Electrical Machines
Speed of DC motor can be controlled by varying

 applied voltage.
 resistance of armature culture.
 its flux oer pole.
 all of these.

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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.
  to neutralize the cross magnetizing effect of the armature reaction.
  mainly to reduce the eddy currents by providing local short circuit.

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