61. Under commutation results in
- sparking at the middle of the brush
- sparking at the leading edge of the brush
- sparking at the trailing edge of the brush
- no sparking
62. In a dc machine, the sparking between brushes and commutator surface may be due to
- under commutation
- over commutation
- too rapid reversal of current
- any of the above
63. The self induced emf in the coil undergoing commutation is called the
- reactance voltage
- statically induced voltage
- dynamically induced voltage
- none of the above
64. The sparking at the brushes, in a dc machine is due to
- armature reaction
- reactance voltage
- presence of commutator
- high resistance of carbon brushes
65. Sparkless commutation can be achieved by employing
- interpoles
- compensating winding
- high resistance carbon brushes
- any one or more of the above methods
66. Current in the coil undergoThe interpoles in dc machines have a tapering shape in order to
- reduce the overall weight
- reduce the saturation in the interpole
- economic on the material required for interpoles and their windings
- increase the acceleration of commutation
67. The compensating windings are placed in the slots in the pole faces to neutralize the
- cross magnetizing effect
- demagnetizing effect
- both the demagnetizing effect and cross-magnetizing effect
- armature reaction mmf under the pole faces and avoid flash over between positive and negative brushes
68. In large sized dc machines, the compensating windings are provided to
- compensate for the decrease in speed due to increase in load
- neutralize the voltage induced in the coil due to high fluctuation of load
- reduce the eddy current losses by providing local short circuits
- none of the above
69. Compensating winding in a dc machine is placed on
- armature in slots
- yoke in the interpolar gap
- yoke in the pole faces
- partly in armature slots and partly in pole faces
70. Compensating windings are employed in dc motors which are likely to operate
- at a constant speed over wide range of load
- with rapidly changing loads of wide range
- over wide range of speed by field excitation control
- both (2) and (3)
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