Analysis of variable-voltage thyristor controlled induction motors

A full analysis is presented of the steady-state performance of a variable-voltage thyristor controlled induction motor system, based on the representation of the rotor as a sinusoidal EMF in series with a resistance-inductance circuit. In the development of this circuit, magnetising current and stator resistance are included and the amplitude and phase of the EMF are determined as functions of motor torque and speed. This enables the full performance of the system to be evaluated for any of three specified conditions: current hold-off angle and speed, thyristor firing angle and speed, torque and speed. Good agreement is shown between computed and measured performance of an industrial drive system. The analysis reveals the influence of motor EMF, which is shown to improve system performance, particularly at small slips