This paper proposes a monitoring scheme to detect the state of the cooling capability for soft-starter-connected induction motors. Based on the estimated stator-winding temperature via dc-signal injection, the cooling capability of the motor can be monitored online using only voltage and current sensors. The thermal parameters of a simplified thermal model of the induction motor are identified using an extended Kalman filter approach, as an indication of the motor cooling capability. The proposed scheme can warn the users for proactive inspection and maintenance in the case of cooling-condition deterioration. The proposed method has been validated from experimental results under three different cooling conditions and variable-load conditions. The errors of thermal-parameter identification are within 3%. The proposed cooling-condition monitoring scheme, combined with the stator-winding-temperature monitoring scheme, can provide complete thermal protection for soft-starter-connected induction motors.
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