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
 field winding was wrongly connected
 there was no residual magnetism in the field circuit

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Electrical Machines
Cogging and crawling are phenomena associated with

 cage induction machines and they are essentially the same.
 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.
 any of the above.

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