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

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

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Electrical Machines
In a DC generator, the armature reaction results in

 demagnetization of centers of poles.
  magnetization of inter poles.
 magnetization of leading and demagnetization of trailing pole tip.
  demagnetization of leading & magnetization of trailing pole tip.

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