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Fault-tolerant Control and Robust Design of Dual-star PMSMs for Power Steering Systems

par Abedini Mohammadi, Abdolmajid
Président du jury Kinnaert, Michel
Promoteur Gyselinck, Johan
Co-Promoteur Pop, Adrian-Cornel
Publication Non publié, 2022-12-13

Thèse de doctorat

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Résumé :

This thesis revolves around reliability and robustness of permanent-magnet synchronous drives designed for Electric Power Steering System (ePS). The first part of the thesis is devoted to reliability. In this regard, dual-star Permanent-magnet Synchronous Machines (PMSM) are adopted. Thanks to the redundancy of the phases, they are inherently more fault-tolerant than their three-phase counterparts. Moreover, unlike e.g., five- or seven-phase machine, they do not need customized converters and can be driven by a pair of off-the-shelf three-phase inverters. The machine model is discussed in the first chapter. In order to take account of the parasitic effects such as saturation and non-uniform airgap, the relationship between flux linkages and currents for different rotor positions is stored in Look-up Tables (LUT)s. The procedure for constructing the LUTs is explained. Several techniques are introduced to reduce the computational burden of the calculations by means of Finite-element (FE) analysis. Two methods are introduced for obtaining the inverse LUTs, in which the currents are outputs, and flux linkages inputs. The inverse LUTs are needed for the numerical solution of the voltage equation. The proposed method has a comparable confidence level as the FE model, however, with much less computational time. Hence, it suits system-level studies. The model is used for several studies, namely a comparative analysis of single- and dual-star machines, the impact of the loss of the inverter signals on the machine operation, and the effect ofconnecting the two star points. Then, the fault-tolerant control of the machine with one or multiple phase Open-circuit (OC) faultsis presented in the second chapter. The conventional methods based on optimization are introduced. Various control strategies which find relevance in the case of OC faults are discussed. Minimum losses, maximum torque, and full-range minimum loss control strategies are introduced. Moreover, the derating factor considering the copper loss is set forward. The model of the faulty machine with one OC is presented and its control according to the proposed model is implemented using simple PI controllers. Starting from the developed model, a generic control structure is proposed which is independent of the number and location of the OCs. It is derived from the healthy control structure with some modifications in terms of de/activating controllers. In the third chapter, the iron losses of dual-star PMSMs under fault-tolerant control are comprehensively investigated. The knowledge of the iron losses in addition to the copper losses is necessary for calculating the derating factor using thermal analysis. Various machine topologies, which are made of stators with either distributed or concentrated winding and rotors with either surface-mounted or interior PMs, are considered. Empirical formulas are used for the calculation of the iron losses.Moreover, eddy-current loss in the PMs is studied.The second part of the thesis deals with the robustness of the motor against manufacturing imperfections. The effect of manufacturing stages, particularly punching, on the magnetic steel properties is identified. First, a local approach for identifying the punching effect is introduced. The expression for the BH curve and iron losses are extended as a function of the distance from the cut edges. The parameters of the model are identified using standard Epstein frame tests. The model is incorporated in the FE equations. The performance of two fractional-kW synchronous machines, namely a PMSM and a Synchronous Reluctance Machine (SynRel) machine, is assessed in terms of torque and iron losses. Moreover, a global approach for the identification of the equivalent BH curve of the magnetic core of a SynRel machine is introduced. It requires the acquisition of the torque and phase voltages obtained from normal operation or dedicated measurements. Chapter five discusses the effect of various types of manufacturing imperfections on the cogging torque of a segmented PMSM. Magnetic steel deterioration, dimensional tolerances, and assembly tolerances are investigated. Statistical studies are employed for identifying the type of imperfections with the dominant contribution.Finally, in the last chapter, the robust design optimization of the machine is addressed. First, a simple and fast method for calculating the cogging torque of the machine with multiple defective stator segments with an arbitrary arrangement is introduced. It helps to reduce the computation burden of the optimization considerably. Then, the robust design optimization is addressed for two different scenarios. In the first case, just the imperfect connection gap between two adjacent segments is considered, while in the second one, the radial displacement of the segments is taken into account as well. Statistical studies on the optimized motors are carried out to show the efficacy of the optimization.