Electrical Machines
The interpoles in dc machines have a tapering shape in order to

 economise on the material required for interpoles and their windings
 increase the acceleration of commutation
 reduce the overall weight
 reduce the saturation in the interpole

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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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