Electrical Machines
The ventilating ducts in most of DC machine are

 radial as well as longitudinal.
 longitudinal.
 none of these.
 radial.

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

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

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