Synchronous motors

21. Synchronous motors are

Not-self starting
Self-starting
Essentially self-starting
None of the above

Detailed SolutionSynchronous motors are

22. If the field of a synchronous motor is underexcited, the power factor will be

zero
unity
lagging
leading

Detailed SolutionIf the field of a synchronous motor is underexcited, the power factor will be

23. If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to

increase in both armature current and power factor angle
increase in back e.m.f. but decrease in armature current
increase in both armature current and power factor which is lagging
increase in torque angle but decrease in back e.m.f.

Detailed SolutionIf excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to

24. In a synchronous motor running with fixed excitation, when the load is increased three times, its torque angle becomes approximately

One-third
Twice
Thrice
Six times

Detailed SolutionIn a synchronous motor running with fixed excitation, when the load is increased three times, its torque angle becomes approximately

25. Which of the following losses is not dissipated by the stator core surface in a synchronous motor ?

Eddy current losses in the conductors
Iron losses in the stator
Copper losses in the slot portion of the conductors
Windage losses

Detailed SolutionWhich of the following losses is not dissipated by the stator core surface in a synchronous motor ?

26. When the voltage applied to a synchronous motor is increased, which of the following will reduce ?

Stator flux
Pull in torque
Both (A) and (B)
None of the above

Detailed SolutionWhen the voltage applied to a synchronous motor is increased, which of the following will reduce ?

27. While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerting torque is produced by

induction motor torque in field winding
induction motor torque in damper winding
eddy current and hysteresis torque in pole faces
reHetance motor torque due to saliency of the rotor E. all of the above methods

Detailed SolutionWhile starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerting torque is produced by

28. The coupling angle or load angle of synchronous motor is defined as the angle between the

rotor and stator teeth
rotor and the stator poles of opposite polarity
rotor and the stator poles of the same polarity
none of the above

Detailed SolutionThe coupling angle or load angle of synchronous motor is defined as the angle between the

29. A synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because

synchronous motor has no slip
stator supply is not required to produce magnetic field
mechanical load on the rotor remains constant
synchronous motor has large airgap

Detailed SolutionA synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because

30. The power factor of a synchronous motor is better than that of induction motor because

stator supply is relieved of responsibility of producing magnetic field
mechanical load on the motor can be adjusted
synchronous motor runs at synchronous speed
synchronous motor has large air gap

Detailed SolutionThe power factor of a synchronous motor is better than that of induction motor because

Page 1Page 2Page 3Page 4Page 5
Test 1Test 2