Electrical Machines
In a DC generator sparking between brushes and commutator surface may be due to

  under commutation.
 over commutation.
  any of the above.
  too rapid reversal of current.

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Electrical Machines
In a DC machine the armature mmf is

  stationary with respect to field poles.
  rotating with respect to field poles as well as armature.
 rotating with respect to field poles.
  stationary with respect to armature.

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