Control of wound rotor induction motor using thyristors in the secondary circuits

Three-phase induction motor operation in the presence of a thyristor-controlled resistive network in each rotor circuit was investigated. 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 was used based on the single-phase equivalent of a balanced, sinusoidal three-phase system. 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). A combination of the circuit input impedance, calculated by the voltage/current ratio, and the input resistance, calculated from the power dissipation, permitted a calculation of a circuit input series inductive reactance caused by current switching delay