Electrical Machines (Armature Reaction in DC Generators)
1. In d.c. generators, armature reaction is
produced actually by
(a) its field current
(b) armature conductors
(c) field pole winding
(d) load current in armature
2. In a d.c. generator, the effect of
armature reaction on the main pole flux is to
(a) reduce it
(b) distort it
(c) reverse it
(d ) both (a) and (b)
3. In a clockwise-rotating loaded d.c. generator,
brushes have to be shifted
(a) clockwise
(b) counter clockwise
(c) either (a) or (b)
(d) neither (a) nor (b).
4. The primary reason for providing
compensating windings in a d.c. generator is to
(a) compensate for decrease in main flux
(b) neutralize armature mmf
(c) neutralize cross-magnetising flux
(d ) maintain uniform flux distribution.
5. The main function of interpoles is to
minimize ............ between the brushes and the commutator when the d.c. machine
is loaded.
(a) friction
(b) sparking
(c) current
(d ) wear and tear
6. In a 6-pole d.c. machine, 90 mechanical
degrees correspond to ............ electrical degrees.
(a) 30
(b) 180
(c) 45
(d) 270
7. The most likely cause(s) of sparking at
the brushes in a d.c. machine is /are
(a) open coil in the armature
(b) defective interpoles
(c) incorrect brush spring pressure
(d) all of the above
8. In a 10-pole, lap-wound d.c. generator, the
number of active armature conductors per pole is 50. The number of compensating
conductors per pole required is
(a) 5
(b) 50
(c) 500
(d) 10
9. The commutation process in a d.c. generator
basically involves
(a) passage of current from moving armature
to a stationary load
(b) reversal of current in an armature coil
as it crosses MNA
(c) conversion of a.c. to d.c.
(d) suppression of reactance voltage
10. Point out the WRONG statement. In d.c. generators,
commutation can be improved by
(a) using interpoles
(b) using carbon brushes in place of Cu
brushes
(c) shifting brush axis in the direction of
armature rotation
(d) none of the above
11. Each of the following statements
regarding interpoles is true except
(a) they are small yoke-fixed poles spaced
in between the main poles
(b) they are connected in parallel with the
armature so that they carry part of the armature current
(c) their polarity, in the case of
generators is the same as that of the main pole ahead
(d) they automatically neutralize not only reactance
voltage but cross-magnetisation as well
12. Shunt generators are most suited for
stable parallel operation because of their voltage characteristics.
(a) identical
(b) dropping
(c) linear
(d) rising
13. Two parallel shunt generators will
divide the total load equally in proportion to their kilowatt output ratings
only when they have the same
(a) rated voltage
(b) voltage regulation
(c) internal IaRa drops
(d) boths (a) and (b)
14. The main function of an equalizer bar
is to make the parallel operation of two over-compounded d.c. generators
(a) stable
(b) possible
(c) regular
(d) smooth
15. The essential condition for stable
parallel operation A Two d.c. generators having similar
characteristics is that they should have
(a) same kilowatt output ratings
(b) dropping voltage characteristics
(c) same percentage regulation
(d) same no-load and full-load speed
16. The main factor which loads to unstable
parallel operation of flat-and over-compound d.c.
generators is
(a) unequal number of turns in their series
field windings
(b) unequal series field resistances
(c) their rising voltage characteristics
(d) unequal speed regulation of their prime
movers
17. The simplest way to shift load from one
d.c. shunt generator running in parallel with another is to
(a) adjust their field rheostats
(b) insert resistance in their armature
circuits
(c) adjust speeds of their prime movers
(d) use equalizer connections
18. Which one of the following types of
generators does NOT need equalizers for satisfactory parallel operation ?
(a) series
(b) over-compound
(c) flat-compound
(d) under-compound.
Answers
1. (d ) 2.
(d ) 3. (a) 4. (c) 5. (b) 6. (d ) 7.
(d ) 8. (a) 9. (b ) 10. (d ) 11. (b ) 12.
(b) 13. (d ) 14. (a) 15. (b) 16. (c) 17. (a) 18. (d)
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