Online stator resistance estimation for thermal monitoring of converter fed induction motors using pseudorandom modified PWM

This paper explains how the electrical impulse response of a VSI fed induction motor can be extracted online from normal working data if the normal PWM signals are slightly modified to introduce a small pseudorandom binary sequence (PRBS), from time to time when measurements are required. It also explains how generic templates can be fitted to the measured impulse responses to obtain adaptive estimates of certain electrical motor parameters. A case study presents simulated and measured results of online rotor speed and stator resistance estimations based on the impulse response method. A novel technique of direct parameter extraction based on certain key points on the impulse response curve is developed to rapidly, yet accurately predict online rotor speed and stator resistance, (under various loads from standstill to full speed), which is then used for sensorless thermal condition monitoring and protection.