What are autotransformers?

Autotransformers are transformers that has only one winding per phase, part of which is common to both the primary and secondary circuits, similar to Buck-Boost transformers in that they are also an economical means of adjusting an output voltage. Autotransformers are designed to adjust the supply voltage when isolation from the line is not necessary and where local electrical codes permit. Units are designed in either a step-up or step-down application and to meet motor inrush currents.