Axial Flux Permanent Magnet Research Articles

Experimental Investigation of Magnet Grouping Technique in Reduction of Cogging Torque and Total Harmonic Distortions in the Axial Flux PM Generator for Direct Drive Wind Turbine

Generators that have been used in direct-drive wind turbines could be produced with a permanent magnet between the power ranges of 1 kW to 1 MW. The cogging torque is the most significant problem of these generators. In this study, the pole grouping method is suggested to reduce the cogging torque of an axial flux permanent magnet (AFPM) synchronous generator with open slotted. The suggested method includes magnetic analyzes and experimental results of the 14-pole open slotted AFPM generator rotor poles that grouped as 7-7, 4-3-4-3, 3-2-2-3-2-2 and named A, B, C, respectively. Besides obtaining the cogging torque (Tc) data, the most considerable part of the study is to get total harmonic distortion (THDV) and output power (Pout) data in which show the electrical performance of the generator for different grouping angles. The optimum ranges for an AFPM generator were determined as 80% reduction for Tc, 15% reduction for Pout, and below 3% for THDV. The optimum grouping angles for A, B, and C were obtained by experimental studies as 32°, 32°, and 30°, respectively. Experimental results and the magnetic analysis have been shown that optimum values are obtained when the ratio of the pole angle to the grouping angle is

The Neutral Voltage Difference Signal as a Means of Investigating Eccentricity and Demagnetization Faults in an AFPM Synchronous Generator

This article investigates the neutral voltage difference signal, VNO signal, for fault diagnosis. The aforementioned signal is the signal of the voltage between the common star point of the stator and the common star point of the load. The under-study faults are demagnetization and static eccentricity faults, while the machine in which the faults are investigated is an axial flux permanent magnet (AFPM) synchronous generator, suitable for wind power applications. This study was conducted using a 3D finite element method (3D-FEM), and the machine’s FEM model was validated through experiments. This method is one of the most accurate methods for electrical machine computation, allowing for a detailed study of electromagnetic behavior. The components that constitute the VNO signal were determined using a 3D-FEM software program (Opera 18R2). Subsequently, further analysis was performed using MATLAB R2022b software, and a fast Fourier transform (FFT) was applied to this signal. In all the investigated faulty cases, new harmonics appeared, and the healthy amplitudes of most of the already existing harmonics increased. These findings can be used for fault identification. The analysis revealed that the harmonic frequency of 1.5fs was the most dominant in the case of demagnetization, while in the case of static eccentricity, the most dominant harmonic was a frequency equal to the machine’s operating frequency, fs. The novelty of this study is that this signal has not previously been used for fault identification, especially in AFPM synchronous machines. This signal depends on EMF voltage and stator phase currents but is less sinusoidal. Consequently, it can detect faults in cases where the aforementioned signals cannot be used for detection.

Equivalent Transformation Between Axial- and Radial-Flux Permanent-Magnet Machines

In this paper, an equivalent transformation approach for permanent-magnet (PM) electrical machines using coordinate transformation theory is presented. With the proposed method, an axial-flux PM (AFPM) machine can be transformed to an equivalent radial-flux PM (RFPM) machine or vice versa. The main purpose of proposing such a transformation method is to simplify the design of AFPM for specific applications. Besides, according to the working space requirement, the shape of PM machines can be changed while maintaining magnetic performances. The proposed transformation theory is based on the basic magnetic theory and the theoretical derivation is presented in detail. The resulted transformation equations for RFPM and AFPM machines with surface-mounted PMs are friendly to engineers. Moreover, an AFPM machine is used to validate the proposed theory, both FEM and experimental results have good agreement with the results of the proposed method. With the equivalent transformation theory, engineers could design, simulate and evaluate the AFPM machines with an exist well-developed machines.

An Improved Magnetic Equivalent Circuit Method for Segmented-Halbach Axial-Flux Permanent Magnet Machines

Much attention is paid to axial-flux permanent magnet (AFPM) machines thanks to the compactness and high torque density. However, the finite element method (FEM) for AFPM machines will cause a large computational burden during simulation. Thus, this paper proposes an improved magnetic equivalent circuit method (MECM) to predict the performances of segmented-Halbach AFPM machines, which can consider the different magnetization directions of Halbach PMs. Magnetic circuit is built in the whole machine. Besides, the air gap is treated as a slide layer to avoid the additional models for position-dependent permeances. Then, the predicted performances are compared with those calculated by FEM and measured in an experiment. As a quasi 3D analytical method, it predicts the same air-gap flux density as the FEM. The only errors are due to the serious flux leakage caused by opening slots. The predicted back EMF is consistent with the measurement in the experiment. The electromagnetic torque is affected by the errors of air-gap flux density, which results in large ripples. However, the trend of torque with current coincides with the measurement. It means the proposed MECM is effective for the segmented-Halbach AFPM machine.

Semianalytical Model of Multiphase Halbach Array Axial Flux Permanent-Magnet Motor Considering Magnetic Saturation

This paper proposes a nonlinear semi-analytical model (SAM) of the multi-phase Halbach array axial flux permanent magnet motor (AFPMM) to speed up the computation of its magnetic field. Compared to the existing analytical models, the proposed nonlinear SAM can directly consider magnetic saturation to obtain more accurate results. To this end, the multi-phase Halbach Array AFPMM is equivalent to several 2-D models by the Quasi-3D method under the Cartesian coordinate system. Then, the nonlinear SAM is developed by using the convolution theorem and the fast Fourier factorization. The proposed nonlinear SAM is studied on a five-phase Halbach array AFPMM with different rotors, and the nonlinear finite element (FE) model and experiment verify its effectiveness. The proposed SAM is computationally efficient and accurate, and it is also applicable to other types of multi-phase Halbach array PM electrical motors in Cartesian coordinates.

Improved Coreless Axial Flux Permanent-Magnet Machine With Nonuniformly Distributed Winding

The windings of coreless axial flux permanent-magnet machine (CAFPM) are exposed to the rotor magnetic field, where each back electromotive force (EMF) harmonic is induced not only by several different spatial field harmonics but also by the winding distribution. In order to reduce the time harmonics of back EMF for CAFPM with non-overlapping winding, one new winding structure with non-uniform distributed coils is proposed in this paper, in which the coils are arranged in a non-uniform way with annular distribution in the stator. Firstly, the relationship between uniform and non-uniform distributed windings is analyzed based on the phasor diagram of each coil EMF, and the analytical expression of back EMF under non-uniform distributed windings is established. Secondly, key performance indexes under different working states, like no-load, are investigated by three dimensional (3-D) finite element analysis (FEA). Finally, comprehensive experiments are carried out to verify related theoretical analysis, which demonstrate that the non-uniform distributed windings can effectively reduce the time harmonics and increase the working efficiency.

Impact of Manufacturing Tolerances on Axial Flux Permanent Magnet Machines with Ironless Rotor Core: A Statistical Approach

Axial Flux Permanent Magnet (AFPM) machines with ironless rotors are an attractive and recently studied solution in low-speed applications, due to their potentially high power/weight ratio, high aspect ratio, and high efficiency. Nevertheless, these machines are prone to be affected by manufacturing tolerance during its fabrication process and consequently, the magnets may move freely inside the rotor structure. This work presents a statistical analysis of manufacturing tolerances of an AFPM machine with an ironless rotor, considering several magnet fault types. A computationally efficient superposition method is developed and implemented to obtain both the cogging torque and rated torque considering several tolerance combinations with acceptable accuracy. The results obtained from a statistical analysis of 10,000 designs of a two-stator one rotor tooth coil winding AFPM (TCW-AFPM) machine allowed us to identify the parameters with the most impact on relevant performance indicators and disclosed a substantial increase in cogging and ripple torque when unavoidable combined tolerances are present.

Research on Electromagnetic System of Large Capacity Energy Storage Flywheel

A large capacity and high power flywheel energy storage system (FESS) is developed and applied to wind farms, focusing on the high efficiency design of the important electromagnetic components of the FESS, such as motor/generator, radial magnetic bearing (RMB) and axial magnetic bearing (AMB). Firstly, a axial flux permanent magnet synchronous machine (AFPMSM) based on soft magnetic composite material (SMC) is designed. Then, considering the structure of FESS and machine, a permanent magnet biased radial magnetic bearing (PMRMB) and a HALBACH axial passive magnetic bearing (PMB) is proposed which have the advantages of low power consumption and large bearing capacity, then the analyses are carried out according to the load capacity requirements. Finally, the flywheel prototype is fabricated and tested. The experimental results verify the relevant theoretical analysis, and finally can achieve stable and reliable operation.

Novel Multiphysics Design Methodology for Coreless Axial Flux Permanent Magnet Machines

This work is devoted to multiphysics design of coreless axial flux permanent magnet machines with concentrated coils. Recently, these machines have been proposed for the propulsion of civil miniature electric unmanned aerial vehicles, which need very high torque and power densities. Such requirements tend to be in contrast with other important features such as high efficiency and resilience, making the design quite challenging. Among the various geometrical parameters, the rotor poles to stator coils ratio plays a key role in their performance. The main contribution of this paper is the development of an original design method hinging upon the said ratio. The 2D electromagnetic model at the heart of the approach is derived at the average radius and accounts for multi-layer and axially thick stator coils. Additional contributions include the concurrent use of thermal modelling in the preliminary design stage and mechanical analyses in the design refinement stage aimed at optimizing torque density and guaranteeing rotor integrity at maximum speed. Comprehensive experimental tests on a full-scale prototype are reported and help build confidence in the proposed methodology.

Analysis of a High-Speed Axial Flux Permanent Magnet Synchronous Motor With Cost-Effective Hybrid Magnets

This paper proposes a novel high-speed axial flux permanent magnet synchronous motor (AFPMSM) with hybrid permanent magnets (PMs) to reduce the magnet cost. Based on basic AFPMSM with sinusoidal NdFeB PMs, the proposed model is designed with an equal quantity of ferrite and NdFeB PMs, wherein the hybrid PMs are divided into parallel and vertical to the magnetization direction of the PM. Furthermore, three kinds of hybrid PM arrangements are adopted for the AFPMSM and the relative analysis of the magnetic circuits is carried out. Then the no-load, load electromagnetic performance and torque cost of the AFPMSMs with three PM arrangements are simulated and compared by the finite element method (FEM). Finally, the demagnetization and stress analysis are performed to verify the stability of the AFPMSM with hybrid PM arrangements.

Comparison of Thermal Characteristics in Various Aspect Ratios for Radial-Flux and Axial-Flux Permanent Magnet Machines

This paper proposes an axial-flux permanent magnet machine (AFPM) that uses a neodymium-bonded permanent magnet (Nd-bonded PM) and a coreless rotor structure and assesses how it performs in industrial applications requiring high efficiency and low cost. The AFPM is able to achieve high efficiency in high-speed regions because its Nd-bonded PM can restrain eddy current loss. Additionally, the AFPM can continuously function at the rated operating point without a cooling system, so the expenses for one can be eliminated. Hence, the AFPM can simultaneously achieve high efficiency and low-cost mass production. In this paper, a comprehensive comparison including the thermal characteristics of the proposed AFPM and a conventional radial-flux permanent magnet machine (RFPM) is shown. Furthermore, this paper also focuses on how the aspect ratio influences the thermal characteristics of both machines. Finally, the effectiveness of the proposed AFPM in industrial applications is assessed using 3D-FEA and experimental results.

Leakage Flux Adjustment and Flux Field Optimization for Coreless Axial Flux Permanent Magnet Synchronous Machine Based on U-shaped Iron

Leakage Flux Adjustment and Flux Field Optimization for Coreless Axial Flux Permanent Magnet Synchronous Machine Based on U-shaped Iron

Stall Torque Performance Analysis of a YASA Axial Flux Permanent Magnet Synchronous Machine

There is a trend to go towards low gear-ratio or even direct-drive actuators in novel robotic applications in which high-torque density electric motors are required. The Yokeless and Segmented Armature Axial Flux Permanent Magnet Synchronous Machine is therefore considered in this work. In these applications, the motors should be capable to deliver high torque at standstill for long periods of time. This can cause overheating of the motors due to a concentration of the losses in a single phase; hence, it becomes necessary to derate the motor torque. In this work the influence of the slot/pole combination, the addition of a thermal end-winding interconnection and the equivalent thermal conductivity of the winding body on the torque performance at standstill will be studied both experimentally via temperature measurements on a prototype stator, and via a calibrated 3D thermal Finite Element model. It was found that both a good choice of the slot/pole combination and the addition of a thermal end-winding interconnection have a significant influence on the torque performance at standstill, and allow up to 8% increase in torque at standstill in comparison to a reference design.

Pole shape design of ring winding axial flux permanent magnet used in a direct‐drive electric boat for cost‐efficient cogging torque alleviation

AbstractThe recently introduced Ring Winding Axial Flux Permanent magnet Motor (RW‐AFPM) is inspired by the yokeless structure of Yokeless and Segmented Armature (YASA) AFPMs and the partitioned‐phases configuration of transverse flux motors. Its potential application for rim‐driven electric ships propulsion has been examined in the previous studies and, here, it will be studied for ultra‐light direct‐drive electric boats. Owing to their specific structural features, the relatively large cogging torque of RW‐AFPMs seems to be a significant challenge for such applications. On the other hand, as investigated in previous studies, cogging torque reduction methods, such as conventional skewing, seem unsuitable for this type of machine. Thus, this paper will address the cogging torque issue via a comprehensive pole shape analysis. The results will be useful for all kinds of partitioned‐phases topologies that have high specific torque capability but suffer from large cogging effects. All analyses are performed via 3D‐FE models, the accuracy of which is pre‐verified through experiments.

Torque Control of an In-Wheel Axial Flux Permanent Magnet Synchronous Motor using a Fuzzy Logic Controller for Electric Vehicles

This paper presents the control design of an in-wheel axial-flux permanent magnet synchronous motor with one stator and one rotor, using a fuzzy logic controller for electric vehicles. In this controller, the surgeon ambiguous inference file is built by two input vectors, the stator current error and the derivative of the stator error. These input variables include five membership functions: Negative Big (NB), Negative Small (NS), Equal Zero (ZE), Positive Small (PS), and Positive Big (PB). The fuzzy logic controller was implemented using a 5×5 matrix to meet the required output stator voltage of the controller. The fuzzy logic torque controller was compared with the PI controller in stator current response, torque, and speed. The proposed controller was evaluated using simulation results from MATLAB/SIMULINK.

A Study on Optimal Design Process of Dual Rotor Axial-Flux Permanent Magnet Synchronous Motors

The core design elements of the motor for the co-robot joint are miniaturization and high torque. In this paper, a dual rotor axial-flux permanent magnet (DRAFPM) motor is proposed to improve the performance of robot joints. DRAFPM motors have the advantage of reducing iron loss and minimizing volume because they can remove stator yoke. When designing a motor with the same volume, it can be designed to increase the height of the fixed ruler by the thickness of the yoke of the fixed ruler and increase the number of turns. In the case of existing axial-flux permanent magnet (AFPM) motors, the shape of the three-dimensional structure is limited by radial laminating of stator. Therefore, considering the production of 3D printing, the shape of stator shoe is designed. The optimal design problem of DRAFPM motor consists of real and integer design variables. In addition, due to the structural characteristics of DRAFPM motors, 3D finite element analysis (FEA) is required, so it takes a long time to interpret. Therefore, this paper proposes an efficient optimal design process to optimize the remaining real design variables after prioritizing the integer design variables. The proposed optimization process is applied to the DRAFPM motor for robot joints, and the optimal design plan satisfying the design function is derived from various design variables to prove the validity of the optimization process.

Demagnetisation optimisation of ring winding axial flux permanent magnet motor by modifying the load line of the magnet

AbstractRecently the new topologies of the permanent magnet (PM) motor attract a lot of attention. These newly introduced PM motors are not mature in design aspects and suffer from different perspectives. Therefore, it is necessary to develop a common design procedure to overcome these drawbacks. One of the common problems with newly introduced PM motors is the phenomenon of irreversible demagnetisation (ID) at high temperatures. So, a solution to this problem will be presented. The recently introduced ring winding axial flux PM (RWAFPM) motor is considered as a case study. First, a modified approach for modelling ID is introduced. Then, the effect of rotor geometry parameters on the ID of the RWAFPM motor is discussed. Finally, developed multi‐level multi‐objective design optimisation considering ID is presented based on the developed Taguchi method.

Investigation of different types of winding in counter-rotating double-rotor axial flux permanent magnet machines

ABSTRACT The choice of the winding type has a significant effect on the performance characteristics of the machine, efficiency, weight of the required wire, ease of winding, etc. The purpose of this paper is to investigate the effect of winding type as well as the polarity of magnets on both sides of the stator core on the back electromotive force (Back-EMF) of the counter-rotating double-rotor axial flux permanent magnet generator (CRDR-AFPMG). The winding type influences the produced voltages of the generator. Hence, in this paper, several types of windings, such as the first type of shifted winding, the second type of shifted winding, and the independent winding are investigated to produce three voltages with the same amplitudes and 120 degrees of phase differences. However, it has been found that, among the studied windings, only the second type of shifted winding and independent winding produces three voltages with similar amplitudes and 120 degrees of phase difference. Finally, the suitable type of winding for the proposed machine will be determined, and the performance of the case study machine will be evaluated by finite element (FE) simulation. JMAG designer software is used for this purpose.

Demagnetization Monitoring and Identification in PM Generators With Concentrated Windings During Transient Conditions

Direct drive permanent magnet machines used in renewables typically have high numbers of poles leading to low speeds. This poses a challenge for detection of faults during transients, while few periods of the operating waveforms can be captured. Moreover, it has been recently shown that the numbers of poles and stator coils determine the demagnetization harmonic generation mechanism and consequently the location of the fault signatures. This leads to different signatures in every permanent magnet machine. Additionally, the traditional signatures may not be clear in the spectrograms due to the low rotor speed. Under such conditions, the monitoring of specific higher harmonics can lead to reliable detection and identification of the demagnetization fault. The authors prove that the fault can be reliably distinguished from other rotor-related faulty conditions with the use of specific higher signatures in the stator current. Finally, the computed harmonics are experimentally verified in the lab, while the detection of demagnetization under transient conditions is achieved in this paper, that studies a double rotor axial flux PM generator as a case subject.

Torque Assessment of Multiphase AFPM Machines With Experimental Validation for YASA Topology

This work presents a theoretical and experimental study on the electromagnetic torque of multiphase axial-flux permanent-magnet (AFPM) machines. Initially, a torque equation is obtained through four different methods. The first method follows the definition of electromagnetic power, the second uses the magnetic shear stress, the third is based on the concept of co-energy, and the fourth applies the principle of Lorentz force. Although these methods are based on different assumptions, they lead to the same generalized torque equation, which depends on four parameters: i) the fundamental air gap induction; ii) the electrical loading defined for an arbitrary rotor radius; iii) the ratio of inner and outer rotor radius; iv) the cubic power of the outer rotor diameter. The torque equation is then validated using finite element (FE) analyses and experimental results obtained with a 32-pole/30-slot yokeless and segmented armature (YASA) machine. For this machine, the coils can be connected according to different arrangements so that the phase number can be chosen as three, five, or fifteen. The results show that the torque equation derived here provides sufficient accuracy in practical cases and that it can be applied to machines with phase numbers higher than three.