Linear Permanent Magnet Vernier Generators for Wave Energy Applications: Analysis, Challenges, and Opportunities

© 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).Harvesting energy from waves as a substantial resource of renewable energy has attracted much attention in recent years. Linear permanent magnet vernier generators (LPMVGs) have been widely adopted in wave energy applications to extract clean energy from oceans. Linear PM vernier machines perform based on the magnetic gearing effect, allowing them to offer high power/force density at low speeds. The outstanding feature of providing high power capability makes linear vernier generators more advantageous compared to linear PM synchronous counterparts used in wave energy conversion systems. Nevertheless, they inherently suffer from a poor power factor arising from their considerable leakage flux. Various structures and methods have been introduced to enhance their performance and improve their low power factor. In this work, a comparative study of different structures, distinguishable concepts, and operation principles of linear PM vernier machines is presented. Furthermore, recent advancements and innovative improvements have been investigated. They are categorized and evaluated to provide a comprehensive insight into the exploitation of linear vernier generators in wave energy extracting systems. Finally, some significant structures of linear PM vernier generators are modeled using two-dimensional finite element analysis (2D-FEA) to compare their electromagnetic characteristics and survey their performance.Peer reviewe

Linear Machines for Long Stroke Applications: a review

This document reviews the current state of the art in the linear machine technology. First,the recent advancements in linear induction, switched reluctance and permanent magnet machines arepresented. The ladder slit secondary configuration is identified as an interesting configuration for linearinduction machines. In the case of switched reluctance machines, the mutually-coupled configuration hasbeen found to equate the thrust capability of conventional permanent magnet machines. The capabilities ofthe so called linear primary permanent magnet, viz. switched-flux, flux-reversal, doubly-salient and verniermachines are presented afterwards. A guide of different options to enhance several characteristics of linearmachines is also listed. A qualitative comparison of the capabilities of linear primary permanent magnetmachines is given later, where linear vernier and switched-flux machines are identified as the most interestingconfigurations for long stroke applications. In order to demonstrate the validity of the presented comparison,three machines are selected from the literature, and their capabilities are compared under the same conditionsto a conventional linear permanent magnet machine. It is found that the flux-reversal machines suffer froma very poor power factor, whereas the thrust capability of both vernier and switched-flux machines isconfirmed. However, the overload capability of these machines is found to be substantially lower than theone from the conventional machine. Finally, some different research topics are identified and suggested foreach type of machine

Permanent Magnet Vernier Machine: A Review

Permanent magnet vernier machines (PMVMs) gained a lot of interest over the past couple of decades. This is mainly due to their high torque density enabled by the magnetic gearing effect. This study will provide a thorough review of recent advances in PMVMs. This review will cover the principle of operation and nature of magnetic gearing in PMVMs, and a better understanding of novel PMVM topologies using different winding configuration as well as different modulation poles and rotor structures. Detailed discussions on the choice of gear ratio, slot-pole combinations, design optimisation and role of advanced materials in PMVMs will be presented. This will provide an update on the current state-of-the art as well as future areas of research. Furthermore, the power factor issue, fault tolerance as well as cost reduction will be discussed highlighting the gap between the current state-of-the art and what is needed in practical applications

Development of high force dense linear generators for wave energy converters

PhD ThesisThe main concern of this thesis is the development of force dense linear generators for a Direct Drive Wave Energy Converter. Linear machines for direct drive power take-off systems are required to deliver very high force in order to harness the significant amount of power from the low velocity oscillation of an ocean wave. Therefore, the linear Vernier Hybrid Machine is investigated for its simple design structure and high force density at low speed, due primarily to the inherent magnetic gearing. Attention is focused on improving the performance of the existing linear Vernier Hybrid Machine and developing new variant topologies with higher force density. An improved E-core stator design, optimised permanent magnet dimensions and new segmented translator structure have been proposed which improve the machine performance in terms of mass and magnet utilisation. The implementation of a pole shifting method is shown to provide a significant reduction in the cogging force. Two cylindrical variants with three-dimensional flux paths are also developed from the improved E-core Vernier Hybrid Machine, which further improves the force with similar magnet mass and current density. Furthermore, a new combination of Halbach magnets arrays and Consequent Pole topology are employed in the flat E-core structure, known later as Halbach Consequent Pole Vernier Hybrid Machine, which significantly improves the flux density by reducing the inherent pole-to-pole leakage and thus further improve the force density and power factor of the machine. A cylindrical variant of the flat Halbach Consequent Pole topology has been designed and analysed to prove the performance improvement of the cylindrical versions compared to the flat. The flat Halbach Consequent Pole and two small scale cylindrical variants of the E-core Vernier Hybrid Machines have been built and tested in the laboratory. The flat prototype is built from laminated steel and both the cylindrical machines are made of Soft Magnetic Composites to allow the three-dimensional flux path. All the experimental results are shown to provide good agreement with the static and dynamic generator performance predictions. Finally, this thesis compares the performance of three flat and three cylindrical Vernier Hybrid Machine topologies for various axial lengths and air-gap diameters and investigate the feasibility of using them for a wave energy devic

Development of a Linear Vernier Hybrid Machine for direct drive wave energy converters

PhD ThesisThe work presented in this thesis concerns the development of linear electric machines for use with wave energy converters. The machine topology selected, the Linear Vernier Hybrid Machine, is extensively investigated, specifically looking at alternative magnet configurations. Topologies are evaluated by their generation capabilities at low velocities, as demanded by Direct Drive Wave Energy Converters. Attention is mainly focused on improving the electromagnetic performance and reducing the magnet mass. A new topology of the Linear Vernier Hybrid Machine is proposed for these purposes, known later as Inset Magnet Consequent Pole machine. Tapered ferromagnetic poles are employed in this topology, which have shown a great impact on minimising the inherent pole-to-pole leakage flux as well as the unwanted cogging force. Further investigation into the Inset Magnet Consequent Pole machine focuses on improving the power factor through modifications made to the machine structure with no increase in the mass magnet, steel or copper used. Two novel variants with the added benefit of flux concentration effect are proposed and described. Finite Element Analysis is used to optimize, analyse and compare the electromagnetic performances for the three investigated machines. Considering the complexity of manufacturing and number of components, two selected topologies are built and tested in the laboratory, the Inset Magnet Consequent Pole machine and V-shape Consequent Pole machine. The experimental results are compared to the simulation results to validate the design. In general, a good agreement is shown between the predicted and measured results. Afterwards, the experimental results obtained from the two prototypes are compared with each other. These results verify that the proposed V-shape Consequent Pole topology is superior in terms of no-load back EMF, force and power factor, while it exhibits lower cogging force in comparison with the Inset Magnet Consequent Pole topology. It is therefore concluded that the V-shape Consequent Pole machine is the best compromise between power factor, efficiency and ease of manufacture. It has half the number of components per pole of the best machine design presented, yet delivers 91% of the force density and 93% of the power factor. The last part of this thesis investigates the feasibility of using the proposed V-shape Consequent Pole machine as an alternative design for an existing wave energy device developed by Uppsala University to assess the effect of employing this sort of machine on Abstract ii the overall machine size and costs. Five variants of the V-shape Consequent Pole machine are described and comparedTechnical and Vocational Training Corporation, Saudi Arabi

Magnetic Material Modelling of Electrical Machines

The need for electromechanical energy conversion that takes place in electric motors, generators, and actuators is an important aspect associated with current development. The efficiency and effectiveness of the conversion process depends on both the design of the devices and the materials used in those devices. In this context, this book addresses important aspects of electrical machines, namely their materials, design, and optimization. It is essential for the design process of electrical machines to be carried out through extensive numerical field computations. Thus, the reprint also focuses on the accuracy of these computations, as well as the quality of the material models that are adopted. Another aspect of interest is the modeling of properties such as hysteresis, alternating and rotating losses and demagnetization. In addition, the characterization of materials and their dependence on mechanical quantities such as stresses and temperature are also considered. The reprint also addresses another aspect that needs to be considered for the development of the optimal global system in some applications, which is the case of drives that are associated with electrical machines

Investigation of Novel Axial Flux Magnetically Geared Machine

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

Advances in Rotating Electric Machines

It is difficult to imagine a modern society without rotating electric machines. Their use has been increasing not only in the traditional fields of application but also in more contemporary fields, including renewable energy conversion systems, electric aircraft, aerospace, electric vehicles, unmanned propulsion systems, robotics, etc. This has contributed to advances in the materials, design methodologies, modeling tools, and manufacturing processes of current electric machines, which are characterized by high compactness, low weight, high power density, high torque density, and high reliability. On the other hand, the growing use of electric machines and drives in more critical applications has pushed forward the research in the area of condition monitoring and fault tolerance, leading to the development of more reliable diagnostic techniques and more fault-tolerant machines. This book presents and disseminates the most recent advances related to the theory, design, modeling, application, control, and condition monitoring of all types of rotating electric machines