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

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

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Electrical Machines
An inverted V curve of synchronous motor shows variation of

  power factor and DC excitation at constant load.
 supply voltage and excitation current at constant load.
 power factor and supply voltage during hunting.
 supply voltage and field current at constant excitation.

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