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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Electrical Machines
The coil spam

 none of above.
 can never be equal to pole pitch.
  may or may not be exactly equal to people pitch.
 must be exactly equal to people pitch.

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