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

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Electrical Machines
In a synchronous motor when the rotor speed exceeds the synchronous speed during hunting

  the damper bars develop synchronous motor torque.
  the damper bars develop induction generator torque.
  negative sequence currents are generated.
  the damper bars develop induction motor torque.

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