Electrical Machines
Sparkless commutation can be achieved by employing

  inter poles.
 compensating winding.
  high resistance carbon brushes.
  any one or more of the above methods.

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

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

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