Direct Torque Control of an IPM-Synchronous Motor Drive at Very Low Speed Using a Sliding-Mode Stator Flux Observer

Direct torque control of interior permanent-magnet synchronous motors is known to deliver fast torque and flux dynamic responses. However, the poor flux estimation at very low speeds has been its largest drawback. A multitude of flux observers have been proposed for flux estimation, but most of them fail to fare in the low-speed region. This paper proposes a sliding-mode stator flux observer for improved flux estimation at very low speeds. Unlike conventional flux observers, this observer does not require any speed adaptation mechanism and is immune to speed estimation error. A novel stator resistance estimator is incorporated into the sensorless drive to compensate the effects of stator resistance variation. Global asymptotic stability of both the flux observer and stator resistance estimator is guaranteed by the Lyapunov stability analysis. DC-offset effects are mitigated by introducing a small integral component in the observer gains. Simulation and experimental results at very low speeds, including 0 and 2 r/min, without signal injection confirm the effectiveness of the proposed method.