CONTROL OF WOUND ROTOR INDUCTION-MOTOR USING THYRISTORS IN THE SECONDARY CIRCUITS

An investigation was made of three-phase induction motor operation in the presence of a thyristor-controlled resistive network in each rotor circuit. The experimental realization of the drive posed some serious problems, notably with regard to the successful firing of thyristors over a wide speed range. Motor performance characteristics of output p ower, stator current, power factor, and efficiency were obtained as fu nctions of speed over the entire range of thyristor firing-angles. An equivalent circuit is used based on the single-phase equivalent of a b alanced, sinusoidal three-phase system. Such an equivalence is not dir ectly justifiable because the per-phase performance of the nonlinear, thyristor controlled star-connected load is not the same as that of th e corresponding single-phase circuit. Proof of the appropriateness of the chosen equivalent circuit approach is in the high level of agreeme nt between the measured and calculated performance characteristics. Th e effective value of the eternal rotor ''resistance'' was calculated u sing the principle of power invariance. This resistance value proved t o be a function of the motor speed and thyristor firing-angle as well as of the resistor values, for the configurations investigated. Analys is of the external network was extended to calculate the total input ' 'impedance'' (i.e., voltage/current ratio). A combination of the circu it input impedance, calculated by the voltage/current ratio, and the i nput resistance, calculated from the power dissipation, permitted a ca lculation of a circuit input series ''inductive reactance'' caused by current switching delay. When both the effective series ''resistance'' and ''reactance'' were included in the motor per-phase equivalent cir cuit, the correlation between measured and calculated results was fair ly good.