Electrical Machines
In a DC machine

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

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Electrical Machines
Compensating windings are used in DC generators

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

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