DC Motor MCQ

21. The air-gap between the yoke and armature in a dc motor is kept small

  1. to achieve a stronger magnetic field.
  2. to avoid overheating of the machine.
  3. to avoid locking of the armature.
  4. to avoid transverse motion.
Answer. a

22. Which one of the following statement is correct for the ideal field excitation during starting in case of shunt and compound motors?

  1. Maximum because motor torque required to overcome friction and load torque.
  2. Maximum, because voltage should be built slowly.
  3. Maximum, because small excitation will lead to low operating speed during starting.
  4. It can be anything because it does not matter in case of these motors.
Answer. a

23. A Separately excited dc motor is started using a 8-phase ac/dc controlled rectifier using ‘Soft starting’. For limiting the starting current, it is required that firing angle should be

  1. Gradually increased from 0° to 180°
  2. Gradually reduced from 180° to 30°
  3. Fixed at 30°
  4. Gradually increased from 30° to 180°
Answer. b

24. A dc motor operated from a type A chopper is switched to type B chopper. How does the motor operate when type A and B are in operation respectively?

  1. Plugging and motoring
  2. Motoring and plugging
  3. Regenerative braking and motoring
  4. Motoring and regenerative braking
Answer. d

25. The armature MMF waveform of a dc machine is

  1. Rectangular
  2. Pulsating
  3. Triangular
  4. Sinusoidal
Answer. c

26. When is the mechanical power developed by a d.c. motor maximum?

  1. Back e.m.f. is equal to applied voltage
  2. Back e.m.f. is equal to zero
  3. Back e.m.f. is equal to half the applied voltage
  4. None of the above
Answer. c

27. Torque developed by the armature of a dc motor is proportional to which one of the following?

  1. (Back emf) × (Armature current)
  2. (Magnetic flux per pole) × (Armature current)
  3. (Back emf) × (Magnetic flux per pole)
  4. (Back emf)/(Magnetic flux per pole)
Answer. b

28. A 5 kW, 200 V dc shunt motor has armature resistance of 1 ohm and shunt field resistance of 100 ohm. At no load, the motor draws 6 A from 200 V supply and runs at 1000 rpm. What is the total copper loss of the machine?

  1. 400 W
  2. 16 W
  3. 36 W
  4. 416 W
Answer. d

29. The field flux method of speed control of dc shunt motor is generally used for which one of the following?

  1. Constant hp drive below the base speed
  2. Constant torque drive above the base speed
  3. Constant torque drive below the base speed
  4. Constant hp drive above the base speed
Answer. d

30. Neglecting all losses, how is the developed torque (T) of a dc separately excited motor, operating under constant terminal voltage, related to its output power (P)?

\\a.\; T\propto\sqrt{P} \\ b. \; T\propto P \\ c. \; T^{2}\propto P^{3} \\ d. \; \text{T is independent of P}

Answer. d
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