Electrical Machines
A dc shunt generator, when driven without any excitation, showed an open circuit voltage of 12 V. When the field winding was excited, the voltage dropped to zero. It happened because

 field resistance was higher than critical resistance
 there was a break in the armature circuit
 field winding was wrongly connected
 there was no residual magnetism in the field circuit

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Electrical Machines
The synchronous motor can be started by

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

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