Electrical Machines
Flashing of DC generator means

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

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Electrical Machines
Cogging and crawling are phenomena associated with

 any of the above.
 the former during starting and the latter at a fraction of its rated speed.
 the former at a fraction of its rate speed and the latter during starting.
 cage induction machines and they are essentially the same.

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