Electrical Machines
Satisfactory commutation of DC machine requires

All of these
 brushes should smoothly run in the holders.
 smooth, concentric commutator properly undercut.
 brushes should be of proper grade and size.

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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)

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

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