Electrical Machines
In a DC machine

 current and emf in armature are unidirectional
 emf in armature conductors and at the terminals is alternating.
 current and emf in armature are alternating.
 current and emf at the terminals are alternating.

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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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