Electrical Machines
The synchronous motor can be started by

  providing damper winding in its rotor circuit.
  coupling it with a small AC induction motor called the pony motor.
  any of the above methods.
  coupling it with a DC compound motor.

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

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