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Regulation of a Transformer

9.11 Regulation of a Transformer

  • Due to the resistances of the windings and leakage reactances voltage drop takes place in a transformer. Accordingly the output voltage under load conditions is different from the output voltage under no-load conditions. Voltage regulation is defined as:

“The change in secondary voltage when rated load at a specified power is removed.

It is specified as a percentage of the rated secondary voltage.

Thus if 0V2 = secondary terminal voltage at no-load.

= E2 = KE1 = KV1 because at no-load the impedance drop is negligible.

+ sign for lagging power factor

- sign for leading power factor

The less this value, the better the transformer, because a good transformer should keep its secondary terminal voltage as constant as possible under all conditions of load. 

In the above definitions of regulation, primary voltage was supposed to be kept constant and changes in secondary terminal were considered. But as the transformer is loaded, the secondary terminal voltage falls (for lagging power factor) ; hence to keep the circuit voltage constant, the primary voltage must be increased. The rise in primary voltage required to maintain rated output voltage from no-load to full-load at a given power factor expressed as percentage of rated primary voltage gives the regulation of the transformer.

If the primary voltage has to be raised from its rated value V1 to V1’ then