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synchronous motors MCQs

Practise focused synchronous motors questions with marked answers for exam preparation and subject revision.

synchronous motors Mcqs
1. If the field winding of an unloaded salient pole synchronous motor is open circuited, the motor will__________?
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A. stop

B. function as static condenser

C. run as induction motor

D. burn with dense smoke

2. The construction of a synchronous motor resembles____________?
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A. a series motor

B. an alternator

C. an induction motor

D. a rotary converter

3. The maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as___________?
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A. pull-in torque

B. pull-out torque

C. none of the above

D. pull-up torque

4. If the synchronous motor, properly synchronised to the supply is running on no load and is having negligible loss then_____________?
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A. the stator current will be very small

B. the stator current will be very high

C. none of the above

D. the stator current will be zero

E. the back e.m.f. will be more than the supply voltage

5. The maximum torque which a synchronous motor will develop at rest for any angular position of the rotor, at rated stator supply voltage and frequency, is known as__________?
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A. reluctance torque

B. pull up torque

C. locked-rotor torque

D. synchronous torque

6. For power factor correction, synchronous motors operate at____________?
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A. no-load and greatly over-excited fields

B. no-load and under-excited fields

C. normal load with minimum excitation

D. normal load with zero excitation

7. The coupling angle or load angle of synchronous motor is defined as the angle between the__________?
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A. rotor and the stator poles of the same polarity

B. none of the above

C. rotor and the stator poles of opposite polarity

D. rotor and stator teeth

8. In a synchronous motor, the breakdown torque is__________?
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A. inversely proportional to applied voltage

B. directly proportional to the square of the applied voltage

C. none of the above

D. directly proportional to applied voltage
9. A synchronous motor which works on a leading power factor and does not drive a mechanical load is called as_________?
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A. condenser

B. static condenser

C. none of the above

D. synchronous condenser
10. A synchronous motor can develop synchronous torque:
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A. below or above synchronous speed

B. only at synchronous speed

C. while over-excited

D. when under loaded

11. The maximum power developed in a synchronous motor will depend on__________?
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A. the rotor excitation, supply voltage and maximum value of coupling angle (90)

B. the rotor excitation and supply volt-age both

C. the supply voltage only

D. the rotor excitation only

E. none of the above

12. A synchronous machine with low value of short-circuit ratio has___________?
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A. good voltage regulation

B. none of the above

C. high stability limit

D. lower stability limit

E. good speed regulation

13. Exciters of synchronous machines are______________?
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A. d.c. compound machines

B. any of the above

C. d.c. shunt machines

D. d.c. series machines

14. The mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motors running at full load, is of the order of_____________?
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A. five degrees

B. two degrees

C. ten degrees

D. zero degree

15. In case one phase of a three-phase synchronous motor is short-circuited the motor will____________?
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A. take less than the rated load

B. not start

C. run at 2/3 of synchronous speed

D. run with excessive vibrations

16. In a synchronous motor, the armature current has large values for____________?
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A. high excitation only

B. low excitation only

C. both high and low excitation

D. none of the above

17. The armature current of the synchronous motor
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A. has large values for low excitation i niy

B. has large values for high excitation only

C. has large values for low and high excitation

D. any of the above

18. A pony motor is basically a_________?
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A. D.C. shunt motor

B. small induction motor

C. D.C. series motor

D. double winding A.C./D.C. motor

19. When the stator windings are connected in such a fashion that the number of poles are made half, the speed of the rotor of a synchronous motor___________?
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A. tends to becomes zero

B. remains same as the original value

C. decreases to half the original value

D. increases to two times the original value
20. The minimum armature current of the synchronous motor corresponds to operation at______________?
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A. 0.707 power factor leading

B. unity power factor

C. zero power factor leading

D. 0.707 power factor lagging

21. Slip rings are usually made of__________?
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A. copper or aluminium

B. silver or gold

C. carbon or graphite

D. brass or steel
22. Synchronous motors are generally not self-starting because:
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A. starting winding is not provided on the machines

B. the direction of rotation is not fixed

C. startes cannot be used on these machines

D. the direction of instantaneous torque reverses after half cycle
23. The power factor of a synchronous motor is better than that of induction motor because
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A. synchronous motor runs at synchronous speed

B. stator supply is relieved of responsibility of producing magnetic field

C. synchronous motor has large air gap

D. mechanical load on the motor can be adjusted

24. When the field of a synchronous motor is under-excited, the power factor will be____________?
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A. zero

B. leading

C. lagging

D. unity

25. Riunting in a synchronous motor takes place___________?
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A. motor is under loaded

B. when load varies

C. when power factor is unity

D. when supply voltage fluctuates

26. In a synchronous motor, the magnitude of stator back e.m.f. & depends on_______________?
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A. d.c. excitation only

B. speed of the motor

C. load on the motor

D. both the speed and rotor flux

27. Change of D.C. excitation of a synchronous motor changes:
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A. applied voltage of the motor

B. all of the these

C. none of the these

D. motor speed

E. power factor of power drawn by the motor
28. A synchronous motor can operate at____________?
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A. unity power factor only

B. leading power factor only

C. lagging, leading and unity power factors

D. lagging power factor only

29. In a synchronous motor it the back e.m.f. generated in the armature at noload is approximately equal to the applied voltage, then
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A. the motor is said to be fully loaded

B. the excitation is said to be hundred per cent

C. the excitation is said to be zero per cent

D. the torque generated is maximum

30. Armature of a synchronous machine is____________?
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A. armature is associated with large power as compared to the field circuits

B. of difficulty of providing high voltage insulation on rotor

C. of reducing number of slip rings on the rotor

D. all of the above reasons
31. The percentage slip in case of a synchronous motor is____________?
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A. 1%

B. zero

C. 0.50%

D. 100%

32. The duration of sudden snort-circuit test on a synchronous motor is usually about_____________?
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A. one hour

B. none of the above

C. one second

D. one minute

33. A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will___________?
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A. none of the above

B. go down

C. improve

D. remain same

34. If load (or torque) angle of a 4-pole synchronous motor is 6 electrical, its value in mechanical degrees is__________?
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A. 6

B. 4

C. 2

D. 3
35. In a synchronous motor, V-curves represent relation between____________?
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A. armature current and field current

B. field current and speed

C. field current and power factor

D. power factor and speed

36. A synchronous machine with large air gap has______________?
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A. a higher synchronizing power which makes the machine less sensitive to load variations

B. a small value of inherent regulation

C. a higher value of stability limit

D. all of the above
37. A rotory converter can also be run as a___________?
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A. induction motor

B. d.c. compound motor

C. d.c. series motor

D. d.c. shunt motor

E. synchronous motor
38. The maximum speed variation in a 3-phase synchronous motor is____________?
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A. 10 per cent

B. 4 per cent

C. 6 per cent

D. 2. per cent

E. zero
39. For V-curves for a synchronous motor the graph is drawn between:_______________?
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A. armature current and power factor

B. power factor and field current

C. terminal voltage and load factor

D. field current and armature current
40. If in a synchronous motor, driving mechanical load and drawing current at lagging power factor from constant voltage supply, its field excitation is increased, then its power factor__________?
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A. none of the above

B. become more

C. become less

D. remain constant

41. An over-excited synchronous motor takes__________?
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A. leading current

B. lagging current

C. both A. and (b)

D. none of the above

42. Slip-rings in a synchronous motor carry____________?
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A. no current

B. direct current

C. alternating current

D. all of the above

43. Which of the following motors will be used in electric clocks?
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A. D.C. series motor

B. A.C. induction motor

C. A.C. synchronous motor

D. D.C. shunt motor

44. Hunting in a synchronous motor cannot be due to___________?
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A. variable frequency

B. variable supply voltage

C. windage friction

D. variable load

45. When the voltage applied to a synchronous motor is increased, which of the following will reduce?
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A. Stator flux

B. Pull in torque

C. Both A. and (b)

D. None of the above
46. A synchronous motor can be made self starting by providing?
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A. damper winding on rotor poles

B. damper winding on stator as well as rotor poles

C. damper winding on stator

D. none of the above
47. The operating speed of a synchronous motor can be changed to new fixed value by___________?
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A. changing the load

B. using brakes

C. changing frequency

D. changing the supply voltage

48. A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. On changing the excitation only, the armature current will have_____________?
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A. lagging power factor with over-excitation

B. leading power factor with over-excitation

C. leading power factor with under-excitation

D. no change of power factor

49. The angle between the rotating stator flux and rotor poles is called _________ angle.
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A. torque

B. obtuse

C. power factor

D. synchronizing

50. In a synchronous motor, damper windings are provided on____________?
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A. rotor shaft

B. none of the above

C. pole faces

D. stator frame