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. For ac motors operated with intermittent periodic duty cycles, conventional techniques largely overestimate the required thermal recovery time in each operating cycle, which results in the reduction of usage of industrial processes. This paper proposes a nonintrusive stator winding resistance and temperature estimation technique for soft-starter-connected ac motors at standstill. 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 usage 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 5°C. 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 usage 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 nonintrusive nature: Only voltage and current measurements are needed; the motor´s operating condition is not interrupted.