A DC Signal Injection-Based Thermal Protection Scheme for Soft-Starter-Connected Induction Motors

This paper proposes a remote and sensorless thermal protection scheme for soft-starter-connected induction motors. A dc signal injection-based method is used to estimate the stator winding resistance and, thus, the stator winding temperature. Based on continuous monitoring of the stator winding temperature, a thermal condition monitoring and protection scheme is proposed using only the motor´s input voltages, currents, motor nameplate information, and the ambient temperature. An adaptive Kalman filter is designed to reduce the stator winding temperature estimation error. In addition, the influence of the cable resistance was investigated, and a compensation method is suggested to improve the accuracy of the estimated temperature. The requirements of the data acquisition system, such as sampling frequency and analog-to-digital conversion resolution, and their affects on the accuracy of the estimated temperature are also discussed in detail. The proposed thermal monitoring scheme has been validated from the experimental results of a 7.5-hp TEFC induction motor under various load conditions. The importance of this new thermal monitoring scheme lies in its remote and sensorless nature.