Electrical Machines
The armature mmf in a dc machine is 1. stationary with respect to field poles 2. stationary with respect to armature 3. rotating with respect to field poles 4. rotating with respect to armature From these, the correct answer is

 2, 4
 2, 3
 1, 4
 1, 3

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Electrical Machines
Cogging and crawling are phenomena associated with

 any of the above.
 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.

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Electrical Machines
In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because

 commutator action is not possible otherwise
 it reduces field structure iron losses
 it gives more uniform air-gap flux distribution
 dc machines are comparatively low rating machines.

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