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

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

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Electrical Machines
What is/are the necessary conditions for voltage build up in self excited generator?

All of these
 Field winding should be properly connected to armature winding.
 Field winding resistance should be less than critical resistance.
 Poles should contain residual flux.

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