Electrical Machines
If residual magnetism of a shunt generator is destroyed accidentally it may be restored by connecting its shunt field

 in reverse.
 to earth.
 to an AC source.
  to a DC source.

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

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