Eddy Current Density Research Articles

Examination of the S–D curve for magnetic stimulation used in the treatment of urinary incontinence using an experimental eddy current waveform

Abstract Strength–Duration Curves (S–D Curves) for the intra-urethral sphincter muscle of the rat were measured using the rectangular waveform and the pseudo-eddy current waveform, which approximate the stimulation current waveform on magnetic stimulation. Finally, the pulse duration for minimum energy using magnetic stimulation is obtained from the measurement results of the S–D curve. The eddy current waveform differs from the rectangular current waveform commonly used in electrical applications. No basic study has examined the effects of magnetic stimulation on threshold values and other aspects of the eddy current waveforms. Magnetic stimulation has been reported to have good efficacy in the treatment of urinary incontinence. It is desirable that the eddy current density produced by a change in the strength of a magnetic field be greater than the excitability threshold of the muscles or nerves to be stimulated. However, effective magnetic stimulation requires the input of a large current into the stimulation coil. The requisite equipment is therefore large and costly, and so it is necessary to develop magnetic stimulators that can more efficiently stimulate tissues and that are more convenient to use than those that are currently available.

電縫溶接現象の電磁伝熱有限要素解析

A three-dimensional high frequency tube welding heat model has been developed by using electro magnetic and heat conduction finite element methods. Eddy current density distribution at a static state is calculated by means of a frequency response analytic law. Heat transfer in the tube circumferential and longitudinal direction is calculated with heating and cooling. This model is verified by experiments with laboratory-scale electric resistance welding mill. The analysis has good agreement with the actual measurement. This model can satisfactorily simulate the eddy current density distribution and the temperature distribution in the high frequency tube welding. The influences of welding frequency, welding speed, and magnetic permeability of impeder on a temperature distribution of a tube are quantitatively clarified.

Analysis of Eddy Current Distribution in High Frequency Coaxial Transformer With Faraday Shield

This paper presents analysis and measurement results of a high-frequency (HF) coaxial transformer with Faraday shield. The simulation results demonstrate that the magnetic flux and eddy current distribution are not impacted by the inclusion of a Faraday shield. The Faraday shield's maximum eddy current density is less than 17% of the maximum density found in either of the other two windings. The experimental results demonstrate that the voltage ratio, coupling coefficient and load characteristic of the HF coaxial transformer with Faraday shield at frequencies up to 1 MHz are in agreement with the turns ratio and load regulation for a transformer in which capacitive effects are neglected

Numerical Analysis of a Dahle-Probe Response to a Hidden Point Flaw

We propose a method to analyze the response of a Dahle-probe to a hidden point flaw. Eddy-current density distribution due to the probe is calculated numerically by using the superposition principle. We modeled the flaw-eddy current interaction by using a pair of circular currents. An analytical expression for the magnetic vector potential produced by a circular current was also developed and the induced voltage measured by the probe pickup coils was numerically calculated. The method has been extended to a pair of point flaws and a long flaw. Finally, we compared the results of numerical calculation with the experimental results which showed a very good agreement. The validity of the method to predict the produced pattern was confirmed by the experiments

Computation of eddy current losses in the mounting rail of a magnetically levitated conveyor vehicle

PurposeThe paper proposes presenting a transient 3D‐FE computation approach of the eddy current losses in the rail and the flux concentrating pieces of a magnetically levitated conveyor vehicle.Design/methodology/approachThe calculation process is started with a coarse mesh in order to reduce computation time without losing accuracy. Then mesh refinement iterations are performed, based on the estimation of the discretisation error. The results of the post processing are the levitation force, the braking force and the eddy current losses.FindingsThe paper finds that by means of adaptive mesh refinement, the error is significantly reduced with a minimum increase of computation time. The hot spots of eddy current losses can be localised by visualizing the eddy current density. At nominal speed, especially the huge amount of eddy current losses in the flux concentrating pieces must be considered during the development process.Research limitations/implicationsFor further development, the linear motor will be modified with the results of FE computations to reduce eddy current losses. Therefore, different materials and a variation of geometry will be considered.Practical implicationsMagnetically levitated systems excite eddy current losses instead of bearing losses. These losses must be taken into account when developing the drive.Originality/valueIt proposes a transient 3D‐FE approach for computing eddy current losses accurately with a minimum increase of computation time.

A study on eddy-current losses in rotors of surface permanent-magnet synchronous machines

In this paper, we investigate the Eddy current (EC) losses in the rotors of surface permanent-magnet (PM) synchronous machines, which have concentrated armature windings. By the finite-element method (FEM) analysis, it is clear that the EC losses in rotors are apparently different at rated currents if the pole and slot combinations are different. The distribution of the EC density in the rotor is closely related with the asynchronous components of the armature magnetomotive force (MMF). If the armature MMF has a low order of spatial harmonic components, which is not synchronous with the rotor, the EC losses are of large amounts.

Improved Concept and Model of Eddy Current Damper

When a conductive material experiences a time-varying magnetic field, eddy currents are generated in the conductor. These eddy currents circulate such that they generate a magnetic field of their own, however the field generated is of opposite polarity, causing a repulsive force. The time-varying magnetic field needed to produce such currents can be induced either by movement of the conductor in the field or by changing the strength or position of the source of the magnetic field. In the case of a dynamic system the conductor is moving relative to the magnetic source, thus generating eddy currents that will dissipate into heat due to the resistivity of the conductor. This process of the generation and dissipation of eddy current causes the system to function as a viscous damper. In a previous study, the concept and theoretical model was developed for one eddy current damping system that was shown to be effective in the suppression of transverse beam vibrations. The mathematical model developed to predict the amount of damping induced on the structure was shown to be accurate when the magnet was far from the beam but was less accurate for the case that the gap between the magnet and beam was small. In the present study, an improved theoretical model of the previously developed system will be formulated using the image method, thus allowing the eddy current density to be more accurately computed. In addition to the development of an improved model, an improved concept of the eddy current damper configuration is developed, modeled, and tested. The new damper configuration adds significantly more damping to the structure than the previously implemented design and has the capability to critically damp the beam’s first bending mode. The eddy current damper is a noncontacting system, thus allowing it to be easily applied and able to add significant damping to the structure without changing dynamic response. Furthermore, the previous model and the improved model will be applied to the new damper design and the enhanced accuracy of this new theoretical model will be proven.

알루미늄 평판의 표면결함에 대한 와전류 신호의 유한요소해석

The detection mechanism of the flaw for the nondestructive testing using eddy current is related to the interaction of the induced eddy currents in the test specimen with flaws and the coupling of these interaction effects with the moving test probe. In this study, the two-dimensional electromagnetic finite element analysis(FEM) for the eddy current signals of the aluminum plate with different depth of surface cracks is described and the comparison is also made between experimental and predicted signals analyzed by FEM. In addition, the characteristics of attenuation of the eddy current density due to the variation of the depth of a conductor are evaluated. The effective parameters for the application of eddy current technique to evaluate surface cracks are discussed by analyzing the characteristics of the eddy current signals due to the variation of crack depths.

Magnetic levitation by height temperature superconductor plates short secondary interacting with translating field: experiences, measurements and modelling

An experimental set-up has been designed and built in order to investigate and evaluate the forces of interaction between a circular primary fed by tree-phase voltage and a high temperature superconducting plate short secondary. This equipment simulates, by analogy, the electromagnetic behaviour of levitated vehicle, with passive superconducting sheets on board, that rides along an active way with translating field. Static and dynamic performances have been evaluated in terms of specific traction thrusts and levitation pressures. Moreover numerical analyses have been carried out by 3D finite element parametric model: the results are presented in terms of thrusts, eddy current and flux density distribution.

Detection of deep-lying defects in carbon fiber composites using SQUID-NDE system cooled by a cryocooler

In this paper, we present our latest progress in the studies on SQUID-NDE for carbon fiber composites (CFCs), including carbon fiber reinforced plastics (CFRPs) and carbon fiber reinforced carbon matrix composites (C/Cs). We have developed a new SQUID-NDE system for the CFCs utilizing a pulse tube cryocooler for more practical and easy-handling use. Electromagnets using U-shaped ferrite cores have been introduced to the system in order to induce enough eddy current density in the relatively low-electric-conductivity CFCs. By using this system, we demonstrated the detection of deep-lying defects in thick and/or multi-layer CFCs specimens, partly including metal layer upon/below CFCs layer. CFRP plates and multi-layer C/C plates with total thickness of 20 mm having deep-lying slots at various depths were prepared. The slots at 15 mm below the surface in the CFRP plate and multi-layer C/C plate were successfully detected. Defect detections on more “real” multi-layer structures, including aluminum plates upon/below C/C plates, were also examined.

3D Magnetic Field Analysis of a Turbine Generator Stator Core-end Region

In this paper we calculated magnetic flux density and eddy current distributions of a 71MVA turbine generator stator core-end using three-dimensional numerical magnetic field analysis. Subsequently, the magnetic flux densities and eddy current densities in the stator core-end region on the no-load and three-phase short circuit conditions obtained by the analysis have good agreements with the measurements. Furthermore, the differences of eddy current and eddy current loss in the stator core-end region for various load conditions are shown numerically. As a result, the facing had an effect that decrease the eddy current loss of the end plate about 84%.

New metrics and scheduling rules for disassembly and bulk recycling

In recent years, growing quantities of end-of-life electronics have increased the amount of attention devoted to product recovery. Research on end-of-life electronics returns has primarily focused on manual disassembly operations. In this paper, we focus on the scheduling problem for a facility with staging, manual disassembly operations, and bulk recycling. In bulk recycling, shredding or grinding reduces the size of the material fragments while magnetic, eddy current or other density separation techniques separate the material fragments. Unlike production, there are often no due dates in materials recovery processing. Recyclers can sell the recovered materials to material commodity buyers at any time. However, recyclers wait to accumulate a shipment of material to reduce transportation costs and meet minimum sales quantities. Another important difference between production and recycling is that manufacturers purchase raw materials while recyclers may be paid to receive products. When due dates do not apply to scheduling products for materials recycling and product receipts generate revenue for recycling services, we propose two new metrics: the staging space turnover and the shipment fill time. We use our metrics to analyze new scheduling rules for disassembly and bulk recycling and to evaluate their performance. Using discrete-event simulation models, we test our scheduling rules on seven product families, where product families are defined based on material composition and separation operations. Of the rules we test, the disassembly scheduling rule which ranks product families based on the ratio of product size to disassembly time (SDT) most quickly empties the staging space. Shipment fill time is less sensitive to our scheduling rules. Our results illustrate how a recycler can reduce incoming product inventory with a new scheduling rule.

Eddy currents and electrical surface charges

AbstractIn eddy current calculations, the displacement current in the non‐conducting space surrounding the eddy current region is usually neglected. This assumption enforces that the electric charge density and the accompanying normal components of the eddy current density on the surface of the eddy current region must vanish.If the field exiting source currents are not accompanied by charges this assumption may yield acceptable results for the eddy current distribution. However, if the field exiting source currents are accompanied by charges, this assumption may lead to totally wrong results for the current distribution in the eddy current region. An example is given which makes plain this point.To obtain correct results it is not necessary to employ the full set of Maxwell's equations capable to describe wave propagation phenomena also outside the eddy current region. It is shown in the paper that by replacing the displacement current density in the field describing equations by a specifically chosen current density function makes it possible to determine eddy currents and surface charges within the quasi‐stationary calculation scheme for arbitrary field exciting source currents which may or may not be accompanied by charges. The solution obtained in this way is shown to be unique. Copyright © 2003 John Wiley & Sons, Ltd.

Model of ferrite-cored probes for eddy current nondestructive evaluation

A model of an axisymmetric probe coil with a cylindrical ferrite core in the presence of a layered conducting half space is developed. The boundary value problem is formulated in terms of the magnetic vector potential, which is expanded in a series of appropriate eigenfunctions. The corresponding eigenvalues are the real roots of a function with the radius and the magnetic permeability of the core as parameters. The unknown coefficients of the series are computed by solving a matrix system, which is formed by applying the usual interface conditions. Expressions are derived for the induced eddy current density and the impedance of the coil. Numerical results for the latter are in good agreement with experimental data.

An electrically conducting sphere in a three-dimensional, alternating magnetic field

The electrical vector and scalar potential as well as the underlying induced eddy current density of a conducting sphere exposed to an external, time-varying magnetic field are calculated. The external magnetic field is assumed to be generated by sinusoidally alternating current density distributions. Admitting arbitrary three-dimensional current density fields, this paper extends previous results on this subject. Among other things, these calculations prove that in a sphere there is no radial component of the induced eddy current density field. As an application of these results the impedance and power absorption of the sphere are derived. The impedance is the key for the contactless, inductive measurement of the electrical conductivity of metallic melts.

A novel hybrid FEM-BEM method for 3D eddy current field calculation using current density J

This paper introduces a novel hybrid FEM-BEM method for calculating 3D eddy current field. In the eddy current region, the eddy current density J is solved by the finite element method (FEM) which is discretized by brick finite element mesh, while in the eddy current free region, the magnetic field intensity H is solved by the boundary element method (BEM) which is discretized by rectangular boundary element mesh. Under the boundary conditions, an algebraic equation group is obtained that only includes J by eliminating H. This method has many advantages over traditional ones, such as fewer variables, more convenient coupling between the FEM and the BEM and wider application to multiply-connected regions. The calculated values of two models are in good agreement with experimental results. This shows the validity of our method.

Impedance evaluation of rectangular coils for eddy current testing of planar media

As for any other eddy current probe coil, the analysis of rectangular ones plays a vital role towards understanding their characteristics and performance. In this paper, closed-form expressions are provided both for the impedance of rectangular coils of rectangular cross-section located above a conducting half-space and for the eddy current density induced therein. The formulation is general and can be easily used for any coil shape.

Finite‐difference time‐domain‐based studies of MRI pulsed field gradient‐induced eddy currents inside the human body

AbstractIn modern magnetic resonance imaging (MRI), patients are exposed to strong, rapidly switching magnetic gradient fields that, in extreme cases, may be able to elicit nerve stimulation. This paper presents theoretical investigations into the spatial distribution of induced current inside human tissues caused by pulsed z‐gradient fields. A variety of gradient waveforms have been studied. The simulations are based on a new, high‐definition, finite‐difference time‐domain method and a realistic inhomogeneous 10‐mm resolution human body model with appropriate tissue parameters. It was found that the eddy current densities are affected not only by the pulse sequences but by many parameters such as the position of the body inside the gradient set, the local biological material properties and the geometry of the body. The discussion contains a comparison of these results with previous results found in the literature. This study and the new methods presented herein will help to further investigate the biological effects caused by the switched gradient fields in a MRI scan. © 2002 Wiley Periodicals, Inc. Concepts in Magnetic Resonance (Magn Reson Engineering) 15: 26–36, 2002.

Numerical model for eddy-current testing of ferromagnetic steel parts

The paper presents an efficient numerical method to simulate eddy-current testing (ECT) of ferromagnetic steel parts. The method is based on the nonlinear integral equation for quasi-stationary electromagnetic field with gauge condition based on tree/cotree decomposition. The eddy-current density in the conductive domain and the magnetization in the ferromagnetic domain are discretized with edge elements and vectorial volume elements, respectively. A simple, but very effective magnetization model of steel in a low field is also presented. The algorithm convergence for solving the forward ECT nonlinear problem with time-periodic boundary conditions is proved.

Computation of 3-D open boundary eddy-current fields in large transformers using T, ψ-φ/sub m/ method coupled with GTM

Three-dimensional distributions of eddy-current densities and loss densities in metals near heavy current leads in a large transformer are calculated by using the T, /spl psi/-/spl phi//sub m/ method coupled with the geometric transformation method. The method is verified through TEAM Problem 21. The local overheating regions can be found out according to the point where the maximum of the loss density is. The loss densities in the ascending flange are analyzed at the different materials and thicknesses of copper screen.