D.C. GENERATORS

Objective Questions

Short notes

Objective questions (Answers are given below)

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 (Grad. I.E.T.E Dec. 1985)

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