Autotransformer is a special variation of transformer in which primary and secondary windings are connected and therefore there is no galvanic separation between circuits. Autotransformers are used to connect networks with different voltage levels. They are also used in starting systems and in systems whereever a lack of electrical separation between primary and secondary circuits is allowed. The undoubted advantage of the autotransformer is that compared to the transformer of the same outgoing power they are smaller, lighter and cheaper.