Variacs are also known as autotransformers and are a speciality type of the electrical transformer that consists only one winding. The winding acts as both primary and secondary sides of the transformer. The ordinary type of transformers have 2 coils instead and they are not electrically connected. The one winding resolution allows for producing a smaller, lighter and cheaper transformers than ordinary 2 winding ones. They also feature lower leakage reactance, lower losses lower excitation current and increased VA rating for a given size and mass. At the other side, the disadvantage is a lack any electrical insulation between circuits. It can result in full input voltage applied to the output.
Variacs/Autotransformers are commonly used for:
Power transmission and distribution – Variacs ensure that voltage at the beginning of the power distribution line is equivalent to the voltage at the end of the line.
Power conversions – Variacs are able to convert common domestic mains voltage bands that are in use across the world. For example 200 V to 250 V or 100 V to 130V.
Audio systems – Autotransformers are used to adapt speakers to constant- voltage distribution and for matching impendence levels.