ELECTRICAL MACHINE DESIGN 2

11. As the number of poles in a dc machine increases the chances of flash over increases because
a) the voltage between the segments increases
b) the voltage between segments decreases
c) the length of the commutator decreases
d) effect of armature reaction increases
ans. a

12. Suitable value of number of poles for a 250 kw , 600 rpm dc motor is
a) 2
b) 4
c) 6
d) 8
ans. d

13. Air gap at the pole tips of a dc machine is kept more than that at the centre of the pole mainly to reduce
a) reactance voltage
b) effect of armature reaction
c) losses in armature core
d) noise of machine
ans. a

14. Number of parallel paths in a dc machine with duplex wave winding will be
a) 2
b) 4
c) 8
d) 16
ans. b

15. The number of parallel paths or a 4-pole duplex lap winding will be
a) 2
b) 4
c) 6
d) 8
ans. d

16. In a dc machine if the number of slot in the armature are more
a) cooling is likely to be poor
b) commutation will be poor
c) cost will be more
d) the flux pulsation will be more
ans. c

17. The number of brush arms is made sometimes equal to the number of poles in case of simplex dc wave winding to
a) avoid sparking commutation
b) reduce armature reaction
c) reduce the length of commutator and to equalise the number of coils in each parallel path.
d) none of the above
ans. a

18. Along with the rate of change of current, reactance voltage in dc machines also varies with
a) number of turns on the coil.
b) square of the number of turns on the coil.
c) square root of the number of turns of the coil.
d) none of the above.
ans. b

19. Width of carbon brush should be equal to
a) less than the width of one commutator segment.
b) the width of 1 to 2 commutator segments.
c) the width of 2 to 3 commutator segments.
d) the width of more than 3 commutator segments.
ans. c

20. The brushes are staggered to
a) avoid eating away of copper due to arcing.
b) avoid ridges and to have even commutator surface.
c) reduce brush contact loss
d) reduce armature reaction.
ans. b