PURPOSE: To make the magnetic flux of a motor suitable for the operating condition of the motor so that a controlling ability which is tolerant to load unbalances can be obtained by comparing the output of a magnetic flux detector with the output of the second magnetic flux which detects the magnetic flux of the motor from the output voltage and output frequency of an inverter and switching the magnetic flux of the motor in accordance with the status of the motor.

CONSTITUTION: The first magnetic flux detector 4 can well detect the magnetic flux ' of each induction motor 1 from an extremely low speed to the maximum speed when the loads of the motors 1 are nearly equal to each other. When a load unbalance exists, however, the detector 4 is not able to detect a decline in magnetic flux though the magnetic fluxes of the motors decline due to the concentration of output currents I1 to a motor receiving a large load and an insufficient current for magnetic flux at each motor. On the other hand, the second magnetic flux detector 5 detects the magnetic flux ' of each motor from the output voltage V1 and output frequency f0 of an inverter. Therefore, the detector 5 can recognize the magnetic fluxes more accurately than the detector 4 when a load imbalance exists, since the detector 5 observes the output voltage V1.