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

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Electrical Machines
In speed control of DC Series motor, speed lower than the rated speed can be obtained by

 Any of the above.
 Connecting an additional resistance in stator circuit.
 Connecting an additional resistance in the armature circuit.
 Connecting an additional resistance in the field circuit.

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