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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.

STARTING LARGE SYNCHRONOUS MOTORS

The earlier part of this unit described how a synchronous motor is started. In general, the starting current for synchronous motors is less than that for squirrel-cage induction motors having the same horsepower and speed ratings. In most cases, it is preferred to start a synchronous motor at full voltage. Motors as powerful as 10,000 or 15,000 hp can be started at full voltage. This type of starting does not harm a motor that is properly designed. Full-voltage starting means that a simpler, less expensive starter is required as compared to reduced-voltage starting.

When the motor starting inrush causes voltage disturbances, reduced-voltage starting current can be used. Approximately 50% to 65% of the rated line voltage is applied to the motor through a reactor or an autotransformer starter.

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

Many automatic motor starters use special relays to energize the field circuit. The circuit must be energized at the correct instant to ensure that the maximum pullin torque is obtained with a minimum amount of disturbance to the system. After the motor is operating properly as a synchronous motor, the dc field excitation current is adjusted to give the desired power factor.

SYNCHRONOUS MOTOR RATINGS

The nameplate data of a synchronous motor contains the same information as the nameplate of an ac generator. The only exception is the output rating because the alternator is rated in kVA and the synchronous motor is rated in horsepower.

The nameplate of a synchronous motor also gives a power factor rating. This type of motor is usually rated at unity power factor, 90% lead power factor, or 80% lead power fac- tor. The motor may be rated at other power factor values for special applications. If a motor is rated at unity power factor, it can be operated with a leading power factor. In this case, the mechanical load must be decreased so that only the rated ac stator current will flow at the reduced power factor. In other words, the mechanical load, in horsepower output, and the electrical load, in leading kilovars, must not exceed the rating of the motor. When a synchronous motor is rated at a leading power factor of 80% or 90%, it will have a larger current capacity for a given horsepower output. This is necessary if the rated horsepower output is to be supplied for the larger current at the reduced power factor.

Synchronous motor losses are the same as those for an ac generator. The losses include the power spent in the separately excited field, the mechanical friction losses con- sisting of the copper losses in the three-phase stator winding and the stray power losses, the windage losses, and the iron losses. 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 MOTORS

The Warren Motor

Small single-phase synchronous motors are used in many timing applications, including electric clocks, time switches, graphical recording instruments, and stroboscope devices. These motors do not require dc field excitation. A widely used motor of this type is the Warren (or General Electric) clock motor. It consists of a laminated stator core with an exciting coil. Normally, this coil is wound for 120-V operation. 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.

A General Electric (Warren) clock motor is shown in Figure 17–12. The rotor consists of several hardened steel discs that are pressed on the small rotor shaft. The discs have a high hysteresis loss. The rotating field acts on the rotor to produce 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.

Operation with Two-Pole Stator

How is a rotating field effect obtained with the two-pole stator? As the flux increases in value, part of the flux attempts to pass through the section of the pole having the copper shading loop. This flux induces a voltage and current in the copper loop. These induced values oppose the flux that produces them. Thus, most of the flux passes through the nonshaded section of each pole. After the flux reaches its maximum value, there is no change in the flux instantaneously. Then, the induced voltage in the loop decreases to zero. The current in the copper loop also drops to zero, as well as the opposing mmf of

the short-circuited coil. A large part of the main field flux now passes through this section of the pole piece. When the main field flux begins to decrease, voltage and current are induced again in the copper loop. This time, the mmf developed attempts to prevent the flux from decreasing in the shaded portion of each pole piece. In this way, a type of rotating field is created. The flux first reaches its maximum value in the nonshaded section of each pole and later reaches its maximum value in the shaded section.

The rotor speed for the single-phase, two-pole synchronous clock motor is 3600 r/min when operated from a 60-Hz source. A gear train is housed in a sealed case containing a light lubricating oil. This train reduces the speed of the motor to the value required for use in various timing devices.

The Holtz Motor

The Holtz motor (Figure 17–13) is another type of single-phase synchronous motor. This motor has the same shaded-pole arrangement as the General Electric (Warren) motor. For the Holtz motor, however, the rotor has six slots that hold a small squirrel-cage

winding. The rotor slots are arranged so that six salient field poles are formed. The Holtz motor starts as a squirrel-cage motor. When operated from a 60-Hz service, the salient poles of the rotor lock with the sections of the field poles each half-cycle. The resulting speed is one-third of the synchronous speed, or 1200 r/min.

SELSYN MOTORS

Selsyn is a contraction of the term self-synchronous. Selsyn motors provide a means of electrically interconnecting two or more remote points of a system. This means that they give automatic synchronized control or indication at one point, with respect to the other point. Selsyn units are extremely flexible and are used widely in industry.

For example, selsyn motors are used to give indications of the positions of remote devices such as generator rheostats, steam turbine governors, waterwheel governors, trans- former tap connections, swing bridges, gates or valves, elevators, and the roll height in steel rolling mills. They are also used in many automatic control systems, signaling systems, and remote control systems.

Figure 17–14 shows the internal wiring of a selsyn motor. The rotor terminals are marked R1 and R2. The terminals marked S1, S2, and S3 are the stator terminals. The rotor field circuit is excited from an external single-phase source. The stator windings consist of a conventional three-phase winding.

Two selsyn motors are shown in Figure 17–15. One motor is called the transmitter, and the other is called the receiver. Stators S1, S2, and S3 of the transmitter are connected to S1, S2, and S3, respectively, of the receiver. The rotors (R1 and R2) are tied together and are connected in common to an alternating-current source.

The transmitter can be turned by either manual or mechanical means. As it turns, the rotor follows at the same speed and in the same direction. The terms transmitter and receiver can be used interchangeably. Either selsyn can be the active transmitter or the passive receiver.

OPERATION

Figure 17–16 shows the vector relationships between the induced voltages of the stator for different positions of the rotor.

Case I

The rotor is lined up with coil S3, resulting in the maximum magnetic coupling. There is a maximum induced voltage in S3 and a partial induced voltage in both S1 and S2. Refer to the vector diagram and note that the resultant field of stators S1, S2, and S3 lines up with the rotor field R1.

Case II

The rotor is now in a position 30° from S3. Therefore, no voltage is induced in S1. A partial voltage is induced in S2 and S3. Again, the resultant field of stators S1, S2, and S3 lines up with rotor field R1.

Case III

The rotor is now lined up on the X axis. The polarity of S1 changes because the south pole of the rotor induces a north pole in S1. In cases I and II, the north pole of the rotor induces a south pole in S1. The change in polarity of S1 means that its vector must be reversed. Thus, the resultant field vector lines up with the rotor field R1.

Figure 17–14 shows that the three legs of each stator winding are connected in wye. The three voltages induced in the legs of the secondary windings are not equal in value. These voltages vary with the position of the rotor.

When the two rotors are in exactly corresponding positions, the voltages induced in the transmitter secondary winding are equal and opposite to those induced in the receiver secondary windings. Therefore, there is no current in the secondary windings.

If the transmitter rotor is moved from its original position, the induced voltages are not equal and opposite. As a result, current flows in the secondary or stator windings. This current sets up a torque. Because of this torque, the receiver rotor moves to the same position as the transmitter rotor.

STARTING LARGE SYNCHRONOUS MOTORS

The earlier part of this unit described how a synchronous motor is started. In general, the starting current for synchronous motors is less than that for squirrel-cage induction motors having the same horsepower and speed ratings. In most cases, it is preferred to start a synchronous motor at full voltage. Motors as powerful as 10,000 or 15,000 hp can be started at full voltage. This type of starting does not harm a motor that is properly designed. Full-voltage starting means that a simpler, less expensive starter is required as compared to reduced-voltage starting.

When the motor starting inrush causes voltage disturbances, reduced-voltage starting current can be used. Approximately 50% to 65% of the rated line voltage is applied to the motor through a reactor or an autotransformer starter.

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

Many automatic motor starters use special relays to energize the field circuit. The circuit must be energized at the correct instant to ensure that the maximum pullin torque is obtained with a minimum amount of disturbance to the system. After the motor is operating properly as a synchronous motor, the dc field excitation current is adjusted to give the desired power factor.

SYNCHRONOUS MOTOR RATINGS

The nameplate data of a synchronous motor contains the same information as the nameplate of an ac generator. The only exception is the output rating because the alternator is rated in kVA and the synchronous motor is rated in horsepower.

The nameplate of a synchronous motor also gives a power factor rating. This type of motor is usually rated at unity power factor, 90% lead power factor, or 80% lead power fac- tor. The motor may be rated at other power factor values for special applications. If a motor is rated at unity power factor, it can be operated with a leading power factor. In this case, the mechanical load must be decreased so that only the rated ac stator current will flow at the reduced power factor. In other words, the mechanical load, in horsepower output, and the electrical load, in leading kilovars, must not exceed the rating of the motor. When a synchronous motor is rated at a leading power factor of 80% or 90%, it will have a larger current capacity for a given horsepower output. This is necessary if the rated horsepower output is to be supplied for the larger current at the reduced power factor.

Synchronous motor losses are the same as those for an ac generator. The losses include the power spent in the separately excited field, the mechanical friction losses con- sisting of the copper losses in the three-phase stator winding and the stray power losses, the windage losses, and the iron losses. 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 MOTORS

The Warren Motor

Small single-phase synchronous motors are used in many timing applications, including electric clocks, time switches, graphical recording instruments, and stroboscope devices. These motors do not require dc field excitation. A widely used motor of this type is the Warren (or General Electric) clock motor. It consists of a laminated stator core with an exciting coil. Normally, this coil is wound for 120-V operation. 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.

A General Electric (Warren) clock motor is shown in Figure 17–12. The rotor consists of several hardened steel discs that are pressed on the small rotor shaft. The discs have a high hysteresis loss. The rotating field acts on the rotor to produce 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.

Operation with Two-Pole Stator

How is a rotating field effect obtained with the two-pole stator? As the flux increases in value, part of the flux attempts to pass through the section of the pole having the copper shading loop. This flux induces a voltage and current in the copper loop. These induced values oppose the flux that produces them. Thus, most of the flux passes through the nonshaded section of each pole. After the flux reaches its maximum value, there is no change in the flux instantaneously. Then, the induced voltage in the loop decreases to zero. The current in the copper loop also drops to zero, as well as the opposing mmf of

the short-circuited coil. A large part of the main field flux now passes through this section of the pole piece. When the main field flux begins to decrease, voltage and current are induced again in the copper loop. This time, the mmf developed attempts to prevent the flux from decreasing in the shaded portion of each pole piece. In this way, a type of rotating field is created. The flux first reaches its maximum value in the nonshaded section of each pole and later reaches its maximum value in the shaded section.

The rotor speed for the single-phase, two-pole synchronous clock motor is 3600 r/min when operated from a 60-Hz source. A gear train is housed in a sealed case containing a light lubricating oil. This train reduces the speed of the motor to the value required for use in various timing devices.

The Holtz Motor

The Holtz motor (Figure 17–13) is another type of single-phase synchronous motor. This motor has the same shaded-pole arrangement as the General Electric (Warren) motor. For the Holtz motor, however, the rotor has six slots that hold a small squirrel-cage

winding. The rotor slots are arranged so that six salient field poles are formed. The Holtz motor starts as a squirrel-cage motor. When operated from a 60-Hz service, the salient poles of the rotor lock with the sections of the field poles each half-cycle. The resulting speed is one-third of the synchronous speed, or 1200 r/min.

SELSYN MOTORS

Selsyn is a contraction of the term self-synchronous. Selsyn motors provide a means of electrically interconnecting two or more remote points of a system. This means that they give automatic synchronized control or indication at one point, with respect to the other point. Selsyn units are extremely flexible and are used widely in industry.

For example, selsyn motors are used to give indications of the positions of remote devices such as generator rheostats, steam turbine governors, waterwheel governors, trans- former tap connections, swing bridges, gates or valves, elevators, and the roll height in steel rolling mills. They are also used in many automatic control systems, signaling systems, and remote control systems.

Figure 17–14 shows the internal wiring of a selsyn motor. The rotor terminals are marked R1 and R2. The terminals marked S1, S2, and S3 are the stator terminals. The rotor field circuit is excited from an external single-phase source. The stator windings consist of a conventional three-phase winding.

Two selsyn motors are shown in Figure 17–15. One motor is called the transmitter, and the other is called the receiver. Stators S1, S2, and S3 of the transmitter are connected to S1, S2, and S3, respectively, of the receiver. The rotors (R1 and R2) are tied together and are connected in common to an alternating-current source.

The transmitter can be turned by either manual or mechanical means. As it turns, the rotor follows at the same speed and in the same direction. The terms transmitter and receiver can be used interchangeably. Either selsyn can be the active transmitter or the passive receiver.

OPERATION

Figure 17–16 shows the vector relationships between the induced voltages of the stator for different positions of the rotor.

Case I

The rotor is lined up with coil S3, resulting in the maximum magnetic coupling. There is a maximum induced voltage in S3 and a partial induced voltage in both S1 and S2. Refer to the vector diagram and note that the resultant field of stators S1, S2, and S3 lines up with the rotor field R1.

Case II

The rotor is now in a position 30° from S3. Therefore, no voltage is induced in S1. A partial voltage is induced in S2 and S3. Again, the resultant field of stators S1, S2, and S3 lines up with rotor field R1.

Case III

The rotor is now lined up on the X axis. The polarity of S1 changes because the south pole of the rotor induces a north pole in S1. In cases I and II, the north pole of the rotor induces a south pole in S1. The change in polarity of S1 means that its vector must be reversed. Thus, the resultant field vector lines up with the rotor field R1.

Figure 17–14 shows that the three legs of each stator winding are connected in wye. The three voltages induced in the legs of the secondary windings are not equal in value. These voltages vary with the position of the rotor.

When the two rotors are in exactly corresponding positions, the voltages induced in the transmitter secondary winding are equal and opposite to those induced in the receiver secondary windings. Therefore, there is no current in the secondary windings.

If the transmitter rotor is moved from its original position, the induced voltages are not equal and opposite. As a result, current flows in the secondary or stator windings. This current sets up a torque. Because of this torque, the receiver rotor moves to the same position as the transmitter rotor.

INDUSTRIAL APPLICATIONS

Synchronous motors rated at 20 horsepower (hp) or more are used for constant speed applications. They are used to drive large air and gas compressors that must be operated at a

fixed speed to maintain a constant output at the maximum efficiency. Synchronous motors are used to drive dc generators, fans, blowers, and large pumps in water-pumping stations.

Some industrial applications use three-phase synchronous motors to drive mechanical loads and correct power factor values. A typical industrial feeder is shown in Figure 17–11. The feeder has a lagging power factor condition due to two induction motors. A synchronous motor is connected to this same feeder and is operated with an overexcited field. The synchronous motor supplies leading reactive kilovars to compensate for the lagging kilovars due to the induction motors or other inductive load on the same three-phase distribution system. The dc field of the synchronous motor can be overexcited enough to supply a value of leading kilovars equal to the lagging kilovars. As a result, the power factor of the distribution system is corrected to unity.

The synchronous motor may be used only to correct the power factor and not for driving any mechanical load. It then has the same function as a bank of capacitors. When used to correct the power factor only, the synchronous motor is called a synchronous capacitor or a synchronous condenser.

The following problems show how a synchronous motor is used to overcome the lagging kilovars due to induction motors on a three-phase distribution system.

PROBLEM 1

Statement of the Problem

A three-phase, 240-V feeder supplies two motors. One motor is a three-phase, wound-rotor induction motor. It takes 40 A at 81% lag power factor. The other motor is a three-phase, synchronous motor that takes 30 A at 65% lead power factor. Determine

1. the watts, volt-amperes, and lagging VARs of the wound-rotor induction motor.

2. the watts, volt-amperes, and leading VARs of the synchronous motor.

3. the total load, in kilowatts, supplied to the two motors.

4. the power factor of the three-phase feeder circuit.

5. the line current of the three-phase feeder circuit.

Solution

3. The total true power, in kilowatts, taken by the two motors is the arithmetic sum of the individual power values for the two motors:

Total watts = 13,452 + 8096 = 21,548 W = 21,548 kW

4. The reactive power for the entire three-phase feeder is the difference between the lagging and leading VARs:

9739 – 9516 = 223 VARs

Thus, there are 245 VARs of lagging quadrature power that are not overcome by the synchronous motor. This value of lagging VARs combined with the total power in watts gives the apparent power in volt-amperes:

The power factor of the entire system is the ratio of the total true power, in kilowatts, to the total apparent power, in kilovolt-amperes:

PROBLEM 2

Statement of the Problem

The load of an industrial plant consists of 600 kVA at a power factor of 0.75 lag. One large motor is to be added to the load and will require 150 kW. Determine the new load, in kVA, and the power factor, given that

1. the motor installed is an induction motor operating at a power factor of 0.85 lag.

2. the motor installed is a synchronous motor operating at a power factor of 0.80 lead.

Solution

1. For the induction motor:

The original load has a quadrature power component of 396.8 lag kilovars. The syn- chronous motor has a quadrature power component of 112.6 lead kilovars. Therefore, the net load in kilovars is 396.8 – 112.6 = 284.2 lag kVAR.

The total load in kVA, including the synchronous motor, is

Compare the kVA load for a synchronous motor, 664 kVA, with that for the induction motor, 774.5 kVA. The actual true power is the same for both motors. The lower kVA value for the synchronous motor indicates that the power factor has improved greatly:

The synchronous motor can supply a leading reactive component of 112.6 kVAR. This value compensates in part for the lagging reactive component of the original load, 396.8 kVAR. A lagging component of 284.2 kVAR remains. It must be sup- plied from the source, as well as the true power component of 600 kW. Therefore, the power factor is still a lagging power factor, although it is improved. Compare this power factor with the system power factor determined for the first part of this problem.

INDUSTRIAL APPLICATIONS

Synchronous motors rated at 20 horsepower (hp) or more are used for constant speed applications. They are used to drive large air and gas compressors that must be operated at a

fixed speed to maintain a constant output at the maximum efficiency. Synchronous motors are used to drive dc generators, fans, blowers, and large pumps in water-pumping stations.

Some industrial applications use three-phase synchronous motors to drive mechanical loads and correct power factor values. A typical industrial feeder is shown in Figure 17–11. The feeder has a lagging power factor condition due to two induction motors. A synchronous motor is connected to this same feeder and is operated with an overexcited field. The synchronous motor supplies leading reactive kilovars to compensate for the lagging kilovars due to the induction motors or other inductive load on the same three-phase distribution system. The dc field of the synchronous motor can be overexcited enough to supply a value of leading kilovars equal to the lagging kilovars. As a result, the power factor of the distribution system is corrected to unity.

The synchronous motor may be used only to correct the power factor and not for driving any mechanical load. It then has the same function as a bank of capacitors. When used to correct the power factor only, the synchronous motor is called a synchronous capacitor or a synchronous condenser.

The following problems show how a synchronous motor is used to overcome the lagging kilovars due to induction motors on a three-phase distribution system.

PROBLEM 1

Statement of the Problem

A three-phase, 240-V feeder supplies two motors. One motor is a three-phase, wound-rotor induction motor. It takes 40 A at 81% lag power factor. The other motor is a three-phase, synchronous motor that takes 30 A at 65% lead power factor. Determine

1. the watts, volt-amperes, and lagging VARs of the wound-rotor induction motor.

2. the watts, volt-amperes, and leading VARs of the synchronous motor.

3. the total load, in kilowatts, supplied to the two motors.

4. the power factor of the three-phase feeder circuit.

5. the line current of the three-phase feeder circuit.

Solution

3. The total true power, in kilowatts, taken by the two motors is the arithmetic sum of the individual power values for the two motors:

Total watts = 13,452 + 8096 = 21,548 W = 21,548 kW

4. The reactive power for the entire three-phase feeder is the difference between the lagging and leading VARs:

9739 – 9516 = 223 VARs

Thus, there are 245 VARs of lagging quadrature power that are not overcome by the synchronous motor. This value of lagging VARs combined with the total power in watts gives the apparent power in volt-amperes:

The power factor of the entire system is the ratio of the total true power, in kilowatts, to the total apparent power, in kilovolt-amperes:

PROBLEM 2

Statement of the Problem

The load of an industrial plant consists of 600 kVA at a power factor of 0.75 lag. One large motor is to be added to the load and will require 150 kW. Determine the new load, in kVA, and the power factor, given that

1. the motor installed is an induction motor operating at a power factor of 0.85 lag.

2. the motor installed is a synchronous motor operating at a power factor of 0.80 lead.

Solution

1. For the induction motor:

The original load has a quadrature power component of 396.8 lag kilovars. The syn- chronous motor has a quadrature power component of 112.6 lead kilovars. Therefore, the net load in kilovars is 396.8 – 112.6 = 284.2 lag kVAR.

The total load in kVA, including the synchronous motor, is

Compare the kVA load for a synchronous motor, 664 kVA, with that for the induction motor, 774.5 kVA. The actual true power is the same for both motors. The lower kVA value for the synchronous motor indicates that the power factor has improved greatly:

The synchronous motor can supply a leading reactive component of 112.6 kVAR. This value compensates in part for the lagging reactive component of the original load, 396.8 kVAR. A lagging component of 284.2 kVAR remains. It must be sup- plied from the source, as well as the true power component of 600 kW. Therefore, the power factor is still a lagging power factor, although it is improved. Compare this power factor with the system power factor determined for the first part of this problem.

LOAD ON A SYNCHRONOUS MOTOR

When the mechanical load is increased on a dc motor or an ac induction motor, there is a decrease in the speed. This decrease results in a reduction of the counterelectromotive force. Thus, the source can supply more current to meet the increased load demands. This method of compensating for an increased load cannot be used with a synchronous motor because the rotor must turn at synchronous speed at all loads. The relative positions of a stator pole and a rotor pole are shown at no load in Figure 17–7A for a synchronous motor. Note that the centers of both poles are in line with each other.

Once the rated load is applied, the relative positions of the stator pole and the rotor pole are as shown in Figure 17–7B. There is now an angular displacement of the rotor pole with respect to the stator pole. The speed is unchanged because the rotor will continue to rotate at synchronous speed. The angular displacement between the centers of the stator and rotor field poles is called the torque angle, a.

Operation with Different Loads

A synchronous motor operating at no load has a torque angle of nearly 0°. The counterelectromotive force in this case is almost equal to the impressed voltage (neglecting no– load motor losses). As the mechanical load increases, the torque angle increases. The phase angle between the impressed voltage and the counter-emf also increases. This increase allows the impressed voltage to cause more current in the stator windings to meet the additional load demands.

In Figure 17–8A, the counter emf is equal and opposite in direction to the impressed voltage at no load. The torque angle here is zero. The counter-emf shown in Fig- ure 17–8B is shifted by the angle a from its no-load position in Figure 17–8A. The shift is due to the addition of the load, which causes the rotor pole centers to shift behind the stator pole centers by the angle a. The line voltage and the counter-emf are not opposite each other at this point. The resultant voltage VR gives rise to the current IS in the stator windings. The stator windings have a high reactance, which causes current IS to lag the resultant voltage VR by nearly 90°. The power input for one phase of the three-phase motor is equal to

The synchronous motor can carry an increased mechanical load by shifting the relative positions of the stator and rotor poles. There is no decrease in the speed of the synchronous motor.

A serious overload will cause the angle between the centers of the stator and rotor poles to become too great. In this case, the rotor will pull out of synchronism. With the aid of the amortisseur winding, the motor will operate as an induction motor. The pull- out torque is the maximum torque value that can be developed by a synchronous motor

without dropping out of synchronism. For most synchronous motors, the pullout torque is 150% to 200% of the rated torque output.

POWER FACTOR

Changes in the dc field excitation do not affect the motor speed. However, such changes do alter the power factor of a synchronous motor. If all of the resistance of the rheostat is inserted in the field circuit, the field current drops below its normal value. A poor lagging power factor results. If the dc field is weak, the three-phase ac circuit to the stator supplies a magnetizing current to strengthen the field. This magnetizing component lags the voltage by 90 electrical degrees. The magnetizing current becomes a large part of the total current input. This gives rise to a low lagging power factor.

If a weak dc field is strengthened, the three-phase ac circuit to the stator supplies less magnetizing current. Because this current component becomes a smaller part of the total current input to the stator winding, the power factor increases. The field strength can be increased until the power factor is unity, or 100%. When the power factor reaches unity, the three-phase ac circuit supplies energy current only. The dc field circuit supplies all of the current required to magnetize the motor. The amount of dc field excitation required to obtain a unity power factor is called normal field excitation.

The magnetic field of the rotor can be strengthened still more by increasing the dc field current above the normal excitation value. The power factor in this case decreases. The circuit feeding the stator winding delivers a demagnetizing component of current. This current opposes the rotor field and weakens it until it returns to the normal magnetic strength.

Interaction between DC and AC Fields

Figure 17–9 shows how the magnetic field set up by the ac windings aids or opposes the dc field. The dc field is assumed to be stationary. The revolving armature is connected to the ac source. (In practice, most synchronous motors have stationary ac windings and a revolving dc field. However, the principle involved is the same.)

The dc excitation current is below its normal value in Figure 17–9A. The ac stator wind- ings supply a magnetizing component of current. This current lags the impressed voltage by 90°. This means that the current reaches its maximum value as shown. For this position, the flux created by the magnetizing component of current aids the weakened dc field. An in-phase component of current creates the torque required by the load. (This in-phase component is not shown in the figure.) The synchronous motor will have a lagging power factor because of the magnetizing component of current.

In Figure 17–9B, the dc excitation current is increased to its normal value. The ac input now supplies an in-phase current only. This current meets the torque requirements of the motor, but there is no magnetizing current component. Therefore, the power factor is unity.

The dc field is overexcited in Figure 17–9C. The ac input supplies a leading quadrature current that is really a demagnetizing component of current. This current sets up an mmf that opposes the mmf of the overexcited dc field. In this way, the dc field flux is limited to its normal full-strength value. Although not shown in Figure 17–8C, there is also an in-phase component of current. This in-phase component supplies the torque requirements

of the motor. When a synchronous motor with an overexcited dc field is supplied with a demagnetizing component of current (quadrature lead current), the motor has a leading power factor.

Typical Characteristic Curves

Typical characteristic curves for a synchronous motor with a constant mechanical load are shown in Figure 17–10. These curves show the changes that occur in the stator current and power factor as the dc field excitation current is varied.

At the normal dc field excitation current, the power factor has a peak value of unity. At the same point, the ac stator current is at its lowest value. As the field current is decreased, the power factor decreases in the lag quadrant. There is a resultant rise in the ac stator current. If the dc field current is increased above the normal field excitation value, the power factor decreases in the lead quadrant and the ac stator current increases.

LOAD ON A SYNCHRONOUS MOTOR

When the mechanical load is increased on a dc motor or an ac induction motor, there is a decrease in the speed. This decrease results in a reduction of the counterelectromotive force. Thus, the source can supply more current to meet the increased load demands. This method of compensating for an increased load cannot be used with a synchronous motor because the rotor must turn at synchronous speed at all loads. The relative positions of a stator pole and a rotor pole are shown at no load in Figure 17–7A for a synchronous motor. Note that the centers of both poles are in line with each other.

Once the rated load is applied, the relative positions of the stator pole and the rotor pole are as shown in Figure 17–7B. There is now an angular displacement of the rotor pole with respect to the stator pole. The speed is unchanged because the rotor will continue to rotate at synchronous speed. The angular displacement between the centers of the stator and rotor field poles is called the torque angle, a.

Operation with Different Loads

A synchronous motor operating at no load has a torque angle of nearly 0°. The counterelectromotive force in this case is almost equal to the impressed voltage (neglecting no– load motor losses). As the mechanical load increases, the torque angle increases. The phase angle between the impressed voltage and the counter-emf also increases. This increase allows the impressed voltage to cause more current in the stator windings to meet the additional load demands.

In Figure 17–8A, the counter emf is equal and opposite in direction to the impressed voltage at no load. The torque angle here is zero. The counter-emf shown in Fig- ure 17–8B is shifted by the angle a from its no-load position in Figure 17–8A. The shift is due to the addition of the load, which causes the rotor pole centers to shift behind the stator pole centers by the angle a. The line voltage and the counter-emf are not opposite each other at this point. The resultant voltage VR gives rise to the current IS in the stator windings. The stator windings have a high reactance, which causes current IS to lag the resultant voltage VR by nearly 90°. The power input for one phase of the three-phase motor is equal to

The synchronous motor can carry an increased mechanical load by shifting the relative positions of the stator and rotor poles. There is no decrease in the speed of the synchronous motor.

A serious overload will cause the angle between the centers of the stator and rotor poles to become too great. In this case, the rotor will pull out of synchronism. With the aid of the amortisseur winding, the motor will operate as an induction motor. The pull- out torque is the maximum torque value that can be developed by a synchronous motor

without dropping out of synchronism. For most synchronous motors, the pullout torque is 150% to 200% of the rated torque output.

POWER FACTOR

Changes in the dc field excitation do not affect the motor speed. However, such changes do alter the power factor of a synchronous motor. If all of the resistance of the rheostat is inserted in the field circuit, the field current drops below its normal value. A poor lagging power factor results. If the dc field is weak, the three-phase ac circuit to the stator supplies a magnetizing current to strengthen the field. This magnetizing component lags the voltage by 90 electrical degrees. The magnetizing current becomes a large part of the total current input. This gives rise to a low lagging power factor.

If a weak dc field is strengthened, the three-phase ac circuit to the stator supplies less magnetizing current. Because this current component becomes a smaller part of the total current input to the stator winding, the power factor increases. The field strength can be increased until the power factor is unity, or 100%. When the power factor reaches unity, the three-phase ac circuit supplies energy current only. The dc field circuit supplies all of the current required to magnetize the motor. The amount of dc field excitation required to obtain a unity power factor is called normal field excitation.

The magnetic field of the rotor can be strengthened still more by increasing the dc field current above the normal excitation value. The power factor in this case decreases. The circuit feeding the stator winding delivers a demagnetizing component of current. This current opposes the rotor field and weakens it until it returns to the normal magnetic strength.

Interaction between DC and AC Fields

Figure 17–9 shows how the magnetic field set up by the ac windings aids or opposes the dc field. The dc field is assumed to be stationary. The revolving armature is connected to the ac source. (In practice, most synchronous motors have stationary ac windings and a revolving dc field. However, the principle involved is the same.)

The dc excitation current is below its normal value in Figure 17–9A. The ac stator wind- ings supply a magnetizing component of current. This current lags the impressed voltage by 90°. This means that the current reaches its maximum value as shown. For this position, the flux created by the magnetizing component of current aids the weakened dc field. An in-phase component of current creates the torque required by the load. (This in-phase component is not shown in the figure.) The synchronous motor will have a lagging power factor because of the magnetizing component of current.

In Figure 17–9B, the dc excitation current is increased to its normal value. The ac input now supplies an in-phase current only. This current meets the torque requirements of the motor, but there is no magnetizing current component. Therefore, the power factor is unity.

The dc field is overexcited in Figure 17–9C. The ac input supplies a leading quadrature current that is really a demagnetizing component of current. This current sets up an mmf that opposes the mmf of the overexcited dc field. In this way, the dc field flux is limited to its normal full-strength value. Although not shown in Figure 17–8C, there is also an in-phase component of current. This in-phase component supplies the torque requirements

of the motor. When a synchronous motor with an overexcited dc field is supplied with a demagnetizing component of current (quadrature lead current), the motor has a leading power factor.

Typical Characteristic Curves

Typical characteristic curves for a synchronous motor with a constant mechanical load are shown in Figure 17–10. These curves show the changes that occur in the stator current and power factor as the dc field excitation current is varied.

At the normal dc field excitation current, the power factor has a peak value of unity. At the same point, the ac stator current is at its lowest value. As the field current is decreased, the power factor decreases in the lag quadrant. There is a resultant rise in the ac stator current. If the dc field current is increased above the normal field excitation value, the power factor decreases in the lead quadrant and the ac stator current increases.

The Synchronous Motor

CONSTRUCTION

The three-phase synchronous motor consists of the following components:

• A laminated stator core with a three-phase armature winding

• A revolving field with an amortisseur winding and slip rings

• Brushes and brush holders

• Two end shields housing the bearings that support the rotor shaft

The stator core and windings of a synchronous motor are similar in construction to those of a three-phase squirrel-cage induction motor or a wound-rotor induction motor. The leads for the stator windings are marked T1, T2, and T3. These leads end in a terminal box, which normally is mounted on the side of the motor frame.

The rotor has salient field poles. The poles are connected to give alternate polarity. There must be the same number of rotor field poles as stator field poles. The field circuit leads are brought out to two slip rings mounted on the rotor shaft. A squirrel-cage (amortisseur) winding is provided as a means of starting the motor. The synchronous motor is not self-starting without this auxiliary winding.

Figure 17–1 shows a rotor having salient poles and an amortisseur winding. The amortisseur winding consists of copper bars embedded in the laminated metal structure of each pole face. The copper bars of this special squirrel-cage winding are brazed to rings mounted on each end of the rotor.

Carbon brushes, mounted in brush holders, contact the two slip rings. The terminals of the field circuit are brought out from the brush holders to a second terminal box, which is mounted on the motor frame. The two leads for the field circuit are marked F1 and F2.

OPERATING PRINCIPLES

The rated three-phase voltage is applied to the stator windings, resulting in a rotating magnetic field. This field travels at synchronous speed. The speed is determined by the same factors that govern the synchronous speed of induction motors. The synchronous speed is found using the following equation:

The rotating magnetic field set up by the stator windings cuts across the amortisseur (squirrel-cage) winding of the rotor. The amortisseur winding is a squirrel-cage winding very similar to the type A winding in a squirrel-cage induction motor. Due to its relatively high resistance, it provides low starting current and high starting torque per ampere of starting current. Voltages and currents are induced in the bars of this winding. A magnetic field is set up in the squirrel-cage windings. This field reacts with the stator field and causes rotation of the rotor. The speed of the rotor increases until it is just below the synchro- nous speed of the stator field. In other words, there is a slight slip of the rotor behind the magnetic field set up by the stator windings. When the synchronous motor is started as an induction motor with the amortisseur windings, the rotor accelerates to about 85% to 97% of the synchronous speed.

The field circuit is excited from an outside direct-current source. Magnetic poles of fixed polarity are set up in the rotor field cores. The fixed magnetic poles of the rotor are attracted to unlike poles of the rotating magnetic field. (This field was set up by the stator windings.)

Figure 17–2 shows how the rotor field poles lock with unlike poles of the stator field. As a result, the rotor speed becomes the same as the speed of the stator field. This speed is the synchronous speed. When the rotor begins turning at synchronous speed, there is no longer any cutting action across the bars of the amortisseur winding. At this point there is no induced voltage in the amortisseur winding, and it therefore has no effect on the operation of the motor.

DC FIELD EXCITATION

The direct current for synchronous motors is obtained from a dc exciter circuit. Such a circuit may supply field excitation to several ac machines. A dc generator may be coupled directly to the synchronous motor shaft. Other installations may use electronic rectifiers to supply the dc excitation current.

The dc connections to a synchronous motor are shown in Figure 17–3. A field rheostat controls the current in the separately excited field circuit. When the field switch is open, the field discharge resistor is connected directly across the field winding.

The Brushless Exciter

Most large synchronous motors use an exciter that does not depend on brushes and slip rings. This is accomplished by adding a separate small alternator of the

armature type on the shaft of the synchronous motor. The armature rotates between wound electromagnets. The dc excitation current is connected to the wound stationary magnets (Figure 17-4). The amount of voltage induced in the armature can be controlled by varying the amount of dc current supplied to the electromagnets. The output voltage of the armature is connected to a three-phase bridge rectifier mounted on the rotor shaft (Figure 17-5). The three-phase bridge rectifier converts the three-phase alternating current produced in the armature to direct current before it is applied to the rotor of the synchronous motor. Because the three-phase armature and rectifier are contained on the rotor shaft, they all turn together and no brushes or slip rings are needed to provide excitation for the rotor of the large synchronous motor. The rotor of a synchronous motor with a brushless exciter is shown in Figure 17–6.

Starting

The dc field circuit is never energized when the synchronous motor is started because the rapidly rotating field produces an alternating torque on the stationary rotor poles. In general, a synchronous motor is started by connecting its armature winding (stator) to the ac line and its field winding (rotor) to a field discharge resistor. The motor is started as an induction motor.

At the instant of startup, the rotating stator field cuts the turns of the dc field coils many times per second. The stator field turns at the synchronous speed and induces a high voltage in the field windings. This voltage may reach 1500 V. This means that the field circuits must be well insulated and enclosed to protect personnel. The field discharge resistor is connected across the field windings so that the energy in the field circuit is spent in the resistor. This arrangement also reduces the voltage at the field terminals, although it is still high enough to be a shock hazard.

Achieving Synchronous Speed

Once the motor accelerates to nearly 95% of synchronous speed, the field circuit is energized from the dc source. The field discharge resistor is then disconnected. The rotor will pull into synchronism with the revolving armature (stator) flux. Thus, the motor will operate at a constant speed. If the load has a high inertia and is hard to start, special automatic equipment is required to apply the field. To ensure that there is a successful transition from induction motor operation to synchronous operation, the field must be applied at the best position of the rotor slip cycle.

Field Discharge Resistor

To shut down the motor, the field circuit is deenergized by opening the field discharge switch. As the field flux collapses, a voltage is induced in the field windings. This voltage may be large enough to damage the insulation of the windings. To prevent such a high volt- age, the field discharge resistor is connected across the field circuit. As a result, the energy stored in the magnetic field is spent in the resistor and a lower voltage is induced in the field circuit.

The Synchronous Motor

CONSTRUCTION

The three-phase synchronous motor consists of the following components:

• A laminated stator core with a three-phase armature winding

• A revolving field with an amortisseur winding and slip rings

• Brushes and brush holders

• Two end shields housing the bearings that support the rotor shaft

The stator core and windings of a synchronous motor are similar in construction to those of a three-phase squirrel-cage induction motor or a wound-rotor induction motor. The leads for the stator windings are marked T1, T2, and T3. These leads end in a terminal box, which normally is mounted on the side of the motor frame.

The rotor has salient field poles. The poles are connected to give alternate polarity. There must be the same number of rotor field poles as stator field poles. The field circuit leads are brought out to two slip rings mounted on the rotor shaft. A squirrel-cage (amortisseur) winding is provided as a means of starting the motor. The synchronous motor is not self-starting without this auxiliary winding.

Figure 17–1 shows a rotor having salient poles and an amortisseur winding. The amortisseur winding consists of copper bars embedded in the laminated metal structure of each pole face. The copper bars of this special squirrel-cage winding are brazed to rings mounted on each end of the rotor.

Carbon brushes, mounted in brush holders, contact the two slip rings. The terminals of the field circuit are brought out from the brush holders to a second terminal box, which is mounted on the motor frame. The two leads for the field circuit are marked F1 and F2.

OPERATING PRINCIPLES

The rated three-phase voltage is applied to the stator windings, resulting in a rotating magnetic field. This field travels at synchronous speed. The speed is determined by the same factors that govern the synchronous speed of induction motors. The synchronous speed is found using the following equation:

The rotating magnetic field set up by the stator windings cuts across the amortisseur (squirrel-cage) winding of the rotor. The amortisseur winding is a squirrel-cage winding very similar to the type A winding in a squirrel-cage induction motor. Due to its relatively high resistance, it provides low starting current and high starting torque per ampere of starting current. Voltages and currents are induced in the bars of this winding. A magnetic field is set up in the squirrel-cage windings. This field reacts with the stator field and causes rotation of the rotor. The speed of the rotor increases until it is just below the synchro- nous speed of the stator field. In other words, there is a slight slip of the rotor behind the magnetic field set up by the stator windings. When the synchronous motor is started as an induction motor with the amortisseur windings, the rotor accelerates to about 85% to 97% of the synchronous speed.

The field circuit is excited from an outside direct-current source. Magnetic poles of fixed polarity are set up in the rotor field cores. The fixed magnetic poles of the rotor are attracted to unlike poles of the rotating magnetic field. (This field was set up by the stator windings.)

Figure 17–2 shows how the rotor field poles lock with unlike poles of the stator field. As a result, the rotor speed becomes the same as the speed of the stator field. This speed is the synchronous speed. When the rotor begins turning at synchronous speed, there is no longer any cutting action across the bars of the amortisseur winding. At this point there is no induced voltage in the amortisseur winding, and it therefore has no effect on the operation of the motor.

DC FIELD EXCITATION

The direct current for synchronous motors is obtained from a dc exciter circuit. Such a circuit may supply field excitation to several ac machines. A dc generator may be coupled directly to the synchronous motor shaft. Other installations may use electronic rectifiers to supply the dc excitation current.

The dc connections to a synchronous motor are shown in Figure 17–3. A field rheostat controls the current in the separately excited field circuit. When the field switch is open, the field discharge resistor is connected directly across the field winding.

The Brushless Exciter

Most large synchronous motors use an exciter that does not depend on brushes and slip rings. This is accomplished by adding a separate small alternator of the

armature type on the shaft of the synchronous motor. The armature rotates between wound electromagnets. The dc excitation current is connected to the wound stationary magnets (Figure 17-4). The amount of voltage induced in the armature can be controlled by varying the amount of dc current supplied to the electromagnets. The output voltage of the armature is connected to a three-phase bridge rectifier mounted on the rotor shaft (Figure 17-5). The three-phase bridge rectifier converts the three-phase alternating current produced in the armature to direct current before it is applied to the rotor of the synchronous motor. Because the three-phase armature and rectifier are contained on the rotor shaft, they all turn together and no brushes or slip rings are needed to provide excitation for the rotor of the large synchronous motor. The rotor of a synchronous motor with a brushless exciter is shown in Figure 17–6.

Starting

The dc field circuit is never energized when the synchronous motor is started because the rapidly rotating field produces an alternating torque on the stationary rotor poles. In general, a synchronous motor is started by connecting its armature winding (stator) to the ac line and its field winding (rotor) to a field discharge resistor. The motor is started as an induction motor.

At the instant of startup, the rotating stator field cuts the turns of the dc field coils many times per second. The stator field turns at the synchronous speed and induces a high voltage in the field windings. This voltage may reach 1500 V. This means that the field circuits must be well insulated and enclosed to protect personnel. The field discharge resistor is connected across the field windings so that the energy in the field circuit is spent in the resistor. This arrangement also reduces the voltage at the field terminals, although it is still high enough to be a shock hazard.

Achieving Synchronous Speed

Once the motor accelerates to nearly 95% of synchronous speed, the field circuit is energized from the dc source. The field discharge resistor is then disconnected. The rotor will pull into synchronism with the revolving armature (stator) flux. Thus, the motor will operate at a constant speed. If the load has a high inertia and is hard to start, special automatic equipment is required to apply the field. To ensure that there is a successful transition from induction motor operation to synchronous operation, the field must be applied at the best position of the rotor slip cycle.

Field Discharge Resistor

To shut down the motor, the field circuit is deenergized by opening the field discharge switch. As the field flux collapses, a voltage is induced in the field windings. This voltage may be large enough to damage the insulation of the windings. To prevent such a high volt- age, the field discharge resistor is connected across the field circuit. As a result, the energy stored in the magnetic field is spent in the resistor and a lower voltage is induced in the field circuit.

COMPARISON BETWEEN SQUIRREL-CAGE AND WOUND-ROTOR INDUCTION MOTORS

The wound-rotor induction motor has the following advantages:

• High starting torque and low starting current if the motor is started with the maximum resistance inserted in the rotor circuit

• Variable speed

• Smooth acceleration under heavy loads

There are several disadvantages to this motor, including the following:

• Greater initial cost

• Higher maintenance and repair costs

• Low efficiency and poor speed regulation (when the motor is operated with resistance inserted in the rotor circuit)

SUMMARY

• A three-phase, squirrel-cage induction motor

1. is simple in construction and is easy to maintain.

2. is smaller in physical size than other motors for a given horsepower rating.

3. has very good speed regulation under varying load conditions.

4. is used for many industrial applications because of its low cost, rugged construction, and operating characteristics.

Other characteristics are as follows:

1. The basic components of a squirrel-cage motor are a stator, a rotor, and two end shields housing the bearings supporting the rotor shaft.

2. The stator is a three-phase winding that is placed in the slots of a laminated steel core. Three single-phase windings are spaced 120 electrical degrees apart. These windings may be connected in wye or delta.

3. The rotor consists of a cylindrical core made from steel laminations. Aluminum bars are mounted near the surface and are connected at the ends to two aluminum end rings.

• The squirrel-cage induction motor is also known as a synchronous speed motor.

1. The synchronous speed is determined by the number of poles in the stator winding and the frequency of the ac source

where

S = synchronous speed, in r/min

f = frequency, in hertz

P = number of poles (not pairs of poles)

2. The following table lists the speeds of a motor at two different frequencies for different numbers of poles:

• Slip of an induction motor:

1. Slip is the difference between the synchronous speed of the motor and the actual speed of the rotor. The rotor always turns at a speed that is slightly less than synchronous speed.

2. The speed performance of an induction motor is usually measured in terms of percent slip. The percent slip for an induction motor is generally in the range of 2% to 5%:

3. The standard squirrel-cage induction motor has excellent speed regulation, from no load to full load. Because of its good speed regulation, an induction motor is considered to be a constant-speed motor:

4. The torque output of an induction motor varies as the square of the terminal volt- age for a given percent slip.

5. The power factor of the induction motor is low at no load and high at full load. At full-load conditions, the in-phase component of current is large and the magnetizing component is small; the opposite is true at no-load conditions.

6. The losses of an induction motor consist of the stray power losses and the copper losses. The stray power losses are nearly constant at all loads. The copper losses increase as the current of the motor increases.

7. The direction of rotation of a three-phase, squirrel-cage induction motor can be changed by reversing two of the three incoming supply leads.

• The torque of an induction motor

1. results from the interaction between the stator flux and the rotor flux.

2. is produced only when the rotor turns at a speed that is less than the synchronous speed of the stator flux pattern.

3. is small at no load because there is only a slight difference between the rotor speed and the synchronous speed of the stator flux.

4. will increase to a larger value as the load on the shaft of the rotor increases. The resulting increase in the rotor current causes a larger torque at a slightly lower rotor speed.

• Rotor frequency:

1. This frequency is an important factor in the operation of the motor. A change in the rotor frequency causes the inductive reactance and the impedance of the rotor to decrease. A change in the frequency will affect the starting and running characteristics of the motor.

2. Rotor frequency is expressed by the following formulas:

• At the instant of start-up

1. the rotor is not turning and there is 100% slip. The rotor frequency is equal to the stator frequency.

2. the inductive reactance of the rotor is very large compared to the effective resistance component. The rotor has a very low lagging power factor and the starting torque is low.

3. the stator field cuts the rotor bars at a faster rate; thus, the induced voltage in the rotor causes a large rotor current and the stator current will also be high at start-up.

In addition:

1. Because of the high starting current, induction motors must have starting protection. This protection may be rated as high as three times the full-load current rating of the motor.

2. Some very large induction motors are started with auxiliary starters. These devices reduce the motor voltage at start-up to limit the starting surge of current. As a result, there is less voltage disturbance on the feeder circuit supplying the motor load.

• The point of the maximum torque output is called the breakdown point.

1. An increase in the load beyond this point means that less torque is developed by the motor and the rotor stops.

2. The breakdown point occurs at 200% to 300% of the rated torque.

• The efficiency of an ac induction motor is given by the following equation:

• Brushes and slip rings are not required on a squirrel-cage induction motor. Thus, this type of motor can be used in locations where there is a possibility of explosion due to arcing, such as in chemical plants and flour or lumber mills.

• There is no practical method of providing stepless speed control for the induction motor.

• The blocked rotor test is used to determine the equivalent resistance of the motor winding, per phase.

1. Two wattmeters are used in this test:

• Stray power losses:

1. Stray power losses include mechanical friction losses, windage losses, and iron losses.

2. To measure these losses, the motor is operated at no load with the rated voltage applied to the motor terminals:

3. The fixed losses at no load are obtained by subtracting the no-load copper losses from the power input at no load:

• Speed control of an induction motor:

1. The synchronous speed of the magnetic field of the stator is determined by the number of stator poles and the frequency of the ac source.

2. Generally, the speed of an induction motor cannot be varied by changing the frequency.

3. The speed of a squirrel-cage motor may be changed if the motor is provided with special stator windings. When these windings are reconnected using special switch controls, different numbers of stator poles are formed. Thus, different speeds can be obtained for a squirrel-cage induction motor connected to an ac source at a fixed frequency.

• Code letter identification for motors:

1. A system of code letters is used to identify certain induction motors. These motors are grouped according to their torque and starting current characteristics.

2. The code letter designates the ratio between the starting and full-load currents.

This letter appears on the nameplate of all squirrel-cage induction motors.

3. By referring to the National Electrical Code, this code letter can be used to deter- mine the current ratings of circuit breakers, fuses, and other overload protective devices.

• A motor nameplate contains the following information:

Full-load horsepower output (hp)

Full-load speed (r/min)

Full-load current amperage (FLA)

Locked-rotor current amperage (LRA)

Manufacturer name

Terminal voltage (volts)

Number of phases (phase)

Frequency (cycles) (Hz)

Temperature rise (rise °C) (or insulation system class and rated ambient temperature)

Time rating (5, 15, 30, 60 min, or continuous)

Code letter

[Note: Other general information may be placed on the motor nameplate.]

• A double-squirrel-cage motor has a low starting current, a strong starting torque, and excellent speed regulation. The starting torque for this type of motor can be as high as 250% of the rated torque.

• Single-phase operation of three-phase induction motors:

1. If a three-phase induction motor is running when it is subjected to single-phase conditions, it will continue to operate but at a greatly reduced capacity.

2. The motor will not have enough torque to start when it is energized from a single-phase source.

3. If the rated load is applied to the motor when it is operating as a single-phase motor, it will overheat and the insulation of the windings may be damaged.

• The wound-rotor induction motor:

1. This motor can be used for variable-speed applications. The squirrel-cage induction motor is a constant-speed motor.

2. The formula used to find the synchronous speed for a squirrel-cage induction motor can also be used for the wound-rotor motor:

3. A wye-connected speed controller provides control current through slip rings to the windings of the rotor.

a. At start-up, all of the resistance of the wye-connected speed controller is inserted in the rotor circuit. This additional resistance causes an excellent starting torque and a large percent slip.

b. As the motor accelerates, steps of resistance are cut out of the wye-connected speed controller.

c. When all of the resistance is cut out, the rotor slip rings are short-circuited.

The motor then operates at the rated speed like a squirrel-cage induction motor.

d. This motor can be operated at heavy loads by cutting in resistance to the rotor circuit to obtain a below-normal speed at a lower motor efficiency.

4. The direction of rotation can be changed by reversing any two leads of the three line leads feeding to the stator windings.

5. A wound-rotor induction motor is used when a strong starting torque and a range of speed control are required. Typical applications include cranes, large compressors, elevators, and pumps.

6. Compared to the squirrel-cage induction motor, the wound-rotor motor has the following advantages:

a. High starting torque and low starting current if it is started with the maximum resistance inserted in the motor circuit

b. Variable speed

c. Smooth acceleration under heavy loads

7. Compared to the squirrel-cage induction motor, the wound-rotor motor has the following disadvantages:

a. Greater initial cost

b. Higher maintenance and repair costs

c. Low efficiency and poor speed regulation (when it is operated with resistance inserted in the rotor circuit)

Achievement Review

1. Explain what is meant by the synchronous speed of a three-phase induction motor.

2. What two conditions determine the synchronous speed of a three-phase induction motor?

3. Explain what is meant by the following terms:

 

	a. b. c.	Revolutions slip Percent slip Rotor frequency
4.	a. b.	What is the rotor frequency of a three-phase, 60-Hz, squirrel-cage induction motor at the instant of start? What is the approximate rotor frequency of a three-phase, 60-Hz, squirrel- cage induction motor when it is operating at the rated load?

5. What is the reason for the poor starting torque of a squirrel-cage induction motor?

6. A six-pole, three-phase, 60-Hz induction motor has a full-load speed of 1140 r/ min. Determine

a. the synchronous speed.

b. the revolutions slip.

c. the percent slip.

d. the rotor frequency.

7. a. Draw characteristic curves for a three-phase, squirrel-cage induction motor for the speed, percent slip, percent efficiency, power factor, and torque.

b. Discuss each of the five characteristic curves developed in part a of this question.

8. For a given value of slip, the torque output of an induction motor varies as the square of the impressed terminal voltage. Explain what is meant by this statement.

9. A 10-hp, 220-V, three-phase, 60-Hz, squirrel-cage induction motor is rated at 28 A per terminal. The full-load speed is 855 r/min, and the full-load power factor is 0.90 lag. The motor has eight poles. At the rated load, determine

a. the synchronous speed.

b. the slip in r/min.

c. the percent slip.

d. the rotor frequency at the rated speed.

10. A three-phase, 60-Hz, four-pole, 220-V, squirrel-cage induction motor takes 52 A per terminal at full load. The power factor is 0.85 lag, and the efficiency is 88%.

The slip is 3.0%. At the rated load, determine

a. the speed in r/min.

b. the horsepower output of the motor.

c. the total losses.

11. A three-phase, 60-Hz, six-pole, 220-V, squirrel-cage induction motor has a full- load output of 15 hp. The full-load efficiency is 87%, and the power factor is 0.88 lag. The windings of the motor are connected in delta. At the rated load, determine

a. the line current.

b. the phase winding voltage.

12. Assuming that the three-phase motor in question 11 is reconnected in wye with the same load and power factor, determine

a. the new rated line voltage.

b. the line current per motor terminal.

13. Explain how the direction of rotation is reversed for

a. a three-phase squirrel-cage induction motor.

b. a three-phase wound-rotor induction motor.

14. Explain why a three-phase squirrel-cage induction motor will not start when energized from a single-phase source.

15. Neither of the following alternating-current, three-phase induction motors is operating properly. Give a possible reason for the motor failure described, and state what should be done to correct each condition.

a. A 15-hp, 220 V, three-phase squirrel-cage induction motor overheats while operating at a normal load. The motor circuit is deenergized. When an attempt is made to restart the motor, it will not turn.

b. A 5-hp, 220-V, three-phase squirrel-cage induction motor stops as soon as the start push button for the across-the-line motor starter is released.

c. A newly installed 10-hp, 220-V, three-phase squirrel-cage induction motor has dual voltage ratings of 220 V and 440 V. The motor is supplied from a 220-V, three-phase source. At no load, the motor operates at a speed that is slightly below synchronous speed. When the rated load is applied, the motor stalls.

16. Show the connections for the nine terminal leads of a wye-connected, three-phase motor rated at 220/440 V for operation at

a. a line voltage of 440 V.

b. a line voltage of 220 V.

17. Compare a three-phase squirrel-cage induction motor and a three-phase wound-rotor induction motor, with regard to

a. construction.

b. starting torque.

c. speed control.

d. initial cost and maintenance.

e. efficiency.

18. a. Explain why the power factor of an induction motor is low at no load.

b. Explain what happens to the power factor as the load on an induction motor is increased.

19. Explain how both a good starting torque and small percent slip can be obtained using a squirrel-cage induction motor with a double-squirrel-cage rotor.

20. How is speed control provided for a squirrel-cage induction motor?

21. The following data are obtained in a prony brake test of a three-phase squirrel- cage induction motor:

Determine

a. the power factor.

b. the horsepower output.

c. the efficiency.

d. the torque output in pound • feet.

22. a. What are the losses in a squirrel-cage induction motor?

b. Which of these losses are constant and which of the losses vary with a change in load?

23. List the nameplate data of a typical squirrel-cage motor.

24. Code letters are included in the nameplate data for three-phase squirrel-cage induction motors. What is the purpose of these code letters?

25. What is the purpose of the protective starting device used on faceplate speed controllers and drum-type speed controllers?

26. The following test data are obtained on a 7.5-hp, three-phase, 220-V, wye- connected, four-pole squirrel-cage induction motor. The data are for a no-load condition and a full-load condition. The effective resistance of each single-phase winding of the wye-connected stator is 0.65 11. Determine the stray power losses of the motor.

27. Use the data given in question 26 and determine

a. the copper losses of the motor at full load.

b. the efficiency of the motor at rated load.

c. the horsepower output at full load.

d. the torque output in pound • feet at full load.

28. A three-phase, squirrel-cage induction motor must carry an additional load. List the sequence of steps, in chronological order, showing how the motor will adjust itself to carry this additional load.