Showing papers in "Compel-the International Journal for Computation and Mathematics in Electrical and Electronic Engineering in 1992"

Transient semiconductor device simulation including energy balance equation

Abstract: An efficient numerical method for the solution of hot‐carrier transport equations describing transient processes in submicrometer semiconductor devices is proposed. The calculations of transient processes in submicrometer MOS transistor were carried out and compared with the results obtained by conventional drift‐diffusion model.

Summary of results for benchmark problem 13 (3‐d nonlinear magnetostatic model)

Abstract: Benchmark problem 13 of the TEAM Workshop consists of steel plates around a coil (a nonlinear magnetostatic problem). Seventeen computer codes developed by twelve groups are applied, and twenty‐five solutions are compared with each other and with experimental results. In addition to the numerical calculations, two theoretical presentations are given in order to explain discrepancies between the calculations and the experiment.

Summary of results for benchmark problem 10 (steel plates around a coil)

Abstract: Benchmark problem 10 of the TEAM workshop consists of steel plates around a coil (non‐linear transient eddy current problem). Seven computer codes are applied, and seven solutions are compared with each other and with experimental results.

Application of finite element modelling in LVDT design

Abstract: Application of finite‐element modelling of magnetic fields in a linear variable differential transformer (LVDT) is demonstrated and electromagnetics of such devices briefly discussed. A finite‐element aided design of a particular LVDT, built and tested at USITT, Southampton, is used to illustrate practical aspects of such modelling.

Numerical computation of the schwarz‐christoffel transformation parameters for conformal mapping of arbitrarily shaped polygons with finite vertices

Abstract: An iterative algorithm is described to compute Schwarz‐Christoffel transformations which map the upper half of a complex plane into the interior of a polygon in another complex plane. An efficient method of numerically integrating the S‐C integral over the singularities is presented. The algorithm is easily programmable in FORTRAN. Convergence rate is high and accuracy is excellent. Examples are provided and wherever possible, analytically obtained results are also presented for comparison. The importance of the algorithm is described and a brief comparison with some of the existing algorithms is made. Potential application of the S‐C transformation are in the solution of Laplace's and Poisson's equation in two‐dimensional domains with polygonal boundary.

A‐posteriori adaptive mesh refinement in the finite element eddy current computation

Abstract: Two a‐posteriori error estimators are presented for adaptive mesh generation in eddy current finite element computation : the discontinuity of the magnetic flux density on the interface between neighboring elements and the energy perturbation between 1st and 2nd order finite element meshes. Validation of the results is being accomplished by application of the adaptive refinement in the case of a T‐shaped slot embedded conductor, a problem several times approached in the literature.

Numerical simulation of electron trajectories for eels

Abstract: Electron energy loss spectrometers (EELS) for surface studies require a high energy resolution (▵E ? 3 meV) monochromatic electron beam with the highest possible current. In order to improve the transmission of such a device, and so the achievable current, a 3D simulation of electron trajectories taking into account space charge effects was performed for typical three lenses cathode systems, electrostatic cylindrical deflector analyzer (CDA 127°) and acceleration lenses. The results of the analysis are presented and their effects on the overall design of the device are discussed.

Some aspects of additional losses in step‐lap joints of transformer core

Abstract: Magnetic properties of step‐lap joints have been investigated experimentally on scale‐models of a three‐phase three‐leg transformer core The influence of core induction, number of steps and overlapping length has been investigated

An extended schwarz‐christoffel transformation for numerical mapping of polygons with curved segments

Abstract: An extension of the Schwarz‐Christoffel transformation is described to formally map polygons which contain curved boundaries. The curved boundaries are divided into small ‘curved elements’ and each element is approximated by a second degree polynomial (higher degree polynomials can also be used). The iterative algorithm of evaluating the unknown constants of the basic S‐C transformation described in a companion paper is applied to the extended S‐C transformation to compute its unknown constants, including the coefficients of the polynomials. Excellent results are achieved as far as accuracy and convergence are concerned. Examples including a practical application, are provided. The mapping of curved polygons is important because they provide a better model of a physical device.

Modelling tooth ripple losses in the solid pole faces of synchronous machines

Abstract: The armature slots produce flux density pulsations in the pole face and hence eddy current losses, the calculation of which has not been fully satisfactory to date. One consideration ignored is that, since the ac field in the pole face oscillates in the presence of a strong dc field, eddy currents are induced by a field whose mean value is at a point on the B/H curve dictated by the dc flux density so that the incremental permeability becomes important. In order to study this effect we require knowledge of both relative (?r) and incremental (pri) permeabilities. Some measurements have been made of ?ri as a function of Hdc in a ring sample and form the basis on which an analytical model of the phenomenon has been developed and some finite‐element studies have been performed.

A comparison of the transmission coefficient and the wigner function approaches to field emission

Abstract: Standard treatments of the field emission problem typically rely on approximations to the evaluation of the Transmission Coefficient (TC). Recently, the Wigner Distribution Function (WDF) has been applied to this problem. In this paper, fast, accurate, and efficient numerical algorithms for each are presented and compared to each other and to traditional WKB and Fowler Nordheim approaches for silicon field emission. As each approach admits a trajectory interpretation, the methods for incorporating each into a larger Ensemble Particle Monte Carlo (EPMC) simulation of quantum transport are briefly discussed.

The estimation of metrological characteristics of instrument trasformers in rated and overcurrent conditions based on the analysis of electromagnetic field

Abstract: Problems of correct estimation of metrological characteristics of current transformers based on constructional data and working conditions are of great importance. Applying the commonly known equivalent circuit of the current transformer requires its parameters to be determined correctly. A method for determining the equivalent circuit parameters of a current transformer, based on its field models, is shown in this paper. The results obtained using field computation and real‐life model tests are compared.

A model for transient impact ionisation

Abstract: A model for impact ionisation allowing for the spatial transient is described. Ionisation rates and phonon scattering rates are adjusted to fit experimental data. To reduce some of the uncertainty, the calculated ionisation rates due to Kane are used.

Electric stress distribution in polymeric insulation containing defect sites and space charge

Abstract: Polymeric insulation is progressively replacing traditional materials in cables, switchgear and machines. The fundamental electricl properties of polymer dielectrics such as conduction and breakdown are strongly influenced by the presence of defect centres (voids and inclusions) and trapped charge. A thorough understanding of the role of trapped charge in the breakdown process is essential if the full potential of these new materials is to be realised.

The fem analysis of nonsymetrical short circuit forces and fields in the window of convertor transformer

Abstract: The paper describes the simulation of a short circuit of one diode in a three‐phase convertor set connected to a 2 winding transformer. The forcing currents are computed with the circuit simulation method. The circuit — field model is solved with the finite‐element method. In the paper is presented the distribution of flux lines and values of short circuit forces (strains) solved during one period every 15 degrees in the window of the convertor transformer. This approach to dynamic phenomena using the method presented has not yet been applied to short circuit research in transformers.

Finite element modelling of inductive sensors

Abstract: Practical aspects of finite‐element modelling of magnetic fields and calculating integral parameters such as a reflected impedance in inductive sensing devices are discussed. The electromagnetics of such sensors is briefly explained and some practical modelling techniques are put forward. Excellent agreement with experiment is reported.

Analysis of control motor with drag-cup rotor transients

Abstract: The time‐stepping finite element method for the analysis of the electromagnetic and electromechanic transients in the control motor with the drag‐cup rotor has been presented. The magnetic field is assumed to be two dimensional and the coil ends in stator winding have been treated as concentrated resistance and inductance elements. The mathematical model of the considered phenomena consists of: (a) field equations, (b) equations which define the scheme of stator winding connection, and (d) equation of motion of the rotor. The equations of this model have been solved simultaneously. The special attention has been paid to the accuracy of the electromagnetic torque calculation. The influence of the nodal density on the results has been investigated.

Computational error in reduced scalar magnetic potential problems

Abstract: The reduced scalar potential representation of magnetic fields is widely believed to be numerically unstable where large permeability contrasts (e.g., 1000:1) prevail. This belief is theoretically unfounded. Computational errors reported in the literature are shown to arise mainly in the process of finite element discretization, where extraneous source densities are introduced when applied magnetic fields are approximated by fields not wholly solenoidal. Simple experiments show local energy density errors of several per cent even in regions of uniform permeability, independently of element order.

Simple method for calculating the peak torque of a switched reluctance motor: a computational investigation

Abstract: The tractive force between two slightly overlapped steel teeth exhibits a remarkably linear relationship with m.m.f. over a considerable range of variables. This occurs despite complex and varying patterns of field at the tooth surfaces, with local flux densities approaching 3 Tesla. A theoretical explanation is not readily available. Results of a computational investigation are reported and analytical approaches considered.

The practical application of fem techniques in transformer design & development

Abstract: Power 4 Distribution Transformers are complex assemblies of metallic and insulating materials which generate equally complex electrostatic and electromagnetic fields.

Error bounds in dual & complementary eddy‐current systems

Abstract: The formulation of electromagnetic field problems using dual and complementary energy methods is now well established, and the advantages of the error bounds to system energy for static systems that it provides are widely realized. When considering eddy‐current problems the picture is less clear, even for the restricted case of time harmonic fields. Several interesting papers have been published that address the particular question of whether or not error bounds can exist for such dissipative systems, but as yet no consensus view has emerged. This contribution is an attempt to help clarify the position. It will be shown that through the use of complementary variational principles it is possible to establish pairs of functionals that do indeed rigourously provide error bounds. Ilustrative examples show that it is usually possible to provide error bounds to eddy‐current system parameters so long as due care is taken to pre‐determine whether or not a particular problem may be regarded as either current driven, or voltage driven, from the circuit point of view.

The effect of a screening oxide on ion implantation studied by monte carlo simulations

Abstract: The effect of a screening oxide layer on 1‐D and 2‐D ion implantation profiles in silicon is investigated using Monte Carlo simulations. Experimental observations of profile broadening by oxide layers are explained by the fact that atoms at lattice positions are less effective in steering ions into channels than atoms at random positions. The influence of the oxide layer on the lateral penetration below a mask is discussed in terms of implantation energy and ion species. A new set of parameters for the electronic stopping of phosphorus and arsenic in silicon is used.

3‐d calculation of magnetic field and inductance of a reactor, with open magnetic core, by integral equations method

Abstract: The reactor with rectangular shape of the winding turns and open magnetic core has been considered in the work. The integral equation has been solved numerically and components of magnetic flux density as well as leakage inductance of the winding have been computed for the object. A computer program for the analysis of magnetically open problems and a procedure, for numerical calculation of inductance, have been worked out. The computer codes were implemented for IBM PC/AT. The calculations were verified by measurements for physical model of the reactor.

Calculation of transient electromagnetic forces in an axisymmetrical electromagnet with conductive solid parts

Abstract: The paper describes the analysis and the calculation of transient response of a voltage fed electromagnet. This calculation is based on the simultaneous solution of the magnetic field equations and the electrical circuit equations. In the modelling of the magnetic fields, eddy currents in solid conductive parts and saturation of magnetic parts are taken into account. This modelling uses Finite Element Method for the calculation of magnetic fields and forces with special quadrilateral elements. Experimental and simulation results for an axisymmetrical electromagnet are presented and compared.

Complementarity in nonlinear magnetostatics: bilateral bounds on the flux-current characteristic

Abstract: The relation between the flux F in a magnetic circuit and the driving current J, or characteristic of the circuit, is described by a curve in the principal quadrant of the J—F plane. We wish to determine this curve, for a given non‐linear b—h law. Standard numerical analysis only allows one to plot some points of the J—F curve, with an error which depends on the grade of the finite‐element mesh, and on which only asymptotic estimates are known. We show that one can do much better: one can “sandwich” the characteristic, i.e., define a narrow strip in the J—F plane within which the characteristic surely lies.

Adaptive algorithms in eddy‐current nondestructive testing

Abstract: Low frequency eddy current technique is being widely used for the nondestructive testing of metal objects. Computer simulation for eddy current nondestructive testing of metal objects is described in this paper.

On the uniqueness of the solution to the drift—diffusion model in semiconductor analysis

Abstract: This paper is concerned with the analysis of global uniqueness of the solution to the drift—diffusion models, for stationary flow of charges carriers in semiconductor devices. Two uniqueness cases are found. Firstly, small applied voltages with a proof introducing new ‘quasi‐monotony condition’ verified for solutions in W and not necessarily in H. Secondly, large applied voltage to the semiconductor with small 2D domain, and not large doping functions. These uniqueness cases allow the construction of algorithms that yield converging sequences of solutions.

Internal behaviour of large coils subject to transient voltage

Abstract: This paper deals with the problem of internal overvoltages within large coils due to fast‐rising surges. Numerical simulations are performed on a lumped‐parameter equivalent circuit, representing a coil of the magnetic system of a thermo‐nuclear fusion experiment. Inductances and capacitances are computed through numerical methods which ensure a good precision even with complex geometries. The effect of the conductive painting on the outer surface of the coil is also taken into account. The simulation results are compared with a number of measurements on a full‐size prototype coil. Turn‐to‐earth, as well as inter‐turn overvoltages, are both computed and measured in many grounding conditions. The experimental results fit well with computation and theoretical prediction.

Analytical prediction of the airgap field distribution in permanent magnet motors accounting for the effect of slotting

Abstract: Although several papers have been published on the calculation of the open‐circuit airgap field of permanent magnet excited radial‐field motors, most analytical models are framed in rectangular coordinates and few account for the effect of stator slotting.

Design, analysis, and simulation of impulse magnetization systems based on solenoid magnetizing fixtures

Abstract: Due to their high magnetizing field requirement, the emergence of rare‐earth based permanent magnets is creating onerous demands on the capacitor‐discharge systems which are used for their initial magnetization, a process which is aggravated by the fact that the transient current pulse induces eddy currents, which inhibit the penetration of the magnetizing field, and causes heating and stressing of the magnetizing fixture. The problems are compounded in multi‐pole and post‐assembly magnetization systems, particularly for fine pole‐pitch fields. However this paper concentrates on the pre‐magnetization of magnets in air‐cored solenoids, which, despite the difficulty in subsequently handling magnetized magnets, remains the most common requirement. It presents a methodology for the design of impulse magnetizing solenoids to produce the amplitude and time to peak of magnetizing field required for a specific generic type and aspect ratio of magnet to be magnetized, and describes a procedure for the subsequent analysis of the complete impulse magnetization system.