Abstract: A vector control method is provided for controlling a rotor speed of an induction motor by an inverter which provides an AC current of varying frequency to the induction motor, and a vector controller which provides control voltage Vu, Vv, Vw, in response to an excitation current command i.sub.1d*. The excitation current command is indicative of an excitation current being fed to the induction motor, a desired rotation speed command .omega..sub.r*, and a primary frequency .omega., for controlling the inverter to vary the rotor speed. The method includes the steps of detecting a primary current being fed to the induction motor, analyzing the detected primary current to obtain an excitation current i.sub.1d and a detected torque current i.sub.1q, multiplying a torque value represented by the detected torque current i.sub.1q by a predetermined motor constant K.sub.m to obtain a slip frequency .omega..sub.s, and adding the obtained slip frequency .omega..sub.s to the rotation speed command .omega..sub.

Abstract: An improved induction motor control system utilizes an inverter supplying a variable frequency AC voltage to drive the induction motor, and volt/frequency (V/F) control section controlling the inverter to produce a suitable voltage and frequency in a predetermined relationship therebetween. A power factor detecting section is included to detect a power factor phase angle between the motor current and the inverter output voltage as indicative of an amount of load imposed on the motor. The detected power factor phase angle is compared with a predetermined phase angle which is a frequency dependent angle indicative of a standard load at a given frequency, in order to provide a control output in response to which the V/F control section alters the inverter output voltage out of the predetermined relationship with the frequency for improving motor efficiency. Thus, the detected load (i.e.