Electrical Machines
Compensating windings are used in DC generators

  to provide path for the circulation of cooling air.
None of these
  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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Electrical Machines
If the applied voltage of a certain transformer is increased by 50% and the frequency is reduced by 50%, the maximum flux density will (assuming that the magnetic circuit remains unsaturated)

 remain the same as the original
 changes to three times the original value
 changes to 0.5 times the original value
 changes to 1.5 times the original value

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