51. In a dc machine, the actual flux distribution depends upon
- size of air gap
- shape of pole shoes
- clearance between tips of the adjacent pole shoes
- all of the above
52. In a dc machine, the space waveform of the air gap flux distribution affects
- torque only
- voltage only
- both the torque and voltage
- neither the torque nor the voltage
53. In a dc machine, the armature mmf is
- stationary with respect to the field poles but rotating with respect to the armature
- rotating with respect to the field poles as well as armature
- rotating with respect to the field poles
- stationary with respect to the armature
54. Armature reaction in a dc shunt generator, running at full load with the brushes not shifted from the geometrical neutral plane and saturation neglected, is
- absent
- cross-magnetizing
- demagnetizing
- magnetizing
55. In a dc machine, on no load the magnetic neutral axis
- moves from geometrical neutral axis in the direction of rotation
- moves from geometrical neutral axis in the opposite direction of rotation
- coincides with the geometrical neutral axis
- none of the above
56. Due to magnetic saturation, the flux per pole in a dc machine without brush shift
- increases in both the generators and the motors with load
- decreases in both the generators and the motors with load
- increases in generator but decreases in motor with load
- decreases in generator and increases in motor with load
57. If the brushes are given a small amount of forward shift in case of a dc generator, the armature reaction effect will be
- totally demagnetizing
- totally cross-magnetizing
- partly demagnetizing and partly cross-magnetizing
- totally magnetizing
58. Air gap at the pole tips of a dc machine is kept more than that at the centre of the pole mainly to reduce
- reactance voltage
- effect of armature reaction
- losses of armature core
- noise of the machine
59. Commutation in a dc machine may be explained as a process
- in which magnetic field is set up
- by virtue of which emf is induced in the armature conductors
- by which current in the coil is reversed during the period it is short-circuited by the brush
- how armature flux affects the main field flux
60. Current in the coil undergoing commutation is not able to get reversed completely by the end of commutation period. It is due to
- reactance voltage induced in the coil
- coil resistance
- armature emf
- none of the above
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