Electrical Machines
The oscillation in a synchronous motor can be damped out by

  providing damper bars in the rotor pole faces.
  maintaining constant excitation.
  running the motor on leading power factors.
  oscillation cannot be damped.

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Electrical Machines
The synchronous reactance( Xs) is the

 reactance due to leakage flux
 reactance either due to armature reaction or leakage flux
 reactance due to armature reaction of the machine
 combined reactance due to armature reaction and leakage flux

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