Improving thermal recovery time for soft-starter-connected AC motors with intermittent periodic duty cycles

The thermal characteristics of de-energized AC motors are largely different from those of operating AC motors. The conventional thermal protection techniques are not capable of providing reliable thermal protection for AC motors operated with intermittent periodic duty cycles. This paper proposes a non-intrusive stator winding resistance and temperature estimation technique for soft-starter-connected AC motors with intermittent periodic duty cycles. By changing the operation of the solid-state power switches in the soft-starter, a DC signal can be intermittently injected into the AC motor when de-energized, with no output torque induced. The stator winding resistance and temperature can therefore be monitored based on the DC model of ac motors, when the motors are de-energized. Based on the monitoring of the stator winding temperature, the required thermal recovery time can be accurately estimated, which can greatly reduce the required time of each operating cycle and improve the efficiency of the industrial process. The proposed technique is validated through experimental testing on a 7.5-hp induction motor. The error in the stator temperature estimation can be within 5degC. Using the proposed stator temperature estimation technique, for ac motors operated under intermittent periodic duty cycles, the thermal recovery time can be reduced by 45% with the overall efficiency of the motor system increased by 70%, compared to the stator temperature estimation techniques used in conventional thermal relays. The importance of the proposed technique lies in its non-intrusive nature: only voltage and current measurements are needed; the motor´s operating condition is not interrupted.