Electrical Machines
In 3 - Φ induction motor some times copper bars are placed deep in the rotor to

  reduce copper loss.
  improve starting torque.
  improve efficiency.
  improve power factor.

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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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