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In this paper, a permanent magnet alternator which presently relies on magnetic slot wedges is redesigned to give lower iron loss. The new segmented stator design is used as the basis for a validated loss study—comparing bonded lamination stacks with those held together with a punched notch. Segmentation is shown to improve the production process, reduce losses, and remove the need for wedges, while not significantly altering the acoustic performance. In the first series of prototypes, however, where notching is used to secure lamination packs, the process is shown to have adversely affected material properties of the nickel iron laminations and overall measured losses increased. In the second prototype, with thinner laminations and glued stacks, iron loss predictions and measurement agree and are less than the present slot wedge design.

This paper focuses on the improved thermal performance of electrical machines by increasing the conductor fill factor. A comparative design study of the integrated starter generator is used as a case study to investigate the mechanical, electrical, and thermal aspects of pressing coils. By performing quasi-static explicit dynamics simulations, deformation of insulation has been investigated. Magnet wires of 1.25 mm diameter are simulated and demonstrated to be compressible up to a 0.73 fill factor without observing insulation failure. Thermal conductivity enhancement of the stator windings at improved slot fill factor is investigated by performing steady-state finite element analysis thermal simulations and short-time thermal transient tests.

In the paper, a coupled dynamic model of the Linear Joule Engine and the embedded permanent magnet linear alternator is presented, which is the first dynamic model alike to bridge mechanical and electrical parts of linear engine generator, and provides better prediction of the performance. The detailed sub-model of the linear alternator responding forces eliminate any underestimation of their impact on the kinetics of the moving mass, i.e. piston, translator and rods, in turn, the electricity generation. The coupled model also enables an integrated design of the Linear Joule Engine and the linear alternator, which has the double acting piston in the compressor mounted with embedded permanent magnets to work as the translator, while the compressor cylinder bore is integrated with coils as the stator. It is predicted that the Linear Joule Engine Generator of the new design produces 1.8kWe with thermal efficiency of 34% and electrical generation efficiency of 30%.

High performance electric motor designs with ferrite magnets have recently gained interest due to the high and volatile price of rare earth magnets. However, due to the relatively poor coercivity of ferrite magnets, these designs are highly susceptible to demagnetization, as a result of which accurate modelling and better understanding of this phenomenon is particularly important. In this paper, the impact of the motor stack length and level of magnetic saturation on the demagnetization risk are studied based on 3-dimensional finite element simulations and a proposed lumped circuit model. It is shown that reducing the stack length can significantly enhance the demagnetization resistance with the effect being more pronounced for designs with a higher level of magnetic saturation. To benchmark the practicality of the concept, a previously presented high-performance ferrite-based design is modified by using a 30% weaker grade of ferrite magnet whilst shortening the stack length. It is shown that the demagnetization withstand capability of the design was significantly enhanced and exceeded the short circuit requirement with a good safety margin. The theoretical findings have been supported by prototype testing.

This paper investigates methods of magnet mass reduction in linear flux switching permanent magnet (LFSPM) machines by introducing tapered consequent poles. Based on the finite element analysis (FEA) three different models are designed, investigated and compared. The obtained results indicate that by adopting the proposed tapered ferromagnetic poles a considerable saving of material cost and weight is obtained without sacrificing the machine performance.

In this paper a novel E-core Linear Variable Reluctance Vernier Hybrid Permanent Magnet (LVR-VHPM) machine with inherent magnetic gearing is proposed for use in the long stroke application of a wave energy converter. The proposed topology is a combination of a doubly salient flux reversal machine, a consequent pole machine and a surface mounted Halbach array Permanent Magnet (PM) machine. The stator teeth interact with horizontally and vertically magnetized PM and ferromagnetic poles in such a way as to maximize the flux density in the machine by minimizing flux leakage. The translator is segmented to ensure the most effective use of material for minimum mass. A performance comparison study between the novel LVR-VHPM machine, a baseline linear Vernier Hybrid Permanent Magnet (VHPM) machine, a linear Halbach VHPM machine and a linear Consequent pole VHPM machine is used to show that the best result is achieved by the combined topology. All four designs consist of the same double sided E-core stators, segmented translators and an equal magnet mass.

The modulated pole machine (MPM) is known to offer high torque density utilizing permanent magnets and a three dimensional flux pattern. However, their construction requires a departure from conventional coil wound teeth formed of lamination stacks which can be a challenge. In this paper three construction techniques for MPM stators are compared: pure soft magnetic composite, pure lamination and a hybrid of the two. Simulation and practical knowledge of the build of each of these machines, including torque capability and mechanical tolerances is presented and discussed. Using the methods proposed here, it is shown that the pure laminate MPM machine has the potential to offer the greatest torque per kg of active material, yet it is the hybrid machine which delivers the highest absolute torque.

More electric is a platform being widely adopted by the current generation of aircraft. It offers significant benefits in terms of fuel savings and flexibility of airframe design but relies on the development of torque dense machines. This paper documents the development of an air-cooled transverse flux machine for use in short-haul aircraft to improve on-ground operational efficiency. If its potential is to be realized, this topology of the machine comes with a new set of manufacturing challenges. Two competing machine layouts with different approaches toward their fabrication are discussed. Experimental results and experiences gained from overcoming the challenges of building the machines are presented. A combined phase topology with soft magnetic composite core-back is shown to give a better rated torque, whereas a separate phase bent lamination concept gives better overload capability.

This paper proposes a new topology of a Linear Veriner Hybrid Permanent Magnet (LVHPM) machine with E-core stators and a segmented chamfered translator, which is shown to give mass reduction and better magnetic flux path compared to existing designs. Linear Permanent Magnet (PM) generators have the capability to reduce the number of moving parts, providing a simpler mechanical drive train when used as an all-electric direct drive power take-off for wave energy converters. The translator can be directly connected to a heaving buoy, or similar, to translate linear motion from the ocean waves into electricity. In this paper, the electromagnetic performance including magnetic flux, back EMF, cogging, thrust force, thrust ripple as well as force density of the new segmented chamfered translator design is analyzed in Finite Element Analysis (FEA). The proposed design is compared with two different models and shown to produce the best performance among those models.

This paper describes the thermal modeling approach of a permanent magnet alternator (PMA), developed to investigate the quantitative effect of thermal paste added into the machine end winding region. A thermal lumped parameter network is proposed to investigate the principal heat flow paths for a flange mounted alternator. The thermal model is implemented in Simulink, which allows many different heat paths to be easily combined. Since addition of a thermal paste introduces new axial heat flow by conduction paths between the stator windings and frame, the developed thermal network considers the detailed heat flow paths when thermal paste is present. Experimental results are presented which validate the results.

This paper describes the effect of alternative power conversion techniques on the performance of a Linear Alternator driven by a Joule Cycle-Free Piston Engine for electrical power generation. The design of a tubular Longitudinal Flux machine with axial magnets is performed using 2-D axisymmetric FEA modelling with transient solver to extract the rated power for three power conversion scenarios: Resistive Loading, Grid Interface, and Unity Power Factor Controlled Converter. The design study is carried out for a specific engine operating force and velocity, taking into consideration geometrical constraints for a compact system. Results are presented and discussed emphasizing the effect of the power conversion method on the idealized resultant machine physical and performance parameters.

Wave energy conversion can be simplified by using direct drive linear generators which may improve reliability. Numerous topologies of direct drive linear generators have been proposed in the last few decades such as the linear vernier hybrid permanent magnet (LVHPM) machine. However, the cost of this kind of machine is driven by the quantity of permanent magnet (PM) material. This paper proposes a linear vernier hybrid permanent magnet machine with consequent pole and concentrated windings for wave energy converters, which requires less magnet material for comparable performance. The key of the proposed design is the modular stator where the unbalanced magnetic circuit caused by consequent pole can be eliminated. Based on finite element analyses (FEA) the present machine is capable of higher thrust force, higher back EMF and lower cogging force as well as force ripple compared to that of a baseline LVHPM machine.

This paper focuses on the improved thermal performance of electrical machines by increasing the conductor fill factor. A comparative design study of integrated starter generator (ISG) considering different pole/slot combinations and magnet topologies is used as a case study to fix slot dimensions. Compressed stator windings coupled with segmented stator structure are introduced to reduce copper losses of the optimized ISG. By performing quasi-static explicit dynamic simulations, deformation of insulation has been investigated regarding mechanical properties of the magnet wire enamels. It is noted that magnet wires at certain cross-sectional area can be compressed up to 0.72 fill factor without observing insulation failure. Thermal conductivity enhancement of the stator windings at improved slot fill factor is demonstrated by performing steady state and transient finite element thermal simulations.

This paper investigates the design of alternative permanent magnet linear machines for use as the generator in a free piston engine. All of the machines are of a modulated pole structure with three dimensional flux flows. In transverse flux machines such as these, the current flows in a different plane to the flux path, eliminating the usual conflict between electric and magnetic loading. For a linear machine, the mass of magnets used is sensitive to whether the magnets are mounted on the stator or translator. Hence this paper compares transverse flux machines with translator mounted magnets and flux switching machines with stator mounted magnets. A three dimensional finite element analysis of each topology is run through an optimization routine in order to investigate the magnet usage, power factor and other performance criteria. It is concluded that flux switching machine is uncompetitive due to the 65% flux leakage even with improved tooth structure, moreover the transverse flux machine is shown to perform better overall, giving a better power factor, better efficiency and lower translator mass as a whole flux switching machine is not match for free piston engine applications.

Electric motors are becoming increasingly utilized in aircraft, replacing traditionally mechanical or hydraulic auxiliary systems to realize improvements in mass, control and efficiency. Aerospace motor design has the unique characteristic that mass and, therefore, torque density is of critical importance. For low maintenance, non-safety critical components, which are only used for short durations of the flight cycle, efficiency also becomes less important, whilst eliminating an external cooling circuit and gearbox will further benefit mass constraints. This paper considers the design of a high torque density machine in order to meet a specification for an aerospace application with a stringent mass target. As mass is reduced, thermal capability becomes critical and detailed thermal models, along with simulation results, are presented for operational characteristics.

This paper discusses three-phase topologies for modulatedpole machines (MPMs). The authors introduce a new threephasetopology, which takes advantage of mutual flux paths; thisis analyzed using 3-D finite-element methods and compared to athree-phase topology using three single-phase units stacked axially.The results show that the new “combined-phase MPM” exhibitsa greater torque density, while offering a reduction in thenumber of components. The results obtained from two prototypesare also presented to verify the concept; the results show that the“combined-phase” machine could provide both performance andconstructional benefits over prior MPM topologies.

This paper presents a case study for multi-variable and multi-modal design optimisation of a doubly fed induction generator (DFIG) based on surrogate-model optimisation algorithm. The DFIG's winding of stator and rotor are optimised to obtain higher efficiency for rewinding purposes. First, a Latin hypercube design is selected as the design of experiments to obtain sampling points. Then, the surrogate model is constructed using Kriging Model (KRG) method based on the Latin hypercube design. Finally, the particle swarm optimisation algorithm is applied in conjunction with the finite element method to achieve the machine design optimisation.

This paper discusses coil pressing through which stator windings are compressed at high pressures to enhance the slot fill factor of electrical machines. Deterioration of insulation in compressed windings is investigated by performing quasistatic explicit dynamics finite element simulations. The results obtained from the proposed method are further investigated experimentally to validate the durability of the compressed coils. It is shown that stator windings can be compressed to a certain degree without observing material failure. In order to demonstrate the thermal advantages of densely packed stator windings, both steady state thermal finite element analysis (FEA) simulations and semi-analytical methods have also been studied. A good agreement between analytical and FEA simulations is obtained. It is noted that steady state thermal finite element modeling could be used to predict effective thermal conductivity of stator windings for different slot fill factors.

Fractional slot, concentrated-winding (FSCW) machines with saliency are being researched extensively for vehicle traction motors. However, they exhibit higher torque ripple than their equivalents without saliency. This paper identifies the origin of the torque ripple and explores potential solutions for its reduction through modification of the stator slot openings. The concept is demonstrated through modification of an in-wheel motor design for high performance electric cars.

The work is focused on investigating the performance of a linear alternator for use with external combustion - free piston engine. This family of engine can be powered either from renewable sources, scavenging waste energy from other sources of heat, or bespoke combined heat and power plants. Finite element analysis optimisation techniques followed by development of a Matlab/Simulink model were used to investigate the effects of electrical machine inductance and the combined electromagnetic loading of the machine on the resulting force. Cogging force is showed to have the most effect on the resultant force fluctuation, whereas machine inductance has showed its effect at loads exceeding (76 %) of machine rating besides reshaping the resulting force profile. The model also accounts for machine loading and total efficiency variation over mechanical cycle. This enables the system designer to investigate the effect of actual machine model thrust force on the engine performance rather than assuming the electrical machine behaves as a simple damper.

With the increased electrification of aircraft, the use of motors to replace traditionally mechanical or hydraulic auxiliary systems has increased. Aerospace motor design has the rather unique characteristics of mass and torque density being extremely important, whereas cost is less so. For low maintenance, non-safety critical components, which are only used for short durations of the flight cycle, efficiency also becomes less important, whilst eliminating an external cooling circuit and gearbox can look very attractive. In order to meet a stringent mass target, this paper considers two high torque density machines suitable for low speed applications. As mass is reduced, thermal capability becomes critical and thermal simulations are presented for operational characteristics.

An induction generator has been designed for use in an open centred tidal turbine. Mechanical constraints in the turbine design enforces a large magnetic gap in the generator, which inherently limits the power factor. The large diameter of the machine means it is likely to be constructed out of a number of segments, which can each be modelled independently and approximated to either linear segments or individual rotary machines with the same pole to air gap ratio. An analytical tool to design an induction machine for this unique set of requirements is presented and compared to results from commercial software. Detailed design options are investigated using finite element analysis before a final design is presented.

In this paper the design of a linear electrical machine for use with a free-piston engine will be discussed. Three conventional permanent magnet machines are compared to a flux switching and modulated pole machine. Each of the topologies is optimised to give a specific force whilst adhering to a limit on the mass of the moving component. Using simple vector analysis, it is shown that machine power factor and efficiency are improved by optimising the machines with an objective function of minimising the MMF. Transient solver results are used to validate the static solver results and provide estimates on efficiency and operating power factor. This method of optimisation is shown to give modulated pole machines with relatively high power factors, which is a known limitation of this class of machine. The modulated pole machine topology could be the best option among all five topologies as it has high efficiency and high power factor. The flux switching machine shows some potential benefits on magnet material use ratio compared with PMs, yet it is not compliant with mass constraint.

A novel V shape magnet motor has been designed in [1] for cost reduction compared to the existing surface-mounted in-wheel motor. The essential concept of this design is to remove the steel ribs on all ends of the magnets minimising the flux leakage. However, this approach poses a significant challenge on the structural aspect of the design. FEA was used to assess its structural integrity under various operating conditions. However, during the motor assembly, unexpected issues were raised. The adhesive, assembly tools and insertion methods were reassessed and a new solution has been proposed.

The work presents a design methodology for a tubular Moving Magnet Linear Alternator for use with an External Combustion - Free Piston Engine (EC-FPE). Modelling and design of three alternative machines of 6 slots / 7 poles composed of feasible PM arrangements have been performed to react the same EC-FPE driving force. Demonstration of geometrical constraints and design methodology for the desired force is primarily presented, from which the Radial machine is found unsuitable for the application under the given design constraints. Then, performance comparison of the remaining designed machines is listed. Comparison using a weighted score system shows that the Quasi-Halbach machine has a lighter translator mass, yet the Axial machine has higher magnet utilisation as well as higher power densities.

A method is presented of estimating the responses of axisymmetric bodies floating in spread irregular seas, using a Laplace transfer-function formulation of a floating body time-domain model. A general-case spread-wave model is formulated, using separate wave excitation transfer functions, and a simplification of this approach is proposed, reducing both model complexity and computation time. Responses are computed using both approaches and a comparison made to assess the circumstances in which the simplified approach may be used effectively. The results are also interpreted to highlight the implications of using an equivalent unidirectional wave as an approximation to a spread wave.

This paper describes the design build and test of a linear test rig for demonstrating novel topology linear machines. The work is aimed at aiding the developments of power take off for wave and tidal energy converters. The rig consists of three parts: prime mover, test machine and converter. Experience and method for the design manufacture and initial testing of each of these components is detailed.

This paper presents for the first time the analysis and experimental validation of a six-slot four-pole synchronous reluctance motor with nonoverlapping fractional slot-concentrated windings. The machine exhibits high torque density and efficiency due to its high fill factor coils with very short end windings, facilitated by a segmented stator and bobbin winding of the coils. These advantages are coupled with its inherent robustness and low cost. The topology is presented as a logical step forward in advancing synchronous reluctance machines that have been universally wound with a sinusoidally distributed winding. The paper presents the motor design, performance evaluation through finite element studies and validation of the electromagnetic model, and thermal specification through empirical testing. It is shown that high performance synchronous reluctance motors can be constructed with single tooth wound coils, but considerations must be given regarding torque quality and the d-q axis inductances.

Modulated pole (transverse flux) machines are known for their torque density and can take advantage of mutual flux paths to further increase the torque density of this class of machine. This paper presents a retrospective design investigation into a combined phase machine conducted in order to reduce the harmonic content of the back EMF and the cogging torque of the machine; two common problems with modulated pole machines. A method is proposed and it is shown that by changing a simple geometry such as the tooth span, cogging torque, torque ripple and harmonic content can all be significantly reduced without a significant effect on the overall torque output.

This paper investigates the design of direct drive linear generators for use in Free Piston Engines. Flux switching, modulated pole and surface mounted magnet machines are individually modelled and optimized to react the force of the engine with minimum magnet mass. The mass of magnet used in each design is found to be heavily dependent on the copper loading in the machine and the flux produced at no load. The modulate pole or transverse flux machine, which in other applications gives an extremely efficient use of magnet, is here shown to be more effective than conventional machines but less effective than the flux switching machine.

This paper investigates magnet topologies and their susceptibility to demagnetisation in an in-wheel motor for an electric vehicle. The analysed motor is a high torque density, low speed, surface-mounted permanent magnet (SMPM) modular motor with fractional slot concentrated winding (FSCW) and an outer rotor. The rich spatial stator MMF harmonics present in the air gap have two effects: they generate AC losses in the magnets and they can increase the peak demagnetising field which the magnets experience. In this work, methods are first developed to give greater insight into the magnet loss mechanisms, thereby indicating when magnet segmentation is beneficial. The methods are then extended to examine how different rotor designs are affected by both loss and demagnetisation fields, showing major differences between rotor topologies.

Variations in the impedance of the acoustic liners in aeroengine intake ducts occur due to the presence of acoustically hard splices, which lead to scattering of the incident fan tone noise into a large number of azimuthal modes, and can significantly reduce the effectiveness of the liner. Currently available computational aeroacoustic packages are capable of modeling the scattering, but are generally impractical for direct application to liner optimization design studies, because they require a substantial quantity of CPU time. A Kirchhoff approximation is applied to a geometry of practical interest, and the predicted field is compared with that computed using a commercially available computational aeroacoustic package. The analytical and computational results are in good agreement for cases both with and without mean flow. A striking result is that the modal power in most of the forward-scattered modes is almost equal, except for a small number of modes close to the rotor-alone mode. The analytical model is found to be accurate for a physically useful combination of parameters, and provides both a method for rapid evaluation of the scattering, and a physical understanding of the process.

Fan noise is one of the principal noise sources in turbofan aero-engines. At supersonic fan speeds, fan tones are generated by the “rotor-alone” pressure field. In general, these tones can be well absorbed by an inlet duct acoustic liner, except at high supersonic fan speeds when the rotor-alone pressure field is well cut-on. In this article an axially segmented liner is proposed, which is predicted to improve the attenuation of tones at high supersonic fan speeds. The analysis is based on locally reacting cavity liners. The axially segmented liner is axisymmetric and consists of two circular sections of different linings joined together. The optimum design consists of two linings with the same face-sheet resistance, but with different cavity depths. The depth of the liner adjacent to the fan is very thin. This means that where the two liners are joined there is a wall impedance discontinuity that can cause acoustic scattering. Fan tones can be modelled in terms of spinning modes in a uniform circular-section duct. The liner is axisymmetric, so modal scattering will be only between different radial modes. The optimum design minimizes the acoustic energy scattered into the first radial mode. This improves the attenuation of fan tones at high supersonic fan speeds, because acoustic energy is scattered into high radial mode orders, which are better absorbed by the lining.

This paper describes a motor for use within the wheel of an electric vehicle. It demonstrates the influence of various rotor parameters on an outer rotor permanent magnet motor (ORPM) with Surface-mounted magnets (SM). The aim of this paper is to reduce the magnet volume, while maintaining the torque performance through the complete operating range. Six different magnet topologies are investigated firstly. Then, Semi Surface-mounted Permanent Magnet (SSPM), I shape tangential PM (IPM) and V shape interior PM (VPM) designs are compared in terms of torque capability at certain magnet volume. The VPM gives highest torque performance. The iron shielding concept in VPM can protect the magnets from the opposing armature flux, thus providing increased resistance to demagnetisation, and hence permitting thinner magnets. Furthermore, a new slot/pole combination with higher torque capability has been studied. However, due to the increased inductance, motor with V shape design needs to work at a poorer power factor and consequently a reduced speed range for a given inverter. Lastly, Cost effective and simple manufacture method of the VPM rotor is also addressed with consideration of mechanical feasibility.

In recent years significant improvements in the field of Modulated Pole Machines (MPMs) have been achieved, especially in electric vehicles applications. MPMs such as Transverse Flux Machines (TFMs) are known for producing high torque densities; though further research and improvements are needed to reduce the inherently high cogging torque and torque ripple. This paper will present a method of reducing these undesirable features of TFMs through simple geometric changes, such as changing the tooth span of the stator teeth. It will be shown that a combination of varying tooth spans rather than a uniform tooth span for the whole machine helps to reduce the cogging torque and torque ripple of the machine without a significant effect on the overall torque output.

OpenHydro is developing open centred submerged turbines for the extraction of energy from ocean tides. Power take off is via a surface-mounted, radial-flux, permanent-magnet generator mounted to the rim of the turbine blades. A number of full scale devices have been deployed globally, upto 2MW and 16m diameter. The aspect ratio of the machines mean that end effects are likely to be significant and hence 3D modelling is required to fully capture them. In this paper, alternative methods for modelling the existing machine design is discussed and compared. It is demonstrated that, although a full scale detailed FEA approach yields the best results, simplified FEA and even analytical approximations are best for the purpose of designing the machines.

This paper will present two new Flux Switching Modulated Pole Machine (FS-MPM) topologies which have a similar structure to a Modulated Pole Machine. It aims to address the complexity of the rotor, by introducing a FS-MPM design in which the magnets are removed from the rotor and placed on the stator. Furthermore the number of magnets in the FS-MPMs is greatly reduced when compared to a standard MPM; reduction in component count and simplification of the rotor will offer a significant benefit in terms of cost and ease of production.

This paper gives details of an ongoing collaborative project in the area of automotive traction drive machine design. The relationship between electrical frequency, torque, pole number and iron loss in modulated pole machines is investigated with respect to fulfilling an industrially relevant specification. A number of alternative topology configurations are simulated, presented and discussed. (6 pages)

This paper presents a new topology for three-phase Modulated Pole Machines. This new topology the “Combined Phase Modulated Pole Machine” is analysed and compared to the more traditional technology of three separate single phase units stacked axially with a separation between phases. Three- dimensional Finite Element calculations are used to compare performance of the machines under the same conditions, it is shown that the new Combined Phase topology produces a greater torque whilst reducing the number of components required to assemble the machine and increasing its mechanical integrity. (6 pages)

Direct drive power take off eliminates the need for a mechanical interface to match the low speed reciprocating motion of a wave device to high speed rotational motion of conventional electrical generators. However, direct drive generators are physically large and heavy due to the low velocities encountered. Iron cored PM machines suffer from magnetic attraction forces which is challenging for the structural and bearing design. An air-cored machine solves the issue of magnetic attraction forces, which will benefit the mechanical design. The performance of an air-cored PM tubular machine is investigated in this paper under displacements typical of a wave energy device.

Wave energy is currently an untapped resource, but has the potential to make a significant contribution to the energy mix. In order to use conventional electrical generators mechanical interfaces are used, such as hydraulic systems and air-turbines. With the electrical generator these interfaces are known as the electrical power take-off and the type used depends upon the wave energy device. A brief description of the different power take offs is provided to show how conventional rotary generators are used in wave devices. Both advantages and disadvantages are highlighted in the paper. Direct drive systems can overcome some of the disadvantages, but there are additional engineering challenges to overcome, in particular physical size and mass. Current and more novel direct machine topologies are discussed in the context of these challenges.

The study describes a power electronic conversion and control scheme for a 8.5 kW, multi-coil, linear, permanent magnet, air -cored generator for direct-drive wave energy conversion developed for a wave energy Test Rig developed for the New & Renewable Energy Centre at Blyth, Northumberland. The proposed scheme is justified against the requirements implied by the features of the linear generator design. Simulation and preliminary test results are presented.

Circumferential variations in the impedance of intake liners occur in aero-engine ducts due to the presence of hard strips or splices, which can cause a significant reduction in liner performance at certain conditions. New intake liner designs are currently aiming to minimise the number and width of splices, and while CAA or CFD methods can be used to model and predict the scattering effects, these are normally impractical for direct application to parametric or liner optimisation design studies, due to the significant CPU time required to solve this 3D problem at realistic frequencies. In the early 1990's at Rolls- Royce, Cargill developed an analytical model for the scattering by liner splices based on the Kirchhoff approximation. Now that robust, accurate CAA methods are becoming available, it is possible to conduct a theoretical validation of the Cargill model, which is the subject of this paper. The CAA code used, ACTRAN, is a commercially available finite element method, which assumes an irrotational mean flow and linear propagation. The ACTRAN and Cargill results agree well for no-flow case, except for some minor discrepancies in the back-scattered modes but these are of secondary importance compared to the forward scattered. A striking feature of these results is that the forward scattered modal sound power is almost equal for all spinning modes except for one or two modes adjacent to the rotor-alone modes. Results for flow Mach numbers up to 0.4 show that agreement between ACTRAN and Cargill is acceptable except again for the field dominated by the back- scattered field. In general, for engineering purposes, Cargill's model agrees well with those obtained from the ACTRAN code, for relevant combinations of parameters. Cargill's model has also helped to improve our understanding of the scattering process and provides a rapid method of evaluating splice scattering effects.

In this paper the authors outline the performance and modelling of a prototype linear tubular permanent magnet machine with an air cored stator. The main application of the machine is in direct drive marine energy conversion systems, where speeds are typically in the region of 0.5 m/s. A simple analytical method was derived to provide an initial design of a prototype machine, which was then modelled using 2D axisymmetric finite element analysis. Experimental results are presented to verify the design, and to investigate the performance of the machine as a generator.

The design of a prototype linear variable reluctance permanent magnet machine is outlined in this paper. It has been built to investigate operation at speeds of 0.5 m/s, which is typical of a wave energy converter. Results are presented to verify the design, and to investigate the performance of the machine. In order to optimise power conversion, methods of compensating for the effects of the high inductance of these machines are discussed.

This study describes the power conversion and control solution used in the electrical power take-off of a 35 kW test rig developed to investigate a linear, direct drive, air-cored, tubular, permanent magnet generator for an offshore wave energy device. The solution proposed is to collect the power extracted directly from individual coils of the generator, which have different induced voltages and cannot easily be connected into a small number of phases. Local energy storage is integrated into the system to smooth the electrical output power and reduce the rating of the downstream inverter for grid interfacing. The solution is demonstrated by analysis, backed up by simulation and test results. This shows the potential and limitations of the proposed conversion technology solution.

Variable reluctance permanent magnet machines exhibit shear stresses higher than conventional machine topologies. However, analysis and design of these machine topologies is not straightforward owing to the nature of the flux paths in the machine. In a transverse flux machine flux flows in all three dimensions and the machine structure is non-conventional. The vernier hybrid machine is another member of this family, but the flux paths are restricted to two dimensions, and can therefore be constructed using laminations. Significant leakage flux paths exist in the vernier hybrid machine, which must be modelled accurately during machine design. The use of flux-linkage maps to determine the performance of a linear vernier hybrid machine is described. Experimental results are presented to verify the model. The technique can be applied to any variable reluctance permanent magnet machine topology.