Microprocessor Control of a Three-Phase Inverter in Induction Motor Speed Control System

The speed control of a three-phase induction motor using the Fairchild F8 microprocessor is described in this paper. In this system, the microprocessor computes the actual motor speed from the pulses received from a shaft encoder, compares the actual and desired speeds, and uses the difference to adjust the frequency of the inverter that feeds the motor. The microprocessor simultaneously varies the inverter voltage through pulsewidth modulation so as to maintain V/f constant up to the rated frequency and V constant at rated voltage thereafter. In addition to controlling the speed, the microprocessor monitors the dc-line current and automatically stops the inverter as soon as the current exceeds a preset upper limit. The control system designed is such that it is applicable not only to fractional horsepower (FHP) but also to integral horsepower (HP) motors.