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
An inverted V curve of synchronous motor shows variation of

 supply voltage and excitation current at constant load.
 supply voltage and field current at constant excitation.
 power factor and supply voltage during hunting.
  power factor and DC excitation at constant load.

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