Electrical Machines
Dummy coils are used for

 mechanical balancing.
 increasing efficiency.
All of these
 reducing armature reaction.

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Electrical Machines
In case of 3 - Φ IM, plugging means

  starting the motor on load which is more than the rated load.
  pulling the motor directly on line without a rotor.
  interchanging two supply phases for quick stopping.
  locking of rotor due to harmonics.

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