Electrical Machines (D.C. Generators)

1. The basic requirement of a d.c. armature winding is that it must be
(a) a closed one
(b) a lap winding
(c) a wave winding
(d) either (b) or (c)

2. A wave winding must go at least ............ around the armature before it closes back where it started.
(a) once
(b) twice
(c) thrice
(d) four times

3. The d.c. armature winding in which coil sides are a pole pitch apart is called ............ winding.
(a) multiplex
(b) fractional-pitch
(c) full-pitch
(d) pole-pitch

4. For making coil span equal to a pole pitch in the armature winding of a d.c. generator, the
back pitch of the winding must equal the number of
(a) commutator bars per pole
(b) winding elements
(c) armature conductors per path
(d) armature parallel paths.

5. The primary reason for making the coil span of a d.c. armature winding equal to a pole pitch is to
(a) obtain a coil span of 180º (electrical)
(b) ensure the addition of e.m.fs. of consecutive turns
(c) distribute the winding uniformly under different poles
(d) obtain a full-pitch winding.

6. In a 4-pole, 35 slot d.c. armature, 180 electrical-degree coil span will be obtained when coils occupy ............ slots.
(a) 1 and 10
(b) 1 and 9
(c) 2 and 11
(d) 3 and 12

7. The armature of a d.c. generator has a 2-layer lap-winding housed in 72 slots with six conductors/ slot. What is the minimum number of commutator bars required for the armature?
(a) 72
(b) 432
(c) 216
(d) 36

8. The sole purpose of a commutator in a d.c. Generator is to
(a) increase output voltage
(b) reduce sparking at brushes
(c) provide smoother output
(d) convert the induced a.c. into d.c.

9. For a 4-pole, 2-layer, d.c., lap-winding with 20 slots and one conductor per layer, the number of commutator bars is
(a) 80
(b) 20
(c) 40
(d) 160

10. A 4-pole, 12-slot lap-wound d.c. armature has two coil-sides/slot. Assuming single turn coils and progressive winding, the back pitch would be
(a) 5
(b) 7
(c) 3
(d) 6

11. If in the case of a certain d.c. armature, the number of commutator segments is found either one less or more than the number of slots, the armature must be having a simplex ............ winding.
(a) wave
(b) lap
(c) frog leg
(d) multielement

12. Lap winding is suitable for ............ current, ............ voltage d.c. generators.
(a) high, low
(b) low, high
(c) low, low
(d) high, high

13. The series field of a short-shunt d.c. generator is excited by ............ currents.
(a) shunt
(b) armature
(c) load
(d) external

14. In a d.c. generator, the generated e.m.f. is directly proportional to the
(a) field current
(b) pole flux
(c) number of armature parallel paths
(d) number of dummy coils

15. In a 12-pole triplex lap-wound d.c. armature, each conductor can carry a current of 100 A.
The rated current of this armature is ............ ampere.
(a) 600
(b) 1200
(c) 2400
(d) 3600

16. The commercial efficiency of a shunt generator is maximum when its variable loss equals ............ loss.
(a) constant
(b) stray
(c) iron
(d) friction and windage

17. In small d.c. machines, armature slots are sometimes not made axial but are skewed. Though skewing makes winding a little more difficult, yet it results in
(a) quieter operation
(b) slight decrease in losses
(c) saving of copper
(d) both (a) and (b)

18. The critical resistance of the d.c. generator is the resistance of
(a) armature
(b) field
(c) load
(d) brushes

Answers
1. (a)    2. (b)    3. (c)    4. (a)    5. (b)    6. (b)    7. (c)    8. (d )   9. (b)    10. (b)
11. (a) 12. (a) 13. (c) 14. (b) 15. (d ) 16. (a) 17. (d ) 18. (b)


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