Electrical Machines
Resistance commutation is used generally in case of

None of these
  fractional kW DC machines.
 DC machines above 10 kW.
 DC machines bellow 5 kW.

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Electrical Machines
The synchronous motor can be started by

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

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