61. Transformer core lamination are made of
a. Cast iron
b. wrought iron
c. silicon steel
d. cast steel
62. 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
63. Skewing of rotors slots helps in
a. improving beat transfer
b. reducing noise
c. suppressing undesirable harmonics
d. all of the above
64. 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
65. The effect of harmonics in rotating machines can be minimised by
a. use of longer air gap
b. skewing the poles
c. use of distributed windings
d. all of the above
66. 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 solient pole machines
d. reducing the number of slot per poles per phase
67. In dc machine the number of poles is generally decided by
a. frequency of flux reversal
b. weight of iron parts
c. weight of copper
d. all of the above
68. 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
69. Transformer action requires a
a. constant magnetic flux
b. increasing magnetic flux
c. alternating magnetic flux
d. alternating electric flux
70. Which component of the no load current of the transformer is opposite inphase to the induced emf?
a. magnetising component
b. core loss component
c. both A and B of the above
d. none of the above
