Electrical Machines
Flashing of DC generator means

None of these
  increasing flux density by providing extra ampere-turns in field.
  creation of residual magnetism by a DC source.
 neutralization of residual magnetism.

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Electrical Machines
A dc shunt generator, when commissioned after a major repair of the field system, failed to build Up voltage. It was because

 there was a break in the armature circuit
 field winding was wrongly connected
 field system lost residual magnetism
 field resistance was higher than critical resistance

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