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

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

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Electrical Machines
A dc shunt generator, when commissioned after a major repair of the field system, failed to build Up voltage. It was because

 field winding was wrongly connected
 field system lost residual magnetism
 field resistance was higher than critical resistance
 there was a break in the armature circuit

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