An autotransformer is a type of transformer that has the primary
winding employed as a secondary winding, or vice versa; This process is
also named a compensator or balancing coil. An autotransformer is a
simple and inexpensive way to correct problems that arise when a
secondary voltage is difficult to obtain or a transformer you have is
either too high or too low.
A transformer that has a part of its winding included in both the input
and output circuit is known as an autotransformer. In a simple
autotransformer connection there is a common portion of the winding
that has no isolation between the input and output circuits, and there
is only one winding.