A Remote and Sensorless Thermal Protection Scheme for Soft-Starter-Connected Induction Motors

A non-intrusive stator winding resistance estimation technique is proposed for soft-starter-connected induction motors. By changing the gate drive signal of the thyristors in the soft-starter, a small dc bias can be intermittently injected to the motor. Based on continuous monitoring of the stator winding resistance, a new thermal monitoring scheme is proposed using only voltage, current, some motor nameplate information and the ambient temperature. An adaptive Kalman filter is designed to reduce the temperature estimation error via the dc injection. In addition, the influence of the cable resistance was investigated, and a compensation method is suggested. The requirements of the data acquisition system, such as sampling frequency and Analog to Digital conversion resolution, are also discussed. The proposed thermal monitoring scheme has been validated by experimental results under various load conditions. The importance of this new thermal monitoring scheme lies in its remote and sensorless nature.