Electrical Machines
Voltage across the open circuited field termninals of a synchorous machine under slip test is

  DC of slip frequency.
 DC.
 A modulated supply frequency AC voltage with slip frequency envelop.
 AC of supply frequency.

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Electrical Machines
In large sized DC machines the compensating windings are provided to

None of these
  neutralize voltage induced in coil due to fluctuation of l
 reduce eddy current losses by providing local short circuits.
 compensate for the decrease in speed due to increase of load.

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