 121 270 5.1 121 330 5.25 120 380 5.5 120 435 5.8 120 480 6.4 120 560

REFERENCES NFPA. 2002. National Electric Code 2002. Natl. Fire Protection Assoc., Boston, MA. NEMA Standards. 2002. MG-1-2002: Information Guide for General Purpose Industrial AC Small and Medium Squirrel-Cage Induction Motor Standards. Natl. Electrical Manufacturers Assoc., Rosslyn, VA. Soderholm, L. H., and H. B. Puckett. 1974. Selecting and Using Electric Motors. USDA Farmer Bulletin No. 2257, Washington, DC.