Electrical Machines
Commutators in DC machines have a role of which converts

 none of these.
 AC to DC.
 Both AC to DC and DC to AC.
 high voltage DC to low voltage DC.

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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

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

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