Reduce Eddy Currents Research Articles

Influence of Remote Laser Cutting on Magnetic Loss and Mechanical Properties in 0.2 mm Silicon Steel

Energy loss due to eddy current generation is a significant factor in reduced efficiency of electrical machines, therefore motor cores with thinner sheets are required due to their effect in reducing eddy currents. However, reducing the thickness of laminations is challenging due to the high tooling costs in blanking due to the small tolerances required, whilst other methods such as conventional laser cutting are too slow to be commercially viable. In this paper the influence of remote laser cutting on thin electrical steel sheets (Cogent/Tata Steel NO20 3.2% Si) has been investigated on the tensile strain to fracture, fatigue life, edge hardness, and energy loss in an AC magnetisation cycle for two laser power levels. The laser power has found to have no statistically significant influence on the bulk mechanical properties of the material, but significantly affects the measured specific loss. The latter was associated with the increased edge hardness at higher laser powers. Based on the parameters selected it is highly questionable whether RLC offers a viable method for lamination cutting, significant refinement of the laser parameters and/or the use an alternative laser such as an ultra-fast pulsed laser is required to be competitive with existing methods. Additionally, it is shown that energy loss increases linearly with respect to length of the cut edge - to the minimum cut spacing tested of 6 mm. These results can be used to optimise laser parameters to reduce the degradation of magnetic properties by decreasing laser power where possible.

Segmented RF shield design to minimize eddy currents for low-field Halbach MRI systems

MRI systems have a thin conducting layer placed between the gradient and RF coils, this acts as a shield at the RF-frequency, minimizing noise coupled into the experiment, and decreasing the coupling between the RF and gradient coils. Ideally, this layer should be transparent to the gradient fields to reduce eddy currents. In this work the design of such a shield, specifically for low-field point-of-care Halbach based MRI devices, is discussed. A segmented double layer shield is designed and constructed based on eddy current simulations. Subsequently, the performance of the improved shield is compared to a reference shield by measuring the eddy current decay times as well as using noise measurements. A maximum reduction factor of 2.9 in the eddy current decay time is observed. The segmented shield couples in an equivalent amount of noise when compared to the unsegmented reference shield. Turbo spin echo images of a phantom and the brain of a healthy volunteer show improvements in terms of blurring using the segmented shield.

Development of ceramic vacuum chamber for collider ring kicker of Circular Electron Positron Collider (CEPC)

The fast-pulsed kicker of the collider ring of Circular Electron Positron Collider (CEPC) uses a ceramic vacuum chamber to reduce eddy currents and maintain the magnetic field waveform at the beam's track. But the image charges induced by beams will accumulate on the insulating ceramic to increase the beam impedance. Eddy current shielding and beam impedance are traded off by coating a discontinuous “pattern” of TiN (titanium nitride) film on the inner wall of the ceramic vacuum chamber. Several potentially effective coating patterns are proposed. The eddy current shielding effect and the beam impedance are analyzed by simulation software. The simulation results show that coating in a ladder-shaped pattern maintains the magnetic field waveform well and helps to reduce the impedance. Properties such as electrical conductivity and SEY (secondary electron yield) of the films are improved in coating experiments and the technical feasibility of coating in a racetrack-shaped ceramic vacuum chamber is verified. Finally, the impedance of a 1200 mm-long ceramic vacuum chamber coated with ladder-shaped pattern film and the effect on the kicker magnetic field are measured.

Geometrical control of eddy currents in additively manufactured Fe-Si

Additive manufacturing has enabled the processing of high silicon electrical steels which have excellent soft magnetic properties. In bulk form, core losses as a result of eddy currents would be too large to allow their use in high-frequency electrical machines, therefore strategies are needed to reduce eddy currents. Additive manufacturing affords high part complexity and provides the opportunity for cross sectional patterns within the material to limit eddy current generation. This study investigates several designs, including a novel hexagonal pattern which is shown to have the lowest eddy current loss coefficient of 0.0005, less than 25% of the bulk material which has an eddy current loss coefficient of 0.0021. Heat treatment is shown to increase the eddy current losses, demonstrating that for high-frequency machines, it may be beneficial to use the material in the as-built state. Physical samples were compared to their intended geometries, showing there are defects in these complex cross sections causing increased eddy currents when compared to simulations, but that geometrical accuracy can be improved by alternative design methodology which experimentally experiences smaller losses. These novel cross sectional designs may be implemented into an electric machine that has a 3D magnetic flux pathway enabled by additive manufacturing, affording more flexibility for electrical engineers to design new motor architectures in the pursuit of higher power density machines.

Effect of ferromagnetic paste on insulating properties of transformer oil

THE PURPOSE. Consider the problem of using a special ferromagnetic paste, which fills the air gaps in the magnetic circuit of an oil power transformer, in order to reduce its losses, by reducing eddy currents in the magnetic circuit. Conduct experiments to determine the dynamics of change of high-voltage breakdown of transformer oil, after adding ferromagnetic paste to it.METHODS. To solve this problem, several samples of ferromagnetic paste have been developed, differing from each other using different binding material for ferromagnetic powder. The samples were placed in containers for several days filled with transformer oil. With the help of electrical installation AIM-90, measurements of high-voltage breakdown of transformer oil of each sample were carried out. Thanks to this method, it was possible to determine how the ferromagnetic paste affects the insulating properties of the transformer oil. RESULTS. For clarity, the results of all samples were plotted. Based on the graph, it became clear that in some samples of transformer oil a significant amount of mechanical impurities formed from ferromagnetic paste appeared, which extremely negatively affected the insulating properties of the oil. There were also samples in which the properties of the oil practically did not change. CONCLUSION. As conclusions, there was a certain vision of which ferromagnetic paste can be used in the design of an oil power transformer, it was also possible to understand which samples of ferromagnetic pastes extremely negatively affect the insulating properties of the transformer oil.

A Winding Structure of Air-Core Planar Inductors for Reducing High-Frequency Eddy Currents

High-frequency converters with high power density have shown promise in recent years. Air-core planar inductors built with printed circuit boards can eliminate problems associated with the use of magnetic materials, such as iron loss and magnetic saturation. However, conventional air-core planar inductors suffer from eddy current loss. Eddy currents affect the resistance and inductance of the inductor. This article proposes a stranded wire structure to reduce eddy currents more than that in the conventional spiral wire structure. Eddy currents can be reduced by splitting the windings as well as currents of each split winding are balanced in the proposed structure. Seven types of air-core planar inductors were built, and their resistances, inductances, and impedances were measured to evaluate the effects of different structures on eddy currents. Furthermore, the temperature distribution of the planar inductors was measured while supplying a sinusoidal voltage with a power amplifier. The experimental results demonstrate the usefulness of the proposed stranded wire structure. Furthermore, the experimental result shows that the stray capacitance of the inductor can be reduced as a by-product of the proposed stranded wire structure.

Laminated Permanent Magnets Enable Compact Magnetic Components in Current-Source Converters

Magnetic components have become one of the primary barriers to high power density converters, especially to current-source converters (CSCs). In CSCs, the inductors/transformers have a predominantly dc flux to store energy, which can be offset by standard permanent magnets (PMs). However, eddy currents in standard PMs induce significant losses and thermal stress for medium-frequency applications. This article proposes using laminated PMs to offset the dc flux while reducing eddy currents, leading to significant reductions in the size, cost, and losses of inductors/transformers. Furthermore, the laminated PMs’ optimal location, orientation, and distribution are investigated to generalize this approach for maximum benefits. Three-dimensional finite-element analysis simulation and hardware experiments are presented to validate the effectiveness of the proposed approach in a medium-frequency transformer (MFT) for CSCs. Compared with standard PMs, the proposed use of laminated PMs reduces aggregate core-plus-PM losses by 85% in experiments, and thus, relaxes the MFT thermal design. Finally, the proposed approach is experimentally validated in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{40}\,\text{kVA}$</tex-math></inline-formula> flyback-type MFT for a soft-switching solid-state transformer. Compared to the traditional design without any PMs, the proposed design increases the saturation current by 46% while inducing only 5% more losses, leading to significant savings in magnetics cost and size.

RF shielding designs for birdcage coils for preclinical MRI at 9.4 T

Birdcage coils are widely used in preclinical MRI as they perform well, allow for quadrature drive, and can provide a homogeneous transmit field. Unlike in larger bore scanners, an RF shield is essential to avoid strong cross-talk with gradient coils that are in close proximity. However, gradient switching induces eddy currents that heat the shield and coil and impair the temporal signal-to-noise ratio (tSNR). The motivation of this study is to investigate the performance of different designs of RF shields on a birdcage coil used for high resolution functional MRI of small primates at 9.4 T. We found the choice of materials for RF shields significantly affected ghosting and tSNR in fMRI scans. Both ultrathin foils and a slotted pattern reduce eddy currents and improve imaging quality. Our results also demonstrate that a 9-um-thick copper foil is sufficiently thin to reduce the eddy current effects for high-resolution fMRI scans and there is no need for high-cost 4-um-thick foil. For slotted shields, our results demonstrate that the number of slots should be carefully considered, and an excessive number of slots can lead to a lower SNR and tSNR. We believe the results from this study can be used as a reference to design future RF coil shields selection for preclinical scanners.

On the Schwarz method for the eddy currents model

We investigate an iterative solver for the reduced eddy currents model set in open domains. Its formulation is the result of the coupling of a variational problem restricted to a truncated domain with a transparent condition at the fictitious boundary. The non-local boundary condition is derived using simple/double layer potentials. Applying a splitting technique yields an iterative algorithm that requires to solve, at each iteration, a local Neumann boundary value problem on the current potential. A re-interpretation of the algorithm as a Schwarz method enables a tractable convergence analysis which is the core subject of the paper. Using suitable variational tools, we establish an exponential convergence of the iterative solver. Some analytical and numerical examples are presented to check the theoretical findings and the relevance of our procedure.

Iron lamination and interlaminar insulation for high-frequency pulsed magnets.

In high-frequency pulsed magnets, such as kickers in particle accelerators, it is essential to reduce eddy currents that could be induced in the magnet core during excitation not to distort and attenuate the magnetic field pulse. A novel iron lamination scheme with additional interlaminar insulation is proposed for the magnet core of such pulsed magnets. A laminated steel sheet core is formed by alternately stacking thin steel and insulation sheets. For application to matched kicker magnets for accelerators, test magnets with the new and conventional iron lamination were designed, assembled, and extensively evaluated. The pulsed magnetic field waveforms of two test magnets with the new lamination successfully matched to below 0.1% over the entire pulse duration, which was significantly better than those with the conventional lamination. Among the applications of the developed high-frequency pulsed magnets, beam injection kickers for the coming next generation light sources and future colliders, where suppression of the transient stored-beam oscillation during beam injection is crucial, are considered to be promising.

Optimization analysis of combing box for air yarn based on computational fluid dynamics

Conventional single card air yarn units create flow vortices in the carding box that affect the fiber structure and thus the yarn performance. Key parameters for airflow characteristics include geometry and spinning parameters (Such as the length of the nozzle, the width of the inlet and outlet, the speed and diameter of the felting roller). This paper proposes to improve the transport channel to reduce eddy currents in the upflow region and to analyze airflow fields using fluid knowledge. This paper examines three designs: a bypass circular channel entrance for air supply, rounded corners at the bottom right in the conveyor channel and combining both modifications. According to the analysis, a nozzle inlet/outlet ratio between 0.5 and 1 gives the lowest swirl and the highest mass flow. Considering the yarn characteristics and the system production efficiency, it is ideal when the barbed roll speed is 8000 rpm and the diameter is about 30 mm. It is possible to dissipate the vortices for three methods.

Composite pressure cell for pulsed magnets.

Extreme pressures and high magnetic fields can affect materials in profound and fascinating ways. However, large pressures and fields are often mutually incompatible; the rapidly changing fields provided by pulsed magnets induce eddy currents in the metallic components used in conventional pressure cells, causing serious heating, forces, and vibration. Here, we report a diamond-anvil-cell made mainly out of insulating composites that minimizes inductive heating while retaining sufficient strength to apply pressures of up to 8GPa. Any residual metallic component is made of low-conductivity metals and patterned to reduce eddy currents. The simple design enables rapid sample or pressure changes, desired by pulsed-magnetic-field-facility users. The pressure cell has been used in pulsed magnetic fields of up to 65T with no noticeable heating at cryogenic temperatures. Several measurement techniques are possible inside the cell at temperatures as low as 500 mK.

Wide-angle visible video diagnostics for JT-60SA utilizing EDICAM

A multi-purpose, wide-angle, visible overview video diagnostic, based on the EDICAM camera, was designed for JT-60SA superconducting tokamak, in order to fulfill both machine protection and scientific observation purposes. The detector is located inside the ‘port plug’ (a ca. 3 m long re-entrant tube), relatively close to the plasma boundary; the EDICAM can tolerate the magnetic field at this location as well as the radiation levels in the early phases of the experiment. A rail-and-carriage system is used to move the detector between the opening of the port plug and the measurement location. A 4-point docking system ensures the precise and robust positioning of the optics, firmly holding the detector also in the event of disruptions or earthquakes. The camera is running under atmospheric condition, separated by a sapphire window from the plasma vacuum. The vacuum window is protected by a water-cooled pin-hole from mechanical impacts and plasma heat; the pin-hole is also the first element of the detector’s optical system. The optics consists of two lens groups and a prism; the housing is made of stainless steel, in order to reduce eddy currents and thus avoid large forces acting on the system in a disruption.

Looking for Energy Losses of a Rotary Permanent Magnet Magnetic Refrigerator to Optimize Its Performances

In this paper, an extensive study on the energy losses of a magnetic refrigerator prototype developed at University of Salerno, named ‘8MAG’, is carried out with the aim to improve the performance of such a system. The design details of ‘8MAG’ evidences both mechanical and thermal losses, which are mainly attributed to the eddy currents generation into the support of the regenerators (magnetocaloric wheel) and the parasitic heat load of the rotary valve. The latter component is fundamental since it imparts the direction of the heat transfer fluid distribution through the regenerators and it serves as a drive shaft for the magnetic assembly. The energy losses concerning eddy currents and parasitic heat load are evaluated by two uncoupled models, which are validated by experimental data obtained with different operating conditions. Then, the achievable coefficient of performance (COP) improvements of ‘8MAG’ are estimated, showing that reducing eddy currents generation (by changing the material of the magnetocaloric wheel) and the parasitic heat load (enhancing the insulation of the rotary valve) can lead to increase the COP from 2.5 to 2.8 (+12.0%) and 3.0 (+20%), respectively, and to 3.3 (+32%), combining both improvements, with an hot source temperature of 22 °C and 2 K of temperature span.

Interslice current change constrained B0 shim optimization for accurate high-order dynamic shim updating with strongly reduced eddy currents.

To overcome existing challenges in dynamic B0 shimming by implementing a shim optimization algorithm which limits shim current amplitudes and their temporal variation through the application of constraints and regularization terms. Spherical harmonic dynamic B0 shimming is complicated by eddy currents, ill-posed optimizations, and the need for strong power supplies. Based on the fact that eddy current amplitudes are proportional to the magnitude of the shim current changes, and assuming a smoothness of the B0 inhomogeneity variation in the slice direction, a novel algorithm was implemented to reduce eddy current generation by limiting interslice shim current changes. Shim degeneracy issues and resulting high current amplitudes are additionally addressed by penalizing high solution norms. Applicability of the proposed algorithm was validated in simulations and in phantom and in vivo measurements. High-order dynamic shimming simulations and measurements have shown that absolute shim current amplitudes and their temporal variation can be substantially reduced with negligible loss in achievable B0 homogeneity. Whereas conventional dynamic shim updating optimizations improve the B0 homogeneity, on average, by a factor of 2.1 over second-order static solutions, our proposed routine reached a factor of 2.0, while simultaneously providing a 14-fold reduction of the average maximum shim current changes. The proposed algorithm substantially reduces the shim amplitudes and their temporal variation, while only marginally affecting the achievable B0 homogeneity. As a result, it has the potential to mitigate the remaining challenges in dynamic B0 shimming and help in making its application more readily available.

The effect of particle size on the core losses of soft magnetic composites

In the field of electrical machines, the actual research activities mainly focus on improving the energetic aspects; for this reason, new magnetic materials are currently investigated and proposed, supporting the design and production of magnetic cores. The innovative aspects are related to both hard and soft magnetic materials. In the case of permanent magnets, the use of NdFeB bonded magnets represents a good solution in place of ferrites. For what concerns the soft magnetic materials, the adoption of Soft Magnetic Composites (SMCs) cores permits significant advantages compared to the laminated sheets, such as complex geometries and reduced eddy currents losses. SMC materials are ferromagnetic grains covered with an insulating layer that can be of an organic or inorganic type. The proposed study focuses on the impact of the particle size and distribution on the final material properties. The original powder was cut into three different fractions, and different combinations have been prepared, varying the fractions percentages. The magnetic and energetic properties have been evaluated in different frequency ranges, thus ranking the best combinations. The best specimens were then tested to evaluate the mechanical performances. The preliminary results are promising, but deeper analysis and tests are required to refine the selection and evaluate the improvements against the original composition taken as a reference.

(Invited) Recent Advances in Porous Silicon Based Microelectronic Devices

Historically, electronics was the first discipline that exploited the properties of porous silicon (PSi) in the 1970’s [1]. Two decades later, a huge development of the studies about PSi has been observed since an efficient photoluminescence of this material was discovered. During this fruitful period, microelectronic devices also benefited from the progresses made in the domain of semiconductor electrochemical etching and from the comprehension of the unique properties of PSi. In particular, it was found that radio-frequency (RF) devices can took advantages of the isolating properties of PSi [2]. Indeed, highly resistive substrates are generally required to reduce eddy currents and capacitive couplings and then, to get high performance passive devices. The insulating properties of PSi, combined with the ability to locate these areas in different resistivity wafers, make this material promising in terms of development of monolithic insulator/semiconductor substrates [3]. In this presentation, we will present the recent advances that have been performed in GREMAN in the field of RF devices that integrate PSi. A specific emphasis will be put on thin and free-standing PSi membranes and 3D structures for high current density applications. In addition, we will describe the way PSi could be used as insulating material in power AC Switch. Many architectures involving PSi peripheries can lead to subsequent performance and reliability improvements [4, 5, 6]. In particular, by forming a relatively thick PS layer in a TRIAC periphery, low leakage currents (<10 μA) have been demonstrated up to a few hundred volts for both bias signs. We will demonstrate that, at present, there is no doubt about benefits PSi can bring to microelectronic devices. Many other applications that require a silicon substrate such as integrated sensors [7], solar cells [8], conductive via [9] or energy micro-sources [10, 11] can also be of great interest for the microelectronics industry. For instance, we will show in details that porous silicon could also be useful for acoustic transducers as an integrated backing material in order to enhance the imaging quality. Nevertheless, until now, PSi is not widely introduced in electronic component manufacturing. This point will also be discussed.

Mechanism of magnetic losses variation in stator‐end structures with windings extensions using space vectors

Electromagnetic and heat issues are more complex in the end region of a generator than in the line segment area. In this study, the electromagnetic field and related challenges are discussed. Stepped cores are considered in the performed calculations. Eddy-current losses variation mechanisms for structures in the end region and iron losses of end stepped cores are investigated under rated load condition, using space vectors. The effects of the end windings stretching out the end cores from the rotor and stator on the electromagnetic losses of the end structures studied in detail. The obtained results can be employed for developing large turbo-generators, and reveal a novel approach for reducing eddy currents and iron losses.

Monolithic MACS micro resonators

Magic Angle Coil Spinning (MACS) aids improving the intrinsically low NMR sensitivity of heterogeneous microscopic samples. We report on the design and testing of a new type of monolithic 2D MACS resonators to overcome known limitations of conventional micro coils. The resonators’ conductors were printed on dielectric substrate and tuned without utilizing lumped element capacitors. Self-resonance conditions have been computed by a hybrid FEM-MoM technique. Preliminary results reported here indicate robust mechanical stability, reduced eddy currents heating and negligible susceptibility effects. The gain in B1/P is in agreement with the NMR sensitivity enhancement according to the principle of reciprocity. A sensitivity enhancement larger than 3 has been achieved in a monolithic micro resonator inside a standard 4mm rotor at 500MHz. These 2D resonators could offer higher performance micro-detection and ease of use of heterogeneous microscopic substances such as biomedical samples, microscopic specimens and thin film materials.

Miniaturized NFC Antenna Design for a Tablet PC With a Narrow Border and Metal Back-Cover

A novel structure of the near-field communication (NFC) antenna design for a tablet PC is proposed. This tablet PC has a narrow border and full metallic back-cover. A miniaturized loop antenna design is achieved by attaching ferrite sheets on both sides of the loop antenna. The ferrite sheets may reduce eddy currents induced on the adjacent metallic back-cover by the loop antenna to improve the communication range of the NFC. Only the edge of the tablet PC allows the antenna to radiate due to the full metallic back-cover. Thus, the NFC antenna needs to be narrow to be installed on the edge of the tablet PC. Therefore, we propose a miniaturized NFC antenna with the dimensions of $41.5 ({L}) \times 7.5 (W) \times 0.45 (T) \hbox{mm}^3$ only. Simulated magnetic field distributions are consistent with measured voltage distributions. This design has a good communication range more than 6 cm in front of the touchscreen panel and reaches 2 cm over the other side above the metal back-cover.