Coaxial Magnetic Gear Research Articles

Comparison of Magnetic Gear Characteristics Using Different Permanent Magnet Materials

In the coaxial magnetic gear, the permanent magnets of each rotor face each other, and the driving frequency of each rotor has different values according to the gear ratio. The magnetic flux that changes with time is transmitted to each permanent magnet. If the permanent magnet has conductivity property, losses by eddy currents are occurred in the permanent magnet. It is a major loss of a magnetic gear. Because the loss causes demagnetization by heat, it is mentioned a fatal problem at high speed. In this paper, the characteristics were analyzed by applying permanent magnets without conductivity. In addition, the analysis of the electromagnetic field as well as the mechanical loss were also considered to show which magnet type is advantageous.

An Asymmetric Pole Coaxial Magnetic Gear With Unequal Halbach Arrays and Spoke Structure

In this paper, a novel asymmetric pole coaxial magnetic gear (CMG) with unequal Halbach arrays and spoke structure is proposed to improve the output torque of CMG. The inner rotor has an asymmetric N-S pole structure, the permanent magnets (PMs) on the inner rotor of the CMG are arranged in Halbach arrays with unequal magnetic poles, and PMs on the outer rotor are Spoke-type. According to the flux modulation effect, the contents of higher harmonics can be reduced and the output torque of the proposed CMG can be significantly improved compared to the conventional CMG with radial magnetization. The proposed CMG topology with 4 inner and 17 outer pole pairs is established. The proportional relationship between N pole and S pole is introduced. Then, the air gap flux density, electromagnetic torque, harmonic component of the CMG are investigated and verified by finite element analysis.

Strucsture and Electromagnetic Characteristics of Pole-Piece Support Materials in Magnetic Gear

Studies on coaxial magnetic gears (CMGs) have been actively conducted. CMGs can replace mechanical gears because they can perform noncontact power transfer minimizing loss and damage from friction. In the design stage of a CMG, 2D electromagnetic finite element analysis (FEA), which is a highly reliable and cost-saving tool, is widely used. However, during prototyping and testing in the review phase of a project, the prototype's pull-out torque is lower than the pull-out torque predicted from 2D electromagnetic FEA, with up to a 40% difference. One explanation for this phenomenon is that the torsional stiffness of the pole-piece structure is low, causing the electrical angle to skew. There are high-frequency magnetic fluxes in the pole-piece structure in which magnetic losses are expected to be high. In many studies, non-metal pole-piece supports that can minimize this magnetic loss were used. The disagreement between analysis and experiment has been demonstrated, but no detailed examination of this effect has been conducted. In this study, metal pole-piece supports are used to remove the torsion possibility. Results from 3D-FEA and experiments are compared, and the results reveal that when torsion is accounted for, the analysis data can be reproduced the experimental data well or not. Furthermore, the magnetic loss on metal supports is analyzed in detail.

Performance analysis of magnetic gear with Halbach array for high power and high speed

In this study, we propose a non-contact 80 kW, 60,000 rpm coaxial magnetic gear (CMG) model for high speed and high power applications. Two models with the same power but different radial and axial sizes were optimized using response surface methodology. Both models employed a Halbach array to increase torque. Also, an edge fillet was applied to the radial magnetized permanent magnet to reduce torque ripple, and an axial gap was applied to the permanent magnet with a radial gap to reduce eddy current loss. The models were analyzed using 2-D and 3-D finite element analysis. The torque, torque ripple and eddy current loss were compared in both models according to the materials used, including Sm2Co17, NdFeBs (N42SH, N48SH). Also, the structural stability of the pole piece structure was investigated by forced vibration analysis. Critical speed results from rotordynamics analysis are also presented.

Loss estimation of magnetic gears

Reduction in losses is essential for the efficiency enhancement of magnetic gears. In this paper, the power losses of a coaxial magnetic gear are estimated, based on a developed convenient approach for loss analysis using a time-dependent finite element model. The results are confirmed by a sophisticated measurement system for magnetic gear testing. Eddy current losses in permanent magnets and magnetic cores, magnetic flux reluctance and harmonic distortion in magnetic materials during magnetic gear operation are determined. The output torque reduction, especially at high rotational speeds and high gear ratios caused by losses, is analyzed. Different permanent magnet material grades are utilized in order to evaluate the eddy currents and the torque transmission losses during the magnetic gear performance.

Magnetic Loss Analysis in Coaxial Magnetic Gears

This paper proposes a procedure for computing magnetic losses in coaxial magnetic gears. These magnetic structures are made of permanent magnets and ferromagnetic poles in relative motion transferring torque between two shafts in a contactless way. The loss computation in magnetic materials is crucial to define the system performance. The flux distribution inside the iron parts is computed by means of the finite element method and a model of iron losses taking into account the rotational nature of the flux loci is applied. The procedure highlights where the major loss sources are present and gives the opportunity to evaluate some corrective measures to reduce their effects. Particular attention is devoted to the 2D modeling in presence of permanent magnets segmentation.

Design and Analysis of Coaxial Magnetic Gear Mechanism with Halbach Permanent-Magnet Array

The disadvantages of mechanical gear can be replaced by the advantages of magnetic gear related to transferring torque without mechanical touch, no gear lubrication, low mechanical loss, low vibration, and intrinsic overload protection. From the types of magnetic gear, coaxial magnetic gear is the most developed type. Halbach permanent-magnet arrangement in coaxial magnetic gear mechanism can offer higher on transmitted torque and also capable of increasing the magnetic field compared to radially magnetized. Then this paper aims to carry out the characteristic of coaxial magnetic gear with Halbach magnetized in steady-state condition. Furthermore, magnetic field and transmitted torque were investigated by commercial software ANSYS/Maxwell. Finally, by using 2D finite element method, the result shows the maximum torque with 2150.76 Nm.

Design and Characteristics Analysis of Coaxial Magnetic Gear for Contra-Rotating Propeller in Yacht

In this article, the design and characteristic research of the coaxial magnetic gear for the contra-rotating propeller in the premium yacht are presented. The scale-down model of 1/25 is reviewed as the research of the applicability of the coaxial magnetic gear. The space harmonic characteristics of the magnetic field in the coaxial magnetic gear with the low gear ratio from 1 to 2 are investigated, and the optimized models with the surface-mounted permanent magnet and quasi-Halbach array are obtained using the response surface method. In the several operating points of the diesel engine, the magnetic loss and the resultant thermal characteristics are analyzed and compared in each model. Finally, the desired coaxial magnetic for the contra-rotating propeller in the premium yacht is achieved.

A Review on Magnetic Gears: Topologies, Computational Models, and Design Aspects

Geared devices are commonly used to match the operating speed and torque of the power source with the second mover. Such geared devices are usually mechanical gears. As counterparts of the conventional mechanical gears, magnetic gears (MGs) are becoming promising devices, mainly due to the merits of physical isolation between moving parts, no gear lubrication, no mechanical fatigue, inherent overload protection and reduced maintenance, etc. Recently, several new topologies have been proposed to achieve better performances. In this paper, a general review of MGs is presented, including a discussion of the most common and upcoming topologies and a description of the working principle. A comparison of different topologies is carried out in terms of gear ratio and torque density. The main computational techniques generally adopted for coaxial MGs are listed and described. Some key aspects concerning the design, the existed challenges, and potential applications are also discussed. This paper aims to provide a comprehensive overview of the MG for readers working in this field.

A Novel HTS Modulated Coaxial Magnetic Gear With Eccentric Structure and Halbach Arrays

In order to improve the high torque density of coaxial magnetic gear, a novel high temperature superconducting (HTS) bulks coaxial magnetic gear (CMG) with an eccentric pole of inner rotor and Halbach arrays of outer rotor is proposed in this paper. According to the Meissner effect, the HTS bulks replace the traditional epoxy resin to improve the flux-modulation effect. Moreover, the HTS bulks CMG has an eccentric structure of permanent magnets (PMs) on inner rotor and has Halbach PMs arrays on the outer rotor. The proposed HTS bulks CMG topology with 4 inner and 23 outer pole pairs is established. The topology performance is assessed and verified by finite element analysis.

Multi-Objective Robust Optimization for a Dual-Flux-Modulator Coaxial Magnetic Gear

The conventional multi-objective deterministic optimization (MODO) design may be less meaningful or even unacceptable when considering the perturbations of design parameters and reliability of the optimization results. In order to overcome the above-mentioned drawback and improve both the torque capability and permanent magnet (PM) utilization efficiency of dual-flux-modulator coaxial magnetic gear (DFM-CMG) simultaneously, a robust optimization design method is presented. In this method, the sigma criteria with the Monte Carlo simulation (MCS) are adopted to obtain the statistic samples addressing the effects of parametric uncertainties of DFM-CMG on the optimization results. Both 2-D and 3-D finite-element (FE) models of a DFM-CMG are first established, and the 3-D FE model is proven more accurate by the experiment and used for further optimization. Through the parametric study, five parameters are selected as the key design variables to establish the quadratic polynomial regression metamodels. Finally, the multi-objective particle swarm optimization algorithm with MCS is employed to conduct the multi-objective robust optimization (MORO) for DFM-CMG. Although the stall torque per PM consumption achieved by MORO with six sigma ( $6\sigma$ ) is little lower than that achieved by MODO, it still has a 21.4% growth than that of the initial design under the same constraint of the stall torque. Furthermore, the reliability and stability of the MORO results are much higher than those of the MODO results. The MORO method is significantly effective as the torque performance and robustness of DFM-CMG could be improved simultaneously.

Parametric Design and Optimization of Magnetic Gears With Differential Evolution Method

Recently, magnetic gears have drawn wide attention due to their inherent merits, such as noncontact operation, high torque density, and intrinsic overload protection. In this paper, an analytical calculation based on magnet magnetomotive force permeance theory is adopted to analyze the modulation effect in a magnetic gear. After that, the effects of some key parameters on magnetic gear performance are investigated, including the air-slot-opening-to-pole-pitch ratio $c_0$ , the magnet-arc-to-permanent-magnet-pole-pitch ratio $\alpha$ , and the permanent magnet thickness $h_m$ . In particular, three types of steel pole pieces are considered: radial slots, parallel teeth, and parallel slots. A global optimization design is then carried out by using the differential evolution algorithm, and some useful suggestions are derived from the optimal Pareto range. Finally, the determination of outer rotor yoke thickness is discussed considering the iron saturation, and the iron loss distribution is investigated. This paper will be helpful for the design and optimization of a coaxial magnetic gear.

Transmitted Torque Analysis of Coaxial Magnetic Gears with Different Pole-Pair Numbers

Transmitted torque is an important performance index of coaxial magnetic gears. This study focuses on the effects of pole-pair numbers on the maximum torque capacity and torque ripple of coaxial magnetic gearsOwing to the magnetic resistance between the pole piece and the medium of air; a torque ripple is generated during magnetic gear operation. In magnetic gear design, maximum torque capacity is considered the higher, the better, and the torque ripple should be the lower, the better. A commercial Finite-element analysis software AN-SYS/Maxwell 2D is applied to simulate the transmitted torque and torque ripple of coaxial magnetic gears by solving the magnetic field, and mechanical motion coupled models. Model parameters, including size and inertia, are designed via the Autodesk/Inventor. The results of finite-element analysis have shown that there are certain relationships between pole-pair numbers and torque performance. It reveals that the sum of the pole-pair numbers is inversely proportional to the maximum torque capacity; the differences of maximum torque between different pole-pair number designs could be up to 82%. However, the torque ripple has not been proven for the relationship. For future research, the torque capability per cost would be necessary for magnetic gear product, especially when the price of rare earth metal is seen rising in the past years. Also, the efficiency of magnetic gear is still important for powertrain system, but it’s not easy to reach both high torque and efficiency, owing to the flux saturation problem. The balance of torque capacity, efficiency, and cost would be the next challenge.

Analysis of High Gear Ratio Capabilities for Single-Stage, Series Multistage, and Compound Differential Coaxial Magnetic Gears

Magnetic gears perform the same fundamental power conversion as mechanical gears. However, magnetic gears have significant potential benefits due to their noncontact operation. This paper compares three different ways to achieve high gear ratios using coaxial magnetic gears by evaluating trends for radial flux coaxial magnetic gears with surface mounted permanent magnets. First, a single-stage design can be used, but the torque density and efficiency both decline as the gear ratio increases. Additionally, the gear ratio achievable with a single-stage coaxial magnetic gear is limited by practical considerations, such as the maximum number of modulators and pole pairs that can be used within the given space. Second, a multistage design can be formed by connecting single-stage designs in series. Multistage designs can achieve much higher net gear ratios with much less of a torque density penalty, especially as the number of stages increases, but this entails greater complexity. Third, the compound differential coaxial magnetic gear (CDCMG) is proposed. The CDCMG is formed by interconnecting two single-stage coaxial magnetic gears and can achieve gear ratios much higher than the product of the gear ratios of the individual stages. However, the circulating power in the CDCMG leads to poor efficiencies.

The Mechanism How Coaxial Magnetic Gear Transmits Magnetic Torques Between Its Two Rotors: Detailed Analysis of Torque Distribution on Modulating Ring

Coaxial magnetic gears (CMGs) have attracted increasing attention in recent years. The field modulation effect arising from modulating ring is considered as the reason why CMG can realize torque transmission between its two rotors. However, the magnetic torque interaction among modulating ring and two rotors is rarely investigated. Therefore, the purpose of this paper is to figure out the detailed torque distribution on the modulating ring, which plays a crucial role as the force/torque medium between the inner rotor and the outer rotor. First, the conceptual model, the operating principle, and the practical configurations of CMGs are introduced. Second, theoretical derivation of torque distribution on each surface of modulating rings with different connecting bridges is conducted by using the Maxwell stress tensor method. The calculation results match very well with those obtained from the traditional “holistic analysis method” that is widely used in previous publications, and this validates our proposed “detailed analysis method.” Third, how the connecting bridges affect the torque distribution on modulating rings is presented intuitively. Finally, three CMGs with inner bridge, outer bridge, and double bridges, respectively, are built for experimental verification.

Development of a Hybrid Analytical Model for a Fast Computation of Magnetic Losses and Optimization of Coaxial Magnetic Gears

This paper focuses on a two-dimensional (2-D) analytical model for a fast computation of magnetic losses due to eddy currents in the permanent magnets as well as iron losses in the ferromagnetic parts within coaxial magnetic gears. The magnetic field distribution is computed in yokes and permanent magnets by solving both Maxwell's equations, whereas for pole pieces, the magnetic field is computed by coupling the previous analytical model with a reluctance network model. Both the eddy current losses and iron losses are determined from this hybrid analytical model. The iron loss model takes into account the temporal and spatial variations of flux density. The eddy current loss model takes into account the magnet splitting. Results of this magnetostatic eddy current loss model are then compared to the results obtained with a 2-D magnetodynamic finite element model. A verification of validity limits is also proposed. The final function of this analytical model is to ensure integration into a set of models in the aim of a global mechatronic optimization of magnetic gears, for their insertion into multimegawatt wind turbines. A preliminary bi-objective, mass-efficiency optimization protocol is subsequently proposed along with an analysis of the computation time reduction via the presented models.

Torque Analysis in Coaxial Magnetic Gears Considering Nonlinear Magnetic Properties and Spatial Harmonics

A coaxial magnetic gear (CMG) consisting of an inner ring, outer ring, and flux modulator exhibits relatively large torque density at low operating cost and low noise level. To guide the selection of soft magnetic materials for the flux modulator, this paper first incorporates the Jiles–Atherton model within a finite-element (FE) framework. The influence of nonlinear magnetic properties on end-effect losses (magnetic flux leakage and fringing) is then investigated with the objective of maximizing the pullout torque. For flux modulators whose initial magnetic permeability and saturation flux density exceed certain threshold value, the pullout torque of a CMG becomes insensitive to magnetic properties and end-effect losses can be efficiently calculated from a 2-D FE model together with a constant torque reduction ratio. This paper presents a systematic torque surface method that can decouple the analysis of average torque and torque ripples. This method has been implemented to evaluate load dependence and rotation speed fluctuation of a CMG.

The use of high temperature superconductor bulk in a co-axial magnetic gear

The torque transmission and angular speed change in machine mechanisms are provided by mechanical gear systems. Recent advances in magnetic system designs, developments in quality of the permanent magnetic and ferromagnetic materials led to possibilities of replacing magnetic gears with their mechanical counterparts. Magnetic gears are free from damages during the overloaded operation since they have no physical contact. However, the torque density in magnetic gears is lower compared to mechanical gears due to the limited magnetic flux densities of the permanent magnets. In this respect, the design alternatives are important from the point of enhancing the torque density in the magnetic gear development. For the design considerations in this paper, the effect of the gear depth on the torque efficiency and the use of high temperature superconductors in the pole pieces are investigated. The finite element analysis performed in COMSOL indicated that the torque values established on the rotors of the magnetic gear increased with increasing the depth without changing the efficiency of the gear mechanism. It was also determined that the use of superconductor in the pole pieces increased the torque transfer. High ripple on the amplitude of the torque values suggests the necessity of the experimental verification and more advance analysis.

Characteristic Analysis of High Speed Coaxial Magnetic Gear by Two-Dimensional Finite Element Analysis

Characteristic Analysis of High Speed Coaxial Magnetic Gear by Two-Dimensional Finite Element Analysis

Analysis of Torque Characteristics according to Non-uniform Air Gaps of Coaxial Magnetic Gear

Recently, studies on magnetic gears have been actively conducted. Magnetic gears can replace mechanical gears as they can perform noncontact power transfer, thereby minimizing loss and damage from friction. A magnetic gear has two rotors with different numbers of poles because of its structural characteristics. For a rotor with a relatively lower number of poles, large torque ripples occur owing to the increase in the permeance variation within the air gaps. Other electric equipment studies conducted thus far have primarily changed the shape of iron core to reduce torque ripples. However, since the magnetic gear has two air gaps, it is necessary to identify the relationship between the two gaps and the ripple before changing the shape of the iron core. In this paper, factors that affect torque ripple are classified and analyzed by examining the magnetic flux density of two air gaps, to propose an air gap structure that benefits the design of magnetic gears.