Sensorless control of a salient pole synchronous reluctance machine for high rotational speeds

Abstract

This thesis describes the design, implementation and optimization of field-oriented control for a salient pole synchronous reluctance machine including flux weakening operation to reachhighrotationalspeeds. Atfirstarotaryencoderisusedforshaftpositionmeasurement. This serves as foundation for the adoption of sensorless field-oriented control where the rotary encoder is substituted by mathematical models. The system’s target application is a variable speed drive capable of four quadrant operation. Standstill operation and slow shaft rotation in sensorless mode are enabled by application of the INFORM method. A dedicated PWM pattern resembling three distinct voltage space vectors is used at low speeds to apply the method directly. If required, a transition to a classical symmetric PWM pattern and a back-electromotive force based estimation method allow for sensorless operation at medium and high shaft speeds. Upon reaching the limit of available DC-link voltage, flux weakening is realized by restricting the reference current space vector alongside the positive or negative MTPA line. In case of sensorless operation, additional constraints result into an allowed region for the reference current space vector that lies between the positive and negative MTPA lines. Mechanical shaft speeds of up to 26000rpm and 24000rpm were achieved in sensor-based and sensorless modes respectively. Chapter 1 motivates this thesis and defines its scope by three topics. Connections to prior and ongoing academical work is established. A patent search report on the subject matter issued by the Austrian Patent Office (ÖPA) is presented. Chapter 2 describes all three major system parts. These are: The machine, the voltage source inverter and the rotary encoder. Important aspects to properly construct the experimental setup are also covered. Chapter 3 describes the targeted digital signal controller TMS320F28335 and applied numerical methods. Advice on how to increase computational performance for that specific device is given. Chapter 4 covers every subsystem that is incorporated into the field-oriented control scheme in detail. Aspects to maintain high voltage reserves and achieve high rotational speeds are highlighted. A flux weakening strategy that distinguishes transient and quasistationary operational states is presented. Chapter 5 completes the system description. The implemented INFORM variant is classified and subsequently assessed with respect to prior set quality targets. Further, the used back electromotive force based method is presented. A transition strategy to link both methods and necessary modifications of the sensor-based system are discussed. Sensorless flux weakening is described by systematically considering voltage, torque and angular error constraints. All discussed subsystems are verified by executing representative speed trajectories. Chapter 6 concludes the thesis, and relates its outcomes to the three topics that were defined in the first chapter.