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This paper proposes a remote and sensorless stator winding resistance estimation method for thermal protection of soft-starter-connected induction motors. By changing the gate drive signals of the thyristors in the soft starter, a small adjustable dc bias can be intermittently injected to the motor for the estimation of the stator winding resistance. Based on online and continuous monitoring of the stator winding resistance, the stator winding temperature can be monitored using only motor voltage and current. In addition, the torque pulsation caused by the injected dc bias is analyzed. It can also be controlled under an acceptable level by adjusting the level of the injected dc signal. The influence of cable resistance is also studied, and a compensation method is proposed. The proposed method has been verified by experimental results from two induction motors. The proposed stator resistance estimation method can provide remote, sensorless, and accurate thermal protection for soft-starter-connected induction motors.
Date of Publication: Oct. 2008