Electrical Machine Design MCQ

21. Normally ……….. winding is used for dc machines having output current more than 400 A.

(a) simplex wave
(b) simplex lap
(c) duplex wave
(d) duplex lap

22. The number of brush arms is made sometimes equal to the number of poles in case of a 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.

23. In dc machines

(a) the number of slots per pole pair should be an odd integer.
(b) the number of slots per pole should be at least 9.
(c) the number of slots per pole usually lies between 9 and 16.
(d) all of the above.

24. In dc machines, the slot pitch usually lies between

(a) 25 to 35 mm.
(b) 50 to 60 mm.
(c) 15 to 25 mm.
(d) 10 to 15 mm.

25. Use of high current density increases

(a) copper cost.
(b) copper losses.
(c) temperature rise.
(d) efficiency.
(e) both (b) and (c).

26. Minimum number of coils for 400 V, 6-pole lap connected dc machine for a maximum voltage of 15 volts between adjacent commutator segments should be

(a) 80
(b) 160
(c) 200
(d) 400 [A.M.I.E. Sec B Elec. Design 1993]

27. The commutator diameter generally lies between …… and …….. times of armature diameter.

(a) 0.25, 5
(b) 0.4, 0.6
(c). 0.6, 0.8
(d) 1.2, 1.5

28. The commutator segment pitch should not be less than

(a) 0.4 mm.
(b) 4 mm.
(c) 2 mm.
(d) 8 mm.

29. The higher commutator peripheral speeds generally leads to

(a) commutation difficulties.
(b) ventilation difficulties.
(c) both (a) and (b).
(d) none of these.

30. Mica is used to insulate the commutator segments because

(a) it has very high specific weight.
(b) it is mechanically strong.
(c) it is cheaper.
(d) both (a) and (b).

31. Commutator segments are insulated from each other by mica layers of thickness varying from

(a) 0.1 to 1 mm.
(b) 0.4 to 1.5 mm.
(c) 1.5 to 2.5 mm.
(d) 2 to 4 mm.

32. Reactance voltage in a dc machine

(a) increases in direct proportion with the increase in number of commutator segments.
(b) increases linearly with the increase in, speed of the machine.
(c) increases with the increase in brush thickness.
(d) all of the above.

33. 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. [A.M.I.E Sec B. 2005]

34. When commutation is obtained by increasing the current density at the leading edge and reducing the same at the trailing edge, it is called the ……….. commutation.

(a) linear
(b) under
(c) over
(d) none of the above

35. The brushes of a dc motor suitable for rotation in any direction should be placed as

(a) radial.
(b) reaction.
(c) trailing.
(d) any of the these. [A.M.I.E. Elec. Design Summer 1993]

36. 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. [A.M.I.E. Sec B. Elec. Design 1993]

37. Which of the following brushes are fragile and cause excessive wear of commutator ?

(a) Hard carbon.
(b) Natural graphite.
(c) Metal graphite.
(d) None of these.

38. ……….. brushes are used for higher values of current density.

(a) Metal graphite
(b) Hard carbon
(c) Natural graphite
(d) Carbon

39. The brush friction loss depends upon

(a) peripheral speed of commutator.
(b) brush pressure.
(c) coefficient of friction between brush and the commutator.
(d) all of the above.

40. The area of each individual brush should be taken so that it does not carry current more than

(a) 20 A
(b) 40 A
(c) 70 A
(d) 100 A