Axial Flux Permanent Magnet Machine Research Articles

Design and Analysis of Oil-Immersed Cooling Stator With Nonoverlapping Concentrated Winding for High-Power Ironless Stator Axial-Flux Permanent Magnet Machines

Ironless stator axial-flux permanent magnet (AFPM) machines provide many attractive electromagnetic performances such as high efficiency in high-frequency mode and strong short-term overload ability, which make them more competitive in flywheel energy storage applications. However, designing high-torque-density AFPM machines, particularly their ironless stators, is with significant challenges and needs great efforts. In this article, a high-strength oil-immersed cooling stator with nonoverlapping concentrated winding is proposed for high-power ironless stator AFPM machine applications. This article explains in detail the design, analysis, and manufacturing process of the proposed oil-immersed cooling stator. With the three-dimensional finite-element analysis (3-D FEA), a 9000-r/m/50-kW 12-slot/10-pole prototype for a flywheel energy storage system is successfully designed and developed using the proposed high-strength oil-immersed cooling ironless stator. The experimental test results demonstrate the effectiveness of the cooling design and mechanical strength.

Designing Rotor Disks of a Coreless Axial Flux Permanent Magnet Machines by Using Simplified FEM and an Approximation Method

The article deals with designing rotor disks of a double-sided axial flux permanent magnet machine, based on their deflection, due to the pull forces between opposite rotor disks, caused by the permanent magnets. Permanent magnets cause the magnetic flux density to flow between the permanent magnets on opposite rotor disks and its size determines the size of the pull force. That is why it is essential to determine the size of the axial component of magnetic flux density in the middle of the clearance between opposite rotor disks (fictitious air gap) in order to calculate the deflection of the rotor disks. The article presents a new method for determining the size of the axial component of magnetic flux density, based on a simplified finite element method and an approximation method as well as modified equations for determining the deflection of rotor disks. The method is used to determine the pull forces between rotor disks and based on the deflection of the disks an optimum rotor disk thickness can be selected in order to reduce the weight of the machine.

A fully coreless Multi-Stator Multi-Rotor (MSMR) AFPM generator with combination of conventional and Halbach magnet arrays

An axial flux permanent magnet (AFPM) generator is known to be a good candidate for both low and high-speed application. In this paper, a new design of fully coreless multi-stator multi-rotor (MSMR) AFPM generator has been presented with conventional and Halbach magnet arrangement combined with an ironless (epoxy) rotor. For MSMR topology of AFPM machine, back iron is still present in the middle rotor and to maintain the same distribution of magnetic flux, magnets are using on both side of the middle rotor. This paper suggests replacing the middle iron rotors with a single epoxy rotor which reduces the weight of the machine, hence increase power density. On the other hand, for the elimination of iron and magnets in the middle rotor, conventional magnet arrays are used to maintain a continuous flux path. In addition, Halbach array is adopted on the external epoxy rotor to reduce flux leakage on the external sides of the machine. The performance of the proposed generator is investigated in terms of voltage, current, power, power density, and torque ripple. The analytical design approach is first presented and subsequently validated using ANSYS Maxwell® electromagnetic finite element analysis (FEA) software. It is found that a fully coreless MSMR AFPM generator with conventional and Halbach magnet arrays gives higher power density and lower torque ripple with a reduced axial length which is favorable in wind power and pico-hydro applications.

Analytical and experimental analysis of Back EMF waveform of a TORUS-type non-slotted axial flux permanent magnet synchronous machine with shifted rotor

In this paper, the effectiveness of rotor shifting technique on the Total Harmonic Distortion (THD) reduction of back electromotive force (Back EMF) of a TORUS type non-slotted axial flux permanent magnet (TORUS NS-AFPM) machine is investigated. The paper’s objective is to conduct this investigation using analytical and experimental methods. Therefore, an analytical formula is proposed for obtaining Back EMF of the machine with shifted rotor. A 3.7 kW and 1400 rpm machine is selected as case study machine and the effectiveness of rotor shifting technique on the THD reduction of Back EMF is confirmed using the proposed formula and experimental results. In addition, an analytical technique for calculating the optimum value of rotor shifting angle for minimizing THD of Back EMF is proposed. The obtained results confirm that the THD of Back EMF of the case study machine decreases from 11.5% to 5% by optimal selection of rotor shifting angle.

Effect of Slot-Pole Combination on the Electromagnetic Performance of Ironless Stator AFPM Machine With Concentrated Windings

Due to the elimination of the magnetic core on the stator, the design of coils is more flexible and more sensitive to the number of rotor poles for ironless stator axial flux permanent magnet (AFPM) machine. The slot-pole combination is a vital aspect of optimization design for the improvement of torque and power density. This article is mainly aimed at investigating the influence of slot-pole combination on the comprehensive performance of ironless stator AFPM machine with non-overlapping concentrated windings. Firstly, the space utilization of coils and winding factor of non-overlapping concentrated winding are deduced in formula form to compare the output capacity. And the copper mass and utilization are analyzed with different numbers of slots and stator diameter. Then, field utilization ratio is defined and derived to analyze magnetic flux leakage of Halbach-array PM dual-rotor for different numbers of poles. The performance comparison of the optimized prototypes is carried out using three-dimensional finite element analysis (3-D FEA). The back-EMF, torque and winding loss are also highlighted. Finally, a 50 kW prototype with 12-slot and 10-pole is designed and manufactured for experimental validations. This paper provides a valuable reference to select the basic configuration for the ironless stator AFPM machine application.

Dual-Skew Magnet for Cogging Torque Minimization of Axial Flux PMSM With Segmented Stator

Reduction of the cogging torque is preferred during the design process of a permanent magnet (PM) synchronous machine (PMSM), especially in the low-speed traction applications. This article presents a dual-skew magnet technique to minimize the cogging torque in the axial-flux permanent-magnet (AFPM) motor with a yokeless and segmented armature (YASA). The theoretical expression of the cogging torque of the AFPM machine is deduced, and the advantages of the dual-skew magnet are verified by comparing with a sector shape magnet and a conventional skew magnet using the 3-D finite-element method (FEM). The simulation results show that, compared with the other two magnet topologies, the machine equipped with the dual-skew magnet shows its advantages in the reduction of cogging torque, torque ripple, and magnet eddy current loss.

Analysis of Motional Eddy Currents in the Slitted Stator Core of an Axial-Flux Permanent-Magnet Machine

This article concerns the modeling and design of a slitted stator core for single-sided axial-flux permanent-magnet machine application. The stator core is specially designed to maximize the magnetic-flux density in the air gap and to minimize the eddy-current losses occurring at high rotational speeds. To reduce the effort needed for computing the motional eddy-current distribution in the presence of nonlinear material characteristics, a novel method is proposed. It combines the harmonic balance method, which is advantageous for simulating in the frequency domain the steady-state periodic response of a nonlinear system under harmonic excitation, together with a source description that introduces a complex magnetization that mimics the displacement of the permanent-magnet array. Following this method, time-domain distributions and losses can be reconstructed accurately with a low number of harmonics. A 3-D periodic model of the slotless axial-flux machine is built in the framework of isogeometric analysis (IGA) and a mixed formulation is employed, which relies on high-order Nedelec edge elements. The proposed model is embedded into a gradient-based optimization problem to determine the optimal shape of the slits in the stator core of the motor. This results in a novel cost-effective solution for improving the efficiency of axial-flux permanent-magnet machines.

3-D Hybrid Model of the Axial-Flux Motor Accounting Magnet Shape

This article presents a generalization of an analytical model of an axial-flux permanent-magnet machine to any magnet shape. It uses an existing model, which computes the 3-D magnetic-flux density by the separation of variables and a finite-difference method. The original magnet shape is modified by adding a radial dependency to the Fourier series description of permanent magnets (PM) magnetization. The model is then developed for a general complex magnet shape. As an example, the model will be computed for a circular magnet shape and will be compared with finite-element analysis.

Minimization of Cogging Torque in Axial Field Flux Switching Machine Using Arc Shaped Triangular Magnets

Axial flux permanent magnet machine (AFPMM) provides high torque characteristics at low speeds without any mechanical gears. AFPMMs have numerous applications in wind energy, electric cars, and direct drive elevator applications. These machines have low cost and improved power to weight ratio. However, single sided AFPM suffers from torque ripples because of its non-sinusoidal back emf, cogging torque, and rotor eccentricity. There are two major components of pulsating torque, namely torque ripples and cogging torque. In PM machine design, the cogging torque is a serious concern because it adds unwanted harmonics to the pulsating torque. Whereas the torque ripples cause noise and vibrations. In order to gain high efficiency, torque ripples should be minimum. The aim of this research is to design “Slotted axial field flux switching permanent magnet machine”. Mathematical models are used to design the machine and Finite element method (FEM) has been used to analyze the machine. In addition, Latin hypercube sampling (LHS) has been used to create the samples. Finally, Kriging Method is used for approximating the model and genetic algorithm has been applied to get the optimum machine. The results showed 61.8 % reduction of the cogging torque in the proposed machine model as compared to the conventional one. Moreover, the optimized model further provided 6.15 % reduction in the cogging torque as compared to the proposed one.

Evaluating the Effects of Electric and Magnetic Loading on the Performance of Single and Double Rotor Axial Flux PM Machines

Axial-flux permanent-magnet machines are used particularly in applications requiring a compact structure. Their disc-shaped topology and axial airgap have led to a variety of configurations including two popular ones: the yokeless and segmented armature (YASA), and the single-stator single-rotor or single-sided machine. In this study, a comprehensive comparative analysis of these configurations is conducted at different magnetic and electric loadings. It is found that at lower loadings, typically employed for air-cooled machines, the torque/ampere characteristics of the YASA machine are almost identical to those of a single-sided machine constructed with half the magnet volume. On the other hand, the single-sided machine outperforms the YASA machine when the magnet volumes in both machines are maintained equal. However, for higher electric loadings, the torque/ampere characteristics of the YASA machine droop significantly less than those of the single-sided machine. This article includes analytical estimations, which are verified with experimentally validated finite element analysis (FEA) simulations. In addition, the impacts of the armature reaction on saturation and the magnetic flux linkage, the magnet losses, and eddy current losses in both machines are also explored.

Systematically Exploring the Effects of Pole Count on the Performance and Cost Limits of UltraHigh Efficiency Fractional hp Axial Flux PM Machines

Optimizing the design of electric machines is a vital step in ensuring the economical use of active materials. The three-dimensional (3-D) flux paths in axial flux permanent magnet (AFPM) machines necessitate the use of computationally expensive 3-D electromagnetic analysis. Furthermore, a large number of design evaluations is required to find the optimum, causing the total computation time to be excessively long. In view of this, a two-level surrogate assisted algorithm capable of handling such expensive optimization problems is introduced, which substantially reduces the number of finite element analysis (FEA) evaluations to less than 200 while conventional algorithms require thousands of designs to be analyzed. The proposed algorithm is employed to optimally design an AFPM machine within a specified envelope, and to identify the limits of cost and efficiency. In order to obtain these limits, the variables' ranges are assigned to be as wide as possible, resulting in a vast design space, the study of which was enabled by the developed special algorithm. Additionally, optimized designs with different rotor polarities are systematically compared in order to form the basis for a set of generalized design rules.

High Torque Density Fractional-Slot Concentrated-Winding Axial-Flux Permanent-Magnet Machine with Modular SMC Stator

In this article, the implementation of a single-stator double-rotor fractional-slot concentrated-winding (FSCW) axial-flux permanent-magnet machine is presented to verify a simple and reliable structure design and oil-immersed cooling of the stator with modular soft magnetic composites (SMC) core. Due to the yokeless and segmented stator, the process of embedding the windings into the stator teeth will probably become simpler. By preforming rectangular copper wire, the shorter end-winding length and higher fill factor can be achieved. In order to eliminate eddy current losses and manufacturing costs, the fiberglass epoxy is used for the supporting components of the fixation for modular SMC stator and PMs, which is a nonmagnetic and nonconductive material. Hence, the robustness of stator and rotor are taken into account due to the stator modularity weakening effect on strength. Meanwhile, a fully sealed stator with oil-immersed cooling is proposed to enable the current density of 20 A/mm2 in the armature windings for stable operation. The electromagnetic properties, including in magnetic field distribution, torque capacity, and flux-weakening ability, are performed based on the three-dimensional finite-element analysis, as well as the mechanical performance. The coupled thermal and computational fluid dynamics model is built to predict the flow and temperature distribution inside stator with oil-immersed forced cooling. The construction and experimental verification of a full-scale 30-kW prototype are elaborated to verify the predicted performances in terms of torque density, structural strength, and cooling.

2-D Analytical Model for Predicting Magnetic Flux Distribution in Slotless Single-sided Axial Flux Permanent-Magnet Synchronous Machines

This paper estimates the magnetic flux density components in the slotless single-sided axial flux permanent-magnet synchronous machines (SAFPMSMs). For this purpose, a 2-D analytical model based on the sub-domain method is utilized in which the cross-section of the presented machine is divided into the seven sub-regions such as stator side exterior, stator, winding, air-gap, permanent-magnets (PMs), mover and mover side exterior. Based on the Maxwell equations, the related partial differential equations (PDEs) of magnetic flux density components are formed in each sub-region which are identified as the essential step for obtaining the machines quantities. According to the superposition theorem, two separate steps are implemented for calculating the magnetic flux density components. In the first step, open circuit analysis includes various type of magnetization patterns, i.e. parallel, ideal Halbach, 2-segment Halbach and bar magnet in shifting direction is investigated and armature currents are zero and in the second step PMs are inactive and the magnetic flux density components are originated due to only armature reaction. Eventually, 2-D finite element method (FEM) is determined to confirm the accuracy of the presented analytical approach and an acceptable agreement between the analytical and FEM models can be observed.

Analysis of end effect in ironless stator AFPM machine via MEC model

The Halbach-array permanent magnet (PM) rotors can achieve higher air-gap flux density due to magnetic flux gathering effect, which contributes to the higher-torque density improvement of the ironless stator axial flux PM (AFPM) machine. However, the end leakage fluxes of the ironless stator AFPM machine are serious due to the large magnetic resistance of the main flux loop in ironless stator machines. Circular arc straight line permeance model is adopted to calculate magnet-to-magnet and magnet-to-rotor end leakage fluxes based on the analysis of the relationship between end leakage fluxes and coils. A magnetic equivalent circuit (MEC) model is established considering end leakage fluxes. The leakage coefficients representing the ratio of end leakage fluxes to main flux and radial coefficients representing effective radial lengths of end leakage fluxes are analysed via the MEC model. The coefficients vary with machine parameters including PM thickness, rotor yoke thickness and air-gap length. On the basis of the analysis results, the influence factors of end leakage fluxes are summarised and the 50 kW ironless stator AFPM machine is optimised. Finally, a 50 kW ironless stator AFPM machine with Halbach-array PM rotors is prototyped to verify the results of the finite element analysis and MEC model.

Magnet shifting for back EMF improvement and torque ripple reduction of a TORUS‐type nonslotted axial flux permanent magnet machine

Magnet shifting for back EMF improvement and torque ripple reduction of a TORUS‐type nonslotted axial flux permanent magnet machine

축자속 영구자석 전동기의 인쇄회로 고정자 권선 형태 분석 및 비교

축자속 영구자석 전동기의 인쇄회로 고정자 권선 형태 분석 및 비교

Design and Optimization of Halbach-Array PM Rotor for High-Speed Axial-Flux Permanent Magnet Machine With Ironless Stator

In this article, a high-speed Halbach-array permanent magnet (PM) rotor for ironless stator axial-flux permanent magnet (AFPM) machine is proposed and researched with consideration of the multiphysics constraints, including the electromagnetic characteristics, mechanical strength, and rotor dynamics. Considering the severity of attractive force between twin rotors, the complex repulsive force among PM segments of Halbach-array and centrifugal force, the new hybrid sleeve, and PM fixation method are proposed to improve the rotor mechanical strength. Then, the rotor stress is analyzed by numerical analysis and three-dimensional (3-D) finite-element analysis (FEA). Furtherly, the sleeve thickness and interference fit are optimized by 3-D FEA. The rotor dynamics and bearing arrangement are analyzed to validate the effectiveness of the high-speed Halbach-array PM rotor. A 50-kW, 9000 r/min ironless stator AFPM machine prototype with Halbach-array PM rotor is manufactured and tested. The measured results show that the output power is up to 53.8 kW at 9000 r/min. The feasibility of high-speed Halbach-array PM rotor and accuracy of analysis method are validated by both simulated and measured results.

Design and analysis of an axial flux permanent magnet motor for the direct drive radial piston pump

Direct drive motor-pumps have attracted a lot of attention from researchers because of high efficiency and compact structure. This paper presents a new type of direct drive motor-pump, which integrates the structure of the radial piston pump and axial-flux permanent-magnet machine. The rotor of the disc motor is fixedly connected to the pump rotor, and the new pump uses a new type of distribution shaft for oil distribution. This structure reduces the intermediate transmission mechanism, and the application of the disc motor further reduces the axial size, the overall volume, and heat dissipation. The efficiency of the entire system has been improved. The three-dimensional finite element method is used to dynamically analyze flow field in the flow channel, which is extracted and simplified. The flow and pressure of the pump meet the design requirements in the simulation. A disc motor was designed for the pump, and the theoretical formula of the disc motor was derived. The initial size of the motor was obtained. The response surface method and three-dimensional finite element are used to optimize the corresponding axial-flux permanent-magnet machine. Loading the torque condition of the pump to the load end of the motor, the response of the motor meets the requirements.

Development of a High-Performance Axial Flux PM Machine With SMC Cores for Electric Vehicle Application

A new axial flux permanent magnet machine (AFPM) with soft magnetic composited (SMC) cores is proposed for electric vehicle (EV) application in this paper. As its windings are wound on the stator ring core, it can be regarded as a toroidally wound internal stator (TORUS) machine. With the adopted SMC material for stator core, this machine has the benefits of 3-D magnetic flux properties. The windings and SMC cores can be designed to form a very compact structure, and thus, the torque density can be improved greatly. To obtain the a good flux concentrating ability, two TORUS machines are designed and analyzed, one is with NdFeB magnet for high-performance EV application and the other is with the cheap ferrite magnet for low-cost application. The 3-D finite-element method is used to analyze the electromagnetic parameter and performance. For performance comparison, a commercial AFPM with yokeless and segmented armature P400 is used.

Design, Analysis, and Prototyping of a Water-Cooled Axial-Flux Permanent-Magnet Machine for Large-Power Direct-Driven Applications

This paper presents a design process and a detailed multiphysics analysis of an axial-flux permanent-magnet synchronous machine for large-power direct-drive applications. The machine in this paper is 130 kW at 26 r/min with a dual-stator inner-rotor structure. A stator core that is assembled with segmented and prewound teeth is first proposed and applied in a large-power axial-flux permanent-magnet machines (AFPMs). Through this method, the challenges of manufacturing large-diameter AFPMs can be solved. A novel water-cooling system is embedded in the machine to transfer the heat. In addition, the assembling procedure and the manufacture process are also proposed, and a novel distributed follower bearing is used to reduce the deformation and stress of the rotor disk. Finally, based on the multiphysics design, a prototype machine is manufactured and tested. The experiment results match well with the finite-element analysis.