Short notes
Objective questions (Answers are given below)
- 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 - 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) - In a clockwise-rotating loaded d.c. generator,
brushes have to be shifted
(a) clockwise
(b) counterclockwise
(c) either (a) or (b)
(d) neither (a) nor (b). - 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. - 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
- In a 6-pole d.c. machine, 90 mechanical degrees
correspond to ………… electrical degrees.
(a) 30 (b) 180
(c) 45 (d) 270 - 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 - 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 - 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 - 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 - 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 - Shunt generators are most suited for stable
parallel operation because of their voltage
characteristics.
(a) identical (b) dropping
(c) linear (d) rising - 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) - 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 - The essential condition for stable parallel operation
A Two d.c. generators having similar
characteristics is that they should have
(a) same kilowatt ouput ratings
(b) droping voltage characterisitcs
(c) same percentage regulation
(d) same no-load and full-load speed - 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 - 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 - 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.
- (d ) 2. (d ) 3. (a) 4. (c) 5. (b) 6. (d ) 7. (d ) 8. (a) 9. (b ) 10. (d ) 11. (b )
- (b) 13. (d ) 14. (a) 15. (b) 16. (c) 17. (a) 18. (d)