A remote and sensorless thermal protection scheme for small line-connected ac machines

An advanced remote and sensorless thermal protection technique for small line-connected AC machines is proposed in this paper. The proposed technique includes an algorithm for detecting abnormal cooling conditions due to reduced motor cooling ability, in addition to a stator resistance (R_s)-based stator temperature (T_s) monitoring scheme. The motor cooling ability monitoring algorithm can be implemented using the R_s estimates of an existing DC-injection-based R_s estimation scheme without parameter information or additional hardware. In addition, the influence of the fuse, circuit breaker, and cable resistances on the R_s estimate is investigated, and methods for compensation are suggested for improving the accuracy of the R_s estimate and, therefore, the reliability of T_s monitoring. Experimental results under a number of abnormal cooling conditions verify that the reduced cooling ability of the motor can be reliably detected and that it can be used in addition to the T_s estimate for complete motor thermal protection.