# ONLINE Multiple Choice Questions – Electric Machine Design PART 6

76. The force exerted by a lifting magnet varies with distance (D) as
(A) D
(B) D2
(C) 1/D
(D) 1/D2

77. The force-stroke curve for a lifting magnet will be of the shape shown in

(A) figure A
(B) figure B
(C) figure C
(D) figure D.

78. When B is the flux density ; A is the area of
the working gap and μ is the permeability, the force exerted by the
attractive type magnets is given by
(A) μA / 2B
(B μ B / A2
(C) A2B1/ 2μ
(D) B A2/ 2μ

79. Transformer-core laminations are made of
(A) cast iron
(B) wrought iron
(C) silicon steel
(D) cast steel.

80. When commutation is obtained by increasing the
current density at the leading edge and decreasing the same at trailing
edge, it is known as
(A) under commutation
(B) over commutation
(C) straight commutation
(D) sinusoidal commutation.

81. Skewing of rotor slots helps in
(A) improving beat transfer
(B) reducing noise
(C) suppressing undesirable harmonics
(D) all of the above.

82. The harmonics in rotating machines are generated due to
(A) non-sinusoidal field form
(B) slotting of the stator core
(C) both (A) and (B) above
(D) none of the above.

83. The effect of harmonics in rotating machines can be minimized by
(A) use of longer air gap
(B) skewing the poles
(C) use of distributed winding
(D) all of the above.

84. Which of the following measure will not help in reducing the effect of harmonics ?
(A) connecting 3 phase winding in star or delta without neutral
(B) use of short pitch coils or fractional slot windings
(C) chamfering the pole shoes of salient pole machines
(D) reducing the number of slots per pole per phase.

85. In dc machines the number of poles is generally decided by
(A) frequency of flux reversals
(B) weight of iron parts
(C) weight of copper
(D) all of the above.

86. In dc machines by increasing the number of poles, all of the following reduce except:
(A) weight of copper
(B) weight of iron parts
(C) frequency of flux reversals
(D) overall size of the machine.

87. Transformer action requires a
(A) constant magnetic flux
(B) increasing magnetic flux
(C) alternating magnetic flux
(D) alternating electric flux.

88. Which component of the no load current of the transformer is opposite in phase to the induced emf ?
(A) magnetizing component
(B) core loss component
(C) both (A) and (B) above
(D) none of the above.

89. In transformers, the cylindrical winding with rectangular conductors is generally used for
(A) low voltage winding
(B) high voltage winding
(C) tertiary voltage winding
(D) any of the above.