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

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

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Electrical Machines
In speed control of DC Series motor, speed lower than the rated speed can be obtained by

 Connecting an additional resistance in the armature circuit.
 Connecting an additional resistance in stator circuit.
 Any of the above.
 Connecting an additional resistance in the field circuit.

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