AUTOTRAFO (Automatic Transformer).

Autotrafo shape is similar to regular transformer, but is rarely found on the transformer primary and secondary windings are separate, single coil windings only wound on an iron core, but there are some terminal settings input voltage and input voltage.
Transformer of this type consists of only one electric coil which continues, with the middle lead. In this transformer, partial primary windings is also a secondary winding. Phase current in the secondary windings are always opposed to the primary current, so for the same power tariff secondary windings can be made with a wire thinner than usual transformer. The advantage of autotransformator is a small physical size and lower loss than other types of two loops. But this type of transformer can not provide electrical isolation between the primary winding with the secondary windings.