Electrical Machine Design MCQ

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

42. Though the cost of single layer winding is less than that of the 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. [A.M.I.E Sec B. 2005]

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

44. 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. [A.M.LE Sec B. 2005]

45. The terminal voltage of a 20 HP, 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 [A.M.I.E Design of Electrical Systems 2005]

46. 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. [A.M.I.E Design of Electrical Systems 2005]

47. Power transformers have ratings above ……… kVA.

(a) 50
(b) 100
(c) 250
(d) 500

48. 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). [A.M.I.E Sec B. 2006]

49. Yokes with rectangular cross section are used for ………… capacity transformers.

(a) small
(b) medium
(c) large
(d) any of these

50. CRGO has …….. permeability in the direction of grain orientation.

(a) nil
(b) maximum
(c) minimum
(d) none of these

51. CRGO has …………. in comparison to hot rolled silicon steel.

(a) better finish
(b) improved space factor
(c) much better magnetic properties
(d) all of the above

52. Cold rolled grain oriented steel

(a) can be worked at much higher flux densities due to improved magnetic properties.
(b) gives much reduced iron losses.
(c) both (a) and (b).
(d) none of these.

54. For transformer laminations ……….. silicon steel is preferred.

(a) cold rolled
(b) hot rolled
(c) grain oriented
(d) any of these

55. Eddy currents are reduced in high content silicon steel as it

(a) increases resistivity.
(b) reduces resistivity.
(c) short circuits.
(d) none of the above.

56. A transformer having constant flux and constant current density, designed for minimum cost must satisfy the following relation

(a) iron loss = copper loss.
(b) weight of iron = weight of copper.
(c) weight of iron/weight of copper = specific cost of copper/ specific cost of iron.
(d) weight of iron/weight of copper = specific cost of iron/specific cost of copper. [A.M.I.E. Elec. Design 1994]

57. The condition for a transformer to be designed for the minimum volume is

(a) iron loss = copper loss
(b) volume of iron = volume of copper
(c) weight of iron = weight of copper.
(d) The volume of iron is minimum.
(e) The volume of copper is minimum.

58. The use of mitred joints for cores and yokes

(a) minimizes core loss and magnetizing current.
(b) simplifies the core construction.
(c) complicates the core construction.
(d) both (a) and (c).
(e) both (a) and (b).

59. 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. [A.M.I.E. Sec B. Elec. Design 1994]

60. When the height of windows is increased in comparison to the width in a transformer

(a) cost of copper will be reduced.
(b) voltage regulation will decrease.
(c) efficiency will decrease due to increase in copper losses.
(d) both (a) and (b).
(e) (a) and (c).