Electrical Machines
In a DC machine

 current and emf at the terminals are alternating.
 emf in armature conductors and at the terminals is alternating.
 current and emf in armature are alternating.
 current and emf in armature are unidirectional

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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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