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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Electrical Machines
In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because

 commutator action is not possible otherwise
 it gives more uniform air-gap flux distribution
 dc machines are comparatively low rating machines.
 it reduces field structure iron losses

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