PURPOSE: To facilitate assembling and standardizing iron cores, by magnetically jointing partial layers of neighboring upper yokes to enhance magnetic characteristics.

CONSTITUTION: Partial layers of upper yokes in a five-leg iron core comprising four combinations of two-leg iron cores 11W14 made of laminated iron cores have joint yokes 18 inserted. By getting a part of magnetic flux flowing in S-phase legs 12b and 13a to flow in the joint yoke 18 so that the magnetic flux in the S-phase legs partially flows into the joint yoke 18, loss caused by magnetic fluxes 12S and 13S flowing in the S-phase legs can be reduced when the S-phase voltage VS becomes zero. Hence, punched plates constituting iron cores more simply formed and easily standardized can be obtained with electrical characteristics such as iron loss excitation being superior.