Electrical Engineering MCQs

Page No. 249

In which of the following motors the stator and rotor magnetic field rotate at the same speed?


aUniversal motor

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bSynchronous motor

✓


cInduction motor

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dReluctance motor

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View Answer Synchronous motor

A synchronous motor running with normal excitation adjusts to load increases essentially by increase in______________?


aback e.m.f.

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barmature current

✓


cpower factor and field current

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dtorque angle

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View Answer armature current only

The efficiency of a properly designed synchronous motor will usually fall in range___________?


a 60 to 70%

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b 75 to 80%

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c 85 to 95%

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d 99 to 99.5%

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View Answer 85 to 95%

The armature current of the synchronous motor has higher values for_____________?


ahigh excitation only

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blow excitation only

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cboth A. and (b)

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d none of the above

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View Answer none of the above

When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop_________________?


a inductor motor torque

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binduction generator torque

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csynchronous motor torque

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dd.c. motor toque

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While starting a synchronous motor by induction motor action, field winding is usually_____________?


aconnected to D.C. supply

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b short-circuited by low resistance

✓


ckept open-circuited

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dnone of the above

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Armature of a synchronous machine is____________?


aof reducing number of slip rings on the rotor

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barmature is associated with large power as compared to the field circuits

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cof difficulty of providing high voltage insulation on rotor

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dall of the above reasons

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


astator supply is relieved of responsibility of producing magnetic field

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b mechanical load on the motor can be adjusted

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csynchronous motor runs at synchronous speed

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dsynchronous motor has large air gap

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If one-phase of a 3-phase synchronous motor is short-circuited, motor will ______________?


awill refuse to start

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b will overheat in spots

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cwill not come upto speed

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dwill fail to pull into step

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View Answer will refuse to start

The maximum speed variation in a 3-phase synchronous motor is____________?


a10 per cent

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b6 per cent

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c4 per cent

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dzero

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View Answer zero

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