CONTROL OF PERMANENT MAGNET SYNCHRONOUS MACHINES WITH SMALL DC-LINK CAPACITOR

This thesis focuses on the control of permanent magnet synchronous machine (PMSM) drive systems with small dc-link capacitor.
In small dc-link capacitor-based PMSM drive systems, the dc-link voltage is subject to fluctuations, and thus, real-time accurate dc-link voltage information is critical. This thesis firstly analyses the challenges posed by dc-link voltage sensor faults, which can lead to significant measurement errors and compromise control accuracy, and then proposes a robust dc-link voltage observer to ensure accurate voltage estimation.
The thesis further investigates the influence of dc-link voltage fluctuation on high-speed sensorless control methods, particularly the extended electromotive force (EEMF)-based method. The results indicate that the dc-link voltage fluctuation can induce the harmonics in the estimated rotor position error, and deteriorate the system stability. A back-EMF harmonic suppression method is proposed to eliminate these rotor position error harmonics and enhance the performance of the EEMF-based sensorless control strategy.
For zero and low-speed sensorless control methods, conventional methods often require the injection of additional voltage signals to extract the rotor position information. However, in small dc-link capacitor-based PMSM systems, the fluctuating dc-link voltage can easily lead to insufficient voltage conditions, which can worsen the sensorless control performance or even lead to the failure of conventional high-frequency signal injection (HFSI)-based sensorless control methods. Two novel sensorless control methods are developed, leveraging inherent current harmonics resulted from the dc-link voltage fluctuation, to accurately determine the rotor position without the need for HFSI. Furthermore, a magnetic polarity detection method which utilizes the inherent current harmonics is also proposed to improve the accuracy and reliability of small dc-link capacitor-based PMSM drive systems under heavy load conditions.
Finally, the thesis analyzes the energy backflow phenomenon in small dc-link capacitor-based PMSM drive systems. This is particularly prevalent at high speeds and under heavy loads, when the amplitude of back-EMF may exceed the minimum dc-link voltage, threatening the system stability and efficiency. Furthermore, the limitations of conventional flux-weakening (FW) control methods under fluctuating dc-link voltage conditions are analyzed. An optimized FW control strategy is proposed to eliminate the energy backflow and further improve the overall system performance.