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Microstructure Role in Permanent Magnet Eddy Current Losses
Abstract
The impact of granular microstructure in permanet magnets on eddy current losses is investigated. A numerical homogenization procedure for electrical conductivity is defined. Then, an approximated simple analytical model for the homogenized conductivity able to capture the main features of the geometrical and material dependences is derived. Finally eddy current losses analytical calculations are given, and the two asymptotic expressions for losses in the stationary conduction limit and advanced skin effect limit are derived and discussed.
... Due to the high supply frequency values used in the considered machines, it is also useful to accurately evaluate the eddy-current losses in the magnets in order to reduce the risk of local overheating which could cause partial demagnetizations. The eddy currents induced in permanent magnets [16,17] can be obtained solving the equation: ...
The full electric aircraft concept requires
accurate design and suitable choice and integration of all the
used components. Obviously, the electric propulsion drive is
one of the most important on-board equipment. Both power
converters and electric motors must have low weight, suitable
overload capacity and high reliability. In this paper, two
possible solutions of electric propulsion motor for unmanned
aerial vehicle are investigated. Sizes, weight and some features
of a three-phase PMSM motor are compared to those of a six-
phase motor obtained by an appropriate rewinding of the
armature, with fixed stator and rotor magnetic circuits. The
losses obtained from the numerical analyzes are compared to
evaluate the convenience of one solution over the other.
Eddy-current analysis is an important research field. This phenomenon occurs in multiple areas and has several applications: electromagnetic braking, repulsive effects, levitation, etc. Thereby, this paper is limited to eddy-current study in rotating electrical machines. In the design process, if the permanent-magnet (PM) loss calculation is very important, the overheating due to eddy-currents must be taken into account. The content of this paper includes sources, calculation methods, reduction techniques, and thermal analysis of PM eddy-current losses. This review aims to act as a guide for the reader to learn about the different aspects and points to consider in studying the eddy-current.
Permanent magnet (PM) motors are rapidly replacing the dominant induction motors in industrial applications including pumps, fans, and compressors. PM motors are also gaining ground in critical sustainable energy applications such as wind systems, photovoltaic pumping systems and electric vehicles. Compared to induction motors, PM have higher efficiency. In this paper, the financial feasibility of replacing induction motors by PM motors at various operating conditions was analyzed on a preliminary basis. The impact of partial demagnetization and full loss of excitation on the feasibility of the replacement was also preliminarily investigated. It is found that the feasibility of replacement was less sensitive to reduction in the life time of PM motors than reduction in efficiency due to partial demagnetization. While detailed and lengthy studies are planned in the future, investigation outcomes suggest that the replacement remains feasible despite risks of demagnetization when utilization rates are above 50%. Details of the investigation are reported in the paper.
Demagnetization in permanent magnet synchronous machine (PMSM) is a special and common fault which greatly reduces the motor's efficiency, and thus an electric vehicle's performance. In this paper, we propose a method to estimate the motor's operating efficiency reduction after a certain flux density degradation. The aim is to reflect the relationship between the PMSM complete demagnetization and the resultant efficiency variation. The demagnetized motor is also introduced into the energy management of a plug-in hybrid electric vehicle (PHEV) based on dynamic programming to investigate its influence on the energy consumption. Simulation results indicate that the fuel economy reduces tremendously when the flux-density degrades by 20% and over.
NdFeB permanent magnets have different life cycles, depending on the applications: from as short as 2–3 years in consumer electronics to 20–30 years in wind turbines. The size of the magnets ranges from less than 1 g in small consumer electronics to about 1 kg in electric vehicles (EVs) and hybrid and electric vehicles (HEVs), and can be as large as 1000–2000 kg in the generators of modern wind turbines. NdFeB permanent magnets contain about 31–32 wt% of rare-earth elements (REEs). Recycling of REEs contained in this type of magnets from the End-of-Life (EOL) products will play an important and complementary role in the total supply of REEs in the future. However, collection and recovery of the magnets from small consumer electronics imposes great social and technological challenges. This paper gives an overview of the sources of NdFeB permanent magnets related to their applications, followed by a summary of the various available technologies to recover the REEs from these magnets, including physical processing and separation, direct alloy production, and metallurgical extraction and recovery. At present, no commercial operation has been identified for recycling the EOL NdFeB permanent magnets and the recovery of the associated REE content. Most of the processing methods are still at various research and development stages. It is estimated that in the coming 10–15 years, the recycled REEs from EOL permanent magnets will play a significant role in the total REE supply in the magnet sector, provided that efficient technologies will be developed and implemented in practice.
The coercive field of permanent magnets decreases with increasing grain size. The grain size dependence of coercivity is explained by a size dependent demagnetizing factor. In Dy free Nd2Fe14B magnets, the size dependent demagnetizing factor ranges from 0.2 for a grain size of 55nm to 1.22 for a grain size of 8300 nm. The comparison of experimental data with micromagnetic simulations suggests that the grain size dependence of the coercive field in hard magnets is due to the non-uniform magnetostatic field in polyhedral grains. (C) 2014 AIP Publishing LLC.
This paper describes how an electrical engineer should take into account the possibility of Permanent Magnet (PM) demagnetisation when designing a PM machine. Different modern PM materials and used magnet models both in parametric models and in Finite Element Method (FEM) models are shortly described. Demagnetisation models and hazardous situations in PM machines are discussed. Finally, the instructions how to check the design against the risk of demagnetisation are discussed.
Eutectic grain boundary diffusion process was applied to Nd–Fe–B hot-deformed magnet using Nd60Tb20Cu20 alloy, which resulted in a large coercivity enhancement from 0.87 T to 2.57 T with a relatively small decrease in remanent magnetization from 1.50 T to 1.38 T. Improved temperature coefficient of coercivity from −0.493%°C⁻¹ to −0.328%/°C⁻¹ led to a high coercivity of 1.47 T at 150 °C. The partial formation of Tb-rich shell on the surface of platelet shaped Nd2Fe14B grains while maintaining their ultra-fine grain size is the reasons for the substantial enhancement of the coercivity. Micromagnetic simulations suggested that a higher coercivity can be obtained when Tb-rich shell covers the c-plane surface interface of the grains than that covering the side surface interfaces. Improvement of the thermal stability of coercivity was found to be due to the exchange decoupling of Nd2Fe14B grains and the formation of (Nd,Tb)2Fe14B shell. In the frame of Kronmüller equation and based on the micromagnetic simulations, the improvement of the thermal stability of coercivity is attributed to the decrease of Neff and increase of α induced by exchange decoupling of grains, as well as the additional decrease of Neff induced by the formation of high-Ha shell.
When designing permanent magnet synchronous machines (PMSM) the losses in the permanent magnets may not be neglected. Especially in high-speed applications these losses contribute a proportion of the total losses which can be significant. In literature existing analytical loss calculation approaches are generally validated by numerical Finite Element Method (FEM) simulations and, however, have hardly been verified by experimental measurements. In this paper the eddy current losses of permanent magnets are investigated by an analytical loss calculation approach which is verified by FEM computations. Furthermore, the results are validated by a measuring setup which determines the AC loss in permanent magnets by a double coil measuring arrangement. The relative conductivity of the permanent magnet is an uncertain parameter for eddy current loss calculations. Therefore it is determined metrologically. The measuring arrangement which is used for this purpose is introduced. As a result important conclusions regarding the calculation and measurement of AC losses in permanent magnets are achieved.
We present a general finite-element solver, escript, tailored to solve geophysical forward and inverse modeling problems in terms of partial differential equations (PDEs) with suitable boundary conditions. Escript's abstract interface allows geoscientists to focus on solving the actual problem without being experts in numerical modeling. General-purpose finite element solvers have found wide use especially in engineering fields and find increasing application in the geophysical disciplines as these offer a single interface to tackle different geophysical problems. These solvers are useful for data interpretation and for research, but can also be a useful tool in educational settings. This paper serves as an introduction into PDE-based modeling with escript where we demonstrate in detail how escript is used to solve two different forward modeling problems from applied geophysics (3D DC resistivity and 2D magnetotellurics). Based on these two different cases, other geophysical modeling work can easily be realized. The escript package is implemented as a Python library and allows the solution of coupled, linear or non-linear, time-dependent PDEs. Parallel execution for both shared and distributed memory architectures is supported and can be used without modifications to the scripts.
A critical survey on some of the principal approaches to the solution of the general three-dimensional eddy-current problem is presented. The two main families of formulations, those magnetic and those electric, are discussed, with reference to both mathematical and computational aspects. Particular attention is paid to the vector potential formulations in view of uniqueness, gauge and interface problems.
Understanding the subtle link between coercivity and microstructure is essential for the development of higher performance magnets. In the case of R–Fe–B (R = rare earth) based materials this knowledge will be used to enable the development of high coercivity, Dy-free permanent magnets, which are relevant for clean energy technologies. A combination of high resolution characterization, molecular dynamics and micromagnetic simulations and model thick film systems has been used to gain valuable new insights into the coercivity mechanisms in R–Fe–B magnets.
Purpose
The purpose of this paper is to consider thermal analysis as part of an automated sizing and design process. The temperature estimation at characteristic points of the machine, and in particular in permanent magnets, is essential to accurately simulate the electromagnetic behavior and avoid irreversible demagnetization.
Design/methodology/approach
In this paper, an electromagnetic dimensioning model, parameterized by finite element analysis, is coupled to a thermal lumped‐parameter model to constitute a fast and efficient design tool for electrical machines.
Findings
A parameterized and hybrid FE‐analytical electromagnetic model, which combines analytical and numerical advantages, to archive a fast and accurate electromagnetic simulation results is combined with a thermal lumped‐parameter model for water‐cooled and passive air‐cooled surface mounted permanent magnet synchronous machines (PMSM).
Practical implications
Sizing, electromagnetic and thermal modeling aspects are integrated into an automated design process. The whole design process is demonstrated on two standard industrial servo motors for passive and active water cooling and afterwards compared with available measurements.
Originality/value
The proposed method allows considering thermal aspects during the iterative automated electromagnetic design process of PMSM.
The magnet shape in permanent-magnet (PM) synchronous motors substantially affects the back-electromotive-force (EMF) waveform and the stator iron losses, which are of particular importance in traction applications, where the energy available in the battery box is limited. This paper presents a methodology based on geometry optimization, providing sinusoidal back-EMF waveform. The method has been applied in a surface PM motor case for electric vehicle, and its validity has been checked by measurements on two prototypes, the first one with constant magnet width and the second one with optimized magnet shape.
For high-speed machines applications, eddy-current losses in the interior permanent magnet of synchronous machine (IPMSM) form a portion of the total losses which can be significant. Indeed, the magnets are exposed to the harmonic fields which rotate with respect to the rotor. The induced losses in the magnets provoke temperature arising that must be limited to avoid the risk of demagnetization. The study carries out a prediction of eddy current losses in PM where the skin effect is considered. A complete analytical model is presented and compared to 3D Finite Element (FE) harmonic computations. The results given by the proposed model are in agreement with the FEA results for local electromagnetic quantities and loss calculations. This approach can be useful for losses estimation in magnets when designing machines by analytical method.
A methodology is presented for the generation and meshing of large-scale three-dimensional random polycrystals. Voronoi tessellations are used and are shown to include morphological properties that make them particularly challenging to mesh with high element quality. Original approaches are presented to solve these problems: (i) “geometry regularization”, which consists in removing the geometrical details of the polycrystal morphology, (ii) “multimeshing” which consists in using simultaneously several meshing algorithms to optimize mesh quality, and (iii) remeshing, by which a new mesh is constructed over a deformed mesh and the state variables are transported, for large strain applications. Detailed statistical analyses are conducted on the polycrystal morphology and mesh quality. The results are mainly illustrated by the high-quality meshing of polycrystals with large number of grains (up to 105), and the finite element method simulation of a plane strain compression of ε = 1.4 of a 3000-grain polycrystal. The presented algorithms are implemented and distributed in a free (open-source) software package: Neper.
Rare earth permanent magnets are very sensitive to temperature as an overheating can lead to a partial or a total demagnetization. It is then necessary to determine the losses in the permanent magnets to design with accuracy an electromagnetic system. In fact, if the permanent magnet experiences a high frequency magnetic field, significant losses appear. In the literature, these losses are calculated assuming the permanent magnet to be a bulk conductive material. In this work, we aim at verifying this assumption and we give a method to determine the equivalent conductivity. Then, we present a comparison between calculated and measured characteristics of two permanent magnet samples.
A new, globally convergent iterative technique for the solution of
3D eddy current problems is described. This technique is based on the
A - V -formulation and requires successive solutions of
the Poisson and Laplace equations. For this reason, this technique can
be carried out with minimal new software development