Design, analysis and application of magnetic variable gears

This paper covers a new emerging class of electrical machines, namely, Magnetic Gears (MGs) and Magnetically Geared Machines (MGMs). This particular kind of gears/machines is capable of either scaling up or down the revolutions-per-minute to meet various load profiles as in the case of mechanical gearboxes, but with physical isolation between the rotating components. This physical isolation between the rotational components leads to several advantages in favor of MGs and MGMs over mechanical gearboxes. Although MGs and MGMs can potentially provide a solution for some of the practical issues of mechanical gears, MGs and MGMs have two major challenges that researchers have been trying to address. Those challenges are the high usage of rare-earth Permanent Magnet (PM) materials and the relatively complex mechanical structure of MGs and MGMs, both of which are a consequence of the multi-airgap design. This paper presents designs that reduce the PM rare-earth content for Electric Vehicles (EVs). Additionally, the paper will ensure having practical designs that do not run the risk of permanent demagnetization. The paper will also discuss some new designs to simplify the mechanical structure.

Magnetic gear is an alternative to mechanical gear, where the magnetic gear has the advantages of no noise, minimum vibration, no maintenance required, increased reliability, overload protection capability, no physical contact between gears, and the resulting torque density is still below the mechanical gear torque density. In this research, the variation of the number of poles and air gap in the amount of torque density produced was carried out. The permanent magnet material used is Neodymium type with a gear ratio of 1:2. In the variation of the number of poles used by pairs of 4 and 8 poles, 8 and 16 poles, 10 and 20 poles, and 12 and 24 poles, in the variation of the air gap used, namely 1 mm, 1.2 mm, 1.3 mm, 1.4 mm and 1.5 mm. Magnetic gear performance can be seen through analysis simulation with the 3D finite element method using Finite Elements Software. The type of simulation used is the magnetostatic analysis method at the processing stage and the transient analysis method at the post-processing stage. From the simulation results, it is concluded that the greater the number of poles, the greater the torque density produced, and the closer the air gap distance will result in a greater torque density. The effect of the number of poles on the torque density is more significant than the effect of the air gap.

NdFeB (Neodymium magnets) rare earth permanent magnet is an advanced magnetic material characterized by a high energy product and excellent magnetic performance, which has broad application prospects. With the rapid development of 3D printing technology, its application in the research and fabrication of rare earth permanent magnet materials has become a research hotspot. This paper reviews the latest research developments of rare earth permanent magnet NdFeB materials in 3D printing. It introduces the basic properties and applications of rare earth permanent magnet materials and highlights the main challenge these materials faced, which is how do we improve their solid content. Additionally, the advantages and challenges of 3D printing technology in preparing rare earth permanent materials are analyzed in this paper. By comparing the variations in magnet properties of ferrite and rare permanent magnet materials with increasing temperature, it is found that the latter have relatively poor magnetic stability at high temperatures. Based on these points, the paper summarizes optimization methods of the current 3D printing process for rare earth permanent magnet materials. Finally, the paper discusses the development prospects of rare earth permanent magnet materials in 3D printing, as well as problems need to be solved and future research directions.

This paper proposes an alternative interior permanent magnet machine (IPMM) design in which the rotor consists of stacks varied in the axial direction. The proposed design aims to reduce the amount of permanent magnet (PM) materials in conventional IPMMs. The PM width and the PM pole arc on the rotor surface for each rotor stack are both optimized to provide a resultant PM flux distribution in the air gap that contains fewer harmonics. Not only is the pulsating torque reduced significantly, the PM materials are also minimized since the PM portion making up the spatial PM flux harmonics is effectively removed. The results of the proposed design concept indicate a significant torque pulsation reduction in comparison with the base-line model 2007 Camry motor. Moreover, a trade-off is discovered between the average torque production and the PM utilization in an IPMM. The improvement made in the proposed designs proves that the PM materials can be used efficiently and the torque pulsation can be further minimized in IPMM designs.

The price fluctuation and the unstable supply chain of rare-earth permanent magnet (PM) have posed potential risks for further application of rare-earth PM motors. To alleviate the consumption of rare-earth PM material, a less-rare-earth PM brushless motor with two types of PM materials is proposed in this paper. The key of this motor is that the rare-earth PM is purposely combined with a non-rare-earth ferrite PM, forming the hybrid PM excitation. To design the proposed motor efficiently, a multi-objective stratified optimization strategy is proposed, where the selected five optimization objectives of output torque, PM cost, cogging torque, torque ripple, and efficiency are divided into two levels. Then, different optimization methods are skillfully combined to meet the specific design requirements. The corresponding electromagnetic performances are analyzed and compared with the baseline rare-earth PM brushless motor. Finally, several experiments of the two motors are carried out for evaluation. Both theoretical analysis and experimental results verify the feasibility and validity of the proposed motor and the optimization method.

Magnetic gears, like mechanical gears, transform power between different speeds and torques; however, magnetic gears' contactless nature provides inherent potential benefits over mechanical gears. A genetic algorithm was used to optimize magnetic gears at different temperatures across a range of gear ratios. Using different magnet material grades on the different rotors and for the tangentially and radially magnetized magnets can slightly increase the specific torque relative to designs with a single magnet material. The high pole count rotor requires a magnet material with higher coercivity than that of the low pole count rotor magnet material, especially for designs with a large gear ratio. While increasing the temperature produces an exponential decay in the achievable specific torque, with a compounding reduction of about 0.4% for each degree Celsius, the temperature does not significantly affect the optimal geometric parameters and primarily affects the optimal materials. The gear ratio significantly affects the optimal geometric parameters and can impact the optimal magnet materials. Additionally, the genetic algorithm was employed to characterize the impact of stack length using 3D finite element analysis. Designs with shorter stack lengths favored thinner magnets and higher pole counts and may be able to use magnet materials with lower coercivities.

Rare earth permanent magnet (PM) materials are used in interior permanent magnet synchronous motors (IPMSM) as mainstream motors in hybrid electrical vehicles (HEV). In mass produced HEVs, the steep rise of price for the recent rare earth materials like Neodymium, Dysprosium, Boron, and etc is one of serious concerns. Therefore, a switched reluctance motor (SRM) without permanent magnets is one of the possible candidates. Design of stator and rotor have been investigated to achieve competitive torque and efficiency profiles at same dimensions of an IPMSM used for mass production in electric and hybrid electric vehicles.

In this paper, an outer rotor interior permanent magnet synchronous machine (IPMSM) with hybrid permanent magnet (PM) material is proposed. The key is to adopt the outer rotor structure with two-layer crescent barriers and hybrid PM material to maximize the output torque and reduce the usage of high-cost rare-earth PM. The ferrite PMs are filled in the middle of the crescent barriers with thicker thickness, while the rare-earth PMs are filled into the bilateral of the crescent barriers with thinner thickness. To verify the effectiveness of the proposed rotor structure, optimizations are first carried out among the conventional rotor structures with single kind of PM material and the proposed rotor structure with hybrid PMs. Then, detailed comparisons have been investigated. Results indicate that the proposed machine has higher torque density than the machine with only ferrite PM and can reduce almost half the usage of rare-earth PM in comparison to the machine with only rare-earth PM. Moreover, the proposed rotor structure is further optimized considering productivity.

In this paper, the partitioned stator flux reversal permanent magnet (PM) (PS-FRPM) machine and the conventional magnetically geared (MG) machine operating in both stator-PM (SPM) and rotor-PM (RPM) modes are comparatively analyzed in terms of electromagnetic performance to provide design guides for a MG machine regarding: (a) a SPM or RPM type machine and (b) a higher or lower gear ratio machine. It is found that a SPM type machine is recommended, since both PS-FRPM and MG machines operating in SPM modes have a higher phase back-EMF and hence torque than their RPM counterparts, respectively, as a result of a similar phase flux-linkage but a higher electric frequency since the iron piece number is larger than the PM pole-pair number. Moreover, a smaller gear ratio machine is preferred from the perspective of a higher power factor and hence a lower inverter power rating, as the conventional MG machines with higher gear ratios suffer from larger flux-leakage, higher synchronous reactance and hence lower power factors, as well as higher iron losses, than the PS-FRPM machines. However, higher gear ratio machines feature lower cogging torques and torque ripples due to the smaller difference between the PM pole-pair number and iron piece number. Both prototypes of PS-FRPM machine operating in SPM mode and MG machine operating in RPM mode are built and tested to verify the FE predicted results.
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Magnetic gear has various advantages related to its capability as contactless moving gear with no lubrication and its overload protection ability. The magnetic gears are commonly used as a transmission that demands high torque characteristics. This paper aims to perceive the effects of the design parameters of Coaxial Magnetic Gear (CMG) on the transmitted torque and the magnetostatic field. Furthermore, the magnetic field and torque characteristics of CMG were inspected by ANSYS/Maxwell commercial software. The proposed CMG was developed using the Taguchi method. This method allows investigating performance under various factors. This paper used four levels in five factors that compose the radial length of the outer and inner iron part, the radial length of outer permanent magnet and inner permanent magnet, and the radial length of pole pieces. ANOVA test showed that the five factors contribute to the transmitted torque. The outer iron part acquires 86.49% contribution, which is the highest contribution, and the outer permanent magnet contributes 1.93%. The percentage of the pole pieces and inner permanent magnet contribution is 2.99% and 6.94%, respectively. Another factor, the inner iron part, was pooled because it did not significantly contribute to the torque. Using the Taguchi method, the transmitted torque prediction was 1861.33 N.m., while the simulation obtained 1853.96 N.m. Finally, the research result can be a consideration in designing transmitted torque demand.

Various studies have been conducted since magnetic gears can replace mechanical gears and the non-contact power transmission characteristics of magnetic gears have emerged. Recent studies use rare earth permanent magnets to represent the high power density of magnetic gears. However, the conductive properties of the rare earth permanent magnets resulted in a decrease in efficiency. Using the ferrite permanent magnets, which have no conductivity, were difficult to compete in size. This paper presents the basic design of magnetic gears using three permanent magnet (NdFeB, Nd-bonded, Ferrite) materials and shows the results of size, efficiency characteristics.

This paper presents a novel coaxial magnetic gear (MG), in which dual consequent-pole permanent magnet rotors are adopted. The MG is attractive for torque conversion in the advanced vehicle thanks to its high torque density. To achieve a comprehensive understanding of the MG, a subdomain model is established to analytically calculate the magnetic field in the MG by solving the vector magnetic potential equations while considering the corresponding boundary and continuity conditions. Consequently, the air-gap field distribution and the electromagnetic performance of the MG can be predicted. The finite element analysis method is also used to validate the accuracy of the proposed analytical method. In comparison with the conventional MG, both methods verify that the proposed MG can offer higher torque transmission density while consume less volume of magnets.

This paper presents a design optimization by Genetic Algorithm (GA) with controlled crossover and mutation by Artificial Neural Network (ANN). Co-axial Magnetic Gear (MG) design has been put under consideration for that research. Optimization process aims the improvement of MGs torques interaction by design sizes variation. Six different optimization parameters are defined which represents the MG design radiuses. ANN is trained for the optimization problem parameters preselection as a part of the GA optimization algorithm. Solution genomes crossover and mutation are controlled during new generation creation. The results obtained for MG design optimization shows the effectiveness of the proposed method, which, we believe, is suitable for wide variety of design optimization problems.

Gear systems in today’s industry are one of the critical pillars providing power transformation and matching speed and torque to application requirements. Reliability, high efficiency, and low operating costs are desirable due to the prevalence of gearboxes. This paper provides a comparative analysis of selected mechanical and magnetic gears, using a specific example to point out the advantages and disadvantages of contactless power conversion. After determining the essence of the operation of the magnetic gear on the base model, two magnetic gear optimization cases are presented, testifying to the application potential. An analysis of the stress distribution in the area of the teeth of the mechanical gearbox and the most stressed element of the magnetic gear – the modulator, was carried out. The effects of temperature and load on losses were measured and simulated, and ultimately, the efficiency characteristics of the two gears were also compared.

A technical innovation for the downsizing of motor system is necessary to realize the saving energy, and roomy and comfortable car interior space in electric vehicles. This research focuses on magnetic gears as a breakthrough because it can achieve maintenance-free, low vibration and low acoustic noise by non-contact power transformation, thus they are effective for the downsizing of system by the high-speed rotation of a motor because of solving drawbacks due to the contact of teeth in mechanical gears. However, there exists some challenges such as the low mechanical strength and the reduction of the gear efficiency on conventional magnetic gears in the high-speed drive. In this research, a novel in-wheel motor system of multiple high-speed motors and Magnetic Multiple Spur Gear (MMSG) for the electric vehicle is proposed. The MMSG can achieve the high mechanical strength and high transmitted efficiency by the structure separating several input rotors even in the speed region of 50000 min-1. Moreover, the drive system realizes the drastic downsizing and the high efficiency by MMSG transforming the input of multiple high-speed motors. In this paper, the characteristics of proposed drive system with MMSG are clarified by the simulation and experiment.