Synchronous Motors MCQs

1. As the load is applied to a synchronous motor, the motor takes more armature current because

(a) the increased load has to take more current
(b) the rotor by shifting its phase backward causes the motor to take more current
(c) the back e.m.f. decreases causing an increase in motor current
(d) the rotor strengthens the rotating field causing more motor current

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Answer. b

2. The synchronous motor is not inherently self-starting because

(a) the force required to accelerate the rotor to the synchronous speed in an instant is absent
(b) the starting device to accelerate the rotor to near synchronous speed is absent
(c) a rotating magnetic field does not have enough poles
(d) the rotating magnetic field is produced by only 50 Hz frequency currents

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Answer. a

3. Mostly, synchronous motors are of

(a) alternator type machines
(b) induction type machines
(c) salient pole type machines
(d) smooth cylindrical type machines

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Answer. c

4. A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased

(a) power factor, as well as armature current, will decrease
(b) power factor, as well as armature current, will increase
(c) power factor will increase but armature current will decrease
(d) power factor will decrease and armature current will increase

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Answer. d

5. The power developed by a synchronous motor will be maximum when the load angle is

(a) zero

(b) 45°
(c) 90°
(d) 120°

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Answer. c

6. A synchronous motor can be used as a synchronous capacitor when it is

(a) under-loaded
(b) over-loaded
(c) under-excited
(d) over-excited

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Answer. d

7. When V is the applied voltage, then the breakdown torque of a synchronous motor varies as

(a) V
(b) V3/2
(c) V2
(d) 1/V

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Answer. a

8. When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected

(a) the motor stops
(b) it runs as a reluctance motor at the same speed
(c) it runs as a reluctance motor at a lower speed
(d) none of the above

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Answer. a

9. Under which of the following conditions hunting of synchronous motor is likely to occur?

(a) Periodic variation of load
(b) Over-excitation
(c) Over-loading for long periods
(d) Small and constant load

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Answer. a

10. A three-phase synchronous motor will have

(a) no slip-rings
(b) one slip-ring
(c) two slip-rings
(d) three slip-rings

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Answer. c

11. A synchronous motor can be started by

(a) pony motor
(b) D.C. compound motor
(c) providing damper winding
(d) any of the above

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Answer. d

12. A synchronous motor can develop synchronous torque

(a) when under loaded
(b) while over-excited
(c) only at the synchronous speed

(d) below or above synchronous speed

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Answer. c

13. A pony motor is basically a

(a) small induction motor
(b) D.C. series motor
(c) D.C. shunt motor
(d) double winding A.C./D.C. motor

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Answer. a

14. In case one phase of a three-phase synchronous motor is short-circuited the motor will

(a) not start
(b) run at 2/3 of synchronous speed
(c) run with excessive vibrations
(d) take less than the rated load

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Answer. a

15. Synchronous motors are generally not self-starting because

(a) the direction of rotation is not fixed
(b) the direction of instantaneous torque reverses after a half cycle
(c) starts cannot be used on these machines
(d) starting winding is not provided on the machines

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Answer. b