Electrical Machines
The residual magnetism of a self-excited dc generator is lost. To build up its emf again

 field winding must be excited by low voltage dc supply
 the field winding must be replaced
 the armature connection must be reversed
 the field winding connection must be reversed

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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.
  coupling it with a small AC induction motor called the pony motor.
  providing damper winding in its rotor circuit.

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Electrical Machines
A dc shunt generator, when commissioned after a major repair of the field system, failed to build Up voltage. It was because

 field resistance was higher than critical resistance
 there was a break in the armature circuit
 field winding was wrongly connected
 field system lost residual magnetism

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