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 power, stator current, power factor, and efficiency were obtained as functions of speed over the entire range of thyristor firing-angles. An equivalent circuit is used based on the single-phase equivalent of a balanced, sinusoidal three-phase system. Such an equivalence is not directly justifiable because the per-phase performance of the nonlinear, thyristor controlled star-connected load is not the same as that of the corresponding single-phase circuit. Proof of the appropriateness of the chosen equivalent circuit approach is in the high level of agreement between the measured and calculated performance characteristics. The effective value of the external rotor “resistance” was calculated using the principle of power invariance. This resistance value proved to be a function of the motor speed and thyristor firing-angle as well as of the resistor values, for the configurations investigated. Analysis of the external network was extended to calculate the total input “impedance” (i.e., voltage/current ratio)