Coaxial Magnetic Gear Research Articles

Two‐dimensional analytical investigation of the parameters and the effects of magnetisation patterns on the performance of coaxial magnetic gears

In this study, a two-dimensional analytical model has been presented for coaxial magnetic gears. Based on Maxwell's equations the extracted model has been obtained via the solution of the Laplace and Poisson equations by using the separation of variable technique for each sub-region (internal and external magnets, internal and external air gaps and slots). Then the integral coefficients of the general solutions have been obtained by using the boundary conditions and continuity between sub-regions. The proposed analytical model can be used as a tool for design and optimisation of magnetic gears. The influences of the number of pole-pairs, the magnetisation patterns, the size of slot-opening and the segment ratio of the two-segment Halbach pattern on the transmitted torque and unbalanced magnetic forces have been analytically investigated. Finally, to illustrate the efficacy of the proposed model, the analytical results of the magnetic field distribution, transmitted torque and unbalanced magnetic force have been compared with those obtained from finite element analyses.

Vibration and noise characteristics of coaxial magnetic gear according to low-speed rotor structure

This paper presents the analysis of the vibration and noise characteristics of Coaxial magnetic gears (CMGs) according to the Lowspeed rotor (LSR) structure. Surface-mounted permanent magnet CMG (SPM_CMG) and Flux concentrated CMG (FC_CMG) are selected for the comparison models. The electromagnetic force distributions in the air-gap adjacent to the LSR are calculated to predict the deformed shape of the LSR. In addition, the electromagnetic forces acting on each pole of the LSR according to the rotation of the rotors and their frequencies are obtained to investigate the vibration and noise characteristics. The mode shapes of the LSR core and their resonant frequencies are obtained through modal analysis to consider the resonance of the analytical models. The vibration and noise analysis result indicates that SPM_CMG has less vibration and noise compared than the FC_CMG.

A Novel Single-PM-Array Magnetic Gear With HTS Bulks

The coaxial magnetic gear is considered as a promising alternative to mechanical gearboxes. This paper proposes a novel single-permanent-magnet-array coaxial magnetic gear with only one layer of permanent magnets, which is stationary. The flux-modulating teeth are made of high-temperature-superconducting (HTS) material. Working principles of the proposed single-permanent-magnet-array coaxial magnetic gear are introduced analytically. A geometric optimization based on the ideal diamagnetic assumption of the superconducting bulks using the finite-element method is conducted to improve its torque capability. The performance is then evaluated by using the nonlinear E-J power law and compared with a single-permanent-magnet-array coaxial magnetic gear using ferromagnetic teeth. The result shows that the proposed topology can successfully magnify the input torque by a given gear ratio. The designed HTS-toothed single-permanent-magnet-array coaxial magnetic gear has a maximal output torque 1.73 times higher than its ferromagnetic-toothed counterpart.

Transfer Torque Performance Comparison in Coaxial Magnetic Gears With Different Flux-Modulator Shapes

Coaxial magnetic gears, which are capable of transferring torque without physical contact or a change in speed, have sufficient advantages, including high efficiency and reliability, allowing them to replace mechanical gears. Such the coaxial magnetic gears have therefore been receiving attention for various applications, including wind power generation and electric vehicles. However, when two rotors of a coaxial magnetic gear, which are attached using permanent magnets (PMs), rotate, a high transfer torque ripple is generated owing to the difference in magnetic resistance of the flux-modulator. In particular, the torque ripple of the inner rotor is relatively higher than that of the outer rotor. These torque ripples should be reduced as much as possible during the design process because they cause vibrations and noises in the machinery. Therefore, in this paper, various flux-modulator shapes are proposed to compare the transfer torque characteristics of a coaxial magnetic gear. The transfer torque characteristics resulting from the proposed shape were derived using nonlinear finite element analysis based on a 2-D numerical analysis.

Analysis of the Vibration Characteristics of Coaxial Magnetic Gear

This paper investigates the vibration and noise characteristics of the coaxial magnetic gear (CMG) by the electromagnetic forces. In the CMG, the vibration and noise can be generated by the exciting forces acting on three parts, namely, low-speed rotor, high-speed rotor, and modulating pieces, respectively. The electromagnetic forces acting on three parts of the CMG and their force frequencies are calculated by using the Maxwell stress tensor method. Also, the mode shapes of the CMG full model and their resonance frequencies are obtained by the modal analysis. Through the electromagnetic forces and the result of the modal analysis, the vibration calculation is performed by using the 3-D-finite element method. Then, the effect of displacement on noise is confirmed by comparing the vibration calculation data and experiment data. The result shows the most affected part among the three parts of the CMG due to the noise is the low-speed rotor.

A Coaxial Magnetic Gear With Consequent-Pole Rotors

In this paper, a novel coaxial magnetic gear (MG) is proposed, in which both permanent magnet and consequent poles are employed on the inner and outer rotors. The MG is expected with improved torque transmission capability by increasing the effective magnetic flux and offering additional reluctance torque. Air-gap field distribution and torque–angle characteristic of this MG are analyzed and compared with the conventional MG, which adopts dual surface-mounted permanent magnet rotors. To maximize the torque density, several key design parameters are investigated using finite-element method. Analysis results indicate that the proposed MG with ferrite magnets can largely improve the pull-out torque. In addition, end effect and power losses with different rotor structures are analyzed. Performance of two optimized MGs is then validated with test. Also, the MG cost effectiveness with different magnet materials is presented.

A Novel Electric Vehicle Powertrain System Supporting Multi-Path Power Flows: Its Architecture, Parameter Determination and System Simulation

In this paper, a novel electric vehicle powertrain system is proposed. In the system, a coaxial magnetic gear (CMG), an electromagnetic clutch, a lock, and two electric machines (EMs) are adopted to achieve the power-split by controlling the states of the clutch and the lock, which enables electric vehicles (EVs) to work in four operation modes. The configuration, power flow paths and operation modes are depicted in detail. A dynamic model is established to help determine the parameters and build simulation models. The simple control strategy is adopted to achieve flexible power-splits. How to determine the relevant parameters to meet the drive requirements in the powertrain system is also elaborated. A dynamic simulation using MATLAB/Simulink is performed to take into account the control strategy and New European Drive Cycle. Finally, the simulation results demonstrate, in theory, the rationality of the determined parameters and the feasibility of the operation modes as well as the control strategy.

Parametric analysis and optimized torque characteristics of a coaxial magnetic gear based on the subdomain analytical model

This paper presents the torque calculation and parametric analysis of a coaxial magnetic gear (CMG). We obtained analytical magnetic field solutions produced by permanent magnets based on a magnetic vector potential. Then, the analytical solutions for magnetic torque were obtained. All analytical results were extensively validated with nonlinear two-dimensional finite element analysis. Finally, using the derived analytical magnetic torque solutions, we carried out parametric analysis to determine the influence of the design parameters on the CMG’s behavior.

A Novel Coaxial Magnetic Gear and Its Integration With Permanent-Magnet Brushless Motor

A magnetic geared machine (MGM) is believed to be a promising candidate for high-torque direct-drive application. One of the key issues for developing MGMs is how to resolve the contradiction between the good performance and the complex structure. This paper aims at proposing a novel coaxial magnetic gear (CMG), which will not increase the mechanical complexity after integration with a permanent magnet (PM) brushless machine. The prominent feature of the proposed CMG is the introduction of the stator with modulating teeth, which function as the same as the modulating pole-pieces in the conventional CMG. The integrated MGM can then be achieved by inserting the armature windings into the stator slots. The configuration, harmonic analysis, and torque capability of the proposed CMG are compared with the conventional CMG. The operating principle and electromagnetic performance of the proposed MGM are investigated by dividing it into one vernier PM machine, one PM brushless machine and one proposed CMG. The results show that the developed integrated MGM exhibits good torque capability and power factor.

Analytical Magnetic Field Calculation of Coaxial Magnetic Gear With Flux Concentrating Rotor

In this paper, the analytical model for coaxial magnetic gear (CMG) having a flux concentrating rotor is presented using 2-D space harmonic analysis (SHA). Comparing the conventional CMG with surface-mounted permanent magnet (SPM) on both rotors, the differences are described in terms of governing equations, magnetizations of PMs, general solutions, and boundary conditions. To apply the superposition principle, the analytical model is divided into two cases, such as model for SPM rotor and model for flux concentrating rotor with inset PM. Based on the proposed SHA, the radial and tangential components of flux densities at both air-gap regions are calculated, and the pull-out torques of both rotors are also determined by the Maxwell stress tensor. The results of SHA are in good agreement with the results obtained from finite-element analysis.

Developments of an Efficient Analytical Scheme for Optimal Composition Designs of Tubular Linear Magnetic-Geared Machines

By adopting the analytical model and magnetic equivalent circuit concepts, a systematic scheme that can be conveniently applied for the optimal component composition designs of an integrated tubular linear magnetic-geared machine (TLM2) will be provided. The TLM2 system can be either adopted to generate electric powers or to supply large mechanical thrust forces. With its complex double-mover and coaxial magnetic gear structures, the associated system performance based on different component parameters and arrangements can be rationally and efficiently evaluated. Based on thorough flux coupling designs and 3-D finite-element analyses, the adequacies of the proposed analytical scheme for assisting the optimal TLM2 system design at specified physical constraints can be confirmed. A laboratory TLM2 prototype has successfully been developed to operate as a generator for demonstration, and it is confident that its linear actuator counterparts can also be conveniently and reasonably designed based on the same scheme.

Comparison of Coaxial Magnetic Gears With and Without Magnetic Conducting Ring

This paper quantitatively compares four coaxial magnetic gears (CMGs) with and without magnetic conducting ring (MCR), which are artfully integrated into a permanent-magnet (PM) machine. In addition, different gear ratios are investigated for both topologies. By using the finite-element method, their torque and loss performances are compared and evaluated. Analysis results show that the CMGs without MCR and low gear ratio can offer higher pull-out torque and lower torque ripple, whereas the CMGs with MCR and low gear ratio have lower PM eddy current and iron loss. Finally, a 9/6 slot-pole CMG machine without MCR is prototyped for verification.

Analytical Investigation on the Power Factor of a Flux-Modulated Permanent-Magnet Synchronous Machine

In recent years, flux modulation is increasingly receiving attention since it is capable of adjusting the pole-pair number and rotational speed of the magnetic field. This principle has already been applied to magnetic gear (MG) to realize the low-speed high-torque operation [1]. Flux-modulated permanent magnet synchronous machine (FM-PMSM) is an integration of a coaxial MG with a conventional outer rotor PM machine. Compared with the coaxial MG, a stator with armature windings is employed in the FM-PMSM to replace the inner rotor of the coaxial MG [2-3], as shown in Fig. 1 (a). A modulation ring with a number of ferromagnetic pole-pieces is sandwiched between the outer rotor and stator, and two air-gaps are thereby formed to separate the above three components from each other. The modulation ring could act as a mediator to match the pole-pairs differential between the two magnetic fields generated by the PMs and armature windings, respectively. The advantage of the FM-PMSM is that it could provide high torque at low speed directly without employing mechanical gearbox, mechanical issues, such as friction loss and mechanical fatigue, are thus avoided and high reliability could be achieved. However, it is found that the power factor of the FM-PMSM is relatively low [4], and few details in the published literatures are provided to explain the intrinsic causes of this problem. Therefore, this paper will develop an analytical model to predict the power factor of the FM-PMSM and investigate the essence of the low power factor. Because of the existence of the flux modulation, many high frequency harmonics are introduced into the magnetic field of the FM-PMSM. It is important to have a good knowledge of the magnetic field distribution in the FM-PMSM in order to accurately predict its power factor. This paper gives an alternative to the commercial finite element method (FEM) to quickly obtain the electromagnetic solution. This approach could provide many physical insights into the electromagnetic solution. In this paper, partial differential equations are used to describe the magnet field behavior in terms of magnetic vector potentials. The whole calculation domain is partitioned into six regions, viz. the stator slot and slot opening (Region I_i and Region II_i), the inner and outer air-gaps (Region III and Region V), the slots between modulation pole-pieces (Region IV_j), and the PMs (Region VI), as shown in Fig. 1 (b). In Region II_i, III, IV_j and V, the magnetic vector potentials are governed by Laplace's equations. And in Region I and Region VI, magnetic vector potentials are of Poisson's equations due to the existence of current and PMs, respectively. By applying the constraints on the interfaces between these regions, the solution of the magnetic field could be derived. Fig. 2 (a) compares the analytically and numerically calculated distribution of the radial flux density in the inner air-gap due to the PMs on the outer rotor. As can be seen, the analytical results have a good agreement with the FEM calculation, which indicates the developed analytical model could be employed as a powerful tool for predicting and analyzing the power factor of the FM-PMSM. With the knowledge of the solution of the magnetic field, the flux linkage of each phase winding could be obtained through the surface integral of the magnetic vector potential in Region I_i. Then, the self and mutual inductances of each winding could be deducted, which are 4.06 mH and 2.02 mH, respectively. Similarly, the analytically and numerically calculated inductances have a good agreement. Substituting the inductances, currents and resistances into the voltage equation, the voltage of each phase winding could be obtained as well. Fig. 2 (b) presents the waveforms of the voltage and current of phase-A winding versus the time at the rated power. Finally, the active and reactive power could be calculated by expressing the flux linkage in terms of impedances, and the predicted power factor of the studied FM-PMSM is 0.6, which is quite low compared with conventional PM machines. And the low power factor will inevitably lead to the increase of the capacity, volume and loss of the inverter.

Design and Analysis of Coaxial Magnetic Gears Considering Rotor Losses

In recent years, coaxial magnetic gears (CMGs) are particularly attractive for applications desiring high torque density, such as electric vehicles and wind generators [1]. They can offer several advantages like low acoustic noise, maintenance free, and inherent overload protection in comparison with conventional mechanical gears. However, because of the modulation effect by the stationary ring, the magnetic fields produced by either the outer or inner rotor permanent magnet (PM) have many sub-harmonics, which lead to high PM eddy current (EC) loss and iron core loss. Notwithstanding the use of silicon steel sheet, the outer rotor loss of CMGs can't be neglected. The purpose of this paper is to propose a novel rotor structure for CMGs to reduce the rotor losses, based on the limitation of sub-harmonics.

Finite-Element Analysis of the Magnetostatic Field of a Coaxial Magnetic Gear Device

This paper presents the magnetostatic field analysis of a coaxial magnetic gear device proposed by Atallah and Howe. The structural configuration and speed reduction ratio of this magnetic gear device are introduced. The 2-dimensional finite-element analysis (2-D FEA), conducted by applying commercial FEA software Ansoft/Maxwell, is performed to evaluate the magnetostatic field distribution, especially for the magnetic flux densities within the outer air-gap. Once the number of steel pole-pieces equals the sum of the pole-pair numbers of the high-speed rotor and the low-speed rotor, the coaxial magnetic gear device possesses higher magnetic flux densities, thereby generating greater transmitted torque.

Closed-Loop Control of Local Magnetic Actuation for Robotic Surgical Instruments

We propose local magnetic actuation (LMA) as an approach to robotic actuation for surgical instruments. An LMA actuation unit consists of a pair of diametrically magnetized single-dipole cylindrical magnets, working as magnetic gears across the abdominal wall. In this study, we developed a dynamic model for an LMA actuation unit by extending the theory proposed for coaxial magnetic gears. The dynamic model was used for closed-loop control, and two alternative strategies—using either the angular velocity at the motor or at the load as feedback parameter—were compared. The amount of mechanical power that can be transferred across the abdominal wall at different intermagnetic distances was also investigated. The proposed dynamic model presented a relative error below 7.5% in estimating the load torque from the system parameters. Both the strategies proposed for closed-loop control were effective in regulating the load speed with a relative error below 2% of the desired steady-state value. However, the load-side closed-loop control approach was more precise and allowed the system to transmit larger values of torque, showing, at the same time, less dependence from the angular velocity. In particular, an average value of 1.5 mN $\cdot$ m can be transferred at 7 cm, increasing up to 13.5 mN $\cdot$ m as the separation distance is reduced down to 2 cm. Given the constraints in diameter and volume for a surgical instrument, the proposed approach allows for transferring a larger amount of mechanical power than what would be possible to achieve by embedding commercial dc motors.

Analysis and Design Optimization of a Coaxial Surface-Mounted Permanent-Magnet Magnetic Gear

This paper presents the analysis and design optimization of a coaxial surface-mounted permanent-magnet magnetic gear. The magnetic field distribution in the coaxial magnetic gear is calculated analytically in the polar coordinate system and then validated by the finite element method (FEM). The analytical field solution allows the prediction of the magnetic torque, which is formulated as a function of design parameters. The impacts of key design parameters on the torque capability are then studied and some significant observations are summarized. Furthermore, the particle swarm optimization (PSO) algorithm is employed to optimize the studied magnetic gear. Given that the torque capability and material cost conflict with each other, both of them are set as the optimization objectives in this paper. Different weight factors may be chosen for the two objectives so that more attention can be placed on one or another. The results shows that the highest torque density of 157 kNm/m3 is achieved with the consideration focusing on the torque capability only, then the highest torque per permanent magnet (PM) consumption could be improved to 145 Nm/kg by taking the material cost into account. By synthesizing the torque capability and material cost, a 124 kNm/m3 of torque density and a 128 Nm/kg of torque per PM consumption could be achieved simultaneously by the optimal design.

Magnetic field analysis of a coaxial magnetic gear mechanism by two-dimensional equivalent magnetic circuit network method and finite-element method

Non-contact magnetic gears offer unique features: no mechanical loss, no mechanical fatigue, overload protection, high efficiency, precise peak torque transmission, tolerance of misalignment, isolation of vibration and low acoustic noise, which overcome the inherent disadvantages of existing mechanical gears. Among the various types of magnetic gear mechanisms, the coaxial magnetic gear mechanism improves the utilization of permanent magnets to generate a high transmitted torque, which has been receiving more and more attention in recent years. This paper presents a two-dimensional (2-D) equivalent magnetic circuit network method to predict the magnetic field distribution of a coaxial magnetic gear mechanism. The topological structure, working principle and speed reduction ratio of an existing coaxial magnetic gear mechanism are introduced first. A 2-D equivalent magnetic circuit network method is then proposed to calculate the radial components of the magnetic flux densities within the inner and outer rotors. 2-D finite-element analysis (FEA), undertaken by applying commercial FEA software Ansoft/Maxwell, confirms the validity and effectiveness of the proposed analytical model. A 2-D equivalent magnetic circuit network method can be provided as an effective tool for the magnetic field analysis and optimal design of coaxial magnetic gear mechanisms.

Cost-Effectiveness Comparison of Coaxial Magnetic Gears With Different Magnet Materials

This paper presents a comparative study between the nonrare-earth PM and rare-earth PM-based coaxial magnetic gears. First, using finite-element analysis, the electromagnetic performances of four coaxial magnetic gears, which are installed with nonrare- or rare-earth PMs, are analyzed and quantitatively compared based on the same structure. Then, the natural magnetic properties of nonrare- and rare-earth PMs are evaluated and discussed. Finally, the cost effectiveness of coaxial magnetic gears adopting different types of PMs is assessed. The results show that the nonrare-earth PMs, especially the aluminum-nickel-cobalt, are preferred for application to coaxial magnetic gears with emphasis on the cost effectiveness.

Design Assessments of a Magnetic-Geared Double-Rotor Permanent Magnet Generator

By adopting the double-rotor structure and the coaxial magnetic gear concept, a permanent magnet (PM) generator is proposed for harvesting the wind energy directly without mechanical conversions. In addition to the physical model realizations, this paper will present the detailed design and implementation concerns of the magnetic-geared double-rotor PM generator along with thorough loss assessments. The comparisons of PM magnetization directions and arrangements, the selections of proper windings, and the operational efficiencies of this machine along with the terminal voltage phase shifts due to different loading conditions will all be summarized. The realization process of a portable 2.5-kVA generator that can be directly coupled to the mechanical driven source without additional mechanical gears will be demonstrated, and the feasibility of such a machine structure can certainly be confirmed from the satisfactory performance results.