Direct Voltage Regulation Method: On the primary winding of the transformer, lead out the tap to connect the tap changer. -Y switching can also be used between the three phases of the primary side to achieve secondary voltage regulation. △-Y switching is a very economical voltage regulation method. Without changing the number of turns on the primary side, the secondary voltage can be reduced to 1/2 of the original. Since each phase winding has 9 taps, plus -Y switching, a total of 2×9 or 18 levels of voltage can be obtained.

Indirect Voltage Regulation Method: The figure shows an indirect voltage regulation method with a series transformer. The main transformer 1 is a three-winding transformer (see multi-system transformer). Winding 1 is a high-voltage winding connected to the power grid; winding 2 is a tapped winding, which is connected in series with the primary winding 4 of the series transformer through a tap changer; winding 3 is connected in series with the secondary winding 5 of the series transformer to supply power to the electric furnace load. By changing the primary voltage of the series transformer through the tap changer, the output voltage of windings 3 and 5 can be changed. Windings 3 and 5 connected to the load generally have only 1 to 2 turns, and are usually made of copper or aluminum plates into 8-shaped coils.