Electrical Engineering MCQs

Page No. 244

In a synchronous motor, damper windings are provided on____________?


a stator frame

✖


brotor shaft

✖


cpole faces

✓


dnone of the above

✖


View Answer pole faces

In a 3-phase synchronous motor, the negative phase sequence exists when the motor is_______________?


asupplied with unbalanced voltage

✓


bunder-loaded

✖


cover-loaded

✖


dnone of the above

✖



Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current?


aHigh field current

✖


bLow short circuit ratio

✖


c High core losses

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dLow field current

✓


View Answer Low field current

A rotory converter can also be run as a___________?


ad.c. shunt motor

✖


bd.c. series motor

✖


cd.c. compound motor

✖


dsynchronous motor

✓


View Answer synchronous motor

In which of the following motors the stator and rotor fields rotate simultaneously?


aD.C. motor

✖


bReluctance motor

✖


cUniversal motor

✖


dSynchronous motor

✓


View Answer Synchronous motor

If the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, then


ait runs at a slower speed

✖


bthe motor stops

✓


c it continues to run at the same speed

✖


dit runs at a very high speed

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View Answer the motor stops

A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because


aspeed of the motor is reduced

✖


bpower factor is decreased

✖


cEb (back e.m.f.) becomes less than V (applied voltage)

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d Er (net resultant voltage) in armature is increased

✓



If in a synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor


awill become more

✖


b will become less

✓


cwill remain unchanged

✖


dnone of the above.

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View Answer will become less

Stability of a synchronous machine:


adecreases with increase in its excitation

✖


b increases with increase in its excitation

✓


c remains unaffected with increase in excitation

✖


dany of the above

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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____________?


aincrease in both armature current and power factor angle

✓


b increase in back e.m.f. but decrease in armature current

✖


cincrease in both armature current and power factor which is lagging

✖


dincrease in torque angle but decrease in back e.m.f.

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