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The availability of limited voltage vectors in a conventional direct torque control drive fed from a two-level inverter causes torque and current ripples. In this study, a new torque ripple reduction technique with a modified look up table incorporating a larger number of synthesised non-zero active voltage vectors is proposed to overcome the limitations of the conventional and duty ratio control switching strategies. The voltage vector selection criteria in the proposed technique are based to prevent the stator flux demagnetisation during the low-speed operation with significant reduction in torque and current ripple. The proposed switching strategy is investigated through simulation and experimentally validated on a test drive.

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In this paper, a simple and accurate method is proposed for online estimation of the rotor permanent magnet (PM) flux linkage for surface mounted PM synchronous machines. By advantageously utilizing an injected zero-voltage vector, the effects of inductance in estimating the PM flux linkage is eliminated. The inverter voltage error and phase resistive voltage drop effects are minimized by repeating the test at different speeds. Therefore, as a result, only the voltage commands are needed for PM flux linkage estimation, resulting in great simplicity. Filtering the current ripple caused by the zero-voltage vector injection is not necessary. The proposed method can be used with sensored or sensorless fieldoriented control strategies. The experimental results have shown a good accuracy of the proposed new method. Index Terms-Permanent magnet (PM) flux linkage estimation, permanent magnet synchronous machines (PMSM), zero-voltage injection.