What is a tap changer?

A tap changer is a device on the outside of Transformers to allow the changing of the primary windings to except different primary voltages. A common one is 7200 - 14400 volts. Look up at the transformers on the poles to your house and you will be able to see this handle with the voltages stamped on the case and the handle in one of the positions. This allows for the transformer to be used on two different primary voltages. What this cuts down on is storage of single primary voltage transformers.

Tap changers are normally incorporated into the high-voltage, rather than the primary, winding of a transformer.