Novel Flux-weakening Control of Permanent Magnet Synchronous Machines with Particular Reference to Stability Issues

For many applications, such as electric vehicles and washing machines, flux-weakening control is required for permanent magnet synchronous machine (PMSM) drives to extend the operation speed range and maximize the power capability under the voltage and current constraints. Voltage magnitude feedback flux-weakening control is widely employed due to its advantages of simple and standard control structure, robustness against parameter variation, both linear and over modulation flux-weakening operation, and automatic flux-weakening operation. However, stability problems are prone to occur in the flux-weakening region since the PMSM drive will operate on the boundary of the voltage limit. In this thesis, based on a non-salient-pole PMSM, the factors that could cause stability problems in the flux-weakening region with voltage magnitude feedback flux-weakening control are investigated and the corresponding solutions are developed. Firstly, based on a d-axis current voltage feedback controller, an adaptive control parameter method is proposed for the PMSM machine without maximum torque per voltage (MTPV) region, which aims to ensure the stability in a wider speed range. Then, a current reference modifier (CRM) and a voltage limit reference modifier (VRM) are incorporated with the conventional voltage feedback controller in order to improve the stability in the over modulation region. As for the PMSM machine with MTPV region, an extra feedback controller is introduced with an MTPV penalty function. The MTPV penalty function is optimized in terms of its effect on the steady-state performance, the dynamic performance, and the stability in the MTPV region. Afterward, the MTPV controller is properly selected and designed. Furthermore, two flux-weakening control methods accounting for MTPV, i.e. dq-axis current based feedback flux-weakening control, and current amplitude and angle based feedback flux-weakening control, are developed and compared in terms of the stability. It shows that the two methods exhibit complimentary merits and demerits in different regions, and consequently, a hybrid feedback flux-weakening control is proposed to combine their synergies and overcome their demerits. As the feedback voltage ripples that origin from the non-ideal drive system can be amplified by a conventional speed PI controller, the oscillation may even occur if a good speed dynamics is required in the flux-weakening region. An adaptive fuzzy logic speed controller is proposed and implemented to reduce the feedback voltage ripples while maintaining good speed dynamics.