Electrical Machines
In DC generators brushes are used for

 collecting of current without any sparkings
 reduce eddy current loss
 convert ac armature current in to dc
 collecting of voltage

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

  by which electric current in a coil is reversed.
  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.

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