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Synchronous Motors

Study 122 Electrical Engineering MCQs with correct answers and explanations.

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21Electrical EngineeringMedium

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________?

Alagging power factor with over-excitation

Bno change of power factor

Cleading power factor with under-excitation

Dleading power factor with over-excitation

Correct answer

D. leading power factor with over-excitation

Explanation

The correct answer is leading power factor with over-excitation.

22Electrical EngineeringMedium

The power developed by a synchronous motor will be maximum when the load angle is________?

A90°

Bzero

C120°

D45°

Correct answer

A. 90°

Explanation

The correct answer is 90°.

23Electrical EngineeringMedium

The percentage slip in case of a synchronous motor is________?

A100%

B0.5%

C1%

Dzero

Correct answer

D. zero

Explanation

The correct answer is zero.

24Electrical EngineeringMedium

In a 3-phase, 4-pole, 50 Hz synchronous motor, the frequency, pole number and load torque all are halved. The motor speed will be________?

A1500 r.p.m.

Bnone of the above

C3000 r.p.m.

D750 r.p.m.

Correct answer

A. 1500 r.p.m.

Explanation

The correct answer is 1500 r.p.m..

25Electrical EngineeringMedium

The maximum power developed in a synchronous motor will depend on________?

Anone of the above

Bthe rotor excitation, supply voltage and maximum value of coupling angle (90°)

Cthe rotor excitation and supply volt-age both

Dthe supply voltage only

Ethe rotor excitation only

Correct answer

B. the rotor excitation, supply voltage and maximum value of coupling angle (90°)

Explanation

The correct answer is the rotor excitation, supply voltage and maximum value of coupling angle (90°).

26Electrical EngineeringMedium

The minimum armature current of the synchronous motor corresponds to operation at________?

Aunity power factor

B0.707 power factor lagging

C0.707 power factor leading

Dzero power factor leading

Correct answer

A. unity power factor

Explanation

The correct answer is unity power factor.

27Electrical EngineeringMedium

In a synchronous motor, the magnitude of stator back e.m.f. & depends on________?

Aspeed of the motor

Bboth the speed and rotor flux

Cd.c. excitation only

Dload on the motor

Correct answer

C. d.c. excitation only

Explanation

The correct answer is d.c. excitation only.

28Electrical EngineeringMedium

When a 3-phase synchronous generator is supplying a zero power factor lagging load, the armature field affects the main field in the following way

Across-magnetises it

Bdirectly opposes it

Cnone of the above

Daugments it directly

Correct answer

B. directly opposes it

Explanation

The correct answer is directly opposes it.

29Electrical EngineeringMedium

If the field winding of an unloaded salient pole synchronous motor is open circuited, the motor will________?

Afunction as static condenser

Bburn with dense smoke

Crun as induction motor

Dstop

Correct answer

D. stop

Explanation

The correct answer is stop.

30Electrical EngineeringMedium

A rotory converter can also be run as a________?

Ad.c. compound motor

Binduction motor

Csynchronous motor

Dd.c. shunt motor

Ed.c. series motor

Correct answer

C. synchronous motor

Explanation

The correct answer is synchronous motor.

The maximum power developed in a synchronous motor will depend on________?

Anone of the above

Bthe rotor excitation, supply voltage and maximum value of coupling angle (90°)

Cthe rotor excitation and supply volt-age both

Dthe supply voltage only

Ethe rotor excitation only

Correct answer

B. the rotor excitation, supply voltage and maximum value of coupling angle (90°)

Explanation

The correct answer is the rotor excitation, supply voltage and maximum value of coupling angle (90°).