A power three-phase transformer with stepless voltage regulation under load has a common magnetocoupler M with subdivision into nine rods arranged in the same plane. The three main rods 2, 5 and 8 are used for winding the primary phase windings W_i . The six additive rods 1, 3, 4, 6, 7 and 9 house the DC magnetizing windings W_d. Secondary phase windings W₂ are located on nine rods: three on the main rods 2, 5, 8 and two on additive rods 1, 3, 4, 6, 7 and 9. The compensating winding WK is wound on all the additive rods. Deep smooth regulation of the secondary voltage of the transformer is carried out by changing the value of the DC current in the six windings of the sub-magnetizing winding W_d.