Abstract: A ferroresonant three-phase constant AC voltage transformer comprising three transformer iron cores with one for each corresponding input supply phase, primary windings and secondary windings formed on each of the transformer iron cores, series reactance components or reactors connected in series, with the primary windings, automatic voltage regulating means for controlling secondary output voltages generated at the secondary windings to a predetermined target value, compensating windings formed so as to be inductively coupled to each of the series reactance components or reactors, and means for connecting the compensating windings in series with each other to form a closed loop circuit. The secondary output voltages are theoretically kept in balanced condition even when the loads or the primary input voltages or both are unbalanced.