Induction motor control system

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., power factor phase angle) can be evaluated, by simple comparison with the standard load level (reference angle), for controlling the inverter to effect the output voltage or motor torque regulation in response to the load imposed. Also, the frequency dependent reference angle can well compensate for a significant phase shifting of the motor current in relation to the inverter output voltage as seen in the low frequency range, enabling a consistent motor control even in the lower frequency range.