a Variable Frequency Control for Three-Phase Induction Motors.

The speed control of a three phase induction motor using a microprocessor controller is discussed. The motor is fed from a three phase inverter, the frequency and voltage of which are controlled by the microprocessor controller. The v/f ratio is kept constant up to the rated frequency, and the voltage is kept constant above the rated frequency. To have satisfactory performance for the motor, the control strategies have to be carefully planned and evaluated. For transient performance of the motor, such as starting and speed changing, the inrush current has to be minimized. In regard to the steady state performance of the motor, the controller should minimize the torque pulsations and heating of the motor and prevent short circuits of the power source. It is also of interest to compare various modulation techniques with regard to the harmonic losses in the motor and the pulsating torques developed. This comparison will lead to an optimized modulation technique which is suitable for the selected speed range. Very little hardware in addition to the basic microprocessor system is required for the implementation of the different control strategies.