science universe

STEPPING MOTORS

Stepping motors are devices that convert electrical impulses into mechanical movement. Stepping motors differ from other types of dc or ac motors in that their output shaft moves through a specific angular rotation each time the motor receives a pulse. Each time a pulse is received, the motor shaft moves by a precise amount. The stepping motor allows a load to be controlled with regard to speed, distance, or position. These motors are very accurate in their control performance. Generally, less than 5% error per angle of rotation exists, and this error is not cumulative regardless of the number of rotations. Stepping motors are operated on dc power but can be used as a two-phase synchronous motor when connected to ac power.

Theory of Operation

Stepping motors operate on the theory that like magnetic poles repel and unlike magnetic poles attract. Consider the circuit shown in Figure 18–39. In this illustration, the rotor is a permanent magnet and the stator winding consists of two electromagnets. If cur- rent flows through the winding of stator pole A in such a direction that it creates a north

magnetic pole, and through B in such a direction that it creates a south magnetic pole, it would be impossible to determine the direction of rotation. In this condition, the rotor could turn in either direction.

Now consider the circuit shown in Figure 18–40. In this circuit, the motor contains four stator poles instead of two. The direction of current flow through stator pole A is still in such a direction as to produce a north magnetic field; the current flow through pole B

produces a south magnetic field. The current flow through stator pole C, however, produces a south magnetic field, and the current flow through pole D produces a north magnetic field. As illustrated, there is no doubt regarding the direction or angle of rotation. In this example, the rotor shaft will turn 90° in a counterclockwise direction.

Figure 18–41 shows yet another condition. In this example, the current flow through poles A and C is in such a direction as to form a north magnetic pole, and the direction of current flow through poles B and D forms south magnetic poles. In this illustration, the permanent-magnet rotor has rotated to a position between the actual pole pieces.

To allow for better stepping resolution, most stepping motors have eight stator poles, and the pole pieces and rotor have teeth machined into them as shown in Figure 18–42. In practice, the number of teeth machined in the stator and rotor determines the angular rotation achieved each time the motor is stepped. The stator–rotor tooth configuration shown in Figure 18–42 produces an angular rotation of 1.8° per step.

Winding

There are different methods for winding stepper motors. A standard three-lead motor is shown in Figure 18–43. The common terminal of the two windings is con-nected to ground of an aboveground–belowground power supply. Terminal 1 is connected to the common of a single-pole, double-throw switch (switch 1), and terminal 3 is connected to the common of another single-pole, double-throw switch (switch 2). One of the stationary contacts of each switch is connected to the positive or aboveground voltage,

and the other stationary contact is connected to the negative or belowground voltage. The polarity of each winding is determined by the position setting of its control switch.

Stepping motors can also be wound bifilar, as shown in Figure 18–44. The term bifilar means that there are two windings wound together. This is similar to a transformer winding with a center-tap lead. A bifilar stepping motor has twice as many windings as does the three-lead type, which makes it necessary to use smaller wire in the windings. This results in higher wire resistance in the winding, producing a better inductive–resistive (L/R) time constant for the bifilar wound motor. The increased L/R time constant results in better motor performance. The use of a bifilar stepper motor also simplifies the drive circuitry requirements. Notice that the bifilar motor does not require an aboveground–belowground power supply. As a general rule, the power supply voltage should be about five times greater than the motor voltage. A current-limiting resistance is used in the common lead of the motor. This current-limiting resistor also helps improve the L/R time constant.

Four-Step Switching (Full Stepping)

The switching arrangement shown in Figure 18–44 can be used for a four-step sequence. Each time one of the switches changes position, the rotor will advance one-fourth of a tooth. After four steps, the rotor has turned the angular rotation of one “full” tooth. If the rotor and stator have 50 teeth, it will require 200 steps for the motor to rotate one full revolution. This corresponds to an angular rotation of 1.8° per step (360°/200 steps 1.8° per step). Figure 18–45 illustrates the switch positions for each step.

Eight-Step Switching (Half-Stepping)

Figure 18–46 illustrates the connections for an eight-stepping sequence. In this arrangement, the center-tap lead for phases A and B are connected through their own separate current-limiting resistors back to the negative of the power supply. This circuit contains four separate single-pole switches instead of two switches. The advantage of this arrangement is that each step causes the motor to rotate one-eighth of a tooth instead of one-fourth of a tooth. The motor now requires 400 steps to produce one revolution, which

produces an angular rotation of 0.9° per step. This results in better stepping resolution and greater speed capability. The chart in Figure 18–47 illustrates the switch position for each step. Figure 18–48 depicts a solid-state switching circuit for an eight-step switching arrangement. A stepping motor is shown in Figure 18–49.

AC Operation

Stepping motors can be operated on ac voltage. In this mode of operation, they become two-phase ac synchronous constant-speed motors and are classified as permanent-magnet induction motors. Refer to the exploded diagram of a stepping motor in Figure 18–50. Notice that this motor has no brushes, slip rings, commutator, gears, or belts. Bearings maintain a constant airgap between the permanent-magnet rotor and the stator windings. A typical eight-stator pole-stepping motor will have a synchronous speed of 72 r/min when connected to a 60-Hz two-phase ac power line.

A resistive–capacitive network can be used to provide the 90° phase shift needed to change single-phase ac into two-phase ac. A simple forward–off–reverse switch can be added to provide directional control. A sample circuit of this type is shown in Figure 18–51. The correct values of resistance and capacitance are necessary for proper opera- tion. Incorrect values can result in random direction of rotation when the motor is started, change of direction when the load is varied, and erratic and unstable operation, as well as failure of the motor to start. The correct values of resistance and capacitance will be different with different stepping motors. The manufacturer’s recommendations should be followed for the particular type of stepping motor used.

Motor Characteristics

When stepping motors are used as two-phase synchronous motors, they have the ability to virtually start, stop, or reverse direction of rotation instantly. The motor will start within about 11⁄2 cycles of the applied voltage and stop within 5 to 25 ms. The motor can maintain a stalled condition without harm to it. Because the rotor is a permanent magnet, no induced current is in the rotor, and no high inrush of current occurs when the motor is started. The starting and running currents are the same. This simplifies the power requirements of the circuit used to supply the motor. Because of the permanent- magnet structure of the rotor, the motor does provide holding torque when turned off. If more holding torque is needed, dc voltage can be applied to one or both windings when the motor is turned off. An example circuit of this type is shown in Figure 18–52. If dc voltage is applied to one winding, the holding torque will be approximately 20% greater than the rated torque of the motor. If dc voltage is applied to both windings, the holding torque will be about 11⁄2 times greater than the rated torque.

SUMMARY

• Types of single-phase motors (split-phase)

1. Single-phase induction motor

2. Repulsion motor

3. Repulsion-induction motor

4. Series motor (universal motor)

5. Shaded-pole motor

• Three-phase motors generally perform better than do single-phase motors. In many instances, however, only single-phase service is available.

• Most single-phase motors have fractional horsepower ratings. In general, their use is limited to commercial and residential applications.

• Resistance-start, induction-run motor

1. A resistance-start, induction-run motor has the following basic parts:

a. A stator (stationary part).

b. A rotor (revolving part).

c. A centrifugal switch, located inside the motor.

d. Two end shields bolted to the steel frame; these shields house the rotor shaft bearings.

e. A cast steel frame; the stator core is pressed into this frame.

2. The stator contains two windings, which are placed 90 electrical degrees apart.

a. The main or running winding

b. The starting or auxiliary winding

3. At start-up, both windings are connected in parallel to the single-phase line. Once the motor accelerates to two-thirds or three-quarters of the rated speed, a centrifugal switch disconnects the starting winding automatically.

4. The rotor of the resistance-start, induction-run motor is the same as that of a three- phase, squirrel-cage induction motor.

a. The rotor contains no windings, brushes, slip rings, or commutator where faults may develop.

b. Fans are provided with the rotor to keep the temperature of the windings at a reasonable level.

5. The centrifugal switch is mounted inside the motor. It disconnects the starting winding after the rotor reaches approximately 66% to 75% of its rated speed.

6. The running winding has a low resistance and a high inductive reactance. Thus, the current of the running winding lags behind the voltage.

7. The current in the starting winding is more nearly in phase with the voltage because the winding has a high resistance and a low inductive reactance.

8. When a current passes through each of these windings, the resulting pulsating field effect gives rise to a rotating field around the inside of the stator core.

9. While traveling at the synchronous speed, S 120 f /P, the rotating field cuts the copper bars of the squirrel-cage winding (rotor). Voltages are induced in the winding and cause currents in the rotor bars.

10. A rotor field is created. This field reacts with the stator field to develop the torque that causes the rotor to turn.

11. As the rotor nears the rated speed, the centrifugal switch disconnects the start- ing winding from the line. The motor then continues to operate, using only the running winding and the current induced in the rotor. The alternating voltages produced generate a torque that keeps the rotor turning.

12. The motor resembles a two-phase induction motor because the currents of the windings are approximately 90 electrical degrees out of phase with each other. However, a single-phase source supplies the motor. Therefore, the motor is called a split-phase motor.

13. If the motor does not start, but a low humming sound is present, then one wind- ing is open. The centrifugal switch should be checked to determine whether its mechanism is faulty or whether the switch contacts are pitted.

14. If the motor operates on its starting winding for more than 60 seconds, the winding may char or burn out.

15. To reverse the rotation of the motor, interchange the leads of either the starting winding or the running winding (preferably the starting winding). Interchanging these leads reverses the direction of rotation of the field.

16. Dual-voltage motors

a. Some single-phase motors have dual-voltage ratings such as 115/230 V. The running winding consists of two sections, each of which is rated at 115 V.

b. The windings are marked “T1 and T2” and “T3 and T4 .”

c. For the higher voltage, the two windings are connected in series across 230 V.

d. For the lower voltage, the windings are connected in parallel across 115 V.

e. The starting winding consists of one winding rated at 115 V.

f. It is connected in parallel across 115 V with the two running windings.

g. For the higher voltage, the starting winding is connected in parallel across one of the series running windings.

17. Speed regulation

a. A resistance-start, induction-run motor has very good speed regulation from no load to full load.

b. The regulation compares to that of a three-phase, squirrel-cage induction motor.

c. The percent slip ranges from 4% to 6%.

18. Starting torque

a. The starting torque of the motor is poor.

b. A phase angle of 30° to 50° is large enough to provide a weak rotating mag- netic field and the starting torque is small.

• Capacitor-start, induction-run motor

1. This type of motor is similar to the resistance-start, induction-run motor. A capacitor is connected in series with the starting winding. The capacitor is mounted in or on the motor.

2. The starting torque is higher than that of a resistance-start motor.

3. The capacitor limits the starting surge of current to a lower value as compared to the resistance-start motor.

4. This motor is used in refrigeration units, compressors, oil burners, small-machine equipment, and any application where split-phase induction motors are used.

5. The basic operation of this motor is nearly the same as that of the resistance-start, induction-run motor. The capacitor is energized for two or three seconds only at start-up. Thus, it cannot improve the power factor.

6. Defective capacitors often cause problems in the motors using them.

a. The capacitor may short-circuit and blow the fuse on the branch motor circuit.

b. If the fuse rating is high, and the fuse does not interrupt the power supply to the motor, the starting winding may burn out.

• Multispeed motors

1. Two basic types of motors are used for multispeed applications:

a. The consequent pole motor.

b. The capacitor-start, capacitor-run motor.

2. The consequent pole motor changes speed by changing the number of stator poles. This causes the synchronous speed of the rotating magnetic field to change.

3. The capacitor-start, capacitor-run (CSCR) type of multispeed motor changes speed by inserting impedance in series with the run winding.

4. Multispeed motors commonly have from two to five ranges of speed.

5. Capacitor-start, capacitor-run motors designed for use as multispeed motors have higher impedance windings than other CSCR motors.

• Repulsion-type motors

1. There are three types of repulsion motors:

a. The repulsion motor

b. The repulsion-start, induction-run motor

c. The repulsion-induction motor

2. These three types of motors differ in their construction, operating characteristics, and industrial applications.

a. Repulsion motor

(1) The stator is usually wound with four, six, or eight poles.

(2) The rotor is a slotted core containing a winding. The rotor is similar to the armature of a dc motor. The coils are connected to the commutator through segments, or bars, parallel to the armature shaft.

(3) Carbon brushes contact the commutator surface. The brush holders are movable. They are connected together by heavy copper jumpers. The brushes are movable so that they can make contact at different points to obtain the correct rotation and maximum torque output.

(4) To reverse the motor, the brushes are shifted 15 electrical degrees to the other side of the stator field pole centers.

(5) This motor has excellent starting torque.

(6) The speed of the motor can be changed by changing the value of the supply voltage.

(7) The motor has very unstable speed characteristics. It may race to a very high speed if there is no mechanical load.

b. Repulsion-start, induction-run motor

(1) This motor is available in two basic types: the brush-lifting motor and the brush-riding motor.

(2) For the brush-lifting motor, a centrifugal device lifts the brushes from the commutator surface at 75% of the rated speed. A short-circuiting necklace shorts out the commutator. The brushes are lifted from the commutator, thus saving wear and tear on all parts.

(3) The brush-riding type of motor has the same short-circuiting neck- lace, but the brushes ride on the commutator at all times.

(4) This motor starts like a repulsion motor. After it reaches 75% of the rated speed, it runs as an induction motor.

(5) To reverse the direction of rotation, follow the steps outlined for a repul- sion motor.

(6) These motors may be designed for dual-voltage connections.

c. Repulsion-induction motor

(1) This type of motor has the same characteristics as a repulsion-start, induction-run motor. It does not have a centrifugal mechanism.

(2) It does have a squirrel-cage winding beneath the slots of the armature.

The squirrel-cage winding improves the speed regulation from no load to full load.

(3) The speed for this type of motor is similar to that of the dc compound motor.

(4) The motor may be designed for dual-voltage windings.

• The same schematic diagram symbol is used for the repulsion-induction motor, the repulsion motor, and the repulsion-start, induction-run motor.

• The dc series or shunt motor cannot be operated from an ac supply.

• Universal motor

1. The field poles of a universal motor are laminated to reduce eddy currents.

2. The voltage loss across the field poles is reduced by using a small number of field turns on a low-reluctance core.

3. A universal motor can be operated on an ac or dc supply.

4. Universal motors are available with fractional horsepower ratings and are commonly used in household appliances.

5. The field windings and the armature of a universal motor are connected in series.

6. There are two versions of the universal distributed field motor: the two-field motor and the single-field compensated motor. The two-field motor has a main winding and a compensating winding to reduce the reactance voltage developed in the armature by the alternating flux.

7. The speed of a universal motor can be controlled by varying the inductance through taps on one of the field poles. The motor may be operated at an excessive speed with no load. Universal motors are permanently connected to the devices being driven.

8. To reverse the direction of rotation, the current flow is reversed through the field circuit or the armature circuit.

• Conductive and inductive compensation

1. Conductive compensation is achieved by placing special windings in slots cut in the pole faces to overcome the armature reaction under load.

a. The strength of this field increases with an increase in the load current.

b. The compensating winding is connected in series with the series field winding and the armature.

c. A motor with conductive compensation has high starting torque and poor speed regulation.

2. An inductively coupled winding is also used to overcome armature reaction in ac series motors.

a. This winding acts like a short-circuited secondary winding in a transformer.

b. The operating characteristics of an inductively compensated motor are similar to those of the conductively compensated motor.

• Shaded-pole induction motor

1. Small fractional horsepower induction motors may be started by shading coils mounted on each of the stator poles.

2. A standard squirrel-cage rotor is used.

3. The motor does not require a starter mechanism such as a centrifugal switch.

4. The shading coil is a low-resistance copper loop.

5. The shading poles distort the main flux pattern, and therefore set up a rotating magnetic field.

6. The torque produced is small.

7. Shaded poles are used only on motors rated at 1⁄10 hp or smaller.

8. A shaded-pole motor is used in applications where a strong torque is not needed, such as for fans and blowers.

• Variable-speed motors

1. Two types of motors are used for variable-speed control:

a. The capacitor-start, capacitor-run motor

b. The shaded-pole induction motor

2. These motors are generally used because they do not require the use of a centrifugal switch.

3. Motors used for variable-speed control have higher impedance windings than do other types of motors not designed for use as variable-speed motors.

4. There are two general methods used to control the speed of variable-speed motors:

a. Variable-voltage control

b. Inserting impedance in series with the motor winding

5. Two common methods of obtaining variable voltage are

a. A solid-state controller using a triac

b. An autotransformer

6. Only triac controllers designed for use as variable-speed motor controllers should be used.

• Stepping motors

1. Stepping motors generally operate on direct current and are used to produce angular movements in steps.

2. Stepping motors are generally used for position control.

3. Stepping motors can be used as single-phase synchronous motors when connected to two-phase ac.

4. Most stepping motors operate at a synchronous speed of 72 r/min when connected to 60-Hz ac.

5. Stepping motors can produce a holding torque by applying a continuous direct current to their stator windings.

6. Full stepping produces an angular rotation of 1.8° per step for a total of 200 steps per revolution.

7. Half stepping produces an angular rotation of 0.9° per step for a total of 400 steps per revolution.

Achievement Review

1. Explain how a motor is started using the split-phase method.

2. Explain why the starting torque of a capacitor-start, induction-run motor is better than that of a resistance-start, induction-run motor.

3. How is the direction of rotation reversed for each of the following?

a. Resistance-start, induction run motor

b. Capacitor-start, induction-run motor

4. The centrifugal switch contacts fail to close when a resistance-start, induction-run motor is deenergized. Explain what happens when the motor is energized.

5. A resistance-start, induction-run motor is started with a heavy overload. The motor fails to accelerate to a speed high enough to open the centrifugal switch contacts. What will happen?

6. A resistance-start, induction-run motor has a dual-voltage rating of 120/240 V. This motor has two running windings, each rated at 120 V, and one starting wind- ing rated at 120 V. Draw a schematic connection diagram for this motor connected for 240 V.

7. Show the connections for a 120-V, capacitor-start, induction-run motor.

8. a. Compare the operating characteristics of a resistance-start, induction-run motor with those of a capacitor-start, induction-run motor.

b. List three applications for

(1) a resistance-start, induction-run motor.

(2) a capacitor-start, induction run motor.

9. Explain the differences in construction and operation between a capacitor-start, induction-run motor and a capacitor-start, capacitor-run motor.

10. Explain how a repulsion motor operates.

11. a. Explain why torque is not developed by a repulsion motor when the brushes are placed directly in line with the stator pole centers.

b. The brushes are shifted 90 electrical degrees from the position described in part a of this question. Is a torque developed by the motor in this case?

c. At what position should the brushes be placed relative to the stator pole centers to obtain the maximum torque?

12. Explain how a repulsion-start, induction-run motor operates.

13. Explain the difference between a repulsion-start, induction-run motor (brush-lift- ing type) and a repulsion-start, induction-run motor (brush-riding type).

14. How are the following motors reversed?

a. A repulsion motor

b. A repulsion-start, induction-run motor

15. Compare the operating characteristics of a repulsion motor with those of a repulsion- start, induction-run motor.

16. List three practical applications for

a. a repulsion motor.

b. a repulsion-start, induction-run motor.

17. Explain the difference between a repulsion-start, induction-run motor and a repulsion- induction motor.

18. Explain why shunt motors cannot be operated satisfactorily on ac service.

19. What is the difference between an ac series motor and a dc series motor?

20. Why are ac series motors with small fractional horsepower ratings called

universal motors?

21. What is the difference between a series motor with conductive compensation and a series motor with inductive compensation?

22. List four practical applications for ac series motors.

23. Describe the construction and operation of a shaded-pole motor.

24. Where is the shaded-pole motor used?

25. A 120-V, two-pole, 60-Hz, capacitor-start, induction-run motor has a full-load speed of 3450 r/min. Determine

a. the synchronous speed.

b. the percent slip.

26. Name two methods of changing the speed of the rotating magnetic field.

27. What type of multispeed motor changes speed by changing the synchronous speed of the rotating magnetic field?

28. Name a major difference between capacitor-start, capacitor-run motors designed for multispeed use and CSCR motors that are not.

29. What two types of motors are used as variable-speed motors?

30. Why are these types of motors used?

31. Name two methods of controlling the speed of variable-speed motors.

32. What solid-state component is used to control the voltage applied to a variable- speed motor?

33. Explain the difference in operation between a stepping motor and a common dc motor.

34. What is the principle of operation of a stepping motor?

35. Why do stepping motors have teeth machined into the stator poles and rotor?

36. When a stepping motor is connected to ac power, how many phases must be applied to the motor?

37. How many degrees out of phase are the voltages of a two-phase system?

38. What is the synchronous speed of an eight-pole stepping motor when connected to a two-phase, 60-Hz ac line?

39. How can the holding torque of a stepping motor be increased?

CONDUCTIVE AND INDUCTIVE COMPENSATION

AC motors larger than 1⁄2 horsepower (hp) are used to drive loads when a high starting torque is required. For these motors, there is excessive armature reaction under load. One method of overcoming the armature reaction is known as conductive compensation. In this method, an additional compensating winding is placed in slots cut in the pole faces. The strength of this field increases with an increase in the load current. Thus, there is a reduc- tion in the distortion of the main field flux by the armature flux.

The compensating winding is connected in series with the series field winding and the armature (Figure 18–29). A motor with conductive compensation has a high starting torque and poor speed regulation. However, resistor-type speed controllers can be used to obtain a wide range of speed control.

A second method of overcoming armature reaction in ac series motors uses an inductively coupled winding (Figure 18–30). This winding acts like a short-circuited secondary winding in a transformer. This winding links the cross magnetizing flux of the armature. (The armature can be compared to the primary winding of a transformer.) Because the magnetomotive force of the secondary is equal in magnitude to the primary magnetomotive

force but is nearly opposite in phase, the compensating winding flux almost neutralizes the armature cross-flux. An ac series motor of this type cannot be used on dc current. Its operating characteristics are similar to those of the universal motor with conductive compensation.

SHADED-POLE INDUCTION MOTOR

If a single-phase induction motor has a small fractional horsepower rating, it may be started by shading coils mounted on one side of each of the stator poles. A standard squirrel-cage rotor is used in this motor, which is known as a shaded-pole induction motor. The motor does not require a starter mechanism such as a centrifugal switch. Thus, there is no possibility of motor failure due to a faulty centrifugal switch mechanism.

Figure 18–31 shows a typical shaded-pole induction motor with four stator poles. The four poles are wound in alternate directions. Note that a shading coil is wound on one section of each of the stator field poles. The shading coil is a low-resistance copper loop.

Action of Flux

As the current in the stator circuit increases, the stator flux also increases. A voltage is induced in each shading coil. The induced current in the shading loop opposes any increase in the main field flux through the loop. As a result, the flux increases in the other section of each pole face (Figure 18–32A).

When the stator current and flux both reach maximum values, there is an instant when no other change occurs in the current or the flux. At this instant, there is no voltage or current in the shading coil. Thus, the shading coil does not set up a magneto- motive force to oppose the stator flux. The resulting stator field is uniform across the pole face (Figure 18–32B). When the stator current and flux decrease, the induced volt- age and current in the shading coil set up a magnetomotive force. This force aids the stator field. As a result, the flux decreases less rapidly in the section of the pole face where the shading coil is mounted than it does in the other part of the pole face (Figure 18–32C).

Figure 18–32 shows that the shading coil causes the field flux to shift across the pole face. This shifting flux can be likened to a rotating magnetic field. The torque produced is small. Thus, the shaded-pole type of starting is used only on small motors rated at no more than 1⁄10 hp. Such motors are used in applications where a strong starting torque is not essential. Typical applications include driving small devices such as fans and blowers.

VARIABLE-SPEED MOTORS

The use of small variable-speed motors has increased greatly because of an increase in demand for products that employ their use. These motors are commonly used to operate light loads such as ceiling fans and blower motors. Two types of motors are used for these applications: the shaded-pole and the capacitor-start, capacitor-run motors. These motors

are used because they operate without having to disconnect a set of start windings with a centrifugal switch. Motors intended to be used in this manner are wound with high-impedance stator windings. The high impedance of the stator limits the current flow through the motor when the speed of the rotor is decreased. Speed control for these motors is accomplished by controlling the amount of voltage applied to the motor or by inserting impedance in series with the stator winding.

Variable Voltage Control

The amount of voltage applied to the motor can be controlled by several methods. One method is to use an autotransformer with several taps (Figure 18–33). This type of controller has several steps of speed control. Notice that the applied voltage, 120 V in this illustration, is connected across the entire transformer winding. When the rotary switch is moved to the first tap, 30 V is applied to the motor. This produces the lowest motor speed for this controller. When the rotary switch is moved to the second tap, 60 V is applied to the motor. This provides an increase in motor speed. When the switch has been moved to the last position, the full 120 V is applied to the motor and it operates at its highest speed.

Another type of variable voltage control uses a triac to control the amount of volt- age applied to the motor (Figure 18–34). This type of speed control provides a more lin- ear control because the voltage can be adjusted from zero to the full applied voltage. At first appearance, many people assume this controller to be a variable resistor connected in series with the motor. A variable resistor large enough to control even a small motor would produce several hundred watts of heat and could never be mounted in a switch box. The

variable resistor in this circuit is used to control the amount of phase shift for the triac. The triac controls the amount of voltage applied to the motor by turning on at different times during the ac cycle.

A triac speed control is very similar to a triac light dimmer used in many homes. A light dimmer, however, should never be used as a motor speed controller. Triac light dimmers are intended to be used with resistive loads such as incandescent lamps. Light- dimmer circuits will sometimes permit one-half of the triac to start conducting before the other half. The waveform shown in Figure 18–35 illustrates this condition. Notice that only the positive half of the waveform is being conducted to the load. Because only positive voltage is being applied to the load, it is dc. Operating a resistive load such as an incandescent lamp with dc will do no damage. Operating an inductive load such as the winding of a motor can do a great deal of damage, however. When direct current is applied to a motor winding, there is no inductive reactance to limit the current. The actual wire resistance of

the stator is the only current-limiting factor. The motor winding or the controller can easily be destroyed if direct current is applied to the motor. For this reason, only triac controllers designed for use with inductive loads should be used for motor control. A photograph of a triac speed controller is shown in Figure 18–36.

Series Impedance Control

Another common method of controlling the speed of small ac motors is to connect impedance in series with the stator winding. This is the same basic method of control used with multispeed fan motors. The circuit in Figure 18–37 shows a tapped inductor connected in series with the motor. When the motor is first started, it is connected directly to the full voltage of the circuit. As the rotary switch is moved from one position

to another, steps of inductance are connected in series with the motor. As more inductance is connected in series with the stator, the amount of current flow decreases. This produces a weaker magnetic field in the stator. Rotor slip increases because of the weaker magnetic field, and the motor speed decreases. A photograph of this type of controller is shown in Figure 18–38.

CONDUCTIVE AND INDUCTIVE COMPENSATION

AC motors larger than 1⁄2 horsepower (hp) are used to drive loads when a high starting torque is required. For these motors, there is excessive armature reaction under load. One method of overcoming the armature reaction is known as conductive compensation. In this method, an additional compensating winding is placed in slots cut in the pole faces. The strength of this field increases with an increase in the load current. Thus, there is a reduc- tion in the distortion of the main field flux by the armature flux.

The compensating winding is connected in series with the series field winding and the armature (Figure 18–29). A motor with conductive compensation has a high starting torque and poor speed regulation. However, resistor-type speed controllers can be used to obtain a wide range of speed control.

A second method of overcoming armature reaction in ac series motors uses an inductively coupled winding (Figure 18–30). This winding acts like a short-circuited secondary winding in a transformer. This winding links the cross magnetizing flux of the armature. (The armature can be compared to the primary winding of a transformer.) Because the magnetomotive force of the secondary is equal in magnitude to the primary magnetomotive

force but is nearly opposite in phase, the compensating winding flux almost neutralizes the armature cross-flux. An ac series motor of this type cannot be used on dc current. Its operating characteristics are similar to those of the universal motor with conductive compensation.

SHADED-POLE INDUCTION MOTOR

If a single-phase induction motor has a small fractional horsepower rating, it may be started by shading coils mounted on one side of each of the stator poles. A standard squirrel-cage rotor is used in this motor, which is known as a shaded-pole induction motor. The motor does not require a starter mechanism such as a centrifugal switch. Thus, there is no possibility of motor failure due to a faulty centrifugal switch mechanism.

Figure 18–31 shows a typical shaded-pole induction motor with four stator poles. The four poles are wound in alternate directions. Note that a shading coil is wound on one section of each of the stator field poles. The shading coil is a low-resistance copper loop.

Action of Flux

As the current in the stator circuit increases, the stator flux also increases. A voltage is induced in each shading coil. The induced current in the shading loop opposes any increase in the main field flux through the loop. As a result, the flux increases in the other section of each pole face (Figure 18–32A).

When the stator current and flux both reach maximum values, there is an instant when no other change occurs in the current or the flux. At this instant, there is no voltage or current in the shading coil. Thus, the shading coil does not set up a magneto- motive force to oppose the stator flux. The resulting stator field is uniform across the pole face (Figure 18–32B). When the stator current and flux decrease, the induced volt- age and current in the shading coil set up a magnetomotive force. This force aids the stator field. As a result, the flux decreases less rapidly in the section of the pole face where the shading coil is mounted than it does in the other part of the pole face (Figure 18–32C).

Figure 18–32 shows that the shading coil causes the field flux to shift across the pole face. This shifting flux can be likened to a rotating magnetic field. The torque produced is small. Thus, the shaded-pole type of starting is used only on small motors rated at no more than 1⁄10 hp. Such motors are used in applications where a strong starting torque is not essential. Typical applications include driving small devices such as fans and blowers.

VARIABLE-SPEED MOTORS

The use of small variable-speed motors has increased greatly because of an increase in demand for products that employ their use. These motors are commonly used to operate light loads such as ceiling fans and blower motors. Two types of motors are used for these applications: the shaded-pole and the capacitor-start, capacitor-run motors. These motors

are used because they operate without having to disconnect a set of start windings with a centrifugal switch. Motors intended to be used in this manner are wound with high-impedance stator windings. The high impedance of the stator limits the current flow through the motor when the speed of the rotor is decreased. Speed control for these motors is accomplished by controlling the amount of voltage applied to the motor or by inserting impedance in series with the stator winding.

Variable Voltage Control

The amount of voltage applied to the motor can be controlled by several methods. One method is to use an autotransformer with several taps (Figure 18–33). This type of controller has several steps of speed control. Notice that the applied voltage, 120 V in this illustration, is connected across the entire transformer winding. When the rotary switch is moved to the first tap, 30 V is applied to the motor. This produces the lowest motor speed for this controller. When the rotary switch is moved to the second tap, 60 V is applied to the motor. This provides an increase in motor speed. When the switch has been moved to the last position, the full 120 V is applied to the motor and it operates at its highest speed.

Another type of variable voltage control uses a triac to control the amount of volt- age applied to the motor (Figure 18–34). This type of speed control provides a more lin- ear control because the voltage can be adjusted from zero to the full applied voltage. At first appearance, many people assume this controller to be a variable resistor connected in series with the motor. A variable resistor large enough to control even a small motor would produce several hundred watts of heat and could never be mounted in a switch box. The

variable resistor in this circuit is used to control the amount of phase shift for the triac. The triac controls the amount of voltage applied to the motor by turning on at different times during the ac cycle.

A triac speed control is very similar to a triac light dimmer used in many homes. A light dimmer, however, should never be used as a motor speed controller. Triac light dimmers are intended to be used with resistive loads such as incandescent lamps. Light- dimmer circuits will sometimes permit one-half of the triac to start conducting before the other half. The waveform shown in Figure 18–35 illustrates this condition. Notice that only the positive half of the waveform is being conducted to the load. Because only positive voltage is being applied to the load, it is dc. Operating a resistive load such as an incandescent lamp with dc will do no damage. Operating an inductive load such as the winding of a motor can do a great deal of damage, however. When direct current is applied to a motor winding, there is no inductive reactance to limit the current. The actual wire resistance of

the stator is the only current-limiting factor. The motor winding or the controller can easily be destroyed if direct current is applied to the motor. For this reason, only triac controllers designed for use with inductive loads should be used for motor control. A photograph of a triac speed controller is shown in Figure 18–36.

Series Impedance Control

Another common method of controlling the speed of small ac motors is to connect impedance in series with the stator winding. This is the same basic method of control used with multispeed fan motors. The circuit in Figure 18–37 shows a tapped inductor connected in series with the motor. When the motor is first started, it is connected directly to the full voltage of the circuit. As the rotary switch is moved from one position

to another, steps of inductance are connected in series with the motor. As more inductance is connected in series with the stator, the amount of current flow decreases. This produces a weaker magnetic field in the stator. Rotor slip increases because of the weaker magnetic field, and the motor speed decreases. A photograph of this type of controller is shown in Figure 18–38.

REPULSION-TYPE MOTORS

There are three general types of repulsion motors: the repulsion motor; the repulsion- start, induction-run motor; and the repulsion-induction motor. These three types of motors differ in their construction, operating characteristics, and industrial applications.

REPULSION MOTOR

Construction

A repulsion motor has the following basic components:

1. A laminated stator core; one winding of the stator is similar to the main (running) winding of a split-phase motor. The stator is usually wound with four, six, or eight poles.

2. A rotor consisting of a slotted core containing a winding. This rotor is called an armature because it is similar to the armature of a dc motor. The coils of this armature winding are connected to the commutator. The axial-type commutator has segments, or bars, parallel to the armature shaft.

3. Two cast steel end shields that act as protection for the motor bearings. These bear- ings are mounted on the motor frame.

4. Carbon brushes that contact with the commutator surface. A brush holder assembly, mounted on one of the end shields, holds the brushes in place. The brushes are con- nected together by heavy copper jumpers. The brush holder assembly can be moved. In this way, the brushes can contact the commutator at different points to obtain the correct rotation and maximum torque output.

5. Two bearings that support and center the armature shaft in the stator core. Either sleeve bearings or ball bearings may be used.

6. A cast steel frame; the stator core is pressed into this frame.

Operation of a Repulsion Motor

When the stator winding is connected to a single-phase line, a field is set up by the current in the windings. A voltage and a resultant current are induced in the rotor windings by this field. By placing the brushes in the proper position on the commutator segments, the current induced in the armature windings causes magnetic poles in the armature. These poles have a set relationship to the stator field poles.

The armature field poles are displaced by 15 electrical degrees from the field poles of the stator windings. Because the instantaneous polarity of the rotor poles is the same as that of the adjacent stator poles, a repulsion torque is formed. This torque causes the motor armature to rotate. This motor is known as a repulsion motor because it is based on the principle that like poles repel.

Positioning the Brushes. Figure 18–20, parts A–D, shows how the positioning of the brushes affects the torque. In Figure 18–20A, there is no torque when the brushes are placed at right angles to the stator poles. In this position, there are equal induced voltages in the two halves of the armature winding. These voltages oppose each other at the connection between the two sets of brushes. Therefore, there is no current in the windings, and no flux is set up by the armature windings.

In Figure 18–20B, the brushes are now under the center of the stator poles. In this position, there is a heavy current in the armature windings. However, a torque is not created. The armature poles due to the current are in line with the stator poles. Thus, there is no twisting force (torque) in either the clockwise or counterclockwise direction.

Figure 18–20C shows the brushes shifted in a counterclockwise direction from the stator pole centers by 15 electrical degrees. The magnetic poles set up in the armature are 15 electrical degrees from the stator pole centers and have the same polarity. Thus, a repulsion torque is formed between the stator and rotor field poles. This torque causes the armature to rotate counterclockwise.

To reverse the motor, the brushes are shifted 15 electrical degrees to the other side of the stator field pole centers (Figure 18–20D). The armature poles formed again have the same polarity but are now 15° in a clockwise direction from the stator pole centers. This torque causes the armature to rotate clockwise.

Torque and Speed. The repulsion motor has excellent starting torque. The speed of this motor can be varied by changing the value of the supply voltage. However, the repulsion motor has unstable speed characteristics. It may race to a very high speed if there is no mechanical load. The repulsion motor can be used for applications requiring a strong starting torque and a range of speed control.

REPULSION-START, INDUCTION-RUN MOTOR

There are two basic types of repulsion-start, induction-run motors. For the brush- lifting type of motor, the brushes lift off the commutator surface once the motor accelerates to nearly 75% of the rated speed. The second type of motor is known as the brush-riding type. In this motor, the brushes ride on the commutator surface at all times.

Construction

A repulsion-start, induction-run motor has the following components:

1. A laminated stator core. This core has one winding that is similar to the running winding of a split-phase motor.

2. A rotor consisting of a slotted core. The core contains a winding that is connected to a commutator. The rotor core and winding are similar to the armature of a dc motor. Thus, the rotor is called an armature.

3. a. For the brush-lifting motor, a centrifugal device lifts the brushes from the commutator surface at 75% of the rated speed. The centrifugal mechanism consists of governor weights, a short-circuiting necklace, spring barrel, spring, push rods, brush holders, and brushes.

b. For the brush-riding motor, a centrifugal device also operates at 75% of the rated speed. This mechanism consists of governor weights, a short-circuiting neck- lace, and a spring barrel. The centrifugal device short-circuits the commutator segments but does not lift the brushes and brush holders from the commutator surface.

4. A special radial-type commutator is used in the brush-lifting motor. The brush-riding motor has an axial-type commutator.

5. a. The brush holder assembly for the brush-riding motor is the same as the one used on a repulsion motor.

b. The brush holder assembly for the brush-lifting motor is designed so that the centrifugal device can lift the brush holders and the brushes from the commutator surface.

6. The end shields, bearings, and motor frame are the same as those used in the repulsion motor.

Operation

A repulsion-start, induction-run motor and a repulsion motor are started in the same way. The repulsion-start, induction-run motor, however, operates as an induction motor once it accelerates to 75% of the rated speed.

Figure 18–21 is an exploded view of an armature, radial commutator, and centrifugal device used on the brush-lifting motor.

Operation of the Centrifugal Mechanism. As the push rods of the centrifugal mechanism move forward, they push the spring barrel forward. The short-circuiting necklace then contacts the bars of the radial commutator and all of the bars are short-circuited. At the same time, the brush holders and brushes are lifted from the commutator surface. In this way, there is no unnecessary wear on the brushes and the commutator surface. Lifting the brushes from the commutator also eliminates a great deal of noise.

By short-circuiting the commutator bars, the armature is converted to a form of squirrel-cage rotor. As a result, the motor operates as a single-phase induction motor. This type of motor starts as a repulsion motor and runs as an induction motor.

Brush-Riding Motor. The brush-riding type of motor uses an axial commutator. The centrifugal mechanism consists of a number of copper segments held in place by a spring (Figure 18–22). This device is mounted next to the commutator. At nearly 75% of the rated speed, centrifugal force causes the short-circuiting necklace to short-circuit the commuta- tor segments. When this happens, the motor operates as an induction motor.

Torque and Speed. The starting torque and speed performances are very good for both types of repulsion-start, induction-run motors. Therefore, these motors are suitable for applications requiring a strong starting torque and a fairly constant speed from no load to full load. Typical uses include commercial refrigerators, compressors, and pumps.

To reverse the direction of rotation of the repulsion-start, induction-run motor, follow the steps outlined for a repulsion motor.

Schematic Diagrams. A schematic diagram of a repulsion-start, induction-run motor is shown in Figure 18–23. The same diagram also represents a repulsion motor.

Repulsion-start, induction-run motors are often constructed for operation on either 120 or 240 V. That is, two stator windings are provided. For 120-V operation, the wind- ings are connected in parallel. For 240-V operation, the windings are connected in series.

A schematic diagram is given in Figure 18–24 for a dual-voltage, repulsion-start, induction-run motor. The connection table provided shows how the leads of this motor are connected for 120- or 240-V operation. The same connections also apply to dual-voltage repulsion motors.

REPULSION–INDUCTION MOTOR

A repulsion-induction motor has nearly the same operating characteristics as a repulsion-start, induction-run motor. However, this type of motor has no centrifugal mechanism. The repulsion-induction motor has the same type of armature and commutator as the repulsion motor, but it does not have a centrifugal mechanism. There is a squirrel-cage winding beneath the slots of the armature of the repulsion-induction motor, as shown in Figure 18–25.

Both the repulsion-induction, motor and the repulsion-start, induction-run motor have very good starting torque because they start as repulsion motors. The squirrel-case winding of the repulsion-induction motor means that it has fairly constant speed regulation from no load to full load. The start-up of this motor and that of the repulsion-start, induction-run motor are similar. This motor is called a repulsion-induction motor to distinguish it from the repulsion-start, induction-run motor having a centrifugal system.

The torque–speed performance of a repulsion-induction motor is similar to that of a dc compound motor.

The repulsion-induction motor is designed for use on 120 or 240 V. The stator wind- ings have two sections that are connected in parallel for 120-V operation and in series for 240-V operation. The markings of the motor terminals and the connection arrangement

for these leads are the same as for a repulsion-start, induction-run motor. The repulsion- induction motor, the repulsion motor, and the repulsion-start, induction-run motor all have the same schematic diagram symbol.

ALTERNATING-CURRENT SERIES MOTOR

Without careful examination, it may appear that the typical dc series or shunt motor can be operated from an ac supply. If the line terminals to the dc motor are reversed, the current and the magnetic flux are reversed in both the field and armature circuits. For a motor operating from an ac source, the net torque will be in the same direction. As a result, it is impractical to operate a dc shunt motor in this manner because the high inductance of the shunt field causes the field current and the field flux to lag the line voltage by almost 90°. The motor thus develops little torque.

Universal Motors

Another reason why the series dc motor should not be operated on ac is the large amount of heat due to eddy currents in the field poles. In addition, there is an excessive voltage drop across the series field windings due to the high reactance. The field poles may be laminated to reduce the eddy currents. The voltage loss across the field poles can be reduced by using a small number of field turns on a low reluctance core. The core is then operated at a low flux density. This type of motor is known as a universal motor and can operate on ac or dc. Universal motors are available with fractional horsepower ratings and are commonly used in household appliances.

A concentrated field universal motor has two salient poles and a winding of few turns connected to give the opposite magnetic polarity. Figure 18–26 shows the laminated field structure of a typical concentrated field universal motor.

The field windings and armature of a universal motor are connected in series, as shown in Figure 18–27.

Universal Motor Type

Another type of universal motor is the distributed field motor. One version of this motor is a single-field compensated motor. A second version is a two-field compensated motor. The field windings of a two-pole, single-field compensated motor resemble the sta- tor winding of a two-pole, split-phase ac motor. A two-field compensated motor has a stator with a main winding and a compensating winding. These windings are spaced 90 electrical degrees apart. When the motor is operated from an ac source, the compensating winding reduces the reactance voltage developed in the armature by the alternating flux.

Universal motors have aluminum, cast iron, and rolled steel frames (Figure 18–28). The field poles are bolted to the frame. The field cores consist of laminations bolted together. The armature core is also laminated. A conventional commutator and brushes are used.

Speed Control

Universal motors operate at nearly the same speed on dc and single-phase ac. These series-wound motors operate at excessive speeds with no load. To overcome this, they are

permanently connected to the devices being driven. The speed regulation for universal motors consists of resistance inserted in series with the motor. Tapped resistors, rheostats and tapped nichrome wire coils, wound over a single-field pole, are used. The speed can also be controlled by varying the inductance through taps on one of the field poles.

Motor Reversal

Any series-wound motor can be reversed by changing the direction of the current in either the field or armature circuit. Universal motors are sensitive to brush position. Severe arcing at the brushes results when the direction of rotation is changed and the brushes are not shifted to the neutral (sparkless) plane.

REPULSION-TYPE MOTORS

There are three general types of repulsion motors: the repulsion motor; the repulsion- start, induction-run motor; and the repulsion-induction motor. These three types of motors differ in their construction, operating characteristics, and industrial applications.

REPULSION MOTOR

Construction

A repulsion motor has the following basic components:

1. A laminated stator core; one winding of the stator is similar to the main (running) winding of a split-phase motor. The stator is usually wound with four, six, or eight poles.

2. A rotor consisting of a slotted core containing a winding. This rotor is called an armature because it is similar to the armature of a dc motor. The coils of this armature winding are connected to the commutator. The axial-type commutator has segments, or bars, parallel to the armature shaft.

3. Two cast steel end shields that act as protection for the motor bearings. These bear- ings are mounted on the motor frame.

4. Carbon brushes that contact with the commutator surface. A brush holder assembly, mounted on one of the end shields, holds the brushes in place. The brushes are con- nected together by heavy copper jumpers. The brush holder assembly can be moved. In this way, the brushes can contact the commutator at different points to obtain the correct rotation and maximum torque output.

5. Two bearings that support and center the armature shaft in the stator core. Either sleeve bearings or ball bearings may be used.

6. A cast steel frame; the stator core is pressed into this frame.

Operation of a Repulsion Motor

When the stator winding is connected to a single-phase line, a field is set up by the current in the windings. A voltage and a resultant current are induced in the rotor windings by this field. By placing the brushes in the proper position on the commutator segments, the current induced in the armature windings causes magnetic poles in the armature. These poles have a set relationship to the stator field poles.

The armature field poles are displaced by 15 electrical degrees from the field poles of the stator windings. Because the instantaneous polarity of the rotor poles is the same as that of the adjacent stator poles, a repulsion torque is formed. This torque causes the motor armature to rotate. This motor is known as a repulsion motor because it is based on the principle that like poles repel.

Positioning the Brushes. Figure 18–20, parts A–D, shows how the positioning of the brushes affects the torque. In Figure 18–20A, there is no torque when the brushes are placed at right angles to the stator poles. In this position, there are equal induced voltages in the two halves of the armature winding. These voltages oppose each other at the connection between the two sets of brushes. Therefore, there is no current in the windings, and no flux is set up by the armature windings.

In Figure 18–20B, the brushes are now under the center of the stator poles. In this position, there is a heavy current in the armature windings. However, a torque is not created. The armature poles due to the current are in line with the stator poles. Thus, there is no twisting force (torque) in either the clockwise or counterclockwise direction.

Figure 18–20C shows the brushes shifted in a counterclockwise direction from the stator pole centers by 15 electrical degrees. The magnetic poles set up in the armature are 15 electrical degrees from the stator pole centers and have the same polarity. Thus, a repulsion torque is formed between the stator and rotor field poles. This torque causes the armature to rotate counterclockwise.

To reverse the motor, the brushes are shifted 15 electrical degrees to the other side of the stator field pole centers (Figure 18–20D). The armature poles formed again have the same polarity but are now 15° in a clockwise direction from the stator pole centers. This torque causes the armature to rotate clockwise.

Torque and Speed. The repulsion motor has excellent starting torque. The speed of this motor can be varied by changing the value of the supply voltage. However, the repulsion motor has unstable speed characteristics. It may race to a very high speed if there is no mechanical load. The repulsion motor can be used for applications requiring a strong starting torque and a range of speed control.

REPULSION-START, INDUCTION-RUN MOTOR

There are two basic types of repulsion-start, induction-run motors. For the brush- lifting type of motor, the brushes lift off the commutator surface once the motor accelerates to nearly 75% of the rated speed. The second type of motor is known as the brush-riding type. In this motor, the brushes ride on the commutator surface at all times.

Construction

A repulsion-start, induction-run motor has the following components:

1. A laminated stator core. This core has one winding that is similar to the running winding of a split-phase motor.

2. A rotor consisting of a slotted core. The core contains a winding that is connected to a commutator. The rotor core and winding are similar to the armature of a dc motor. Thus, the rotor is called an armature.

3. a. For the brush-lifting motor, a centrifugal device lifts the brushes from the commutator surface at 75% of the rated speed. The centrifugal mechanism consists of governor weights, a short-circuiting necklace, spring barrel, spring, push rods, brush holders, and brushes.

b. For the brush-riding motor, a centrifugal device also operates at 75% of the rated speed. This mechanism consists of governor weights, a short-circuiting neck- lace, and a spring barrel. The centrifugal device short-circuits the commutator segments but does not lift the brushes and brush holders from the commutator surface.

4. A special radial-type commutator is used in the brush-lifting motor. The brush-riding motor has an axial-type commutator.

5. a. The brush holder assembly for the brush-riding motor is the same as the one used on a repulsion motor.

b. The brush holder assembly for the brush-lifting motor is designed so that the centrifugal device can lift the brush holders and the brushes from the commutator surface.

6. The end shields, bearings, and motor frame are the same as those used in the repulsion motor.

Operation

A repulsion-start, induction-run motor and a repulsion motor are started in the same way. The repulsion-start, induction-run motor, however, operates as an induction motor once it accelerates to 75% of the rated speed.

Figure 18–21 is an exploded view of an armature, radial commutator, and centrifugal device used on the brush-lifting motor.

Operation of the Centrifugal Mechanism. As the push rods of the centrifugal mechanism move forward, they push the spring barrel forward. The short-circuiting necklace then contacts the bars of the radial commutator and all of the bars are short-circuited. At the same time, the brush holders and brushes are lifted from the commutator surface. In this way, there is no unnecessary wear on the brushes and the commutator surface. Lifting the brushes from the commutator also eliminates a great deal of noise.

By short-circuiting the commutator bars, the armature is converted to a form of squirrel-cage rotor. As a result, the motor operates as a single-phase induction motor. This type of motor starts as a repulsion motor and runs as an induction motor.

Brush-Riding Motor. The brush-riding type of motor uses an axial commutator. The centrifugal mechanism consists of a number of copper segments held in place by a spring (Figure 18–22). This device is mounted next to the commutator. At nearly 75% of the rated speed, centrifugal force causes the short-circuiting necklace to short-circuit the commuta- tor segments. When this happens, the motor operates as an induction motor.

Torque and Speed. The starting torque and speed performances are very good for both types of repulsion-start, induction-run motors. Therefore, these motors are suitable for applications requiring a strong starting torque and a fairly constant speed from no load to full load. Typical uses include commercial refrigerators, compressors, and pumps.

To reverse the direction of rotation of the repulsion-start, induction-run motor, follow the steps outlined for a repulsion motor.

Schematic Diagrams. A schematic diagram of a repulsion-start, induction-run motor is shown in Figure 18–23. The same diagram also represents a repulsion motor.

Repulsion-start, induction-run motors are often constructed for operation on either 120 or 240 V. That is, two stator windings are provided. For 120-V operation, the wind- ings are connected in parallel. For 240-V operation, the windings are connected in series.

A schematic diagram is given in Figure 18–24 for a dual-voltage, repulsion-start, induction-run motor. The connection table provided shows how the leads of this motor are connected for 120- or 240-V operation. The same connections also apply to dual-voltage repulsion motors.

REPULSION–INDUCTION MOTOR

A repulsion-induction motor has nearly the same operating characteristics as a repulsion-start, induction-run motor. However, this type of motor has no centrifugal mechanism. The repulsion-induction motor has the same type of armature and commutator as the repulsion motor, but it does not have a centrifugal mechanism. There is a squirrel-cage winding beneath the slots of the armature of the repulsion-induction motor, as shown in Figure 18–25.

Both the repulsion-induction, motor and the repulsion-start, induction-run motor have very good starting torque because they start as repulsion motors. The squirrel-case winding of the repulsion-induction motor means that it has fairly constant speed regulation from no load to full load. The start-up of this motor and that of the repulsion-start, induction-run motor are similar. This motor is called a repulsion-induction motor to distinguish it from the repulsion-start, induction-run motor having a centrifugal system.

The torque–speed performance of a repulsion-induction motor is similar to that of a dc compound motor.

The repulsion-induction motor is designed for use on 120 or 240 V. The stator wind- ings have two sections that are connected in parallel for 120-V operation and in series for 240-V operation. The markings of the motor terminals and the connection arrangement

for these leads are the same as for a repulsion-start, induction-run motor. The repulsion- induction motor, the repulsion motor, and the repulsion-start, induction-run motor all have the same schematic diagram symbol.

ALTERNATING-CURRENT SERIES MOTOR

Without careful examination, it may appear that the typical dc series or shunt motor can be operated from an ac supply. If the line terminals to the dc motor are reversed, the current and the magnetic flux are reversed in both the field and armature circuits. For a motor operating from an ac source, the net torque will be in the same direction. As a result, it is impractical to operate a dc shunt motor in this manner because the high inductance of the shunt field causes the field current and the field flux to lag the line voltage by almost 90°. The motor thus develops little torque.

Universal Motors

Another reason why the series dc motor should not be operated on ac is the large amount of heat due to eddy currents in the field poles. In addition, there is an excessive voltage drop across the series field windings due to the high reactance. The field poles may be laminated to reduce the eddy currents. The voltage loss across the field poles can be reduced by using a small number of field turns on a low reluctance core. The core is then operated at a low flux density. This type of motor is known as a universal motor and can operate on ac or dc. Universal motors are available with fractional horsepower ratings and are commonly used in household appliances.

A concentrated field universal motor has two salient poles and a winding of few turns connected to give the opposite magnetic polarity. Figure 18–26 shows the laminated field structure of a typical concentrated field universal motor.

The field windings and armature of a universal motor are connected in series, as shown in Figure 18–27.

Universal Motor Type

Another type of universal motor is the distributed field motor. One version of this motor is a single-field compensated motor. A second version is a two-field compensated motor. The field windings of a two-pole, single-field compensated motor resemble the sta- tor winding of a two-pole, split-phase ac motor. A two-field compensated motor has a stator with a main winding and a compensating winding. These windings are spaced 90 electrical degrees apart. When the motor is operated from an ac source, the compensating winding reduces the reactance voltage developed in the armature by the alternating flux.

Universal motors have aluminum, cast iron, and rolled steel frames (Figure 18–28). The field poles are bolted to the frame. The field cores consist of laminations bolted together. The armature core is also laminated. A conventional commutator and brushes are used.

Speed Control

Universal motors operate at nearly the same speed on dc and single-phase ac. These series-wound motors operate at excessive speeds with no load. To overcome this, they are

permanently connected to the devices being driven. The speed regulation for universal motors consists of resistance inserted in series with the motor. Tapped resistors, rheostats and tapped nichrome wire coils, wound over a single-field pole, are used. The speed can also be controlled by varying the inductance through taps on one of the field poles.

Motor Reversal

Any series-wound motor can be reversed by changing the direction of the current in either the field or armature circuit. Universal motors are sensitive to brush position. Severe arcing at the brushes results when the direction of rotation is changed and the brushes are not shifted to the neutral (sparkless) plane.

CAPACITOR-START, INDUCTION-RUN MOTOR

Construction

Physically, the capacitor-start, induction-run motor is similar to the resistance-start, induction-run motor. However, the capacitor-start motor has a capacitor connected in series with the starting windings. Generally, the capacitor is mounted in any convenient external location on the motor frame. In some cases, the capacitor is mounted inside the motor housing. With regard to the starting torque, the capacitor-start motor has a higher torque than the resistance-start motor. The capacitor also limits the starting surge of current to a lower value as compared to the resistance-start motor.

The capacitor-start, induction-run motor is used in refrigeration units, compressors, oil burners, small-machine equipment, and in any application where split-phase induction motors are used. Because of its improved starting torque characteristics, the capacitor-start motor is replacing the split-phase motor in many applications.

Principle of Operation

A typical capacitor-start, induction-run motor is shown in Figure 18–10. At start-up, both the running and starting windings are connected in parallel across the line voltage while the centrifugal switch is closed. The starting winding is also connected in series with the capacitor. When the motor reaches 75% of the rated speed, the centrifugal switch opens. The starting winding and the capacitor are disconnected from the line. The motor then operates on the running winding as a single-phase induction motor.

Starting Torque. As stated previously, the starting torque is due to to a revolving magnetic field that is set up by the stator windings. By adding a capacitor of the cor- rect value in series with the starting winding, the current in this winding will lead the running winding current by 90 electrical degrees.

The angle between the starting winding current and the running winding current is almost 90° (Figure 18–11). The magnetic field set up by the stator windings is almost iden- tical to that of a two-phase induction motor. Therefore, the starting torque for the capacitor- start motor is much better than the torque for a resistance-start, induction-run motor.

Capacitor Starting. This motor starts with the capacitor in the starting winding circuit. For normal running, only the running winding is energized and the motor operates as a single-phase induction motor. The capacitor is used to improve the starting torque. Because it is energized for just two or three seconds at start-up, the capacitor cannot make any improvement in the power factor.

Defective capacitors often cause problems in the motors using them. For example, the capacitor may short-circuit and blow the fuse on the branch motor circuit. If the fuse rating is high so that the fuse does not interrupt the power supply to the motor, the starting winding may burn out. Starting capacitors may short-circuit if the motor is turned on and off many times in a short period of time. To prevent failures, many manufacturers recommend that this type of motor be started no more than twenty times per hour.

Paper or oil filled capacitors are used in these motors. If the motor is started too often in a short period of time, the current surge at start-up gradually damages the dielectric of the capacitor. Eventually, the dielectric breaks down and shorts (short- circuits) the capacitor plates. The capacitor-start motor is recommended for use only in those applications where there are few starts in a short period of time.

Speed Regulation. The speed regulation for a capacitor-start, induction-run motor is very good. The percent slip at full load is in the range from 4% to 6%. This means that the speed performance is the same as that of a resistance-start, induction-run motor.

Reversing the Motor. To reverse the direction of rotation of this motor, the leads of the starting winding circuit are reversed. The magnetic field developed by the start windings in the stator then rotates around the stator core in the opposite direction. Thus, the rotation of the rotor is reversed. The direction of rotation can also be reversed by interchanging the leads of the running windings.

The circuit connections for the capacitor-start motor are shown in Figure 18–12. In Figure 18–13, the starting winding leads are interchanged to reverse the direction of rotation of the motor.

Dual-Voltage Ratings. Capacitor-start, induction-run motors can also have dual-voltage ratings of 120 V and 240 V. The connections in this case are the same as those for split- phase induction motors.

CAPACITOR-START, CAPACITOR-RUN MOTOR

In the capacitor-start, capacitor-run (CSCR) motor, the starting winding and the capacitor are connected into the circuit at all times. This motor has a very good starting torque. Because the capacitor is used at all times, the power factor at the rated load is 100%, or unity.

Operation

Several different versions of the CSCR motor are available. In one type of motor, two stator windings are placed 90 electrical degrees apart. The running winding is connected directly across the rated line voltage. A capacitor is connected in series with the starting winding. This winding is also connected across the rated line voltage. Because the starting winding is energized as long as the motor is operating, a centrifugal switch is not required. Figure 18–14 gives the internal connections for a CSCR motor.

Using One Capacitor. The motor shown in Figure 18–14 is quiet in operation. It is used on oil burners, fans, and small woodworking and metalworking machines. The direction of rotation of this motor is reversed by interchanging the lead connections of either winding.

Using Two Capacitors. A second type of motor uses two capacitors. At start-up, a large value of capacitance is connected in series with the starting winding, as shown in Figure 18–15.

At this moment, the two capacitors shown are in parallel. When the motor accelerates to nearly 75% of the rated speed, the centrifugal switch disconnects the larger capacitor. The motor then operates with the small-value capacitor connected in series with the start- ing winding.

This type of motor has a very good starting torque, good speed regulation, and a power factor of almost 100% at the rated load. It is used on furnace stokers, refrigerators, and compressors, and in other applications where its strong starting torque and good speed regulation are required.

Autotransformer and One Capacitor. Another version of the CSCR motor uses an autotransformer. One capacitor is used to obtain a high starting torque and a large power factor.

The internal connections for this motor are shown in Figure 18–16. As the motor starts, the centrifugal switch connects winding 2 to point A on the tapped autotransformer. The capacitor is connected across nearly 500 V. A large leading current is formed in wind- ing 2 and a strong starting torque results.

When the motor reaches nearly 75% of the rated speed, the centrifugal switch disconnects the starting winding from point A and reconnects it to point B on the autotransformer. In this way, less voltage is applied to the capacitor. The motor operates with both windings energized. The capacitor maintains the power factors at nearly unity at the rated load.

The starting torque of this motor is very good, and its speed regulation is satisfactory. This motor is used in refrigerators, in compressors, and in other applications in which strong starting torque and good speed regulation are required.

MULTISPEED MOTOR

There are two basic types of multispeed motors. One type is known as the consequent pole motor. The other type is generally a capacitor-start, capacitor-run motor.

The Consequent Pole Motor

The speed of the rotating magnetic field of an ac induction motor can be changed in either of two ways:

1. Change the frequency of the ac voltage.

2. Change the number of stator poles.

The consequent pole motor changes motor speed by changing the number of its stator poles. The run winding in Figure 18–17 has been tapped in the center. If the ac line is connected to each end of the winding as shown, current flows through the winding in only one direction. Therefore, only one magnetic polarity is produced in the winding. If the winding is connected as shown in Figure 18–18, current flows in opposite directions in each half of the winding. Because current flows through each half of the winding in opposite directions, the polarity of the magnetic field is different in each half of the winding. The run winding now has two polarities instead of one. If the windings of a two-pole motor were to be tapped in this manner, the motor could become a four-pole motor. The synchronous speed of a two-pole motor is 3600 r/min, and the synchronous speed of a four-pole motor is 1800 r/min.

The consequent pole motor has the disadvantage of having a wide variation in speed. When the speed is changed, it changes from a synchronous speed of 3600 r/min to 1800 r/min. The speed cannot be changed by a small amount. This wide variation in speed makes the consequent pole motor unsuitable for some loads such as fans and blowers.

The consequent pole motor, however, does have some advantages over the other type of multispeed motor. When the speed on the consequent pole motor is reduced, its torque increases. For this reason, the consequent pole motor can be used to operate heavy loads.

Multispeed Fan Motors

Multispeed fan motors have been used in industry for many years. These motors are generally wound for two to five steps of speed and are used to operate fans and squirrel-cage blowers. A schematic drawing of a three-speed motor is shown in Figure 18–19. Notice that the run winding has been tapped to produce low, medium, and high speed. The start

winding is connected in parallel with the run winding section. The other end of the start lead is connected to an external oil-filled capacitor. This motor obtains a change in speed by inserting inductance in series with the run winding. The actual run winding for this motor is between the terminals marked “High” and “Common.” The windings shown between “High” and “Medium” are connected in series with the main run winding. When the rotary switch is connected to the medium-speed position, the inductive reactance of this coil limits the amount of current flow through the run winding. When the current of the run winding is reduced, the strength of the magnetic field of the run winding is reduced and the motor produces less torque. This causes the motor speed to decrease.

If the rotary switch is changed to the low position, more inductance is connected in series with the run winding. This causes less current to flow through the winding and another reduction in torque. When the torque is reduced, the motor speed decreases again.

Common speeds for a four-pole motor of this type are 1625, 1500, and 1350 r/min. Notice that this motor does not have the wide range between speeds that the consequent pole motor does. Most induction motors would overheat and damage the motor windings if the speed were to be reduced to this extent. This motor, however, has much higher imped- ance windings than do most motors. The run windings of most split-phase motors have a wire resistance of 1 to 4 f!. This motor will generally have a resistance of 10 to 15 f! in its run winding. It is the high impedance of the windings that permits the motor to be operated in this manner without damage.

Because this motor is designed to slow down when load is added, it is not used to operate high-torque loads. This type of motor is generally used to operate only low-torque loads such as fans and blowers.

CAPACITOR-START, INDUCTION-RUN MOTOR

Construction

Physically, the capacitor-start, induction-run motor is similar to the resistance-start, induction-run motor. However, the capacitor-start motor has a capacitor connected in series with the starting windings. Generally, the capacitor is mounted in any convenient external location on the motor frame. In some cases, the capacitor is mounted inside the motor housing. With regard to the starting torque, the capacitor-start motor has a higher torque than the resistance-start motor. The capacitor also limits the starting surge of current to a lower value as compared to the resistance-start motor.

The capacitor-start, induction-run motor is used in refrigeration units, compressors, oil burners, small-machine equipment, and in any application where split-phase induction motors are used. Because of its improved starting torque characteristics, the capacitor-start motor is replacing the split-phase motor in many applications.

Principle of Operation

A typical capacitor-start, induction-run motor is shown in Figure 18–10. At start-up, both the running and starting windings are connected in parallel across the line voltage while the centrifugal switch is closed. The starting winding is also connected in series with the capacitor. When the motor reaches 75% of the rated speed, the centrifugal switch opens. The starting winding and the capacitor are disconnected from the line. The motor then operates on the running winding as a single-phase induction motor.

Starting Torque. As stated previously, the starting torque is due to to a revolving magnetic field that is set up by the stator windings. By adding a capacitor of the cor- rect value in series with the starting winding, the current in this winding will lead the running winding current by 90 electrical degrees.

The angle between the starting winding current and the running winding current is almost 90° (Figure 18–11). The magnetic field set up by the stator windings is almost iden- tical to that of a two-phase induction motor. Therefore, the starting torque for the capacitor- start motor is much better than the torque for a resistance-start, induction-run motor.

Capacitor Starting. This motor starts with the capacitor in the starting winding circuit. For normal running, only the running winding is energized and the motor operates as a single-phase induction motor. The capacitor is used to improve the starting torque. Because it is energized for just two or three seconds at start-up, the capacitor cannot make any improvement in the power factor.

Defective capacitors often cause problems in the motors using them. For example, the capacitor may short-circuit and blow the fuse on the branch motor circuit. If the fuse rating is high so that the fuse does not interrupt the power supply to the motor, the starting winding may burn out. Starting capacitors may short-circuit if the motor is turned on and off many times in a short period of time. To prevent failures, many manufacturers recommend that this type of motor be started no more than twenty times per hour.

Paper or oil filled capacitors are used in these motors. If the motor is started too often in a short period of time, the current surge at start-up gradually damages the dielectric of the capacitor. Eventually, the dielectric breaks down and shorts (short- circuits) the capacitor plates. The capacitor-start motor is recommended for use only in those applications where there are few starts in a short period of time.

Speed Regulation. The speed regulation for a capacitor-start, induction-run motor is very good. The percent slip at full load is in the range from 4% to 6%. This means that the speed performance is the same as that of a resistance-start, induction-run motor.

Reversing the Motor. To reverse the direction of rotation of this motor, the leads of the starting winding circuit are reversed. The magnetic field developed by the start windings in the stator then rotates around the stator core in the opposite direction. Thus, the rotation of the rotor is reversed. The direction of rotation can also be reversed by interchanging the leads of the running windings.

The circuit connections for the capacitor-start motor are shown in Figure 18–12. In Figure 18–13, the starting winding leads are interchanged to reverse the direction of rotation of the motor.

Dual-Voltage Ratings. Capacitor-start, induction-run motors can also have dual-voltage ratings of 120 V and 240 V. The connections in this case are the same as those for split- phase induction motors.

CAPACITOR-START, CAPACITOR-RUN MOTOR

In the capacitor-start, capacitor-run (CSCR) motor, the starting winding and the capacitor are connected into the circuit at all times. This motor has a very good starting torque. Because the capacitor is used at all times, the power factor at the rated load is 100%, or unity.

Operation

Several different versions of the CSCR motor are available. In one type of motor, two stator windings are placed 90 electrical degrees apart. The running winding is connected directly across the rated line voltage. A capacitor is connected in series with the starting winding. This winding is also connected across the rated line voltage. Because the starting winding is energized as long as the motor is operating, a centrifugal switch is not required. Figure 18–14 gives the internal connections for a CSCR motor.

Using One Capacitor. The motor shown in Figure 18–14 is quiet in operation. It is used on oil burners, fans, and small woodworking and metalworking machines. The direction of rotation of this motor is reversed by interchanging the lead connections of either winding.

Using Two Capacitors. A second type of motor uses two capacitors. At start-up, a large value of capacitance is connected in series with the starting winding, as shown in Figure 18–15.

At this moment, the two capacitors shown are in parallel. When the motor accelerates to nearly 75% of the rated speed, the centrifugal switch disconnects the larger capacitor. The motor then operates with the small-value capacitor connected in series with the start- ing winding.

This type of motor has a very good starting torque, good speed regulation, and a power factor of almost 100% at the rated load. It is used on furnace stokers, refrigerators, and compressors, and in other applications where its strong starting torque and good speed regulation are required.

Autotransformer and One Capacitor. Another version of the CSCR motor uses an autotransformer. One capacitor is used to obtain a high starting torque and a large power factor.

The internal connections for this motor are shown in Figure 18–16. As the motor starts, the centrifugal switch connects winding 2 to point A on the tapped autotransformer. The capacitor is connected across nearly 500 V. A large leading current is formed in wind- ing 2 and a strong starting torque results.

When the motor reaches nearly 75% of the rated speed, the centrifugal switch disconnects the starting winding from point A and reconnects it to point B on the autotransformer. In this way, less voltage is applied to the capacitor. The motor operates with both windings energized. The capacitor maintains the power factors at nearly unity at the rated load.

The starting torque of this motor is very good, and its speed regulation is satisfactory. This motor is used in refrigerators, in compressors, and in other applications in which strong starting torque and good speed regulation are required.

MULTISPEED MOTOR

There are two basic types of multispeed motors. One type is known as the consequent pole motor. The other type is generally a capacitor-start, capacitor-run motor.

The Consequent Pole Motor

The speed of the rotating magnetic field of an ac induction motor can be changed in either of two ways:

1. Change the frequency of the ac voltage.

2. Change the number of stator poles.

The consequent pole motor changes motor speed by changing the number of its stator poles. The run winding in Figure 18–17 has been tapped in the center. If the ac line is connected to each end of the winding as shown, current flows through the winding in only one direction. Therefore, only one magnetic polarity is produced in the winding. If the winding is connected as shown in Figure 18–18, current flows in opposite directions in each half of the winding. Because current flows through each half of the winding in opposite directions, the polarity of the magnetic field is different in each half of the winding. The run winding now has two polarities instead of one. If the windings of a two-pole motor were to be tapped in this manner, the motor could become a four-pole motor. The synchronous speed of a two-pole motor is 3600 r/min, and the synchronous speed of a four-pole motor is 1800 r/min.

The consequent pole motor has the disadvantage of having a wide variation in speed. When the speed is changed, it changes from a synchronous speed of 3600 r/min to 1800 r/min. The speed cannot be changed by a small amount. This wide variation in speed makes the consequent pole motor unsuitable for some loads such as fans and blowers.

The consequent pole motor, however, does have some advantages over the other type of multispeed motor. When the speed on the consequent pole motor is reduced, its torque increases. For this reason, the consequent pole motor can be used to operate heavy loads.

Multispeed Fan Motors

Multispeed fan motors have been used in industry for many years. These motors are generally wound for two to five steps of speed and are used to operate fans and squirrel-cage blowers. A schematic drawing of a three-speed motor is shown in Figure 18–19. Notice that the run winding has been tapped to produce low, medium, and high speed. The start

winding is connected in parallel with the run winding section. The other end of the start lead is connected to an external oil-filled capacitor. This motor obtains a change in speed by inserting inductance in series with the run winding. The actual run winding for this motor is between the terminals marked “High” and “Common.” The windings shown between “High” and “Medium” are connected in series with the main run winding. When the rotary switch is connected to the medium-speed position, the inductive reactance of this coil limits the amount of current flow through the run winding. When the current of the run winding is reduced, the strength of the magnetic field of the run winding is reduced and the motor produces less torque. This causes the motor speed to decrease.

If the rotary switch is changed to the low position, more inductance is connected in series with the run winding. This causes less current to flow through the winding and another reduction in torque. When the torque is reduced, the motor speed decreases again.

Common speeds for a four-pole motor of this type are 1625, 1500, and 1350 r/min. Notice that this motor does not have the wide range between speeds that the consequent pole motor does. Most induction motors would overheat and damage the motor windings if the speed were to be reduced to this extent. This motor, however, has much higher imped- ance windings than do most motors. The run windings of most split-phase motors have a wire resistance of 1 to 4 f!. This motor will generally have a resistance of 10 to 15 f! in its run winding. It is the high impedance of the windings that permits the motor to be operated in this manner without damage.

Because this motor is designed to slow down when load is added, it is not used to operate high-torque loads. This type of motor is generally used to operate only low-torque loads such as fans and blowers.

Single-Phase Motors

INTRODUCTION

There are a number of types of single-phase motors. The types covered in this unit are the single-phase induction motor, the repulsion motor, the repulsion-induction motor, the series motor, and the shaded-pole motor. Three-phase motors generally perform better than single-phase motors. However, in many instances, only single-phase service is avail- able. Most single-phase motors have fractional horsepower ratings. In general, their use is limited to commercial and residential applications.

SINGLE-PHASE INDUCTION MOTORS

There are two main types of single-phase induction motors: the resistance-start, induction-run motor and the capacitor-start, induction-run motor. These motors have fractional horsepower ratings. The resistance-start induction motor is used in appliances and with other small loads where a strong starting torque is not required. The capacitor- start induction motor is used on refrigerators, compressors, and similar loads. Both types of motors are low in cost, are rugged, and have good performance. These induction motors are also called split-phase motors. That is, the capacitor or resistance is used to obtain a phase change for one winding, resulting in a rotating field.

RESISTANCE-START, INDUCTION-RUN MOTOR

Construction

The basic parts of a resistance-start, induction-run motor are

• the stator (stationary part).

• the rotor (revolving part).

• a centrifugal switch, located inside the motor.

• two end shields bolted to the steel frame; these shields house the rotor shaft bearings.

• a cast steel frame; the stator core is pressed into this frame.

Stator. The stator consists of two windings, held in place in the slots of a laminated steel core. The windings are made up of insulated coils. The coils are placed so that they are 90 electrical degrees apart. One winding is the main (running) winding. The other winding is the auxiliary (starting) winding.

The running winding is made of a heavy insulated copper wire. It is located at the bottom of the stator slots. A small wire size is used in the starting winding. This winding is placed near the top of the slots above the running winding.

At start-up, both windings are connected in parallel to the single-phase line. Once the motor accelerates to two-thirds or three-quarters of the rated speed, a centrifugal switch disconnects the starting winding automatically.

Rotor. The rotor of the resistance-start, induction-run motor (Figure 18–1) is the same as that of a three-phase, squirrel-cage induction motor. The cylindrical core of the rotor

consists of steel laminations. Copper bars are mounted near the surface of the rotor and are brazed or welded to two copper end rings. The rotor may also be a one-piece case aluminum unit. This type of rotor requires very little maintenance. It contains no windings, brushes, slip rings, or commutator where faults may develop. Fans are provided with the rotor to keep the temperature of the windings at a reasonable level.

Centrifugal Switch. The centrifugal switch is mounted inside the motor. It disconnects the starting winding after the rotor reaches a predetermined speed. The switch has a station- ary part that is mounted on one of the end shields. This part has two contacts that act like a single-pole, single-throw switch. The centrifugal switch also has a rotating part that is mounted on the rotor. A typical centrifugal switch is shown in Figure 18–2. This switch is commonly used on split-phase induction motors.

The operation of a centrifugal switch is shown in Figure 18–3. When the rotor is not moving, the pressure of a spring on the fiber ring of the rotating part of the switch keeps the contacts closed. At three-quarters of the rated motor speed, the centrifugal action of the

rotor causes the spring and fiber ring to release its pressure and opens the switch contacts. As a result, the starting winding circuit is disconnected from the line.

Frame and End Shields. The stator core is pressed directly into the cast steel frame. The two end shields are bolted to the frame. The shields contain bearings that support the rotor and center it in the stator. Thus, the shaft rotates with little friction and does not strike or rub against the stator core.

Principle of Operation

At the instant the motor circuit is closed, both the starting and running windings are energized. Because the running winding uses large-size wire, it has a low resistance. But the running winding is placed at the bottom of the stator core slots. Thus, its inductive reactance is high. Because of its low resistance and high inductive reactance, the current of the running winding lags behind the voltage.

The current in the starting winding is more nearly in phase with the voltage. Small wire is used in this winding, resulting in a high resistance. Because the winding is near the top of the stator slots, the mass of iron surrounding it is small. This means that its inductive reactance is small. Because the starting winding has a high resistance and a low inductive reactance, its current is more nearly in phase with the voltage.

Figure 18–4 shows the relationship between the currents in the windings and the volt- age. The current of the main winding (IM ) lags the current of the starting winding (IS) by nearly 90 electrical degrees. When a current passes through each of these windings, the resulting pulsating field effect gives rise to a rotating field around the inside of the stator core. The speed of this rotating magnetic field is determined in the same way it was found for a three-phase induction motor.

Synchronous Speed. Consider a single-phase, resistance-start, induction-run motor with four poles wound into both the main windings and the starting winding. If this motor is energized from a 60-Hz source, the synchronous speed of the revolving field is

While traveling at the synchronous speed, the rotating field cuts the copper bars of the squirrel-cage winding. Voltages are induced in the windings and cause currents in the rotor bars. A rotor field is created. This field reacts with the stator field to develop the torque that causes the rotor to turn.

As the rotor accelerates to approximately 80% of the rated speed, the centrifugal switch disconnects the starting winding from the line. The connections for the centrifugal switch are shown in Figure 18–5. At start-up, the switch is closed. As the motor accelerates to its normal running speed, the centrifugal switch opens. The motor then continues to operate, using only the running winding.

Once the motor is running, current is induced in the rotor for two reasons: (1) the alternating stator flux induces “transformer voltage” in the rotor, and (2) “speed voltage” is induced in the rotor bars as they cut across the magnetic field of the stator. The combined effect of these alternating voltages produces a torque that keeps the rotor turning.

Energized Windings. A resistance-start, induction-run motor must have the starting winding and the main winding energized at start-up. The motor resembles a two-phase induction motor because the currents of the windings are approximately 90 electrical degrees out of phase with each other. However, a single-phase source supplies the motor. Therefore, the motor is called a split-phase motor because it starts like a two-phase motor from a single-phase line. Once the motor accelerates to a value near the rated speed, it operates on the running winding as a single-phase induction motor.

If the centrifugal switch mechanism fails to close the switch contacts when the motor stops, the starting winding circuit will be open. This means that when the motor circuit is reenergized, the motor will not start. Both the starting and running windings must be energized at the instant the motor circuit closes if the necessary starting torque is to be formed. If the motor does not start, but a low humming sound is present, then one winding is open. The centrifugal switch should be checked to determine whether its mechanism is faulty or the switch contacts are pitted.

If only one stator winding is energized, an alternating field, rather than a rotating field, is formed. If the rotor is at rest, this field induces an alternating current in the rotor winding. This current acts as the secondary of a transformer. Rotor poles are developed by this induced current. These poles are exactly aligned with the stator poles. Thus, no starting torque is developed in either direction of rotation.

If a split-phase motor is started with too great a mechanical load, it may not accelerate enough to open the centrifugal switch. Also, if a low terminal voltage is applied to the motor, the motor may fail to reach the speed required to operate the centrifugal switch.

Starting Winding. The starting winding uses a small size of wire, resulting in a large resistance. The starting winding is designed to operate across the line voltage for just three or four seconds as the motor accelerates to the rated speed. Therefore, the starting winding must be disconnected from the line by the centrifugal switch as soon as the motor accelerates to three-quarters of the rated speed. If the motor operates on its starting winding for more than sixty seconds, the winding may char or burn out.

To reverse the rotation of the motor, simply interchange the leads of either the starting winding or the running winding, preferably the starting winding. Interchanging these leads reverses the direction of rotation of the stator field (and the rotor).

Dual-Voltage Motor

In many cases, single-phase motors have dual voltage ratings such as 120 V and 240 V. The running winding consists of two sections, each of which is rated at 120 V. One section of the running winding is marked T1 and T2. The other section is marked T3 and T4. The starting winding consists of only one 120-V winding. The leads of the starting winding are marked T5 and T6. To operate the motor on 240 V, the two 120-V sections of the running winding are connected in series across the 240-V line. The starting winding is paralleled with one section of the running winding.

To operate the motor on 120 V, the two 120-V sections of the running winding are connected in parallel across the 120-V line. The starting winding is connected in parallel with both sections of the running winding.

Figure 18–6 shows the circuit connections for a dual-voltage motor connected for 120-V operation. The connections for 240-V operation are shown in Figure 18–7. In this case, the jumpers are changed in the terminal box so that the two 120-V running windings are connected in series. These sections are then connected across the 240-V line. Note that the 120-V starting winding is connected in parallel with one section of running winding. If the voltage drop across this section of the running winding is 120 V, then the voltage across the starting winding is 120 V. The starting winding may also consist of two sections for some dual-voltage, resistance-start, induction-run motors. As before, the two running winding sections are marked T1 and T2 and T3 and T4. The two starting winding sections are marked “T5 and T6” and “T7 and T8” (Figure 18–8).

The table in Figure 18–8 shows the correct connections for 120-V operation and for 240-V operation.

Speed Regulation. A resistance-start, induction-run motor has very good speed regulation. From no load to full load, the speed performance of this type of motor is about the same as that of a three-phase, squirrel-cage induction motor. The percent slip on most fractional horsepower split-phase motors ranges from 4% to 6%.

Starting Torque. The starting torque of the motor is poor. Although the main winding consists of large wire, it has some resistance. Also, the starting winding has an inductive reactive component, even though it is placed near the top of the stator slots. As a result, the current in the main winding does not lag the line voltage by a full 90° because of the resistance. The current in the starting winding is not in phase with the line voltage because of the inductive reactance.

Figure 18–9 shows the phase angle between the current in the main winding and the current in the starting winding for a typical resistance-start, induction-run motor. A phase angle in the order of 30° to 50° is large enough to provide a weak rotating magnetic field. As a result, the starting torque is small.

Single-Phase Motors

INTRODUCTION

There are a number of types of single-phase motors. The types covered in this unit are the single-phase induction motor, the repulsion motor, the repulsion-induction motor, the series motor, and the shaded-pole motor. Three-phase motors generally perform better than single-phase motors. However, in many instances, only single-phase service is avail- able. Most single-phase motors have fractional horsepower ratings. In general, their use is limited to commercial and residential applications.

SINGLE-PHASE INDUCTION MOTORS

There are two main types of single-phase induction motors: the resistance-start, induction-run motor and the capacitor-start, induction-run motor. These motors have fractional horsepower ratings. The resistance-start induction motor is used in appliances and with other small loads where a strong starting torque is not required. The capacitor- start induction motor is used on refrigerators, compressors, and similar loads. Both types of motors are low in cost, are rugged, and have good performance. These induction motors are also called split-phase motors. That is, the capacitor or resistance is used to obtain a phase change for one winding, resulting in a rotating field.

RESISTANCE-START, INDUCTION-RUN MOTOR

Construction

The basic parts of a resistance-start, induction-run motor are

• the stator (stationary part).

• the rotor (revolving part).

• a centrifugal switch, located inside the motor.

• two end shields bolted to the steel frame; these shields house the rotor shaft bearings.

• a cast steel frame; the stator core is pressed into this frame.

Stator. The stator consists of two windings, held in place in the slots of a laminated steel core. The windings are made up of insulated coils. The coils are placed so that they are 90 electrical degrees apart. One winding is the main (running) winding. The other winding is the auxiliary (starting) winding.

The running winding is made of a heavy insulated copper wire. It is located at the bottom of the stator slots. A small wire size is used in the starting winding. This winding is placed near the top of the slots above the running winding.

At start-up, both windings are connected in parallel to the single-phase line. Once the motor accelerates to two-thirds or three-quarters of the rated speed, a centrifugal switch disconnects the starting winding automatically.

Rotor. The rotor of the resistance-start, induction-run motor (Figure 18–1) is the same as that of a three-phase, squirrel-cage induction motor. The cylindrical core of the rotor

consists of steel laminations. Copper bars are mounted near the surface of the rotor and are brazed or welded to two copper end rings. The rotor may also be a one-piece case aluminum unit. This type of rotor requires very little maintenance. It contains no windings, brushes, slip rings, or commutator where faults may develop. Fans are provided with the rotor to keep the temperature of the windings at a reasonable level.

Centrifugal Switch. The centrifugal switch is mounted inside the motor. It disconnects the starting winding after the rotor reaches a predetermined speed. The switch has a station- ary part that is mounted on one of the end shields. This part has two contacts that act like a single-pole, single-throw switch. The centrifugal switch also has a rotating part that is mounted on the rotor. A typical centrifugal switch is shown in Figure 18–2. This switch is commonly used on split-phase induction motors.

The operation of a centrifugal switch is shown in Figure 18–3. When the rotor is not moving, the pressure of a spring on the fiber ring of the rotating part of the switch keeps the contacts closed. At three-quarters of the rated motor speed, the centrifugal action of the

rotor causes the spring and fiber ring to release its pressure and opens the switch contacts. As a result, the starting winding circuit is disconnected from the line.

Frame and End Shields. The stator core is pressed directly into the cast steel frame. The two end shields are bolted to the frame. The shields contain bearings that support the rotor and center it in the stator. Thus, the shaft rotates with little friction and does not strike or rub against the stator core.

Principle of Operation

At the instant the motor circuit is closed, both the starting and running windings are energized. Because the running winding uses large-size wire, it has a low resistance. But the running winding is placed at the bottom of the stator core slots. Thus, its inductive reactance is high. Because of its low resistance and high inductive reactance, the current of the running winding lags behind the voltage.

The current in the starting winding is more nearly in phase with the voltage. Small wire is used in this winding, resulting in a high resistance. Because the winding is near the top of the stator slots, the mass of iron surrounding it is small. This means that its inductive reactance is small. Because the starting winding has a high resistance and a low inductive reactance, its current is more nearly in phase with the voltage.

Figure 18–4 shows the relationship between the currents in the windings and the volt- age. The current of the main winding (IM ) lags the current of the starting winding (IS) by nearly 90 electrical degrees. When a current passes through each of these windings, the resulting pulsating field effect gives rise to a rotating field around the inside of the stator core. The speed of this rotating magnetic field is determined in the same way it was found for a three-phase induction motor.

Synchronous Speed. Consider a single-phase, resistance-start, induction-run motor with four poles wound into both the main windings and the starting winding. If this motor is energized from a 60-Hz source, the synchronous speed of the revolving field is

While traveling at the synchronous speed, the rotating field cuts the copper bars of the squirrel-cage winding. Voltages are induced in the windings and cause currents in the rotor bars. A rotor field is created. This field reacts with the stator field to develop the torque that causes the rotor to turn.

As the rotor accelerates to approximately 80% of the rated speed, the centrifugal switch disconnects the starting winding from the line. The connections for the centrifugal switch are shown in Figure 18–5. At start-up, the switch is closed. As the motor accelerates to its normal running speed, the centrifugal switch opens. The motor then continues to operate, using only the running winding.

Once the motor is running, current is induced in the rotor for two reasons: (1) the alternating stator flux induces “transformer voltage” in the rotor, and (2) “speed voltage” is induced in the rotor bars as they cut across the magnetic field of the stator. The combined effect of these alternating voltages produces a torque that keeps the rotor turning.

Energized Windings. A resistance-start, induction-run motor must have the starting winding and the main winding energized at start-up. The motor resembles a two-phase induction motor because the currents of the windings are approximately 90 electrical degrees out of phase with each other. However, a single-phase source supplies the motor. Therefore, the motor is called a split-phase motor because it starts like a two-phase motor from a single-phase line. Once the motor accelerates to a value near the rated speed, it operates on the running winding as a single-phase induction motor.

If the centrifugal switch mechanism fails to close the switch contacts when the motor stops, the starting winding circuit will be open. This means that when the motor circuit is reenergized, the motor will not start. Both the starting and running windings must be energized at the instant the motor circuit closes if the necessary starting torque is to be formed. If the motor does not start, but a low humming sound is present, then one winding is open. The centrifugal switch should be checked to determine whether its mechanism is faulty or the switch contacts are pitted.

If only one stator winding is energized, an alternating field, rather than a rotating field, is formed. If the rotor is at rest, this field induces an alternating current in the rotor winding. This current acts as the secondary of a transformer. Rotor poles are developed by this induced current. These poles are exactly aligned with the stator poles. Thus, no starting torque is developed in either direction of rotation.

If a split-phase motor is started with too great a mechanical load, it may not accelerate enough to open the centrifugal switch. Also, if a low terminal voltage is applied to the motor, the motor may fail to reach the speed required to operate the centrifugal switch.

Starting Winding. The starting winding uses a small size of wire, resulting in a large resistance. The starting winding is designed to operate across the line voltage for just three or four seconds as the motor accelerates to the rated speed. Therefore, the starting winding must be disconnected from the line by the centrifugal switch as soon as the motor accelerates to three-quarters of the rated speed. If the motor operates on its starting winding for more than sixty seconds, the winding may char or burn out.

To reverse the rotation of the motor, simply interchange the leads of either the starting winding or the running winding, preferably the starting winding. Interchanging these leads reverses the direction of rotation of the stator field (and the rotor).

Dual-Voltage Motor

In many cases, single-phase motors have dual voltage ratings such as 120 V and 240 V. The running winding consists of two sections, each of which is rated at 120 V. One section of the running winding is marked T1 and T2. The other section is marked T3 and T4. The starting winding consists of only one 120-V winding. The leads of the starting winding are marked T5 and T6. To operate the motor on 240 V, the two 120-V sections of the running winding are connected in series across the 240-V line. The starting winding is paralleled with one section of the running winding.

To operate the motor on 120 V, the two 120-V sections of the running winding are connected in parallel across the 120-V line. The starting winding is connected in parallel with both sections of the running winding.

Figure 18–6 shows the circuit connections for a dual-voltage motor connected for 120-V operation. The connections for 240-V operation are shown in Figure 18–7. In this case, the jumpers are changed in the terminal box so that the two 120-V running windings are connected in series. These sections are then connected across the 240-V line. Note that the 120-V starting winding is connected in parallel with one section of running winding. If the voltage drop across this section of the running winding is 120 V, then the voltage across the starting winding is 120 V. The starting winding may also consist of two sections for some dual-voltage, resistance-start, induction-run motors. As before, the two running winding sections are marked T1 and T2 and T3 and T4. The two starting winding sections are marked “T5 and T6” and “T7 and T8” (Figure 18–8).

The table in Figure 18–8 shows the correct connections for 120-V operation and for 240-V operation.

Speed Regulation. A resistance-start, induction-run motor has very good speed regulation. From no load to full load, the speed performance of this type of motor is about the same as that of a three-phase, squirrel-cage induction motor. The percent slip on most fractional horsepower split-phase motors ranges from 4% to 6%.

Starting Torque. The starting torque of the motor is poor. Although the main winding consists of large wire, it has some resistance. Also, the starting winding has an inductive reactive component, even though it is placed near the top of the stator slots. As a result, the current in the main winding does not lag the line voltage by a full 90° because of the resistance. The current in the starting winding is not in phase with the line voltage because of the inductive reactance.

Figure 18–9 shows the phase angle between the current in the main winding and the current in the starting winding for a typical resistance-start, induction-run motor. A phase angle in the order of 30° to 50° is large enough to provide a weak rotating magnetic field. As a result, the starting torque is small.

SELSYN DIFFERENTIAL SYSTEM

A selsyn differential system consists of a transmitter, a differential, and a receiver, Figure 17–17. The differential selsyn resembles a miniature wound-rotor induction motor. There are three rotor windings, usually connected in wye. The windings are brought out through three slip rings to external terminals R1, R2, and R3. The stator windings are also connected in wye. They are brought out to terminals S1, S2, and S3. The differential selsyn is really a single-phase transformer. Three-phase voltages and currents are not present.

The stator windings serve as the primary side. The rotor windings serve as the second- ary side. The differential selsyn is used to modify the electrical angle transmitted by the transmitter selsyn. This means that the selsyn receiver will take up a position that is either the sum or the difference of the angles applied to the selsyn transmitter and the differential selsyn. Thus, two selsyn generators (transmitters) can be connected through a differential selsyn and turned through any angle. The differential selsyn will indicate the difference between the two angles.

For the differential system shown in Figure 17–17, the voltage distribution in the pri- mary winding is the same as that in the secondary winding of the excited selsyn. If any one of the three selsyns is fixed in position, and a second selsyn is displaced by a certain angle, the third selsyn will turn through the same angle. The direction of rotation can be reversed by reversing any pair of leads to the rotor or stator windings of the differential selsyn. If any two of the selsyns are rotated at the same time, the third selsyn will rotate through an angle equal to the algebraic sum of the movement of the first two selsyns. The algebraic sign of the angle depends on the physical direction of rotation of the rotors and the phase rotation of the windings.

The differential selsyn is not connected directly to the excitation source. The excitation current for the differential must be furnished through one, or both, of the standard selsyn units. Generally the excitation current is supplied to the primary (stator) windings of the differential selsyn from the exciter selsyn. For this reason, the exciter selsyn is always designed to be larger in capacity than the selsyn transmitter or receiver.

SUMMARY

• The synchronous motor

1. It is a three-phase ac motor that operates at a constant speed from no load to full load.

2. It is similar to a three-phase generator.

3. It has a revolving field. This field is separately excited from a direct-current source, which can be varied to obtain a wide range of lagging and leading power factor values.

4. The synchronous motor is used in many industrial applications because

a. it has a fixed speed from no load to full load.

b. it has a high efficiency.

c. the initial cost is low.

d. it can improve the power factor of three-phase ac industrial circuits.

5. Construction

a. The three-phase synchronous motor consists of

(1) a laminated stator core with a three-phase armature winding.

(2) a revolving field with an amortisseur winding and slip rings.

(3) brushes and brush holders.

(4) two end shields that house the bearings supporting the rotor shaft.

b. The leads for the stator winding are marked T1, T2, and T3.

c. The field is marked F1 and F2.

6. Operating principles

b. A three-phase voltage applied to the stator windings causes a rotating mag- netic field.

(1) This field travels at the synchronous speed.

(2) The field cuts across the amortisseur (squirrel-cage) winding of the rotor.

(3) A magnetic field is set up in the rotor. This field reacts with the stator field and causes the rotation of the rotor.

(4) The rotor speed reached 95% to 97% of synchronous speed.

c. A synchronous motor is started by connecting its armature winding (stator) to the ac line and its field windings (rotor) to a field discharge resistor. The motor is started as an induction motor.

(1) The voltage induced in the field windings may reach 1500 V. Therefore, the field circuits must be well insulated and enclosed to protect personnel.

(2) The field discharge resistor is connected across the field windings so that the energy in the field circuit is spent in the resistor.

d. Once the motor accelerates to nearly 95% of synchronous speed

(1) the field circuit is energized from the dc source.

(2) the field discharge resistor is disconnected.

(3) the rotor pulls into synchronism with the revolving armature (stator) flux. Thus, the motor will operate at a constant speed.

e. To shut down the motor, the field circuit is deenergized by opening the field discharge switch. The field discharge resistor is connected across the field circuit to reduce the induced voltage in the field as the field flux collapses. The energy stored in the magnetic field is spent in the resistor, and a lower voltage is induced in the field circuit.

• Load on a synchronous motor

1. Once the rated load is applied, there is an angular displacement of the rotor pole with respect to the stator pole.

2. The speed is unchanged because the rotor continues to rotate at the synchronous speed.

3. The angular displacement between the centers of the stator and rotor field poles is called the torque angle and is represented by the Greek letter alpha, a.

a. At no load, the torque angle is nearly 0°.

b. As the mechanical load increases, the torque angle increases. The phase angle between the impressed voltage and the counter emf also increases. This increase means that the impressed voltage will cause more current in the sta- tor windings to meet the additional load demands.

c. A serious overload causes too large a value of the phase angle. In this case, the rotor pulls out of synchronism. With the aid of the amortisseur winding, the motor will operate as an induction motor.

d. The pullout torque is the maximum torque value that can be developed by a synchronous motor without dropping out of synchronism. The pullout torque is usually 150% to 200% of the rated torque output.

• Power factor of a synchronous motor

1. Changes in dc field excitation alter the power factor of a synchronous motor.

a. Increasing the resistance of the field rheostat lowers the field current. A poor lagging power factor results. As the field current decreases, more magnetizing current is supplied by the ac stator windings. This ac current lags the voltage by 90° and causes a low lagging power factor.

b. As the field current increases, the stator supplies a smaller magnetizing com- ponent of current. Thus, the power factor increases.

2. The field strength can be increased until the power factor is unity or 100%.

3. When the power factor reaches unity, the three-phase ac circuit supplies energy current only.

4. The amount of dc field excitation required to obtain a unity power factor is called the normal field excitation.

5. Overexciting the dc field produces a demagnetizing component of current called the quadrature lead current. The motor has a leading power factor.

• Industrial applications

1. Synchronous motors rated at 20 hp or more are used for constant-speed applications.

2. They are used to drive large air and gas compressors that are operated at fixed speeds to maintain a constant output at the maximum efficiency.

3. They are also used to drive dc generators, fans, blowers, and large pumps in water- pumping stations.

4. Three-phase synchronous motors can be used to drive mechanical loads and correct power factor values.

5. When the synchronous motor is used to correct the power factor only, it is called a synchronous capacitor or a synchronous condenser. It then has the same function as a bank of capacitors.

• Formulas for synchronous motors

• Generally, a synchronous motor is started at full voltage. Motors rated as high as 10,000 or 15,000 hp may be started at full voltage.

1. When the starting inrush current is high enough to cause voltage disturbances, reduced voltage starting current can be used.

2. Fifty percent to 65% of the rated line voltage is applied to the motor through a reactor or an autotransformer starter.

3. At a reduced voltage, the motor starts as an induction motor. It accelerates to a speed close to the synchronous speed. Then the autotransformer, or reactor starter, is shifted into the running position. At this point, the dc field circuit is energized.

• Nameplate data

1. The nameplate data for the synchronous motor is the same as that for an ac generator. A horsepower rating is given for the synchronous motor instead of a kVA rating.

2. The mechanical load, in horsepower output, and the electrical load, in leading kilovars, must not exceed the rating of the motor.

• Synchronous motor losses are the same as those for an ac generator.

• The efficiency of the three-phase synchronous motor is slightly higher than that of an induction motor having the same speed and horsepower ratings.

• Small single-phase synchronous motor

1. This type of motor is used in many applications, including electric clocks, time switches, graphical recording instruments, and stroboscope devices.

2. These motors do not require dc field excitation.

3. The Warren (or General Electric) clock motor is a commonly used motor of this type.

a. It consists of a laminated stator core with an exciting coil. Normally, this coil is wound for 120-V operation.

b. The coil has two poles, and each pole is divided into two sections. One turn of heavy copper wire is placed over half of each pole to produce a rotating field effect.

c. The rotor consists of several hardened steel discs that are pressed on the small rotor shaft. The rotating field acts on the rotor to produce a strong torque. The rotor accelerates to a speed near synchronous speed. It then locks into synchronism with the rotating field. The flux of the rotating field seeks the path of minimum reluctance (resistance) through the two small rotor crossbars.

4. The Holtz motor

a. This type of motor has the same shaded-pole arrangement as the Warren motor.

b. The rotor has six slots that hold a small squirrel-cage winding.

c. The motor starts as a squirrel-cage motor. The salient poles of the rotor then lock with the sections of the field poles twice each cycle. The resulting speed at 60 Hz is one-third of the synchronous speed, or 1200 r/min.

5. Selsyn motors

a. Selsyn is a contraction of the term self-synchronous.

b. Selsyn motors are used to interconnect two or more remote points of a system by electrical means.

c. A selsyn system indicates the positions of remote devices such as generator rheostats, steam turbine governors, waterwheel governors, transformer tap connections, swing gates or valves, elevators, and the roll height in steel roll- ing mills. Selsyn motors are also used in automatic control systems, signaling systems, and remote control systems.

d. The rotor terminals are marked R1 and R2. The stator terminals are marked S1, S2, and S3.

e. The rotor field circuit is excited from an external single-phase source.

f. The stator windings consist of a conventional three-phase winding.

g. One motor of a selsyn system is called the transmitter and the other motor is called the receiver.

(1) The stator terminals of one motor are wired to the stator terminals of the other motor in sequence.

(2) The rotors (R1 and R2 ) are tied together and are connected in common to an alternating-current source. Turning the rotor of one motor causes the rotor of the other motor to be displaced an equal amount.

6. Selsyn differential system

a. A differential unit is added between the transmitter and the receiver of the common selsyn system.

b. The differential resembles a miniature wound-rotor induction motor.

There are three rotor windings. Leads R1, R2 , and R3 are brought out through three slip rings. The stator windings are marked S1, S2, and S3.

c. The differential selsyn is used to modify the electrical angle transmitted by the transmitter selsyn. The differential selsyn indicates the sum or difference between the two selsyns.

Achievement Review

1. What are the factors that determine the speed of a synchronous motor?

2. Explain how the operation of a synchronous motor is different from that of a squirrel-cage induction motor.

3. What is the purpose of the amortisseur winding in a synchronous motor?

4. Explain how a synchronous motor adjusts its electrical input with an increase in the mechanical output.

5. What is the purpose of the field discharge resistor that is used with the separately excited dc field circuit of a synchronous motor?

6. A motor generator set is used for frequency conversion from 60 Hz to another frequency. A three-phase, 60-Hz synchronous motor with 24 poles is used as the prime mover. The ac generator has 20 poles.

a. Find the speed of the synchronous motor.

b. Find the frequency of the output voltage of the ac generator.

7. When is a synchronous motor underexcited? When is it overexcited?

8. Describe a method for starting a large three-phase synchronous motor.

9. A three-phase, three-wire, 208-V feeder supplies two motors. One motor is a three-phase, squirrel-cage induction motor and requires 50 A at 0.80 lagging power factor. The other motor is a three-phase synchronous motor that requires 40 A at a leading power factor of 0.75.

a. For the squirrel-cage induction motor, determine

(1) the apparent power, in kilovolt-amperes.

(2) the true power, in kilowatts.

(3) the reactive power, in kVARs.

b. For the three-phase synchronous motor, determine

(1) the apparent power, in kilovolt-amperes.

(2) the true power, in kilowatts.

(3) the reactive power, in kVARs.

10. For the circuit given in question 9, determine

a. the total true power, in kilowatts, supplied by the feeder circuit.

b. the total apparent power, in kVA, supplied by the feeder circuit.

c. the power factor of the feeder circuit. Indicate whether it is a leading or lag- ging power factor.

d. the quadrature power, in kVARs, supplied by the feeder circuit.

e. the total current, in amperes, supplied by the three-phase, three-wire feeder to operate both motors.

11. A three-phase, three-wire, 2400-V feeder supplies an industrial plant that has a 500-kVA load operating at a lagging power factor of 0.75. A 2400-V, three- phase synchronous motor is to be installed. This motor will require 200 kVA at a power factor of 0.80 lead. With the synchronous motor in operation, determine

a. the total load, in kW, on the 2400-V feeder.

b. the total load, in kVA, on the 2400-V feeder.

c. the power factor on the 2400-V feeder. Indicate whether it is a leading or lag- ging power factor.

12. What are some practical applications for three-phase synchronous motors?

13. Explain the operation of a General Electric (Warren) clock.

14. Explain the operation of a single-phase Holtz motor.

15. Draw a circuit diagram for a selsyn circuit consisting of a selsyn transmitter and a selsyn receiver.

16. Explain the operation of the selsyn circuit in question 15.

17. Draw a circuit diagram for a selsyn circuit, including a selsyn transmitter, a differential selsyn, and a selsyn receiver.

18. Explain the operation of the differential selsyn system in question 17.

19. List some practical uses for selsyn units.