Electrical Machines
Flashing of DC generator means

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

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Electrical Machines
A dc shunt generator, when driven without any excitation, showed an open circuit voltage of 12 V. When the field winding was excited, the voltage dropped to zero. It happened because

 field resistance was higher than critical resistance
 there was a break in the armature circuit
 there was no residual magnetism in the field circuit
 field winding was wrongly connected

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