Single Sheet Tester Research Articles

Identification and analysis of static and dynamic magnetization behavior sensitive to surface laser treatments within the electromagnetic field diffusion inside GO SiFe electrical steels

Surface laser treatment of soft magnetic materials, such as GO SiFe electrical steels, is mainly used to reduce iron losses by modifying the static and the dynamic magnetic properties. Laser treatment effect on mesoscopic electromagnetic properties of soft magnetic materials is presented. An experimental procedure using the Single Sheet Tester is performed on a magnetized sheet with specific exciting induction level and frequency. Transient average magnetic flux density through the sample cross-section and its corresponding applied magnetic field are measured. Data are identified in the time-domain with numerical results obtained by solving the diffusion equation using a 1-D discretization approach. The identification strategy requires a material law that includes both non-linear static and dynamic properties and describes the magnetic behavior due to domains and walls dynamics. Next, parametric and physical studies are performed on materials submitted to different laser treatments in order to determine and interpret their effects on the identified magnetic properties and the time response in the sheet’s depth. The results show that the static and the dynamic properties can be simultaneously improved. This analysis help to understand the impact of laser treatments on the static and the dynamic behaviour in order to improve the material magnetic performances within magnetic circuits inside electrical machines and transformers.

The international round robin test of magnetostriction measurement of grain-oriented electrical steel by means of a single sheet tester and an optical sensor

This paper reports the results of the round robin test (hereafter RRT) of magnetostriction measurement using the test apparatus following IEC 60404-17 from April 2019 to August 2019. The main purpose of the RRT is to identify factors affecting the magnetostriction measurement, and then to estimate the reproducibility of the measurements. Nine anonymous laboratories, whose test apparatuses are a little different from the draft standard, participated in the RRT. In the progress of the RRT, we identified factors affecting the magnetostriction measurement. Finally, the reproducibility of the measurements was estimated. Doshisha Univ. acts as the reference laboratory.

Correlating magnetic properties of ferritic NO electrical steel containing 2.4 m.%Si with hot strip microstructure

Understanding the interdependencies of the processing steps of high silicon containing non-oriented (NO) electrical steel is a key factor for improving its final magnetic properties. Moreover, every process step in the production has a significant impact on the properties. Steels with a high silicon content are ferritic (α-Fe). Accordingly, the microstructure and texture are inherited from one processing step to the next. The presented research relates the hot strip microstructure and texture to the magnetic properties of the final annealed material. To investigate the impact of hot strip microstructure and texture for a ferritic 2.4 m.%Si silicon containing steel, the material was hot rolled from an initial thickness of 64 mm down to a thickness of 1 mm whereby different microstructures and textures were achieved by varied hot rolling conditions. Subsequently, the material was cold rolled to 0.25 mm and annealed at 800 °C or 1000 °C for 3 min. Finally, samples of each processing step were analyzed by means of light microscopy and XRD texture measurements. Samples after hot rolling and final annealing were tested in a single sheet tester for specific losses and magnetic polarization. Findings show an influence of the initial hot strip microstructure on the microstructure and magnetic properties of the final annealed samples.

Acceleration technique of flux waveform control with FPGA

The magnetic flux density waveform is controlled to a sinusoidal wave for quantitative measurement in AC magnetic property measurement. The magnetic characteristics are nonlinear and it is difficult to make the magnetic flux density waveform sinusoidal. Therefore, the magnetic flux density waveform is made into a sinusoidal wave by feedback control (Kaido et al., 1998). Feedback control is performed by differential control, but in the non-linear region of high magnetic flux density, the convergence of the waveform is deteriorated and measurement time is increased.In this research, we succeeded in shortening the control time by using FPGA (Field-Programmable Gate Array) as the magnetic flux density waveform control method. The waveform control method sets various initial conditions in the PC, and is sent to the FPGA together with the target voltage waveform data. The FPGA starts feedback control from the information.The magnetic flux density was measured from 0.1 T to 2.0 T by using a single sheet tester and setting the amplitude ratio and the strain ratio to 0.5% or less using 50A1300 and 50A470. As a result, it has succeeded in significantly speeding up compared with the device which controls the waveform only with the conventional software.

Blanking induced damage in thin 3.2% silicon steel sheets

The cores of electrical motors and transformers are made by blanking, piercing and stacking of thin metallic sheets having various features cut from the original blank. The material experiences local plastic deformation near the cut edge due to the blanking operation. The quality and efficiency of the produced products are directly affected by the mechanical and magnetic properties of the blanks at the cut edge. The effects of the blanking process on deformation evolution in thin sheets of high Si electrical steels was investigated. In-situ blanking experiments together with the digital image correlation (DIC) technique were used to quantify local deformation evolution during thin sheet blanking operations. Magnetic hysteresis losses were measured using a purpose-built single sheet tester and linked to the measured deformation maps. The residual stresses were qualitatively assessed by means of nano-hardness measurements while the local microstructural properties and dislocation generations were determined using EBSD analysis of the blanked parts. The results indicated that for the tested materials with 0.1t blanking clearance, electrical steel sheets with 0.2 mm thickness experiences larger deformation prior to fracture during blanking compared with samples having 0.35 mm thickness. This has a direct relationship with the measured hysteresis losses. However, the dislocation maps indicated that dislocations of GNDs are more pronounced for thicker samples that aligns with the effect of dislocations on magnetic power losses rather than hysteresis losses measured in this research.

Non-Oriented Silicon Iron Alloys Used in Efficient Electrical Motor Production

In this paper four non-oriented electrical steel grades M400-50A, M400-65A, M300-35A and NO20, utilizing a laboratory single sheet tester, were analyzed. For the magnetic measurements the peak magnetic polarization of 1 T was chosen. The frequency was variated between 3 Hz and 400 Hz. The isotropic electrical steel sheets were cut through punching technology, along the rolling direction. The experimentally determined energy losses were decomposed, using the loss separation theory, into hysteresis, classical and excess components. The relative magnetic permeability was analyzed and its parts, real and imaginary permeability were computed. The non-oriented alloy magnetic properties’ variation as a function of frequency was plotted and explained in the paper.

State-of-the-art and future trends in soft magnetic materials characterization with focus on electric machine design – Part 1

Abstract The first of this two-part article illustrates the state-of-the-art in soft magnetic materials characterization starting with very early developments and giving a retrospective view of research carried out in this field. One famous measurement setup is the Epstein frame, named after Josef Epstein. Around 1900, he published a German article about “The magnetic testing of laminated steel”. As it is highly likely interesting to researchers worldwide, the authors reconsidered his contributions in English language. Consequently, the most common measurement setups are introduced. Besides the Epstein frame, these are the single sheet tester and setups analyzing ring-shaped specimens. These setups allow for a 1-D characterization of the material, while it usually features anisotropic properties and cross-coupling of the magnetization axes. For this reason, 2-D and even 3-D setups are considered and presented here. This article provides a thorough introduction to the field of material characterization. It is the basis for modeling the material characteristics, which is presented in the second part, which also includes recent developments for considering manufacturing impact. It is of particular interest when dealing with mass-produced electric machines. Eventually, these articles shall constitute valuable references for both engineers new to this field as well as experienced researchers.

Influence of distortion factor of induced voltage in B-coil on magnetic property measured with a single sheet tester

Influence of distortion factor of induced voltage in B-coil on magnetic property measured with a single sheet tester

High-resolution measurement method for magnetostriction of electrical steel sheets using an open-type single sheet tester

One of the main cause of acoustic noise from iron core in electrical machines is magnetostriction. However, good reproducibility of magnetostriction measurement cannot be expected due to its ppm-order value. To measure the magnetostriction accurately

Iron Loss Estimation Method for Silicon Steel Sheet Taking Account of DC-Biased Conditions

One of the issues of an iron loss estimation is the modeling of an excess loss. This paper investigates the modeling method for the excess loss under dc-biased conditions. The proposed approach is based on the 1-D finite-element analysis for the silicon steel sheet in the thickness direction considering hysteretic property and skin effect. In addition, the magnetic flux density waveform is divided into the outline loop and minor loops and estimate the excess loss of each loop using the eddy current loss correction factor $\kappa $ taking account of dc-biased conditions. To demonstrate the effectiveness of the proposed method, we compare the estimated loss with measured one for the harmonic wave in single sheet tester.

Rotational Single Sheet Tester for Multiaxial Magneto-Mechanical Effects in Steel Sheets

A detailed design of a new rotational single sheet tester device which allows comprehensive multiaxial magneto-mechanical analysis of ferromagnetic sheets is proposed. The challenges that arose during the mechanical and magnetic design phases are addressed. The applicability of the device is tested by performing magneto-mechanical measurements on an M400-50A electrical steel sheet. Results under several multiaxial magneto-mechanical loadings with circular and alternating magnetic flux densities are reported. It is shown that the effect of multiaxial stress on iron losses can be much more significant than that of uniaxial stress.

An Improved Method for Measuring the Magnetic Properties of Silicon Steel With Double Sheets at High Frequency

Single sheet tester (SST) is a traditional method for measuring magnetic properties of silicon steel. In order to make the measurement more accurate, this paper presents an improved SST with double yokes and double sheets. The electromagnetic simulation demonstrates that the magnetic field strength H between the two sheets that are in symmetric magnetic path approximately equal to H of two sheets, proving that the improved SST has higher test accuracy than the traditional double-yoked SST. A novel control method is proposed for the measurement system to make sure the symmetry of magnetic path. In this experiment, the magnetic field strength H and the magnetic flux density B of the tested samples are measured by an induction coil method, and the maximum working frequency of this apparatus can be up to 20 kHz.

Numerical and Experimental Determination of Local Building Factors of a Three-Phase Transformer Core Package

The analyses of flux distributions and losses in transformer cores need highest expenditure if performed in experimental ways. This paper presents a novel combined methodology for the evaluation of local and global loss building factors BF. As a first step, the local patterns of the dynamic induction vector B(t) were calculated, by means of a novel numerical “magnetic anisotropic circuit calculation (MACC) methodology. The latter considers the core materials non-linear permeability functions in rolling direction and transverse direction, as well as non-linear effects of overlaps in corners and T-joints. In a second step, the calculated induction patterns were exactly simulated on the hexagonal Vienna rotational single sheet tester and the corresponding local losses p were measured electrodynamically. Finally, the values p were allocated to the corresponding core positions, taking into account the entire core geometry. BF of the core was estimated from the total of loss values weighted to the corresponding volume portions. The method was applied for a three-phase core of highly grain-oriented material, magnetized with BNOM = 1.7 T. The procedure shows the local distributions of losses, yielding also information on those of eddy currents and rotational magnetization (RM). As an example, the T-joint, as the area of highest RM, yields a local loss increase of about 50%. Apart from RM, the local induction variations-including waveform distortions, caused mainly by circulating magnetization-prove to have a high impact on losses, locally yielding 50% as well. These highly convincing results indicate that the complicated sensor studies of cores can partly be replaced by: 1) numerical modeling of induction distributions and 2) subsequent allocations of loss values as detected by RSST simulations.

Effects of Annealing under Tensile Stress on the Magnetic Properties of FeSiB Amorphous Magnetic Material

In order to improve magnetic properties of Fe-based amorphous magnetic materials (Fe-Si-B), effects of tensile stress applied to specimens during heat treatment were studied. The heat treatments were implemented to 60 mm ×350 mm sheet samples with an electric furnace. The temperature was controlled to be a reference temperature lower enough than the crystallization temperature and the retention time was changed. The tensile stress depending on the attached weights was also changed from 9.8 MPa to 49.0 MPa per 9.8 MPa step. After and before the heat treatments under tensile stress, the magnetic properties of the samples were measured by using a single sheet tester (SST). As a result, it was found that the iron loss was decreased and the permeability was increased by the heat treatment under tensile stress. However excessive tensile stress showed weak effect on the magnetic property improvement.

Extent of embossing-related residual stress on the magnetic properties evaluated using neutron grating interferometry and single sheet test

Due to the permanently increasing maximum speed of modern electric machines, the mechanical load on the electrical steel rises. The punched flux barriers of reluctance machines play an increasingly key role in the mechanical design. The weakening of the material and the notching effect can cause local failure due to centrifugal force. In this paper, a method is presented which induces residual stresses by embossing electrical steel laminations, which contribute to the guidance of the magnetic flux and cause a negligible mechanical weakening of the sheet. The aim of this paper is to demonstrate the effectiveness of these embossed flux barriers. This is achieved by using neutron grating interferometry, that allows the density of the magnetic domains to be resolved locally. Furthermore the influence of the embossing is mapped globally by single sheet tests. The finite element analysis of the forming process derives the residual stress distribution which is responsible for the measured magnetic behavior. In order to isolate the influence of the residual stress, non-embossed and stress-relief annealed specimens are used as a reference for both measurement methods.

Can Circular Rotational Losses of Non-Oriented Soft Magnetic Materials Be Estimated From Alternating Losses?

Non-oriented (NO) electrical steels are widely used as core materials in rotating electrical machines, whereas for measurements under alternating magnetization (AM), standardized methods exist; measurements under rotational magnetization (RM) prove to represent a significant problem. In this paper, we performed measurements of both circular rotational and alternating losses in rolling and transverse directions for four types of NO materials of different Si contents and thicknesses by means of a hexagonal rotational single-sheet tester at different frequencies and flux density values. The losses were measured by the electrodynamical method and for confirmation also by the thermal (rise-of-temperature) method. The estimated rotational losses by means of established well-known methods are correlated with the measured ones. Literature reports attempt to estimate losses $P^{\mathrm {(RM)}}$ for circular RM on the basis of losses $P^{\mathrm {(AM)}}$ for AM. A loss ratio factor $\Gamma = P^{\mathrm {(RM)}}/P^{\mathrm {(AM)}}$ is defined which is reported to be close to $\Gamma = 2$ . In this paper, $\Gamma $ factors were determined for three different grades of material. The results indicate a rather weak dependence of $\Gamma $ on the grade, however, a strong one on the intensity $B$ of induction. In approximation, $\Gamma = 2$ is valid for rather weak induction of the order $B = 0.6$ T. Induction increases in steps of $\Delta B = 0.1$ T yield decreases of loss ratio by about $\Delta \Gamma = 3$ %. This enables a rough estimation of rotational losses. However, the possibility of estimation is restricted to the circular case of constant modulus and angular velocity of the $B$ vector. Other cases are characterized by different values of both eddy current losses and hysteresis losses, RM involving completely different domain reconstructions compared to AM. For increased frequency, the decrease in $\Gamma $ becomes weaker due to more dominant eddy current losses.

Modeling the stress effect on the measurement of magnetostriction in electrical sheets under rotational magnetization

The magnetostriction in electrical steel under rotational magnetization is usually measured with cross-shaped samples. However, the inhomogeneity of the magnetization and stress in the sample might hinder the measured results. In this paper, we investigate this phenomenon by using a magneto-mechanically coupled energy-based model to simulate the sample in a single sheet tester measurement setup, and compare the simulations and measurements. The results show that some anomalies in the measured magnetostriction can be explained by the inhomogeneous magnetization in the sample and the form effect, which result in inhomogeneous stresses and thus affect the observed quantities. The validity of the model as well as the presented statements are ascertained through experiments on the single sheet tester. The backgrounds of the used modelization technique are also detailed.

Impact of Punching Parameter Variations on Magnetic Properties of Nongrain-Oriented Electrical Steel

The efficiency and performance of an electrical machine depend on the magnetic properties of its magnetic core, i.e., the rotor and stator core made of stacked and processed electrical steel laminations. Especially a mechanical cutting, also known as punching or blanking, deteriorates the magnetic material properties next to the cutting line due to induced residual stress. The extent of the deterioration depends on the amount of induced stress, which is affected by the mechanical material properties and the process parameters used during punching. For this reason, in this paper, the effect of punching parameter variations on the magnetic properties of eight different nongrain-oriented electrical steels is investigated by using a single-sheet tester. The magnetic property deterioration is discussed based on the specific loss and the maximum field strength increase. In addition, the impact of punching parameter variations on the efficiency and the local flux and loss distribution of a permanent-magnet synchronous machine is shown based on a finite-element analysis. This allows one to tailor the punching parameters toward energy-loss minimization.

Effects of Heat Treatment Under Strong Magnetic Field of 1 T or More on Magnetic Properties of Non-Oriented Electrical Steel Sheet

In order to improve magnetic properties of non-oriented electrical steel sheets, effects of heat treatment in a magnetic field to control crystallographic orientation were studied. In this paper, we report on the effects of the amplitude and direction of the magnetic field applied during the heat treatment in comparison to magnetic properties of samples, which the longitudinal direction is parallel to the rolling direction (RD) or rolling transverse direction (TD). To clarify the effects of heat treatment in a magnetic field on the magnetic properties of the electrical steel sheets, heat treatment without magnetic field was firstly carried out and a change rate, defined as a value obtained by dividing the iron loss value after the heat treatment by the iron loss value before the heat treatment, was used in the evaluation. Fig. 1 shows a schematic view of the heat treatment apparatus. The heat treatment apparatus consisted of an electric furnace at inner diameter of a superconducting magnet (10 T - CSM) and a quartz glass tube inserted into the furnace. The quartz tube was depressurized to less than $10 ^{-3}$ Pa with a turbo molecular pump. The heat treatment temperature was controlled at 1023 K, 1123 K, and 1273 K. 10 mm $\times 50$ mm sized samples were prepared by cutting a non-oriented electrical steel sheet, 50A470 (JIS C 2552), with electrical discharge machining. Samples were cut so that their longitudinal direction was parallel to the rolling direction or rolling transverse direction. The samples were placed at the center of the magnetic field in the quartz tube using a holding jig made of quartz in a way that the direction of the magnetic field would be parallel to the longitudinal direction of the samples. The strength of the applied magnetic field was set to a maximum of 10 T. Fig. 2 shows iron loss change rate of the samples heat treated at 1273 K with respect to iron loss before heat treatment. In Fig. 2, for example, “RD-10T” means the sample with longitudinal direction parallel to the rolling direction and applied magnetic field during the heat treatment of 10 T. The vertical axis represents the change rate and the horizontal axis represents the amplitude of the excitation magnetic flux density controlled by the sinusoidal wave of 50 Hz with a small-sized single sheet tester. As shown in Fig. 2, both the RD- and TD-samples heat-treated without magnetic field (0 T) under the temperature condition of 1273 K showed a remarkable change in the magnetic properties. The grain growth was accelerated by the heat treatment, the iron loss was reduced, and the permeability under low excitation conditions was improved. Contrarily, the permeability at 1.6 T or higher decreased. It was also found that the effect of the heat treatment on the reduction of the iron loss was greater in the TD-samples. As for the results of heat-treatment in magnetic field, the iron loss of the RD-sample was reduced after heat treatment in 10 T. On the other hand, the iron loss of the TD-sample increased after heat treatment in 10 T. It was also found that application of the strong magnetic field of 10 T throughout the heat treatment, i.e., during the heating and cooling, caused increase of the iron loss and decrease of the magnetic permeability regardless of the direction of the material. The cause of this phenomenon is possibly the contraction of the polycrystalline specimen in the strong magnetic field. The details will be discussed at the presentation and in the full version of this paper.

1-D and 2-D Loss-Measuring Methods: Optimized Setup Design, Advanced Testing, and Results

Accurate measurements of magnetic losses in laminations are a prerequisite for their theoretical assessment, as well as for satisfying calculations of energy dissipation in engineering systems. The standardized and universally applied measurement method, used as a reference for the definition of the material quality in the specification standards, is based on the Epstein test frame magnetizer. Its success relies on the reproducibility of the performed measurements. Its limitations come, on the one hand, from cumbersome sample preparation and, on the other hand, from a certain divergence of the measured loss figures from the true loss figures. Similar systematic differences between measured and true loss values are also observed with the standard single-sheet tester (SST) method. In both cases, measurements under bidimensional induction are or cannot be envisaged. The design of new measurement setups and magnetizers overcoming the drawbacks of the Epstein and SST methods and possibly becoming recognized standards in the future is welcome, but challenging. This paper is devoted to a comprehensive discussion of the state of the art in the alternating and 2-D measurements of energy losses in soft magnetic materials for electrical applications. We will summarize, in particular, measuring solutions proposed in the current literature, and we will discuss in detail recent developments achieved in the authors' laboratories regarding 1-D measurements with compensated permeameters and 2-D measurements at high inductions and high frequencies.