Construction of three phase auto transformer: The coils connected in star are placed on electromagnetic cores; each phase of auto-transformer consists of a single continuous winding common to primary and secondary circuit. The limbs (electromagnetic cores) are made of laminations (sheet steel with silicon). The output terminal connections are gang / simultaneously operated to get identical tapings to all phases and are brought out on the insulated plate. The variable voltage can be obtained with tapings to which the output terminals are connected as required.
Operation: Working principle is based on self-induction. When three-phase ac supply is given to star connected three windings, flux is produced and gets linked with each phase winding. The emf is induced in it according to self-induction. As only one winding per phase is available, part of it acts as secondary between variable terminal and neutral. Depending upon the position of variable terminal, we get variable AC voltage at the output.