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

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Electrical Machines
Which of the following factors improve commutation in a dc machine ? 1. High contact resistance of brushes 2. High inductance of the coils undergoing commutation 3. Shifting of brushes in the direction of rotation for a generator and in the opposite direction For a motor 4. Narrow width for the commutation bars Select the correct answer using the codes given below : Codes :

 1, 2, and 3
 2, 3 and 4
 1, 2 and 4
 1 and 3

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Electrical Machines
For a given torque, reducing diverter resistance of a DC series motor causes

 increase in speed demanding more armature current.
 increase in speed but armature currnet remains the same.
 decrease in speed but armature current remains the same.
 decrease in speed demanding less armature current.

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