Torque-Performance Improvement for Direct Torque-Controlled PMSM Drives Based on Duty-Ratio Regulation

This article proposes a new duty-ratio regulation-based strategy to improve the torque performance for the direct torque control (DTC) of three-phase permanent magnet synchronous motors (PMSMs). The conventional DTC schemes utilize two hysteresis regulators that are hard to be tuned to satisfy proper torque performance for wide speed ranges because of the contrary change in the positive and the negative torque deviations of the converter's voltage vectors with the rotational speed variations. In contrast, the proposed method uses a duty-ratio regulator that considers the operating speed impact on the torque deviation of the active voltage vectors and avoids triggering those of them that produce high torque deviations, therefore reducing the torque ripple of the DTC system. Moreover, it proposes a virtual reference generator to mitigate the steady-state torque error. The proposed method provides enhanced torque control performance, meanwhile maintaining the main advantages of the conventional DTC techniques, including simple structure, fast transient response, and good robustness. The feasibility and effectiveness of the proposed strategy are verified through a detailed comparative assessment with the conventional DTC scheme and two existing duty regulation-based methods using experimental results obtained from a 0.75-kW PMSM drive system.