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 commissioned after a major repair of the field system, failed to build Up voltage. It was because

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

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