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Wind energy, as one of the most promising renewable energy sources, has been well developed in the past 30 years. However, the improvement in reliability and availability of wind turbine systems is still an important topic to both academia and industry due to the high operation and maintenance cost. As two of the key components of a wind turbine system, the wind generator and gearbox, have the longest downtime although they are less prone to failure than the power converters and their associated control units. Therefore, it is necessary to study the faults of wind generators and try to reduce their downtime by detecting their faults at an early stage and then implementing effective maintenance. Among all the faults of these wind generators, the winding fault, as the second most frequent fault, has attracted significant interest. It has been reported by the Electrical Apparatus Service Association (EASA) that there are five major winding faults, namely (1) inter-turn (turn-to-turn) short circuit (ITSC), (2) coil-to-coil short circuit, (3) open circuit of one phase, (4) phase-to-phase short circuit, and (5) coil-to-ground short circuit. Amongst all the faults, the ITSC fault, often regarded as one of the root causes of other winding faults, has attracted increasing attention from researchers over the last few years. This thesis will mainly focus on modelling and analysis of permanent magnet machines under ITSC fault for wind power application.

Offshore wind power has huge potential to be one of the main renewable energy sources. Larger wind turbines can significantly reduce the overall cost of offshore wind farms, making wind energy a competitive source of clean energy. However, because of the harsh environment, reliability and maintenance have been major challenges for the offshore wind market. The gearbox is considered to be one of the weakest links in the whole drive train system, creating serious reliability issues. A direct-drive system enables the complete elimination of the gearbox by directly coupling the generator with the shaft of turbine blades. Therefore, in recent years, high power direct-drive wind generators have become very popular especially for offshore wind applications. However, high power at low speed equates to high torque and hence increases the volume and mass of the direct-drive generators. It is broadly recognized that permanent magnet (PM) synchronous machines are the most attractive candidate for direct drive applications due to their high power density, efficiency and easy scalability. Although the high cost of permanent magnet machines has been a concern, these machines can still be competitive when the long-term costs including maintenance, efficiency and design simplifications are considered. In an attempt to further improve the torque density of the permanent magnet machines, Vernier machines have emerged as a potential candidate. Vernier machines have high torque density, simple structure and an inherently low torque ripple which are desirable features for direct-drive applications. However, the high torque density of Vernier machines comes as a trade-off with their poor power factors which result in increased power converter rating and cost. Hence it would be valuable to investigate the impact of the high torque density and the poor power factor features of Vernier machines on the overall system-level performance of the multi-MW direct-drive power train system. The objective of this research work is threefold. Firstly, to investigate the suitability of the conventional surface-mounted permanent magnet Vernier (SPM-V) machines for off-shore wind power application by comparing the overall system-level performance (including generator and converter) with the existing conventional SPM machines. This will include the scaling study of Vernier machines ranging from a few kWs to multi-MW power levels using analytical models, 2D FEA and experimental validations. Secondly, to propose a novel Vernier machine topology suitable for off-shore wind power applications that can improve the torque density compared to the conventional SPM-V machines. Finally, the proposed Vernier machine will be compared with the conventional SPM and SPM-V machines for the Levelized Cost of Energy (LCOE) considering the annual energy production, capital expenditure and operation cost for the lifetime of the wind turbine at a 10MW power level.

This thesis investigates some novel current harmonic injection methods to improve the electromagnetic performance of doubly salient synchronous reluctance machines (DS-SRMs). These machines will have different winding configurations, slot/pole number combinations and phase numbers. The theoretical analyses (both static and dynamic) are carried out based on Fourier Series analysis, and validated by 2-dimensional finite element method and also experiments using several prototype machines. Based on the analytical torque model in abc-axis frame, a powerful insight into the mechanism of torque generation of the DS-SRMs with pure sinewave current supply can be achieved. The electromagnetic torque (both magnitude and phase angle) produced by each order of inductance harmonic can be predicted, which allows us to obtain the dominant torque ripple components for such machines. Therefore, the appropriate current harmonic (3rd, 5th and 7th) can be injected to generate torque ripple components in order to compensate that produced by the fundamental current, and hence to achieve an overall reduced torque ripple. On the other hand, the average torque of the DS-SRMs can also be improved by properly selecting the current harmonics in terms of harmonic order, amplitude and phase angle. However, it is found that the current harmonics, although can improve torque performance, will often cause extra losses (both copper and iron losses) and undesirable distortion in the phase voltages, which could lead to negative impact on the machine efficiency and dynamic performance. Therefore, in order to fully evaluate the potential of the proposed harmonic current injection method, comprehensive studies about losses, efficiency and dynamic performances such as torque-speed curves of 3-phase and multi-phase DS-SRMs have been carried out. In order to simplify the investigation of dynamic performance analyses such as the torque speed curves and efficiency maps, novel analytical torque model in dq0-axis frame has also been proposed. The findings in this thesis can provide some useful guidelines for torque performance improvement of DS-SRMs using harmonic current injections.

This thesis describes an investigation of high frequency permanent magnet generators for use in a novel power generation system for aerospace applications. The system consists of a high frequency generator (in the 10s of kHz range) which feeds a full-wave rectifier to produce to a high frequency pulse train as input to a pulse-density modulated soft-switched converter. Various topologies of flux-switching, flux-reversal and Vernier machines are investigated using electric-circuit coupled finite element analysis. Having demonstrated the limitations of these topologies, a comprehensive design study into a single-phase, surface mounted permanent magnet machine based on a single turn serpentine winding is described. This study covers both internal and external rotor machines with pole numbers of 192 and 96 which correspond to generator fundamental frequencies of 32kHz and 16kHz at the rated speed of 20,000rpm. Several aspects of the machine design are optimised through extensive use of finite element modelling, including mechanical analysis of the rotor containment. This study includes a detailed consideration of iron loss, including consideration of iron powder based cores. This study has resulted in a down-selected design based on a low permeability but high resistivity powdered iron core. The manufacture of a demonstrator is described including the need to re-design the machine to employ ultra-thin Nickel Iron laminations because of the difficulties encountered in the machining of a powdered iron core. The performance of this Nickel Iron variant is investigated and a final design established. The numerous challenges involved in manufacturing this novel machine are described.

As axial flux permanent magnet (AFPM) machines are currently the most appropriate topologies for limited axial space applications, a novel axial flux magnetically geared permanent magnet (AFMGPM) machine is investigated in this thesis. Based on a yokeless and segmented YASA machine, a new AFMGPM topology was designed and studied. The proposed AFMGPM machine consists of stator segments equipped with concentrated windings and two PM rotors with different pole-pair numbers: a high speed rotor (HSR) and low speed rotor (LSR). The proposed AFMGPM offers the merit of simple mechanical structure and is suitable for applications with limited axial space. Two possible rotor pole combinations were selected and designed with the same stator segments: MG12/5-7 with HSR pole pairs of 5 and LSR pole pairs of 7, and MG12/4-8 with HSR pole pairs of 4 and LSR pole pairs of 8. These were optimised for maximum torque capability. Performance comparisons at no-load and on-load conditions using 3D-finite element analysis (FEM) reveal that the machine torque performance is sensitive to the PM dimensions and better performance can be obtained with the MG12/5-7 topology. It is demonstrated that the MG machines are a valid alternative to the conventional planetary gear function in HEVs. Combining the conventional PM machine with the MG machine has made it possible to replace the power split components using only one electrical device. Additionally, the proposed machine can work as a conventional magnetic gear (MG) and a generator. It is shown that the new AFMGPM machine can realise the function of power split devices in conventional HEVs, as a mechanical planetary gear, motor and generator. It is further shown that the rotor manufacturing tolerance has a significant effect in terms of stator/LSR misalignment on the no-load and on-load performances of the machine. Finally, a performance comparison between the novel machine and the conventional axial flux YASA machine is performed. To validate the predicted results of finite element analysis, a prototype of the new topology and a conventional YASA machine are manufactured and tested. It has, showing that with the benefit of two rotors with different torques and speeds, the new AFMGPM machine has superior performance at all load conditions.

This thesis describes the design, construction and testing of an axial field permanent magnet machine for an aero-engine variable guide vane actuation system. The electrical machine is used in combination with a leadscrew unit that results in a minimum torque specification of 50Nm up to a maximum speed of 500rpm. The combination of the geometry of the space envelope available and the modest maximum speed lends itself to the consideration of an axial field permanent magnet machines. The relative merits of three topologies of double-sided permanent magnet axial field machines are discussed, viz. a slotless toroidal wound machine, a slotted toroidal machine and a yokeless axial field machine with separate tooth modules. Representative designs are established and analysed with three-dimensional finite element method, each of these 3 topologies are established on the basis of a transient winding current density of 30A/mm2. Having established three designs and compared their performance at the rated 50Nm point, further overload capability is compared in which the merits of the slotless machine is illustrated. Specifically, this type of axial field machine retains a linear torque versus current characteristic up to higher torques than the other two topologies, which are increasingly affected by magnetic saturation. Having selected a slotless machine as the preferred design, further design optimization was performed, including detailed assessment of transient performance. A key feature of this design is the use of a solid (i.e. non-laminated) toroidal stator core. This provides a stator with increased mechanical robustness, improved heat transfer and a ready means of incorporating fixing points into the core. However, these advantages are gained at the expense of a significant eddy currents in the stator core. A series of three-dimensional, magneto-dynamic finite element simulations were performed. Although the resulting eddy current losses are excessive for continuous operation, the reduction in transient performance which results from the eddy currents is shown to be manageable. The loss analysis is supplemented by transient three-dimensional finite element thermal modelling. Three-dimensional mechanical analysis is performed in combination with analytical equation to analyse the stator and rotor plate deflection subject to axial attractive force. The construction of a prototype double-sided axial field machine is described in this thesis which contains several interesting design features including a profiled rotor core to reduce mass, radially magnetised rotor magnets to produce torque from the axially oriented conductors on the inner edge of the toroidal winding. The testing of the machine is performed under a series of load points up to 75Nm to validate the predicted torque versus current density characteristics.

This thesis investigates the electromagnetic performance of the novel doubly salient synchronous reluctance machines (DSRMs) with different winding configurations, excitation methods, stator and rotor structures, and slot/pole number combinations. Both the wellestablished DSRMs with double layer conventional and mutually-coupled windings, as well as the fully-pitched winding have been compared against those proposed DSRMs. Different current waveforms such as rectangular wave and sinewave excitations have been considered in the comparison. It is worth mentioning that with rectangular wave excitation, the DSRM is in effect a classic switched reluctance machine (SRM). With rectangular wave excitation, different conduction angles such as unipolar 120° elec., unipolar/bipolar 180° elec., bipolar 240° elec., and bipolar 360° elec., have been adopted and the most appropriate conduction angles have been obtained for the SRMs with different winding configurations. In order to achieve improved machine performance, simpler manufacturing process and better fault tolerant capability, several novel modular SRMs have been proposed with different slot/pole number combinations, flux gap widths, and winding configurations. With appropriate conduction angles, the modular SRMs with higher rotor pole number than stator slot number are found to produce similar average torque, but much lower torque ripple, iron loss and radial force when compared with non-modular SRMs. With sinewave excitation, both the static and dynamic performances (torque-speed curve, efficiency map, etc.) of the DSRMs with different winding configurations have been investigated in-depth. In order to obtain the maximum efficiency, appropriate winding configuration can be selected for different speed range applications. In order to compare with the conventional synchronous reluctance machines (SynRMs) having flux barriers inside the rotor, the appropriate rotor topologies together with different winding configurations and slot/pole number combinations for maximum average torque generation have been obtained. Moreover, for better understanding of the nature of the DSRMs and distinguishing the double/single layer conventional and mutually-coupled winding configurations, analytical models on the basis of magneto-motive force (MMF) and airgap permeance have been developed to analyse the contribution of airgap flux density harmonics to average torque and torque ripple. Furthermore, some prototypes have been built with different winding configurations, stator structures (modular and non-modular) and slot/pole number combinations to validate the predictions.

This thesis investigates the stator mounted permanent magnet machines from the point of view of fault tolerant capability. The topologies studied are switched flux (and its derivatives C-Core, E-Core and modular), doubly salient and flux reversal permanent magnet machines. The study focuses on fault mode operation of these machines looking at severe conditions like short-circuit and irreversible demagnetization. The temperature dependence of the permanent magnet properties is taken into account. A complex multi-physics model is developed in order to assess the thermal state evolution of the switched flux machine during both healthy and faulty operation modes. This model couples the electro-mechanical domain with the thermal one, thus being able to consider a large range of operating conditions. It also solves issues such as large computational time and resources while still maintaining the accuracy. Experimental results are also provided for each chapter. A hierarchy in terms of fault tolerant capability is established. A good compromise can be reached between performance and fault tolerant capability. The mechanism of the magnet irreversible demagnetization process is explained based on magnetic circuit configuration. It is also found that the studied topology are extremely resilient against the demagnetizing influence of the short-circuit current and the magnet demagnetization is almost only affected by temperature.

This thesis is focused on the novel modular permanent magnet (PM) machines having flux gaps (FGs) in alternate stator teeth. Three research topics are included: 1) the design of multiphase modular PM machines, 2) cogging torque and torque ripple reduction methods for modular PM machines, 3) the manufacturing tolerance of modular PM machines. First of all, the influence of FGs on the multi-phase modular PM machines are summarized and some general rules are established which can be used as the design guidelines for the multiphase modular PM machines. It is worth noting that the 4- and 5-phase modular stator PM machines are only studied through the simulation. Secondly, two cogging torque and torque ripple reduction methods by the slot-opening shift and also the employment of the C-core stator segments are introduced. The proposed methods can effectively reduce the resultant cogging torque and torque ripple of the modular PM machines. Thirdly, the manufacturing tolerances of modular PM machines are also studied in this thesis and it provides an insight into the influence of manufacturing tolerance on the modular PM machines performance. Three possible manufacturing tolerance scenarios, e.g. stator segment radial or circumferential displacement and the PM defects, are investigated. The assessment of the manufacturing tolerance withstand capability of modular PM machines has also been carried out in this thesis. The prototype machines have been built and the numerical results calculated by both 2D and 3D finite element (FE) have been validated. It is worth mentioning that although the research carried out in this thesis based on the small size modular PM machines (for experimental validation purpose), the conclusions obtained in this thesis may be extended to other large modular PM machines.

This thesis focuses on the investigation of novel modular dual 3-phase permanent magnet (PM) machines with redundant 2-pole machine for wind power application by finite element analyses and experiments. The process of obtaining the novel modular machines is systematically introduced. In essence, it is equivalent to the combination of a conventional modular machine and a simple redundant one for modularity purpose. A 42-slot/32-pole (42S/32P) electrical machine is chosen as the example to present the features of this novel modular machine. Compared with its counterpart conventional modular machine (36S/30P) this novel modular machine shows comparable electromagnetic performance, while the redundant teeth contributed by the 6S/2P PM machine make it easier to adopt a modular structure. The coils located at the end slots of each segment can be protected effectively by these redundant teeth during the transportation and assembly process. Then, the available slot and pole number combinations for constructing this type of novel modular machine are summarised. The four prototype machines with 39S/32P, 39S/44P, 42S/32P and 42S/44P are used to demonstrate the influence of slot and pole number combinations on electromagnetic performance. Subsequently, the comparative study between the 42S/32P novel modular machine and the conventional modular machines with overlapping windings is performed. Since the dual 3-phase winding of this type of novel modular machine can adopt either thirty or zero degree phase shift between two sets of windings, they are compared with the 192S/32P and 96S/32P PM machines, respectively. Both normal operation with two sets of windings and faulty operation with only one set of winding are investigated. The merit of good fault-tolerant capability can be seen for the novel modular machines. Finally, the influence of different magnetic slot wedges (symmetrical magnetic slot wedges, two kinds of asymmetric magnetic slot wedges, fully closed slots and fully open slots) is investigated. It provides the guidance on how to select the appropriate magnetic slot wedges for electromagnetic performance improvement in different aspects.

Three novel types of permanent magnet linear machines have been investigated in this thesis, viz. partitioned stator flux reversal permanent magnet (PS-FRPM) planar machine, partitioned stator permanent magnet (PS-PM) tubular machines and partitioned stator single phase short stroke permanent magnet (PS-SPSSPM) tubular machines. PS-PM machines are distinguished by having two active stators and one passive mover. Such configurations offer better inner space utilization as well as enhanced electrical and magnetic loadings. A novel PS-FRPM linear machine is introduced by adopting the idea of PS-PM machine. The machine structure and operation principle are described. In addition, the force production mechanism of the proposed machine is analysed. Furthermore, the machine performance in terms of open circuit and load characteristics, losses and demagnetization withstand capability are investigated. To validate finite element predicted results, a prototype is manufactured and tested. Linear machine with tubular configuration is of great interest in terms of no end winding effect and no net attractive force between the stator and the mover. PS-PM tubular machines, i.e. PS-SPM and PS-IPM are designed and optimized for maximum thrust force capability. Moreover, the influence of the major design parameters on the machine thrust force is investigated. A comparison between both PS-PM tubular machines is carried out. Furthermore, PS-PM tubular machines having single-layer winding configuration are designed and compared to that with double-layer winding configuration counterparts. It is found that better performance can be delivered by single-layer winding PS-PM tubular machines. The interest of single phase short stroke permanent magnet tubular machines (SPSSPMTMs) is considerably increased over last few years, and they have been employed in healthcare, wave energy extraction, and household appliance. Novel PS-SPSS-PMTMs, named as PS-SPSS-SPMTM and PS-SPSS-IPMTM, are investigated. The optimized machine performances are analysed and compared. It is shown that the machines have the advantages of light mover mass and low eddy current loss. Finally, comparisons between the proposed machines and their conventional counterparts are carried out. It is found that the proposed PS-FRPM linear and PS-PM tubular machines exhibit higher thrust force density, better PM utilization, lower magnet eddy current loss and better demagnetization withstand capability compared to their conventional counterparts, i.e. FRPM, SFPM linear machines and SFPM tubular machine, respectively.