ELECTRICAL MACHINE DESIGN 3

21. Though the cost of single layer winding is less than that of double layer winding, single layer winding is not generally used for dc machines because it
a) leads to high reactance voltage and sparking commutation.
b) increases the effect of armature reaction under pole.
c) reduces the efficiency of operation.
d) leads to less number of segments.
ans. a

22. In a dc generator, emf commutation can be achieved by
a) keeping the brushes in the gap.
b) shifting the brushes ahead of gap.
c) shifting the brushes forward.
d) shifting the brushes backward.
ans. c

23. The mmf of an interpole is proportional to
a) armature current
b) field current
c) product of armature and field currents.
d) ratio of armature and field currents.
ans. a

24. The terminal voltage of a 20hp,230 V dc series motor under rated speed and no load condition is
a) 0 V
b) 230 V
c) about 5 to 20 V.
d) 250 V
ans. b

25. Modern starters used in dc shunt motors
a) are manual
b) use time delay relays.
c) use back emf sensing relays.
d) use time delay relays or back emf sensing relays.
ans. c

26. Magnetic material used for large transformers is
a) cast steel
b) hot rolled silicon steel
c) cold rolled grain oriented steel.
d) either b or c
ans. c

27. Thickness of core laminations of tranformers is
a) 18 cm
b) 25 cm
c) 20 cm
d) 15 cm
ans. c

28. In order to reduce hysteresis loss
a) core many be laminated.
b) silicon steel may be used as the core material
c) core may be constructed with any permanent magnet material such as alnico.
d) core may be impregnated with varnish.
ans. b

29. Multi-step core is used in a transformer to
a) increase the output.
b) decrease the cost of core material
c) decrease the cost of copper.
d) increase the efficiency.
ans. c

30. Gross window area in a transformer is 750 cm-sq. The approximate height of window is
a) 50 cm
b) 75 cm
c) 30 cm
d) 25 cm
ans. b