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

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

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Electrical Machines
If the applied voltage of a certain transformer is increased by 50% and the frequency is reduced by 50%, the maximum flux density will (assuming that the magnetic circuit remains unsaturated)

 remain the same as the original
 changes to 1.5 times the original value
 changes to 0.5 times the original value
 changes to three times the original value

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