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An enhanced active DC-flux injection based approach for thermal monitoring of induction machines with direct torque control schemes

This paper proposes an enhanced approach to estimate the stator winding temperature based on active DC flux injection for induction machines with two representative direct torque control (DTC) schemes, the conventional hysteresis-based DTC and the deadbeat DTC. In this method, the stator resistance is regarded as the indicator of its temperature, and is estimated by applying offsets to the flux controllers in the DTC schemes thus injecting DC currents and voltages into the stator windings for short periods of time. To compensate the low-frequency pulsating torque caused by the DC fluxes and currents that is inherent in conventional DC-signal-injection-based methods, second-order harmonic fluxes that generate the opposite low-frequency pulsating torque are injected into the stator windings at the same time in both the conventional hysteresis-based and deadbeat DTC schemes. Experimental results show that the proposed method can accurately estimate the stator winding temperatures and effectively eliminate the pulsating torque thus minimizing the impacts of DC-signal injection on the normal operation of the induction motors.