Electrical Machines
Which of the following is/are the advantages of carbon brush over the copper brush?

 They are not hard as copper brush
 They are self lubricating in nature
 In case of any sparking they will be less damaged than copper brushes
All of these

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

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