Hajime Tsuboi

Computation of Three-Dimensional Electric Field Problems by a Surface Charge Method and its Application to Optimum Insulator Design

This paper describes an improved surface charge method for computation of three-dimensional electric field distribution and its application to optimum insulator design. In this method, each curved surface on which the charge is distributed is divided into many curved surface elements instead of planar elements. After computing numerically the charge distribution, the distributions of both potential and electric field are obtained. Because the use of many curved surface elements provides a good approximation of the insulator contour, the correction of insulator contour to achieve optimum insulator design can be performed smoothly.

Three-dimensional analysis of eddy current distributions by the boundary element method using vector variables

This paper describes a boundary element method using vector variables for three-dimensional analysis of eddy current distributions. In the boundary element method, electric field vectors and magnetic flux density vectors are used as unknown vector variables on the boundaries of two materials. The formulation is performed by using the vector Greens theorem. After determining electric field vectors and magnetic flux density vectors on the boundaries, eddy current distributions in the conductor are computed. The computation results of a conducting sphere model agreed exactly with analytical solutions. As an example of three dimensional eddy current problems, the analysis of a conducting cube model was done.

Eddy current and deflection analyses of a thin plate in time-changing magnetic field

Eddy current and deflection analysis of a thin-plate model in a time-changing magnetic field is described. The model is solved as a coupled problem in which the time-changing magnetic field induces eddy currents and the eddy currents cause deflection of the thin plate by the Lorentz force. The eddy current analysis and deflection analysis are performed by an integro-differential method using a current vector potential and a structural finite element method using beam elements, respectively. The formulations of the motional electromotive force and the Lorentz force for the thin-plate model are presented. In addition, the applicability of the proposed method is verified by using a cantilevered-beam model. >

The optimum design of electrode and insulator contours by nonlinear programming using the surface charge simulation method

A new method is presented for optimizing electrode and insulator contours. The contours are modified by using the iteration methods of nonlinear programming until the desired electric field distribution is obtained. The Gauss-Newton, quasi-Newton, conjugate gradient, or steepest descent method is used for the iteration. The electric-field distributions are computed by means of the surface charge simulation method. It is shown that the Gauss-Newton method gives very fast convergence. >

Improved Structure Avoiding Local Field Intensification on Spacers in SF 6 Gas

Problems concerning local electric field intensification on a cone-type spacer which is fitted between flanges in SF6-gas-insulated apparatuses were investigated. Conventional structures, in which flat surfaces of the spacer come in contact with rounded corners of the flange, sometimes cause flashovers at considerably low voltages because of local field intensification. In the improved structure proposed by the authors, surface shape of the spacer and contact position are slightly changed in order to avoid local field intensification. Field calculations and experiments verified that the improved structure is effective for actual use.Problems concerning local electric field intensification on a cone-type spacer which is fitted between flanges in SF6-gas-insulated apparatuses were investigated. Conventional structures, in which flat surfaces of the spacer come in contact with rounded corners of the flange, sometimes cause flashovers at considerably low voltages because of local field intensification. In the improved structure proposed by the authors, surface shape of the spacer and contact position are slightly changed in order to avoid local field intensification. Field calculations and experiments verified that the improved structure is effective for actual use.

Finite element model of natural crack in eddy current testing problem

Eddy current testing (ECT) is one of the nondestructive inspection techniques to detect cracks and eddy current analysis is important for ECT. However, it is difficult to model a natural crack in finite element method (FEM), because of the narrow gap and the complex shape. In this paper, a finite element model for the natural crack is proposed. The conductivity for the finite element model is expressed by a tensor. In the proposed finite element, the change in crack shape and contact condition are simulated by changing the coefficients of the conductivity tensor. The proposed model is applied to a conductor plate model with a crack and a coil. The computation results are compared with the computation results of an artificial crack model. The validity of the proposed method is shown by the computation results.

Design of the RF antenna for MRI

A RF antenna for MRI (magnetic resonance imaging) must emit very uniform electromagnetic fields over the imaging space as well as have a high efficiency in order to obtain high quality images. The authors have developed a program in which the moment method using a point-matching formulation is used. It is demonstrated, using the moment method, that the uniformity of the current distribution on the RF antenna can be improved by using adaptive series capacitors. This allows the adaptive dimensions of the multiturn RF antenna to be calculated to make the electromagnetic field over the imaging space uniform, on the assumption that uniform current flows on the antenna. >

A simplification of boundary element model with rotational symmetry in electromagnetic field analysis

A simplification method for the boundary element model with rotational symmetry is described. When the boundary element model has a rotational symmetry, the region to be treated for boundary integrations can be reduced to the fundamental boundary surface. This reduction is possible because the coefficient matrix of the final simultaneous equations for the model can be transformed to a block diagonal matrix by a transformation matrix using spatial eigenmodes. The simplification reduces the computation time and storage capacity because the coefficient matrix of the final simultaneous equations of the boundary element method is dense. Computation results for a four-wire method demonstrate the applicability of the proposed simplification method. >

Three-dimensional eddy current analysis by the boundary element method using vector potential

A boundary-element method using a magnetic vector potential for eddy-current analysis is described. For three-dimensional (3-D) problems, the tangential and normal components of the vector potential, tangential components of the magnetic flux density, and an electric scalar potential on conductor surfaces are chosen as unknown variables. When the approximation is introduced so that the conductivity of the conductor is very large in comparison with the conductivity of air, the number of unknowns can be reduced; also, for axisymmetric models the scalar potential can be eliminated from the unknown variables. The formulation of the boundary-element method using the vector potential, and computation results by the proposed method, are presented. >

Fast simulation method for eddy current testing

In eddy current testing (ECT) analysis, it is necessary to develop a high performance computing method for the simulation of the testing signal. The testing signal is obtained from the changes of the eddy current around a crack. In this paper, a fast computation technique for the finite element model with a small crack is proposed. In the proposed method, the region where the current changes is selected by a basis function, and only the changes of the eddy current are solved. Furthermore, a formulation of integral form to calculate the impedance of the ECT model including magnetic material is shown.