DC Generator MCQ

31. A self-excited d.c. shunt generator, driven by its prime-mover at the rated speed fails to build up voltage across its terminals at no-load. What reason can be assigned for this?

  1. The field circuit resistance is higher than the critical resistance.
  2. The initial shunt field mmf does not assist the residual magnetism.
  3. One of the inter-pole connections is reversed.
  4. The brush-axis shifts slightly from the geometrical neutral axis of the machine.
Answer. b

32. Consider the following statements in respect of d.c. generators:

  1. Compensating windings are located in teeth cut out in main poles.
  2. Compensating windings are series-connected so that the armature reaction is aided at any load.
  3. Compensating windings are series-connected for cancellation of the armature reaction at any load.
  4. Compensating windings are shunt-connected to aid the main poles at any load.

Which of these statements are correct?

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

33. A 6-pole lap wound d.c. machine armature has 720 conductors and it draws 50 A from supply mains. What is the flux distribution produced by the armature reaction per pole?

  1. Rectangular in wave shape with a peak of 500 AT
  2. Rectangular in wave shape with a peak of 100 AT
  3. Triangular in wave shape with a peak of 500 AT
  4. Triangular in wave shape with a peak of 1000 AT
Answer. c

34. In a d.c. compound generator, “flat-compound” characteristic, required for certain applications, may be obtained by connecting a variable resistance:

  1. Across the series field
  2. In series with the series field
  3. In parallel with the shunt field
  4. In series with the shunt field
Answer. a

35. The armature resistance of a 6-pole lap wound d.c. machine is 0.05 Ω. If the armature is rewound as a wave-winding, what is the armature resistance?

  1. 0.45 Ω
  2. 0.30 Ω
  3. 0.15 Ω
  4. 0.10 Ω
Answer. a

36. Consider the following statements:

In a d.c. machine, iron loss occurs in

  1. armature core
  2. yoke
  3. pole cores
  4. pole shoes

Which of these statements are correct?

  1. i and iv
  2. i and ii
  3. i and iii
  4. ii and iii
Answer. a

37. In a d.c. machine, for the same values of Φ, Z and N; which one of the following statements is correct?

  1. Armature e.m.f is more with wave winding than with lap winding
  2. Armature e.m.f is less with wave winding than with lap winding
  3. Armature e.m.f. depends on whether the machine is running as a motor or a generator
  4. Armature e.m.f is the same as long as the flux density in the air gap remains the same
Answer. a

38. The dummy coils in d.c. machines are useful to

  1. Increase the efficiency
  2. Improve the commutation
  3. Reduce the cost of the machine
  4. Maintain mechanical balance of armature
Answer. d

39. A dc shunt generator when driven with without connecting field winding shows an open circuit terminal voltage of 12 V. When field winding is connected and excited, the terminal voltage drops to zero because

  1. field resistance is higher than critical resistance
  2. there is no residual magnetism in the field circuit
  3. field winding has got wrongly connected
  4. there is a fault in armature circuit
Answer. d

40. Consider the following statements:

The use of interpoles in dc machines is to counteract the

  1. reactance voltage
  2. demagnetizing effect of armature mmf in the commutating zone
  3. cross-magnetizing effect of armature mmf in the commutating zone

Which of these statement(s) is/are correct?

  1. i and ii
  2. ii and iii
  3. i and iii
  4. iii alone
Answer. c
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