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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 (Rs)-based stator temperature (Ts) monitoring scheme. The motor cooling ability monitoring algorithm can be implemented using the Rs estimates of an existing DC-injection-based Rs estimation scheme without parameter information or additional hardware. In addition, the influence of the fuse, circuit breaker, and cable resistances on the Rs estimate is investigated, and methods for compensation are suggested for improving the accuracy of the Rs estimate and, therefore, the reliability of Ts 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 Ts estimate for complete motor thermal protection.
Date of Publication: Sept.-Oct. 2003