Electrical Machines
DC generator will not build up voltage

 if field resistance is more than critical resistance.
  if it has lost residual magnetism.
 due to wrong direction of rotation.
 any one of above.

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

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

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