Electrical Machines
Satisfactory commutation of DC machine requires

 smooth, concentric commutator properly undercut.
All of these
 brushes should be of proper grade and size.
 brushes should smoothly run in the holders.

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Electrical Machines
Cogging and crawling are phenomena associated with

 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.
 cage induction machines and they are essentially the same.

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