What is an autotransformer and when is it used?

An autotransformer has a single winding with a tap, where part of the winding is common to both primary and secondary circuits. It is smaller and cheaper than a two-winding transformer for the same VA rating because it only transforms part of the power electromagnetically (the rest is conducted). Advantages include lower cost, better regulation, and higher efficiency. Disadvantages include no electrical isolation and higher fault current. Used for motor starting, voltage adjustment, and when isolation is not required.