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
Compensating windings are used in DC generators

None of these
  to provide path for the circulation of cooling air.
  mainly to reduce the eddy currents by providing local short circuit.
  to neutralize the cross magnetizing effect of the armature reaction.

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