Synchronous Motor MCQ

1. An electric motor in which rotor and stator fields rotate simultaneously is called ________ motor.

1. DC
2. Induction
3. Synchronous
4. Universal

2. A 3-phase synchronous motor has been provided with damper winding. It can be started as a

1. Simple phase synchronous motor
2. 3-phase squirrel cage induction motor
3. 1-phase induction motor
4. 3-phase alternator

3. In an AC machine, the effect of distributing the turns in different slots, results in a further reduction of generated EMF by the factor K_d. This factor is called

1. distribution/spread factor
2. coil pitch factor
3. winding factor
4. generation factor

4. The EMF equation e = Nω_rφsinω_rt is applicable to

1. AC systems with time-variant field flux
2. DC systems with time-variant field flux
3. both AC and DC systems with time-invariant field flux
4. both AC and DC systems with time-variant field flux

5. A synchronous motor with negligible armature resistance runs at a load angle of 20° at the rated frequency. If the supply frequency is increased by 10%, keeping other parameters constant, the new load angle will be:

1. 20°
2. 22°
3. 12°
4. 14°

6. A synchronous motor is operating with normal excitation. With the increase in load, the armature current drawn from the supply mains increase due to

1. increase in resultant voltage across the armature
2. Increase in power factor
3. Increase in back emf
4. Fall in motor speed

7. For constant power, the current drawn by a synchronous motor will be minimum when the power factor is:

1. Leading
2. Lagging
3. Unity
4. Zero

8. A Synchronous motor having 5 poles is running with a supply frequency of 40 Hz. What is the control operating speed of the motor?

1. 1000 rpm
2. 1200 rpm
3. 600 rpm
4. 960 rpm

9. Which among the following statements is true about a 3-phase synchronous motor?

1. The speed of stator MMF is always more than that of rotor MMF
2. The speed of stator MMF is always less than that of rotor MMF
3. The speed of stator MMF is synchronous speed while that of rotor MMF is zero
4. Rotor and stator MMF are stationary with respect to each other

10. Which of the following is true for a synchronous condenser?

1. It is a synchronous motor with capacitor connected across stator terminals to improve power factor.
2. It is a synchronous motor operating at full load with leading power factor.
3. It is an over-excited synchronous motor partially supplying mechanical load, and also improving power factor of the system to which it is connected.
4. It is an over-excited synchronous motor operating at no-load with leading pf used in large power stations for improvement of power factor.

11. In a synchronous alternator which of the following coils will have emf closer to sine waveform?

1. concentrated winding in full pitch coils
2. distributed winding in full pitch coils
3. distributed winding in short pitch coils
4. concentrated winding in short pitch coil

12. The V curves of a synchronous motor show the relationship between.

1. field current and supply voltage
2. armature current and supply voltage
3. field current and armature current
4. armature current and power factor

13. Change in excitation of synchronous motor result

1. Change in motor speed
2. Change in power factor
3. Both (a) & (b)
4. None of the above

14. Synchronous capacitor is

1. An ordinary static capacitor bank
2. An over-excited synchronous motor driving mechanical load.
3. An over-excited motor running without mechanical load.
4. None of the above

15. Damper winding and pony brake are the methods of starting of

1. Slip ring motors
2. Synchronous motors
3. Schrage motors
4. Squirrel cage motors

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

1. the increased load has to take more current
2. the rotor by shifting its phase backward causes the motor to take more current
3. the back e.m.f. decreases causing an increase in motor current
4. the rotor strengthens the rotating field causing more motor current

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

1. the force required to accelerate the rotor to the synchronous speed in an instant is absent
2. the starting device to accelerate the rotor to near synchronous speed is absent
3. a rotating magnetic field does not have enough poles
4. the rotating magnetic field is produced by only 50 Hz frequency currents

18. Mostly, synchronous motors are of

1. alternator type machines
2. induction type machines
3. salient pole type machines
4. smooth cylindrical type machines

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

1. power factor, as well as armature current, will decrease
2. power factor, as well as armature current, will increase
3. power factor will increase but armature current will decrease
4. power factor will decrease and armature current will increase

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

1. zero
2. 45°
3. 90°
4. 120°

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

1. under-loaded
2. over-loaded
3. under-excited
4. over-excited

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

1. V
2. V^3/2
3. V²
4. 1/V

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

1. the motor stops
2. it runs as a reluctance motor at the same speed
3. it runs as a reluctance motor at a lower speed
4. none of the above

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

1. Periodic variation of load
2. Over-excitation
3. Over-loading for long periods
4. Small and constant load

25. A three-phase synchronous motor will have

1. no slip-rings
2. one slip-ring
3. two slip-rings
4. three slip-rings

26. A synchronous motor can be started by

1. pony motor
2. D.C. compound motor
3. providing damper winding
4. any of the above

27. A synchronous motor can develop synchronous torque

1. when under loaded
2. while over-excited
3. only at the synchronous speed
4. below or above synchronous speed

28. A pony motor is basically a

1. small induction motor
2. D.C. series motor
3. D.C. shunt motor
4. double winding A.C./D.C. motor

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

1. not start
2. run at 2/3 of synchronous speed
3. run with excessive vibrations
4. take less than the rated load

30. Synchronous motors are generally not self-starting because

1. the direction of rotation is not fixed
2. the direction of instantaneous torque reverses after a half cycle
3. starts cannot be used on these machines
4. starting winding is not provided on the machines