DC Generator MCQ

11. Assuming a uniform distribution of current over the armature surface conductors, the shape of the resultant armature m.m.f. in space of a loaded dc machine is a symmetrical

  1. Triangular wave with its peak along the interpolar axis
  2. Triangular wave with its peak along the pole axis.
  3. Rectangular wave with its central axis coinciding with the inter-polar axis
  4. Rectangular wave with its central axis coinciding with the pole axis.
Answer. a

12. The induced emf of a dc machine running at 750 rpm is 220V. The percentage increase in field flux for generating an induced emf of 250V at 700 rpm would be

  1. 7%
  2. 11.25%
  3. 21.7%
  4. 42.4%
Answer. c

13. Consider the following statements:

The armature reaction mmf in a dc machine is

  1. Stationary with respect to the field poles.
  2. Rotating with respect to the field poles
  3. Rotating with respect to the armature.

Which of these statements are correct?

  1. i, ii and iii
  2. i and ii only
  3. i and iii only
  4. ii and iii only
Answer. c

14. Determine the flux per pole for 6-pole dc machine having 240 wave connected conductors, which generates an open circuit voltage of 500 volt, which is running at 1000 rpm.

  1. 0.129 Wb
  2. 0.021 Wb
  3. 0.042 Wb
  4. 7  mWb
Answer. c

15. A 6-pole dc armature has simplex lap-connected 720 conductors, 3 turns per coil and 4 coil-sides per slot. Determine the number of slots in the armature and state whether equalizers can be employed or not.

  1. 60 slots and not possible
  2. 30 slots and possible
  3. 60 slots and possible
  4. 30 slots and not possible
Answer. c

16. A 4-pole lap wound dc generator has a developed power of P watt and voltage of E volt. Two adjacent brushes of the machine are removed as they are worn out. If the machine operates with the remaining brushes, the developed voltage and power that can be obtained from the machine are

  1. E and P
  2. E/2 and P/2
  3. E and P/4
  4. E and P/2
Answer. c

17. At 1200 rpm the induced emf of a dc machine is 200 V. For an armature current of 15 A the electromagnetic torque produced would be

  1. 238 N-m
  2. 23.8 N-m
  3. 3000 N-m
  4. 2000 N-m
Answer. b

18. A triangular mmf wave is produced in the air-gap of an electric machine. Such a wave is produced by

  1. stator of an induction machine
  2. rotor of a synchronous machine
  3. stator of a do machine
  4. rotor of a dc machine
Answer. d

19. The overall efficiency of a dc shunt generator is maximum when its variable loss equals

  1. the iron loss
  2. the stray loss
  3. mechanical loss
  4. constant loss
Answer. d

20. A 4-pole dc generator is running at 1500 rpm. The frequency of current in the armature winding is

  1. 150 Hz
  2. 50 Hz
  3. 100 Hz
  4. 25 Hz
Answer. b