PURPOSE: To know a correct magnetic flux component and a torque component by increasing a discriminating divisions equally divided from the circumference of magnetic flux vector direction discriminating means by a microprocessor as large as possible to calculate.

CONSTITUTION: Magnetic vector direction discriminating means 8 outputs any of 12 types as magnetic vector position signal i according to the clockwise rotating angle from the d-axis of a magnetic flux vector . Control signal generating means 9 selects the optimum voltage vector according to a magnetic flux increase/decrease signal x from magnetic flux comparing means 6 aud a torque increase/decrease signal τx from torque comparing means 7, and turns ON, OFF switching elements so that a 3-phase PWM inverter 2 generates it.