Epstein Frame Research Articles

Effect of Annealing Temperature and Time on the Magnetic Properties and Magnetic Anisotropy of a Temper-Rolled, Semi-processed Non-oriented Electrical Steel

The effect of final annealing temperature and time on the core loss, magnetic permeability, and magnetic anisotropy of a temper-rolled, semi-processed 0.5 wt.% Si non-oriented electrical steel was investigated. The magnetic properties of the steel sheets at 50–400 Hz and 0.5–1.50 T were measured by the Epstein frame method on strips cut along both the rolling (RD) and transverse directions (TD). Optimal magnetic properties were obtained when the annealing temperature was at 800–825°C, and the annealing time was 2–4 h. Relatively large magnetic anisotropy between the RD and TD was observed in samples after recrystallization (~ 10% in core loss and ~ 70% in relative permeability), while deformed and non-recrystallized samples showed small anisotropy in magnetic properties. Regardless of the processing state of the steel, i.e., temper-rolled, recovered, or recrystallized, the core loss followed quadratic polynomial functions with respect to both the frequency and magnetic flux density, while the relative magnetic permeability followed cubic polynomial functions with respect to both the frequency and magnetic flux density. The microstructure and texture of selected samples were characterized by electron backscatter diffraction, which revealed the correlations between the magnetic properties of the steel and the microstructure and texture.

Calculation of core hysteresis loss resistance for Buntenbach’s analogy hysteresis model

The value for core hysteresis loss (CHL) resistance in Buntenbach’s analogy hysteresis model does not consider the impact of frequency variations. In order to address this issue, this paper proposes a calculation expression for CHL resistance that takes into account the influence of frequency variation through a variable coefficient. The paper analyzes changes in CHL resistance values at different frequencies using hysteresis loss characteristics and circuit theory. To validate the proposed calculation method for CHL resistance, Buntenbach’s analogy hysteresis model is applied to model the Epstein frame and magnetically saturated controllable reactor. The comparative results reveal that the proposed calculation expression for CHL resistance is effective.

An Experimental Setup for Power Loss Measurement up to 1 kHz using an Epstein Frame at CMI

Abstract This paper describes an experimental setup used at the Czech Metrology Institute (CMI) to measure the specific power loss of oriented and non-oriented electrical steel sheets up to 1 kHz using an Epstein frame. Special attention is given to a) a description of the hardware that is used, b) a description of the feedback control and measurement software, and c) an analysis of the sources of uncertainty and validation. Calibration expanded uncertainty of (0.5 up to 1.6)% for k = 2 can be achieved with this setup.

High Temperature Magnetic Properties of Selected Steel Grades

In order to effectively control the microstructure of steel strip during manufacture it is vital that techniques be developed to monitor microstructural changes in the steel at high temperatures. Electromagnetic non-destructive testing is increasingly being used in these applications as it is non-contact, robust, relatively inexpensive and has the potential to provide real-time feedback to control the manufacturing process. However, if the data produced by these systems is to be fully exploited it is necessary to accurately measure the magnetic properties of steel grades of interest at temperatures up to the Curie point. This work details measurements carried out to assess the magnetic properties of selected steels grades, supplied by Tata Steel, up to the Curie temperature using a novel test rig based on a modified Epstein frame designed by the University of Manchester with high temperature trials at the University of Warwick. Several steel grades are studied with the full magnetic hysteresis (BH) loops reported during heating and cooling, along with extracted magnetic parameters including coercivity, magnetic saturation and magnetic permeability. The magnetic parameters of the selected steel grades are compared and implications of the results on electromagnetic monitoring of steel strip during manufacture considered. Results show the developed system has the potential to provide valuable data to inform online electromagnetic monitoring systems.

Influence of the geometric parameters of a vertical rotational single-sheet tester on sample field homogeneity

PurposeThe horizontal rotational single-sheet tester (RSST) suffers from weaknesses such as the reduced size of test samples, measurement disturbances due to magnetic flux leakage and nonhomogeneity of field in the measurement area. Although the vertical RSST allows to overcome the first two aforementioned drawbacks, the heterogeneity of the field in the test sample remains an issue. In addition, there is still a lack of device standardization to ensure test repeatability, as already is well established with the Epstein frame. This paper aims to investigate the influence of several parameters on the field homogeneity in the test sample.Design/methodology/approachA fully 3D finite element model of a vertical RSST is developed and used to perform a sensibility study on several geometrical parameters.FindingsThe influence of several parameters on the field homogeneity in the test sample, such as the geometrical dimensions of the yokes, the presence or not of holes drilled inside the test sample for B-coil placement as well as the size of the H-coils and B-coils, is addressed.Originality/valueIt is expected that this study will contribute to the optimization and standardization vertical RSSTs.

Application analysis of self-bonded electrical steel sheet in high power density PMSM for all-electric aircraft

Driving motor for all-electric aircraft (AEA) needs high power density and high torque density, resulting in the magnetic load and electrical load of driving motor in a limit state. Therefore, it is important to analyze the accurate calculation of core loss and the technology of loss suppression. Firstly, B35A230 self-bonded electrical steel sheet is used as core, which can reduce core loss caused by machining process. Then the ratio-loss curves of B35A230 at different frequencies and different flux densities are measured by Epstein frame and ring sample method, and the increment of core loss after machining is obtained, and the influence of axial welding paths on stator core loss is analyzed. According to experimental results, Based on the classical stator core loss model, machining factors are considered to improve the accuracy of stator core loss, and a core loss model is adopted to improve accuracy, considering processing factor, alternating and rotating magnetization modes and harmonic magnetic field. Finally, an outer rotor SPMSM for AEA is designed and manufactured, and the no-load loss is tested experimentally, which verified that the self-bonded electrical steel sheet has certain advantages in high power density driving motor.

Effect of grain size on the core loss of nanocrystalline Fe86B13Cu1 prepared by ultra-rapid annealing

Nanocrystalline alloy samples with the same chemistry (Fe86B13Cu1) but with a range of mean grain sizes were prepared in order to investigate the effect of the grain size (D) on the core losses in nanocrystalline alloys. The core loss was measured on an Epstein frame in a frequency range between 10 Hz and 30 kHz. The measured core loss was separated into 3 parts, i.e. the hysteresis loss, classical eddy current loss and the excess loss. The classical eddy current loss is only a minor component due to the small thickness and moderately high electrical resistivity of the samples. Both the coercivity and the cycle hysteresis loss are found to follow roughly a D3 dependence. The excess loss at the maximum polarization (Jm) between 0.6 and 1.0 T is described universally by a simple power dependence of (Jm·f)n with an exponent of about 1.4 over a wide frequency (f) range between 10 Hz to 30 kHz. The excess loss at low frequencies appears almost independent of the grain size. Thus, the grain size effect on the total loss at low frequencies is attributable to the change in the hysteresis loss which reflects the static coercivity. However, the excess loss at 30 kHz shows a clear increase with an increase in D, indicating that the excess loss also becomes influential to the grain size dependence of the total core loss at high frequencies.

Temperature-Dependent Ferromagnetic Loss Approximation of an Induction Machine Stator Core Material Based on Laboratory Test Measurements

The accurate measurement and modeling of ferromagnetic material losses are vital issues during the design and analysis of electrical machines. Higher loss values can describe the manufactured rotor and stator machine plates better than the catalog data obtained by standardized measurements using the Epstein frame. In this paper, different temperature-dependent models based on the loss-separation principle are introduced and compared with the measurements. The model parameters are computed from customized laboratory and standardized measurements. The customized measurements based on the stator part of an induction machine in the range of the automotive industry standard, i.e., in [−40 °C, ⋯, 180 °C]. The proposed model and measurement process can be used in the post-processing stage of numerical field analysis to obtain electromagnetic losses according to the agreement between measured and simulated results. During a numerically expensive optimization process, this model can be used to consider the temperature dependence of the losses more accurately. The study shows that more than 50% of loss increase can be measured, compared with the catalog data, if we use the manufactured, stator-based, customized measurements based on the estimation of the iron loss parameters.

Investigation and Application of Magnetic Properties of Ultra-Thin Grain-Oriented Silicon Steel Sheets under Multi-Physical Field Coupling

Nowadays, energy shortages and environmental pollution have received a lot of attention, which makes the electrification of transportation systems an inevitable trend. As the core part of an electrical driving system, the electrical machine faces the extreme challenge of keeping high power density and high efficiency output under complex workin g conditions. The development and research of new soft magnetic materials has an important impact to solve the current bottleneck problems of electrical machines. In this paper, the variation trend of magnetic properties of ultra-thin grain-oriented silicon steel electrical steel (GOES) under thermal-mechanical-electric-magnetic fields is studied, and the possibility of its application in motors is explored. The magnetic properties of grain-oriented silicon steel samples under different conditions were measured by the Epstein frame method and self-built multi-physical field device. It is verified that the magnetic properties of grain-oriented silicon steel selected within 30° magnetization deviation angle are better than non-grain-oriented silicon steel. The magnetic properties of the same ultra-thin grain-oriented silicon steel as ordinary non-oriented silicon steel deteriorate with the increase in frequency. Different from conventional non-grain-oriented silicon steel, its magnetic properties will deteriorate with the increase in temperature. Under the stress of 30 Mpa, the magnetic properties of the grain-oriented silicon steel are the best; under the coupling of multiple physical fields, the change trend of magnetic properties of grain-oriented silicon steel is similar to that of single physical field, but the specific quantitative values are different. Furthermore, the application of grain-oriented silicon steel in interior permanent magnet synchronous motor (IPM) for electric vehicles is explored. Through a precise oriented silicon steel motor model, it is proved that the magnetic flux density of stator teeth increases by 2.2%, the electromagnetic torque of motor increases by 2.18%, and the peak efficiency increases by 1% after using grain-oriented silicon steel. In this paper, through the investigation of the characteristics of grain-oriented silicon steel, it is preliminarily verified that grain-oriented silicon steel has a great application prospect in the drive motor (IPM) of electric vehicles, and it is an effective means to break the bottleneck of current motor design.

Manufacturing-Induced Cogging Torque in Segmented Stator Permanent-Magnet Machines With Respect to Steel Punching

There are applications like electric-power-assisted steering system in which strict requirements on the cogging torque of the machine should be fulfilled. Numerous techniques and methods are introduced to minimize the inherent cogging torque of the machine, but manufacturing tolerances and imperfections still can cause extra cogging torque. In this article, the impact of punching on the manufacturing-induced cogging torque of segmented permanent-magnet synchronous machines is investigated. The punching model is implemented by modifying the BH curve of the steel as a function of distance from the cut edges. The parameters of the new BH curve are identified by means of standard Epstein frame measurements using steel strips with different widths. Then, the punching model is incorporated into the finite element (FE) model of a 10-pole 12-tooth segmented stator machine designed for power steering application. It is observed that deviation among the magnetic properties of the teeth caused by punching can introduce extra low-order harmonics in the cogging torque of the machine. The amplitude of the new orders can be comparable or even much bigger than the inherent orders, where not only the magnitude of deviation but also the arrangement of teeth with given punching parameters play a role.

Modeling of Magnetic Anisotropy in Electrical Steel Sheet by Means of Cumulative Distribution Functions of Gaussians

This article proposes a methodology for modeling magnetic anisotropy in ferromagnetic materials. This approach was initially used to represent the isothermal remanent magnetization (IRM) acquired by natural mineral assemblages and based on the use of the cumulative distribution function (CDF) of a probability density function (PDF). For the purpose of this work, the anisotropy of electrical steel sheets is modeled by means of CDFs of Gaussians. The methodology is validated by comparisons between numerical and experimental data obtained from an Epstein frame and a Rotational Single Sheet Tester. The outcomes of the study are twofold: first, the parameters defining the CDF follow a polynomial trendline, and second, they can easily be calculated from a limited amount of experimental data which is a key point from an engineering point of view.

Loss Parameter Identification After Cutting for Different Non-Oriented Electrical Steel Grades

The magnetic properties of nonoriented (NO) electrical steel sheet are sensitive to industrial cutting processes. Decreased operation point efficiency and an increase in local iron loss in the magnetic circuit of an electrical machine are the consequences. During the design of electrical machines, the cut-edge effect and its consequences can only be considered and assessed, if the effect is locally modeled. In fact, the cut-edge effect is often neglected as standardized magnetic characterization is performed, where the sheet material is measured on an Epstein frame or single sheet tester (SST) and gentle sample preparation is specified. In order to parameterize a local material model, a large number of samples and their detailed magnetic characterization at various magnetizing frequencies are necessary. In this article, the loss parameters of nine different industrial NO electrical steels are identified and analyzed in order to study the course of the parameters and possible interrelation with material properties. The sheet thickness, alloying content of silicon and aluminum, and grain sizes are considered as they are the dominant influences on the iron loss from the material side. The presented results help to increase the understanding of the cut-edge effect and its consequences on the iron loss modeling. Furthermore, they help to identify possibilities to decrease the measurement effort significantly.

Computing Frequency-Dependent Hysteresis Loops and Dynamic Energy Losses in Soft Magnetic Alloys via Artificial Neural Networks

A neural network model to predict the dynamic hysteresis loops and the energy-loss curves (i.e., the energy versus the amplitude of the magnetic induction) of soft ferromagnetic materials at different operating frequencies is proposed herein. Firstly, an innovative Fe-Si magnetic alloy, grade 35H270, is experimentally characterized via an Epstein frame in a wide range of frequencies, from 1 Hz up to 600 Hz. Parts of the dynamic hysteresis loops obtained through the experiments are involved in the training of a feedforward neural network, while the remaining ones are considered to validate the model. The training procedure is accurately designed to, firstly, identify the optimum network architecture (i.e., the number of hidden layers and the number of neurons per layer), and then, to effectively train the network. The model turns out to be capable of reproducing the magnetization processes and predicting the dynamic energy losses of the examined material in the whole range of inductions and frequencies considered. In addition, its computational and memory efficiency make the model a useful tool in the design stage of electrical machines and magnetic components.

Indirect yoke-based B-H hysteresis measurement method determining the magnetic properties of macroscopic ferromagnetic samples part I: Room temperature

This work describes an indirect determination method for magnetic flux density-field strength.(B-H) hysteresis measurement. Several electromagnetic measurement methods determining and analysing specific material properties are already established, but these are meant for laboratory measurements using small samples having only limited application possibilities. Such methods are the vibrating sample magnetometer, ring sample method, Epstein frame and other indirect methods, which are until now under continuous development. In the present work, a combination of the mentioned measurement setups based on a yoke setup was designed to measure magnetic behaviour of industrial samples in an indirect measurement setup having a wide range of application possibilities. This was realised using various corrections and a reluctance model for the magnetic circuit. Using a min. 70 vol-% pure iron particle filled flux conducting paste at the contact positions, the H-field loss called demagnetisation and caused by stray fields became insignificant. Interestingly, finite element model and experimental estimation result in about half of demagnetisation of the setup as the validation indicated. For validation purposes, a comparison with the results of a direct electromagnetic measurement method was established. Using the adjusted demagnetisation, the results were reasonably in line with the validation.

Analysis of the grain oriented electrical steel heterogeneities and their influence on the Epstein frame loss measurements

The Epstein frame is a well-known standardized system used to characterize soft magnetic materials. The users of this device usually consider that all the strips of a same grade placed in the frame are homogeneous, and they do not take into account the potential impact of the heterogeneity of the sheets on the quality of the characteristics deduced from the measurements. The aim of this paper is to quantify the individual heterogeneity of Epstein samples of the same FeSi grade (BH curve, losses, permeability), and to analyze, by both experimental and numerical ways, if the layout of the sheets can impact the Epstein core losses.

Examination of magnetic properties of nonoriented electrical steels using ring-type specimens

The Epstein frame test is commonly used for investigating the magnetic properties of electrical steels because of the convenient sample preparation. This test requires the samples to be cut half-parallel and half-perpendicular to the direction of rolling to account for the magnetic anisotropy of the sheet. An alternative method to evaluate the magnetic properties of electrical steels is the ring core test. This test more closely represents the magnetic cores in rotating machinery such as motors and generators because the sample shape for this test resembles the practical rotating applications. The measured results may vary according to the test methods. In this study, the differences between the results of the Epstein frame tests and ring core tests were investigated for the various grades of NO steels. For the magnetic induction measurements, results of the ring core test showed a trend of lower values than in the results of the Epstein frame tests. The core losses measured by the ring core tests are generally greater than those of the Epstein frame tests. The difference in results due to the test methods can be attributed to the anisotropy parameter, A(h), which can be obtained from the crystallographic texture of the sheet sample. The difference becomes significant when an area fraction of the texture component shows a high 1-A(h) value by the Epstein test but a low value by the ring test.

Effective magnetic path length in Epstein frame test of electrical steels.

The effective magnetic path length lm for a 25cm Epstein frame test is studied on the basis of a relevant demagnetizing model and determined by a comparative measurement of dc and ac hysteresis loops at each value of peak flux density Bp, with the field recorded by H-coils placed above the mid-section of the limbs or calculated from the magnetizing current. The obtained Bp and frequency dependences of lm for testing core loss Ps and peak field Hp of an electrical steel are explained in terms of differential susceptibility along the measured loops and demagnetizing and eddy-current effects on local magnetization processes.

Sheet thickness dependence of magnetization properties based on domains and walls within the non-linear diffusion-like equation for grain-oriented electrical steels

The definition and development of a dynamic or transient magnetic formulation compatible with the Finite Element Method, able to take the dynamic hysteresis and classical and extra losses into account (not a posteriori but a priori), and using the most adapted state variable that makes the problem well defined and easily convergent requires the knowledge of dynamic behavioral properties of soft magnetic materials whatever the thickness and the working conditions. The dynamic hysteresis of soft magnetic materials corresponds to excess iron losses, due to dynamic magnetization reversal processes within magnetic domains and especially to microscopic eddy currents around the magnetic walls in motion and inside rotating domains. The model properties used are the internal quasi-static permeability μ and the dynamic magnetization property Λ that lumps the magnetization mechanisms (domain walls displacement, bowing, fusion, nucleation and multiplication). The latter are involved in the magnetic field damping due to microscopic eddy currents within the field diffusion that renders the magnetic behavior geometry dependent. This model does not separate the field diffusion process from the magnetization reversal mechanisms. In this paper, the sheet thickness ζ dependence of the dynamic magnetization property Λ(ζ) for a Grain Oriented Electrical Steel (GOES: steel made of 3% Silicon and Iron: SiFe) is analyzed in the Rolling Direction (RD). To this extend, it is proposed to carry out and interpret magnetic measurements on GOES samples with the Epstein frame for four different thicknesses (ζ = 0.23, 0.27, 0.30 and 0.35 mm), but with similar metallurgical and crystallographic properties for the whole specimens. Magnetic properties are first identified at low induction with linear assumptions and at higher induction with non-linearities. It makes it possible to re-compute the dynamic hysteresis loops of the material and to predict the losses whatever the working conditions with frequencies from 50 to 800 Hz. It is found that μ does not depend significantly on the sheet thickness whereas Λ depends on it significantly. Microscopic observation of the magnetic domains width are then performed thanks to the MOIF (Magneto-Optical Indicator Film) technique. It helps us discriminate between three simultaneous origins for the dependence of iron losses to the sheet thickness: a skin-like effect, a domains’ refinement and changes on the walls’ mobility. Results are discussed taking the dipolar magnetic effects, closure domains, the grains and texture and the manufacturing and coating residual stress on the metal surface into account.

A study of Jiles-Atherton and the modified arctangent models for the description of dynamic hysteresis curves

This paper considers the identification of hysteresis parameters for two different hysteresis models, operated in quasi-static and dynamic behaviors. The extended Jiles-Atherton model via the Bertotti approach and the improved arctangent model are tested and compared. The hysteresis measurement was performed via standard Epstein frame on automotive parts for different magnetization levels and frequencies. The robustness of the Levenberg-Marquardt algorithm is assessed via a comparison of measured, modeled hysteresis loops, and relative permeability profiles, at various frequencies.

Effect of Electrical Steel Punching on the Performance of Fractional-kW Electrical Machines

The effect of electrical steel degradation due to punching on the performance of fractional-kW electrical machines in terms of torque and iron losses is investigated. The local degradation of the steel is considered in the BH curve and iron losses model by continuous variation of the magnetic proprieties as a function of the distance from the cut edges. The newly introduced parameters are identified by fitting the proposed relations for the BH curve and iron losses with measured values obtained via standard Epstein frame test. The position-dependent BH curve is incorporated into magnetostatic FE equations. Then, the performance of two different synchronous machines, i.e. with permanent magnets and reluctance rotor, is analyzed by means of FE simulations. Considering punching, the average torque decreases by 0.5-2 %, while the iron losses can undergo an increase of 30-40 %. Moreover, in order to check the validity of the model, the magnetic friction torque of two permanent-magnet motors, one with an annealed stator and the other one with a non-annealed stator, is measured and compared with calculated values based on the FE model