Electrical Machines
Compensating windings are used in DC generators

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

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Electrical Machines
Interpoles in a DC motor must be

 Series excited and should have the same polarity as that of the behind main pole in the direction of rotation of armature.
 Shunt excited and should have the same polarity as that of the ahead main pole in the direction of rotation of armature.
 Series excited and should have the same polarity as that of the ahead main pole in the direction of rotation of armature.
 Shunt excited and should have the same polarity as that of the behind main pole in the direction of rotation of armature.

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