Flux Permanent Magnet Machine Research Articles

PM Eddy-Current Loss Reduction of Axial Flux PM Wheel Motor With Arc Incompletion Segmentation

The high permanent magnet (PM) eddy-current loss will inevitably cause the risk of irreversible demagnetization, and the PM complete segmentation (CS) method is a common method to reduce the PM eddy-current loss. However, PM CS design will increase the cost and difficulty of assembly process, especially for axial flux PM (AFPM) machines. To address this issue, a new arc incompletion segmentation (AIS) method is proposed to reduce the PM eddy current loss and cost, including double-side AIS and single-side AIS designs. Then, some of the key parameters are optimized. In addition, the PM eddy current loss, cost, and torque performance are studied and compared with different segmentation methods under different conditions based on 3-D finite element method (FEM). Finally, two different 25kW prototypes are manufactured and tested to validate the proposed method and analysis.

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.

Nonlinear Semianalytical Model for Axial Flux Permanent-Magnet Machine

In this article, we propose a novel nonlinear semianalytical model (AM) for the magnetic field calculation of electric machines. The nonlinear properties and local saturation effect of the iron part are taken into consideration in Cartesian coordinates, which is the main contribution of the proposed model. Thus, high accuracy of electromagnetic field results can be obtained with the low computational time cost. The model is developed based on the harmonic modeling technique by solving Maxwell's equations. The detailed theoretical derivations, which use the complex Fourier series and the Cauchy product, are presented. To verify the proposed model, an axial flux permanent-magnet (PM) machine is selected to be investigated. Both finite-element model and experimental results agree well with that of the proposed model. Moreover, the nonlinear AM has potential application for other types of PM electrical motor in Cartesian coordinates, such as flat PM linear machines.

Two-dimensional Harmonic Modelling for Electro-magnetic Solution in Cartesian Coordinates

This paper first presents a general two-dimension (2D) harmonic analytical solution for the magnetic field of electric machines in the Cartesian coordinates. In this solution, the relative permeance is directly considered in Laplace and Poisson’s equations, and the particular solutions in Cartesian coordinates are solved. By applying the complex Fourier separation method, with the boundary and interface conditions, the magnetic field in the inhomogeneous region is solved from system equations. Numerical examples validate the presented method and the obtained results have a satisfactory agreement with the finite element analysis. The proposed model in this paper has a significant value for modelling electric machines, such as linear permanent magnet (PM) machines and axial flux PM machines.

Innovative Construction of the AFPM-Type Electric Machine and the Method for Estimation of Its Performance Parameters on the Basis of the Induction Voltage Shape

The article presents an innovative construction of the Axial Flux Permanent Magnet (AFPM) machine designed for generator performance, which provides the shape of induced voltage that enables estimation of the speed and rotational angle of the machine rotor. Design solutions were proposed, the aim of which is to limit energy losses as a result of the occurrence of eddy currents. The method of direct estimation of the value of the rotational speed and rotational angle of the machine rotor was proposed and investigated on the basis of the measurements of induced voltages and machine phase currents. The advantage of the machine is the utilization of simple and easy-to-use computational procedures. The acquired results were compared with the results obtained for estimation performed by using the Unscented Kalman Filter (UKF).

Comparison Between Dual-Armature Linear Switched Flux Permanent Magnet Machine and Linear Surface-Mounted Permanent Magnet Machine Considering Thermal Conditions

It has been proved that dual-armature permanent magnet (PM) machines have higher power density than conventional PM machines under the same copper loss. However, the thermal conditions of these machines are different. This paper makes a fair comparison between a dual-armature linear switched flux PM machine (DALSFPMM) and a linear surface-mounted PM machine (LSPMM) by considering both electromagnetic performances and thermal conditions. First, the topologies of two machines and platform are introduced. Then, the 2D and 3D finite element (FE) models are employed for the electromagnetic and thermal analyses, respectively. During the machine optimization, the thermal analysis is used to determine the copper loss under the fixed machine dimensions, while the electromagnetic analysis is used to optimize the machine dimensions under the fixed copper loss. After iterations, two machines are optimized and their performances are compared. Under the limitations of 80 °C maximum PM temperature and 120 °C maximum winding temperature, the DALSFPMM can exhibit 38% higher thrust force density for the continuous duty and up to 43% higher thrust force density for the short-time duty. Finally, a prototype machine of DALSFPMM is tested to validate the analyses.

Modeling, Investigation, and Minimization of AC Winding Loss in Slotless PM Machines

The AC winding loss can be the primary source of losses in slotless permanent magnet (PM) machines. This paper presents the simplified model, investigation, and reduction method of the AC winding loss in slotless radial flux PM machines. Preferentially, the open-circuit AC winding loss is calculated in the rotor reference frame based on the resistance limited. Subsequently, a fast optimization model, including the AC winding loss, is developed by solving the field equations only once for a specific geometry. As a result, the influence of the design parameters on the AC winding loss can be investigated by taking advantage of the fast computational model. It shows that the concentrated winding with multiples of 2-pole/3-coil is the best selection to achieve the low AC winding loss due to the possibility of using multiple parallel paths. Finally, the finite element (FE) and experimental results demonstrate the validity of the proposed analysis.

Various parameters influence on field distribution in eccentric disc-type permanent magnet machine based on analytical method

ABSTRACT Eccentricity fault is a common fault in axial flux permanent magnet (PM) machines that can be destructive. The finite element method (FEM) has been already used for modelling eccentricity fault in disc type PM machine. Applying three-dimensional (3D) FEM to this motor is essential. However, 3D-FEM is a very time-consuming technique. Therefore, it is more appropriate to use an analytical technique to investigate the impact of different parameters upon the motor’s performance. The existing analytical methods usually represent a qualitative overview of the faulted machine. In this paper, a precise and quick analytic method is introduced, reflecting the impact of various important parameters, including rotor angle and fault position on the motor’s performance. This method enables the calculation of the induced voltage in a search coil. Comparing the results obtained from the proposed method and FEM, in both the no-load and full-load conditions, confirms the method’s accuracy. Finally, the impact of the search coil’s various parameters on dynamic and static eccentricity faults is examined. In addition, choosing the proper search coil pitch for eccentricity faults detection is discussed.

Optimization Design of a Dual-Stator Switched Flux Consequent Pole Permanent Magnet Machine With Unequal Length Teeth

This article proposes a dual-stator switched flux consequent pole permanent-magnet (DS-SFCPM) machine with unequal length teeth and carries out optimal design of the DS-SFCPM machine. Compared with a conventional dual-stator switched flux permanent-magnet (SFPM) machine, it can be regarded as a combination of two different dual-stator machines, so as to significantly enhance the torque density, and has good flux-regulating capability (FRC) with field winding. First, the basic principle of the proposed DS-SFCPM is explained, and the torque-generating mechanism and flux-regulating performance are analyzed based on the superposition of the above two dual-stator machines. Then, the influence of the unequal length teeth on the torque density and FRC is analyzed, the evaluation indexes of optimization are also introduced, and the parameters of unequal length teeth and permanent magnets (PMs) are optimized by sensitivity analysis. Various finite-element analysis (FEA) simulation, optimization, and comparison studies of the proposed machine are performed. The results verified the effectiveness of the proposed structure.

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.

An Inverse Thermal Modeling Approach for Thermal Parameter and Loss Identification in an Axial Flux Permanent Magnet Machine

Despite the use of state-of-the-art thermal modeling tools in the design stage, the measured thermal behavior of prototype electrical machines can differ significantly from the modeled ones. This paper shows how a thermal model, based on the finite-element method, of an electric machine can be improved using inverse modeling techniques. In a thorough study, a forward high fidelity finite-element thermal model of a 4-kW axial flux permanent magnet (PM) machine is introduced and improved using inverse modeling techniques via noncollocated thermal sensors. Parametric model order reduction of the high fidelity finite-element thermal model based on the moment matching method is performed to make the recovery of the actual thermal parameters characterizing the thermal behavior of the axial flux PM machine tractable. Furthermore, the same reduced order model is used to identify the different power loss components in the machine. Experimental results confirm that the presented two-stage approach is capable of identifying the thermal parameters and losses with high accuracy.

A Systematic Study on the Effects of Dimensional and Materials Tolerances on Permanent Magnet Synchronous Machines Based on the IEEE Std 1812

In the process of designing and manufacturing an electrical machine, a systematic study of dimensional and material tolerances is of the utmost importance. This paper proposes a systematic method by which the effect of design specification variations on permanent magnet (PM) synchronous machine performance may be identified and quantified. The method combines design of experiments techniques, open-circuit and short-circuit physical measurements, and virtual test simulations conducted based on the recently approved IEEE Std 1812 testing guide. Three case studies, two provided by a spoke-type PM radial field machine configuration, in two designs with different electromagnetic loading, and an axial flux PM machine are discussed. It is shown that based on the output performance, out of specification tolerances for magnet remanence, steel grade, as well as dimensional variables, and stator to rotor eccentricity, may be identified under certain conditions. It is also exemplified that the ratings, magnetic loading, and configuration of the machine play critical roles and should be thoroughly considered as part of the studies.

Characterization of a Variable Flux Machine for Transportation Using a Vector-Controlled Drive

Variable flux machines (VFMs) have one more dimension for the control and performance improvement of the drive. The magnet flux in the VFM can be controlled to optimize motor performance. Software packages used in the design and performance evaluation of variable flux permanent magnet (PM) machine use a single demagnetization curve and recoil the operating point based on a linear recoil line parallel to the original demagnetization curve. Since the magnets like AlNiCo that is used in VFM have a nonlinear demagnetization curve, performance prediction of a VFM designed using finite-element software needs experimental verification. This paper presents a method based on vector control for the characterization of a VFM. An existing drive is used to measure the dq inductances of the machine at several magnetization levels, including the cross-magnetization effects. The same technique is extended to measure the torque-ripple and the torque-angle characteristics of the machine at different magnetization levels. Experimental results are provided for a 5-hp variable flux PM machine. The proposed method uses an existing drive to perform the tests, requiring no additional test setup.

Overview of novel magnetically geared machines with partitioned stators

This study overviews recent development of a new class of magnetically geared machine topologies, i.e. partitioned stator (PS) machines. They are developed from magnetic gears and magnetically geared machines, as well as stator permanent magnet (PM) machines (switched flux, flux reversal, and doubly saliency PM), wound field, or hybrid PM and field winding machines. Based on the operating principle, i.e. magnetic gearing effect/air-gap field modulation and flux switching by the salient rotor iron poles, various PS machine topologies are developed. All have features of two stators, two air-gaps, and one segmented ferromagnetic rotor identical to a magnetic gear's modulating rotor. Their inherent relationships are revealed, while their electromagnetic performance is compared. Both PM and wound field PS machines are discussed, together with hybrid excited PS-PM machines and Vernier machines. It shows that all of these PS machines share the same torque production principle and the differences are mainly in PM configurations and relative positions of two stators. All PS machines have higher torque density per copper loss compared with their counterparts of single-stator machines. PS switched flux PM machines can produce the highest torque density per copper loss.

A New Algorithm for Optimum Design of Slotless Axial Flux Permanent Magnet Motor

In this paper, sizing equations of an Axial Flux Permanent Magnet (AFPM) machine based on analytical method are presented. Dimensions of the stator and rotor cores are calculated. It is shown that the magnetic flux densities throughout these cores remain closed to the flux density of B-H curve knee point of the ferromagnetic material characteristics. A new algorithm is proposed to determine dimensions of different parts of machine and it is used to calculate height of permanent magnet precisely. To show the suggested algorithm capability, a sample AFPM machine is designed based on sizing equations and Finite Element Analysis (FEA) is employed to validate these design formulas. A complete simulation study is done and some of the results are presented to confirm the accuracy of the sizing equations.

Analysis of Magnetic Gearing Effect in Partitioned Stator Switched Flux PM Machines

The partitioned stator switched-flux permanent magnet (PM) (SFPM) (PS-SFPM) machine is a novel stator-PM machine in which PMs and armature windings are separately placed in two different stators. Compared with the conventional SFPM machine with a single stator in which both PMs and armature windings are located, the PS-SFPM machine can generate higher torque density due to the utilization of the machine's inner space. In this paper, the magnetic gearing effect in the PS-SFPM machine is investigated in terms of air-gap field harmonics based on a simple magnetomotive force (MMF)-permeance model. It is found that the PS-SFPM machine is one type of magnetically-geared machine, in which the modulations of rotor iron pieces to the open-circuit PM and armature reaction fields are similar to those found in the magnetic gear and the conventional magnetically geared machine. The pole-pair numbers and rotating speeds of the air-gap field harmonics obtained by the MMF-permeance model are verified by finite element (FE) analysis. More than 93% of the average electromagnetic torque is contributed by the main air-gap field harmonics having pole-pair numbers $(2i - 1){\boldsymbol p}$ PM and $\rm{| N}_{\rm{r}} \,\pm \, $ (2 i -1) p PM| ( i = 1,2,3) in a PS-SFPM machine with p PM-pole-pair PMs and Nr -rotor-pole. A prototype machine is fabricated and tested to verify the FE analysis.

Direct Liquid Cooling in Low-Power Electrical Machines: Proof-of-Concept

This paper evaluates the feasibility of a direct liquid cooling approach in the thermal management of an axial flux permanent-magnet machine. To demonstrate the cooling method, a test motor was fitted with helical tooth coil windings formed from a hybrid conductor comprising a stainless steel coolant conduit tightly wrapped with a stranded Litz wire. The motor is a 100-kW permanent-magnet, axial-flux, double-stator, single-rotor machine. The proof of concept integrated the motor with a closed liquid coolant loop, appropriate instrumentation, and a data acquisition system. The general performance of the motor was examined at various power levels using polyalphaolefin oil as the cooling fluid. The results show the proposed cooling method to be feasible, and furthermore, to provide significant improvements to the machine thermal management.

Analytical Modeling of Static Eccentricities in Axial Flux Permanent-Magnet Machines with Concentrated Windings

The aim of this paper is to calculate the static eccentricity (SE) of a double rotor axial flux permanent magnet (AFPM) machine by using a general analytical model. The flux density in the air gap under healthy conditions is calculated firstly, where the axial and circumferential magnetic flux densities are obtained using a coupled solution of Maxwell’s equations and Schwarz-Christoffel (SC) mapping. The magnetic flux densities under SE conditions are calculated afterwards using a novel bilinear mapping. Some important electromagnetic parameters, e.g., back electromotive force (EMF), cogging torque and electromagnetic (EM) torque, are calculated for both SE and healthy conditions, and compared with the finite element (FE) model. As for the double rotor AFPM, SE does not contribute much effect on the back EMF and EM torque, while the cogging torque is increased. At each calculated section, FE models were built to validate the analytical model. The results show that the analytical predictions agree well with the FE results. Finally, the results of analytical model are verified via experimental results.

Comparative study of biased flux permanent magnet machines with doubly salient permanent magnet machines considering with influence of flux focusing

Different from the doubly salient permanent magnet (PM) machines (DSPMMs), the stator/rotor pole number combination (Ns/Nr) in biased flux PM machines (BFPMMs) is flexible and Nr can be any integer except the phase number and its multiples. When Ns/Nr differ by one, 6-stator pole BFPMM exhibits bipolar phase flux-linkage and symmetrical phase back-electromotive force waveforms. Based on the optimal 6/7 Ns/Nr and the rated copper loss, the torque density of BFPMMs is enhanced by 19%, 18.8% and 40.3% respectively when employing the flux-focusing structures with inner-type, outer-type and combined-type. Meanwhile, the PM utilization efficiency is also enhanced. Moreover, based on the inner-type flux-focusing structure, the optimized 6/7 Ns/Nr BFPMM exhibits about 18% higher average torque and 80% lower torque ripple than the optimized 6/4 Ns/Nr DSPMM. Further, the unbalance between phases which is observed in DSPMMs is overcome in the BFPMMs. The analyses are validated experimentally on a prototype machine.

Parasitic Currents in Stray Paths of Some Topologies of YASA AFPM Machines: Trend With Machine Size

In some topologies of yokeless and segmented armature (YASA) axial flux permanent-magnet (AFPM) machines, the particular frame structure required to fasten floating teeth can originate stray paths where parasitic currents may circulate. This paper investigates how the amplitude of these currents and the related additional Joule losses depend on the machine size. Reference is made to wind energy generators, rated from some tens of kilowatts up to a few megawatts. The developed model is mainly based on wind turbine basic data of vendor data sheets, and on the quantities evaluated and tested in two YASA AFPM prototypes, rated 50 and 200 kW.