Electrical Machines
Commutation in a DC machine may be explained as a process

  in which magnetic field is set up.
 how alternate flux affects the main field flux.
 by virtue of which emf is induced in the armature conductors.
  by which electric current in a coil is reversed.

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Electrical Machines
An alternator is feeding an infinite busbar. Its prime mover is suddenly shutdown. The alternator will

 start working as induction motor.
 continue to work as alternator but the direction of rotation will reverse.
 come to stand still.
 continue to work as synchronous motor with same direction of rotation.

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