synchronous motors MCQs – Page 2

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

synchronous motors Mcqs
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