Electrical Machines
In DC generators, the cause of rapid brush wear may be

 imperfect contact.
 rough commutator surface.
 any of above.
 severe sparking.

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Electrical Machines
In a DC machine

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

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