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

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

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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

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

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Electrical Machines
A belt-driven cumulatively-compound dc generator is delivering power to the dc mains. If the Belt snaps then the machine will run as a

 differentially compounded motor in the same direction
 cumulative compounded motor in the same direction
 cumulative compounded motor in opposite direction
 differentially compounded motor in the opposite direction

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