## Multiple Choice Questions (MCQ) on Synchronous Motors for Electrical Engineering

1. Synchronous motors are generally not self-starting because
(a) the direction of rotation is not fixed
(b) the direction of instantaneous torque reverses after half cycle
(c) starter cannot be used on these machines
(d) starting winding is not provided on the machines

Ans: b

2. In case one phase of a three-phase synchronous motor is short-circuited the motor will
(a) not start
(b) run at 2/3 of synchronous speed
(c) run with excessive vibrations
(d) take less than the rated load

Ans: a

3. A pony motor is basically a
(a) small induction motor
(b) D.C. series motor
(c) D.C. shunt motor
(d) double winding A.C./D.C. motor

Ans: a

4. A synchronous motor can be started by
(a) pony motor
(b) D.C. compound motor
(c) providing damper winding
(d) any of the above

Ans: d

5. A three-phase synchronous motor will have
(a) no slip-rings
(b) one slip-ring
(c) two slip-rings
(d) three slip-rings

Ans: c

6. When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected
(a) the motor stops
(b) it runs as a reluctance motor at the same speed
(c) it runs as a reluctance motor at a lower speed
(d) none of the above

Ans: a

7. Mostly, synchronous motors are of
(a) alternator type machines
(b) induction type machines
(c) salient pole type machines
(d) smooth cylindrical type machines

Ans: c

8. The synchronous motor is not inherently self-starting because
(a) the force required to accelerate the rotor to the synchronous speed in an instant is absent
(b) the starting device to accelerate the rotor to near synchronous speed is absent
(c) a rotating magnetic field does not have enough poles
(d) the rotating magnetic field is produced by only 50 Hz frequency currents

Ans: a

9. As the load is applied to a synchronous motor, the motor takes more armature current because
(a) the increased load has to take more current
(b) the rotor by shifting its phase backward causes motor to take more current
(c) the back e.m.f. decreases causing an increase in motor current
(d) the rotor strengthens the rotating field causing more motor current

Ans: b

10. The working of a synchronous motor is similar to
(a) gear train arrangement
(b) transmission of mechanical power by shaft
(c) distribution transformer
(d) turbine
(e) none of the above

Ans: b

11. For V-curves for a synchronous motor the graph is drawn between
(a) field current and armature current
(b) terminal voltage and load factor
(c) power factor and field current
(d) armature current and power factor

Ans: a

12. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on
(a) d.c. excitation only
(b) speed of the motor
(d) both the speed and rotor flux

Ans: a

13. The back e.m.f. of a synchronous motor depends on
(a) speed
(d) all of the above

Ans: c

14. A synchronous motor can be made self starting by providing
(a) damper winding on rotor poles
(b) damper winding on stator
(c) damper winding on stator as well as rotor poles
(d) none of the above

Ans: d

15. The speed regulation of a synchronous motor is always
(a) 1%
(b) 0.5%
(c) positive
(d) zero

Ans: d

16. The percentage slip in case of a synchronous motor is
(a) 1%
(b) 100%
(c) 0.5%
(d) zero

Ans: d

17. The standard full-load power factor ratings for synchronous motors are
(b) unity or 0.8 lagging
(d) unity or zero

Ans: c

18. A synchronous motor running with normal excitation adjusts to load increases essentially by increase in
(a) back e.m.f.
(b) armature current
(c) power factor
(d) torque angle

Ans: b

19. Slip rings are usually made of
(a) carbon or graphite
(b) brass or steel
(c) silver or gold
(d) copper or aluminium

Ans: b

20. When the voltage applied to a synchronous motor is increased, which of the following will reduce?
(a) Stator flux
(b) Pull in torque
(c) Both (a) and (b)
(d) None of the above

Ans: d

21. Slip-rings in a synchronous motor carry
(a) direct current
(b) alternating current
(c) no current
(d) all of the above

Ans: a

22. The armature current of the synchronous motor has higher values for
(a) high excitation only
(b) low excitation only
(c) both (a) and (b)
(d) none of the above

Ans: c

23. Which of the following methods is used to start a synchronous motor ?
(a) Damper winding
(b) Star-delta starter
(c) Damper winding in conjunction with star-delta starter
(d) Resistance starter in the armature circuit

Ans: c

24. When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop
(a) inductor motor torque
(b) induction generator torque
(c) synchronous motor torque
(d) d.c. motor toque
(e) none of the above

Ans: b

25. While starting a synchronous motor by induction motor action, field winding is usually
(a) connected to D.C. supply
(b) short-circuited by low resistance
(c) kept open-circuited
(d) none of the above

Ans: b

26. Stability of a synchronous machine
(a) decreases with increase in its excitation
(b) increases with increase in its excitation
(c) remains unaffected with increase in excitation
(d) any of the above

Ans: b

27. If the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, then
(a) it runs at a slower speed
(b) the motor stops
(c) it continues to run at the same speed
(d) it runs at a very high speed

Ans: b

28. The maximum speed variation in a 3-phase synchronous motor is
(a) 10 per cent
(b) 6 per cent
(c) 4 per cent
(d) 2. per cent
(e) zero

Ans: e

29. In a 3-phase synchronous motor, the negative phase sequence exists when the motor is
(a) supplied with unbalanced voltage
(d) none of the above

Ans: a

30. The induced e.m.f. in a synchronous motor working on leading power factor will be
(a) more than the supply voltage
(b) less than the supply voltage
(c) equal to the supply voltage

Ans: a

31. The net armature voltage of a synchronous motor is equal to the
(a) vector sum of Eb and V
(b) arithmetic sum of Eb and V
(c) arithmetic difference of Eb and V
(d) vector difference of Eh and V

Ans: d

32. The ratio of starting torque to running torque in a synchronous motor is
(a) zero
(b) one
(c) two
(d) infinity

Ans: a

33. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on
(b) d.c. excitation only
(c) both the speed and rotor flux
(d) none of the above

Ans: b

34. A 3-phase synchronous motor is running clockwise. If the direction of its field current is reversed (a) the motor will stop
(b) the motor continue to run in the same direction
(c) the winding of the motor will burn
(d) the motor will run in the reverse direction
(e) none of the above

Ans: b

35. The torque angle, in a synchronous motor, is the angle between
(a) the supply voltage and the back e.m.f.
(b) magnetising current and back e.m.f.
(c) the rotating stator flux and rotor poles
(d) none of the above

Ans: c

36. In a synchronous motor, the damping winding is generally used to
(a) prevent hunting and provide the starting torque
(b) reduce the eddy currents
(c) provide starting torque only
(d) reduce noise level
(e) none of the above

Ans: a

37. The maximum value of torque that a synchronous motor can develop without losing its synchronism, is known as
(a) slip torque
(b) pull-out torque
(c) breaking torque
(d) synchronising torque

Ans: d

38. A synchronous machine with low value of short-circuit ratio has
(a) lower stability limit
(b) high stability limit
(c) good speed regulation
(d) good voltage regulation
(e) none of the above

Ans: a

39. The coupling angle or load angle of synchronous motor is defined as the angle between the
(a) rotor and stator teeth
(b) rotor and the stator poles of opposite polarity
(c) rotor and the stator poles of the same polarity
(d) none of the above

Ans: b

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