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.
 how alternate flux affects the main field flux.
  by which electric current in a coil is reversed.

ANSWER DOWNLOAD EXAMIANS APP

Electrical Machines
In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because

 it gives more uniform air-gap flux distribution
 commutator action is not possible otherwise
 dc machines are comparatively low rating machines.
 it reduces field structure iron losses

ANSWER DOWNLOAD EXAMIANS APP