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

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Electrical Machines
The introduction of interpoles in between the main poles improves the performance of a dc machine, because

 the interpoles produce additional flux to augment the developed torque
 a counter-emf is induced in the coil undergoing commutation
 the inequality of air-gap flux on the top and bottom halves of the armature is removed
 the flux waveform is improved with reduction in harmonics

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