Transformer with Load Tap Changing

a power transformer that permits changes in the transformation ratio—and, consequently, in the amplitude of the secondary voltage—without interrupting the load circuit. Such transformers are used mainly in cases of necessity of redistributing power, both real and reactive, among various consumers (the power is redistributed by changing the voltage of the supply current).

The most common transformers of this type change the secondary voltage in steps either by switching sections of the windings (that is, by changing the number of turns in the windings) or by switching into the load circuit an auxiliary, or booster, transformer where secondary voltage is also adjustable in steps. The process of switching sections is usually completely automated. A smooth voltage change is obtained by moving a current pickup contact along a bare part of a winding (as in adjustable laboratory autotransformers) or by shifting the windings and the elements of the core relative to one another.