PURPOSE: To sustain the switching voltage pattern in the preceding flux declination area even if the flux declination area of primary interlinkage flux is switched when the primary interlinkage flux has a magnitude between maximum and minimum.

CONSTITUTION: If the magnitude of primary interlinkage flux vector operated at an operating circuit 6 is between preset maximum value max and minimum min at the time of switching the flux declination region thereof, a latch circuit 26 latches the data of switching voltage pattern in the preceding flux declination region by latching the address of preceding flux declination region at the time of gaining access to a switching table 8.