Electrical Machines
Short circuiting is caused in the armature winding due to

 failure of insulation between two commutator bars.
  grounding of two or more turns of the same coil.
  any of above.
  failure of insulation between two turns of coil.

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

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

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