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Minimization of steady-state torque tracking error in direct-torque controlled PM brushless AC drives

In conventional direct-torque controlled permanent-magnet brushless AC motor drives, the stator flux-linkage and electromagnetic torque are regulated using hysteresis controllers. Unlike the analog counterpart, a digital hysteresis controller operates at fixed sampling period and no control action can be taken between any two sampling instants. Consequently, the controlled variable in a discrete time hysteresis controller may exceed the predefined hysteresis band limits. When the conventional direct torque control scheme is employed, the stator flux-linkage and torque feedbacks are found to exceed the hysteresis band limits. Although the feedback stator flux-linkage goes beyond the band limits, its average tracks the commanded value relatively well. However, due to a significant difference between the rising and falling rates of the feedback electromagnetic torque, a steady-state torque tracking error is resulted. The steady-state torque tracking error can be minimized by employing an improved torque regulator with asymmetric hysteresis band limits.