A Nonintrusive Thermal Monitoring Method for Induction Motors Fed by Closed-Loop Inverter Drives

Closed-loop induction motor drives have wide applications in electric traction system and industrial processes. Accurate thermal monitoring not only protects the induction machines from overheating, but also boosts the usage and performance of the overall drive system. State-of-the-art thermal monitoring schemes for induction machines utilize thermal models, which are inaccurate for drive-fed machines and are not adaptive to varying cooling conditions. This paper proposes a nonintrusive thermal monitoring scheme for induction motors fed by closed-loop inverter drives. By applying offsets to the current controllers in the motor drive, dc currents are injected into the stator windings of the machine. The accurate estimation of the injected dc voltage is achieved by carefully analyzing and compensating for inverter nonidealities. The stator winding temperature can thus be monitored based on the estimated dc stator resistance. The method is nonintrusive because it eliminates the thermocouples, requires no hardware change to the existing motor drive, and has minimal impacts on the normal operation of the motor. The proposed method is practically implemented in a programmable inverter drive and is validated both by simulation and real-time experimental results.