Torque ripple reduction methods for vector controlled induction motor with current preview control

The authors describe a method for direct high-speed control of the secondary magnetic flux and torque of an induction motor supplied with power via a transistor inverter. The control system is totally digitized using digital signal processing. Since operation time leads to control delay in such a system, the random-frequency torque ripples involved are more significant than with conventional PWM (pulse-width modulation) techniques. This difficulty is relieved considerably by using current preview control. Two methods for further improvement are proposed. The usefulness of these techniques is demonstrated by showing the results of a simulation, and results of measurements on a 7.5 kW induction motor are presented. In particular, when the motor velocity is zero (rotor locked), the motor produces torque ripples with twice the inverter frequency, which are believed to be due to imbalance of the motor magnetic circuit. These torque ripples are generated in nearly equal quantities when conventional PWM is used and when a low-frequency sine wave power supply is used