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This paper compares two variable reluctance permanent magnet machines – a flux reversal and vernier hybrid variant. The machine, which is being developed in conjunction with Mocean Energy, is required to operate at very low speed. The paper describes the application and electrical topology development, with a focus on the optimization routine used. Results show there is a conflict between peak torque and cogging torque, but also that the peak induced voltage and cogging torque are highly dependent on and sensitive to the rotor tooth width, rather than if the machine is vernier or flux reversal type.

In low-speed applications, variable-reluctance permanent magnet machines are often proposed due to their efficient use of magnet material and high torque density. This becomes even more important in large linear applications, where the translator is longer than the stator. Often, however, very low power factors are experienced by this class of machines. This paper proposes a V-shaped flux concentrated version of a consequent pole linear Vernier hybrid permanent magnet machine and compares it to a surface-mounted magnet variant. Using finite-element analysis validated by two laboratory prototypes, it is shown that the flux-concentrated version increases the airgap flux density, which potentially leads to an improvement in the force density and efficiency or can be used to increase the operating power factor.

This work presents a new wave energy converter power take off (PTO) system that combines two recent developments in drive train technology, the consequent pole linear vernier hybrid permanent magnet machine and the linear magnetic gear. The combined system is fully realised in three-dimensional finite element method and compared to a similar direct drive machine. While the resulting system is lighter, reducing the active material mass by 30%, and more efficient, the system requires larger amounts of high cost permanent magnet material and increased PTO complexity. It is summarised that while such a system could be recommended in certain conditions, it is not viable for general application.

This article proposes a new topology of double-sided linear Vernier permanent magnet (LVPM) machine with segmented translator structure. A combination of a typical Halbach array consists of two horizontally magnetised permanent magnets (PMs) sandwiched between a salient iron tooth and adjacent vertically magnetised PM pole with the aim of improving its electromagnetic performance. The coreless partitioned translator contributes to a significant reduction in mass and the associated cost. With the aid of finite element analysis (FEA), two LVPM machines with the same PM, and stator volume have been investigated, analysed, and compared. The results imply that the proposed LVPM is capable of producing higher force density, while it exhibits lower cogging force and force ripple compared to its counterpart.

In some wave energy converters, there is a need for linear electrical machines where all the active parts are mounted on the stator. For this reason the linear Vernier hybrid machine has been investigated. In other research, it is analyzed in 2D and assumed to have a flat cross section. In this paper a cylindrical version of the Vernier hybrid machine is presented and analyzed. A geometrical model is used to investigate potential mass savings of the cylindrical version, and a finite element analysis design study is used to demonstrate a potential improvement in performance. A proof of concept laboratory scale model has been built and tested to validate the predictions.

This paper considers the use of transverse flux machines (TFMs) as an alternative to radial flux machines (RFMs) in an in-wheel application for an electric vehicle. The base motor is an existing outer rotor surface-mounted permanent magnet machine with fractional slot concentrated windings and liquid cooling. Two topologies of TFM are compared against this base machine. The first topology is a mutual flux path (TFM-MP), while the second is a claw pole machine (TFM-CP). First, the proposed TFM topologies are studied using FEA to understand the torque capability of both topologies. Second, a comparison between two different pole numbers of each topology is compared with the performance of the benchmarking machine (BMM). The machines will be compared in terms of torque density, losses, efficiency, and power factor (PF). Finally, it is presented as a comparison on the torque capabilities when constraining the magnet mass. It is shown that the TFMs selected can deliver higher torques than the base motor during continuous operation, but have failed to deliver the required overload torque. Furthermore, low PF and low efficiency make these machines unsuitable for the application of this in-wheel traction motor.

Different topologies of Vernier permanent magnet (PM) machines have been proposed for direct drive low-speed applications due to their high thrust and power density. However, this type of machine can suffer from low power factor (PF). In this study, a flux concentrated doubly salient linear permanent magnet machine with V-shape PM arrays is proposed and compared to a consequent pole (CP) type machine. Using the configuration of V-shaped PMs allows the flux to be concentrated in iron spacers located between two pieces of PMs. This feature is shown to result in an improvement of the airgap flux density, which allows a reduction in electrical loading and hence an improvement in power factor. In order to achieve a fair comparison, two doubly salient linear permanent magnet machines with different PMs’ configurations have been analysed and compared using transient FEA. The investigated machines have the same overall machine and, crucially, magnet volume. The results confirm that the proposed V-shape machine is capable of higher thrust force, higher no-load back EMF and can achieve a higher PF of 42% compared to the consequent pole machine, while it consumes the same amount of the PM material.

This paper presents the testing of an in-wheel permanent magnet Halbach array machine developed for automotive traction. A machine with a Halbach array rotor is experimentally compared to results from a surface mount variant with the same magnet mass and the finite element analysis (FEA) simulations done during its design. A battery emulator and dynamometer is used to mimic the performance in a vehicle. The Halbach version is shown to require between 6% and 15% less current to deliver the same torque across the operational speed range and delivers 20.6% more power during the continuous operation. High static cogging torque was found during testing and an approach to its solution is shown.

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.

Design of direct drive power take off in renewable energy generators tends to focus on force density, naturally leading to the use of relatively expensive permanent magnet machines. Elsewhere in the industrial landscape, induction machines traditionally rule as they are low cost and robust. In this study, the design of a generator for use in an open centred tidal turbine is presented as a case study to directly compare induction and permanent magnet machines. Mechanical constraints in the turbine design enforce a large magnetic gap in the generator, which inherently limits the power density and power factor. A comparison between the permanent magnet design and the induction design in terms of performance and economy has been presented.

In a free piston engine, the translator of a linear generator is coupled directly to the moving cylinder of a thermodynamic engine. For a linear machine to have a constant active area, either the translator or the stator must be oversized. The length of the translator effects its mass, which in turn effects the engine performance. This paper investigates the effect of translator length on the overall power density of a free piston engine coupled to a cylindrical permanent magnet machine.

This paper discusses a field weakening method for permanent magnet electrical machines. The proposed method can be an alternative to complex mechanical field weakening methods for safety critical applications. The developed concept is based on a split rotor in which a permanent magnet rotor is split into two parts to achieve two states of field excitation; either maximum flux output or the least flux output, depending on alignments between the rotor parts, having a degree of freedom to rotate on the shaft. The stator of the permanent magnet (PM) machine exposes two magnetic fluxes by the split rotor sections. Thus, the fixed nature of the PM excitation can be altered by this concept to achieve a wide speed range, which is a key concern in PM electrical machines at higher speeds. A number of 3D finite element (FE) simulations and a Simulink/Matlab model have been implemented to investigate the dynamic behavior of the system. Although it is challenging to computationally investigate the behavior of the split rotor at different operating conditions due to complex torque interactions, altering the relative angular position of the free-to-rotate rotor on a shaft, the proposed method might eliminate the complex implementation of mechanical field weakening apparatus for PM electrical machines. Thus, a PM machine with split rotor would be used over a wide speed range without excessive voltages at high speeds in fault tolerant aerospace applications.

This paper presents a linear variable reluctance permanent magnet (LVRPM) machine where the armature winding and inset Halbach array permanent magnets (PMs) are mounted on E-core stators. The Halbach array consists of a vertically magnetised PM pole and a salient ferromagnetic iron pole (split tooth) sandwiched between two horizontal PMs. The translator consists solely of laminations with salient iron poles. As a variable reluctance machine, it exhibits inherent magnetic gearing and a higher force and power density than the conventional synchronous PM machines. Finite element analysis (FEA) has been used to investigate the effect of machine parameters. The logical development of the LVRPM from the more conventional Vernier Hybrid machine is presented, including a near 50% reduction in magnetic flux leakage and a higher flux per unit magnet mass.

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.

In this paper, 28 slot − 18 pole and 24 slot − 20 pole permanent magnet alternators (PMA) with soft magnetic composite slot wedges are designed and compared. Power lane separation required due to fault tolerance requirements, means the proposed machines initially do not produce balanced phase voltage and currents, resulting in high ripple on the rectified power output. The alternators are optimised using 2-D finite element analysis (FEA) to get smoother DC power waveform. It is shown that the power ripple might be reduced by about 12%and un-balanced windings can be significantly balanced by thickening up the SMC slot wedges between the sub-machines. Furthermore, no load loss analysis has been conducted at rated speed and it is shown that 28 slot − 18 pole machine generates lower loss on no load condition compared to 24 slot − 20 pole single layer PMA.

Vernier hybrid machines (VHMs) are known for their simple salient translator structure and inherent magnetic gearing, making them a strong candidate for low-speed, high-force applications. As both the armature coils and the permanent magnets (PM) are mounted on the stator, linear versions of this machine are attractive for applications with a large stroke amplitude. In most configurations, single-sided or double-sided flat cross sections are utilized, giving a two-dimensional flux path. In this article, a cylindrical variant of the double-sided flat configuration is presented, where the flux path is now three dimensional. Three-dimensional finite-element analysis is used to design, analyze, and compare both flat and cylindrical topologies for a fixed PM mass and current density. Details and test results of a soft magnetic composite based laboratory prototype of the cylindrical VHM are given, which demonstrate the concept and validate the simulation results.

This paper proposes a new flux concentrated version of a consequent pole linear Vernier hybrid permanent magnet machine. The Vernier hybrid machine stator has alternatively magnetised adjacent magnets directly in contact with each other. A salient rotor then switches the direction of flux past the stator coils. Alternate magnets can be removed to make the consequent pole version, where inset rectangular magnets are magnetized in a common direction and separated by soft magnetic poles. In a flux concentrated consequent pole configuration, each magnet pole comprises of two magnets forming a V -shape. This topology hence possesses a combination of consequent poles, flux concentration and flux switching machine topologies. Magnetic flux concentration increases the airgap flux density, which potentially leads to an improvement of the force density and operating power factor. This paper compares the inset and V -shape consequent pole topologies using finite element analysis simulations. Both machines have the same dimensions, magnet volume, rated speed and current density. The simulation results imply that the V-shape machine offers higher electromagnetic performance. To confirm this, two linear prototypes are currently under construction, where the key parts that already produced are presented in this paper.

This paper compares a Halbach array to a more conventional surface mounted permanent magnet arrangement for an in wheel traction motor. Demagnetisation, physical air-gap and torque capability at continuous and overload operation is considered. Without increasing the machine operating space, using one transition per pole was found to increase rated torque by 5% and overload torque by 3 %. A second comparison was done using 3 transitions per pole and this topology was found to give an increase in the overload torque of 4%. Finally, a novel trapezoidal Halbach array arrangement has been proposed to reduce the demagnetisation of the transition magnets and increase their permeance coefficient.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.