Direct Torque Control of Permanent Magnet Synchronous Machines with Applications to Motor Drives and Wind Energy Conversion Systems

Direct torque control (DTC) is a promising control scheme for high-performance AC motor control systems. Contrary to field-oriented control (FOC), DTC controls the electromagnetic torque and the stator flux linkage directly so that a faster dynamic response can be achieved. The permanent magnet synchronous machine (PMSM) has attracted extensive attention due to its distinctive advantages, such as high power density, efficiency, and reliability. The feasible applications of the PMSM include, but are not limited to, transportation, renewable energy generation, and industrial motion control. Usually, a high sampling frequency is desired for the digitally implemented DTC on the PMSM motor control system, meaning the increased hardware costs cannot be ignored. Moreover, the use of hysteresis comparators in conventional DTC leads to irregular and unpredictable torque and stator flux ripples. Therefore, the main objective of the research for this dissertation is to develop novel digital DTC strategies for PMSMs to reduce torque, flux, and current ripples with a relatively low hardware demand while retaining the essential advantages of the conventional DTC as much as possible. In this work, the root causes of ripple production in the PMSM systems controlled by digital DTC schemes are explicitly analyzed and demonstrated. Based on the analysis, three improved DTC schemes are developed to effectively reduce the torque, flux and current ripple levels afflicting the conventional DTC. The improvement strategies in the proposed DTC schemes are generally classified into two categories. First, the space vector modulation (SVM) is integrated into DTC using fewer machine parameters and no proportional-integral (PI) regulators. Second, the hysteresis controller in conventional DTC is modified to develop new switching-table-based DTC following the idea of the conventional DTC. All of the proposed DTC schemes can effectively solve the problems associated with the conventional DTC, i.e., the large ripple levels and high-demand hardware, and provide viable and cost-effective solutions for various industrial PMSM-based applications. The inherent properties of the conventional DTC, such as fast dynamic response, no PI and current regulators, and no coordinate transformations are well preserved