16. In a commutator
(A) copper is harder than mica
(B) mica and copper are equally hard
(C) mica is harder than copper
(D) none of the above
Ans: C
17. In D.C. generators the pole shoes are fastened to the
pole core by
(A) rivets
(B) counter sunk screws
(C) brazing
(D) welding
Ans: B
18. According to Fleming's right-hand rule for finding the
direction of induced e.m.f., when middle finger points in the direction of
induced e.m.f., forefinger will point in the direction of
(A) motion of conductor
(B) lines of force
(C) either of the above
(D) none of the above
Ans: B
19. Fleming's right-hand rule regarding direction of induced
e.m.f., correlates
(A) magnetic flux, direction of current
flow and resultant force
(B) magnetic flux, direction of motion
and the direction of e.m.f. induced
(C) magnetic field strength, induced
voltage and current
(D) magnetic flux, direction of force and
direction of motion of conductor
Ans: B
20. While applying Fleming's right-hand rule to And the
direction of induced e.m.f., the thumb points towards
(A) direction of induced e.m.f.
(B) direction of flux
(C) direction of motion of the conductor
if forefinger points in the direction of generated e.m.f.
(D) direction of motion of conductor, if
forefinger points along the lines of flux
Ans: D
21. The bearings used to support the rotor shafts are
generally
(A) ball bearings
(B) bush bearings
(C) magnetic bearings
(D) needle bearings
Ans: A
22. In D.C. generators, the cause of rapid brush wear may be
(A) severe sparking
(B) rough commutator surface
(C) imperfect contact
(D) any of the above
Ans: D
23. In lap winding, the number of brushes is always
(A) double the number of poles
(B) same as the number of poles
(C) half the number of poles
(D) two
Ans: B
24. For a D.C. generator when the number of poles and the
number of armature conductors is fixed, then which winding will give the higher
e.m.f. ?
(A) Lap winding
(B) Wave winding
(C) Either of (A) and (B) above
(D) Depends on other features of design
Ans: B
25. In a four-pole D.C. machine
(A) all the four poles are north poles
(B) alternate poles are north and south
(C) all the four poles are south poles
(D) two north poles follow two south
poles
Ans: B
26. Copper brushes in D.C. machine are used
(A) where low voltage and high currents
are involved
(B) where high voltage and small
cur-rents are involved
(C) in both of the above cases
(D) in none of the above cases
Ans: A
27. A separately excited generator as compared to a
self-excited generator
(A) is amenable to better voltage
con-trol
(B) is more stable
(C) has exciting current independent of
load current
(D) has all above features
Ans: D
28. In case of D.C. machines, mechanical losses are primary
function of
(A) current
(B) voltage
(C) speed
(D) none of above
Ans: C
29. Iron losses in a D.C. machine are independent of
variations in
(A) speed
(B) load
(C) voltage
(D) speed and voltage
Ans: B
30. In D.C. generators, current to the external circuit from
armature is given through
(A) commutator
(B) solid connection
(C) slip rings
(D) none of above
