Direct Torque Control of Dual Three Phase Permanent Magnet Synchronous Machine Drives with Particular Reference to Reduction of Current Harmonics and Torque Ripples

This thesis is focused on the direct torque control (DTC) for dual three phase permanent magnet synchronous machines (DTP-PMSMs), with particular reference to the reduction of current harmonics and torque ripples in switching-table-based DTC (ST-DTC) and space-vector-modulation-based DTC (SVM-DTC). In order to reduce the current harmonics in ST-DTC for DTP-PMSMs, the back electromotive force (back-EMF) distortion, which is one of the major causes of current harmonics, is considered and investigated. The virtual voltage-vector (VV) algorithm is a widely used method in DTC to reduce current harmonics for DTP-PMSM. However, it is found that when the back EMF distortion is considered, setting the amplitudes of current harmonics to zero is not the optimal solution for the current harmonic reduction in the VV algorithm. Consequently, two novel current harmonic compensation VV methods considering back-EMF distortion are proposed. The first one utilizes two groups of space vectors to synthesize VVs so that only the one selective current harmonic (e.g. either the 5th or 7th) can be eliminated. The second one utilizes three groups of space vectors to synthesize VVs so that two current harmonics (e.g. both the 5th and 7th) can be suppressed. Furthermore, a current harmonic reduction method in SVM-DTC is also developed. Space vector modulation strategy in DTP-PMSM drives and current controllers in the harmonic-related subspaces are investigated and introduced to reduce the current harmonics. Torque ripple reduction is also a challenge in ST-DTC. The conventional DTC can achieve torque ripple reduction in the high-speed region but has a limited capability for torque ripple reduction in the low-speed region. In this thesis, the effects of each group in all possible six groups of VVs on torque variation are firstly investigated. Then, a multiple VV DTC based on only three groups of VVs is proposed and the corresponding optimal selections of three VV groups and level of torque regulator are presented. The proposed DTC not only can effectively reduce the torque ripples in the full speed region, but also is simple and easy to implement. Finally, DTC for DTP-PMSM in postfault operation (one phase-open) is investigated. Based on the investigation of effects of each VV on flux and torque variations in one phase open condition, an improved VV selection strategy is proposed for fault-tolerant DTC with one phase open. Compared to the DTC extension from the conventional normal operation, the proposed DTC based on the improved VV selection strategy can effectively reduce the torque ripples and enhance the torque performance.