Electrical Machines
Dummy coils are used for

 reducing armature reaction.
All of these
 increasing efficiency.
 mechanical balancing.

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

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

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