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The maximum force capability of conventional active magnetic bearings is limited since the magnetic flux density in the iron is limited due to saturation. We introduce in this paper a new concept of active magnetic bearings that exploits high bearing force from a number of parallel ironless stator and permanent-magnet rotor discs. We optimized the design of the bearing by solving tens of thousands of finite-element models in nested loops. We commissioned an experimental rig comprising a set of interleaved bearing discs and a 3 degrees-of-freedom mechanical platform to verify its force production capability.

Insulator contamination remains an important technical problem because contamination-caused insulator flashovers may jeopardize the security of high voltage power systems. This paper updates an insulator selection guide with respect to contamination first published 25 years ago. As in that guide, this update relates primarily to contamination conditions in North America. Results of research that have led to a better understanding of contamination-caused flashovers, particularly for some types of nonceramic insulators are summarized. Assessment techniques for site severity are discussed. Even though they have improved significantly, no leading method has emerged. Artificial contamination test methods have been standardized. Mitigation techniques are discussed for extreme contamination-caused dangers, including important new knowledge regarding ice and snow flashover voltage reduction. caused.

High electric field strength may cause problems, such as corona and deterioration, on nonceramic insulators. Therefore, control of the electric field strength (E) in the vicinity of such insulators is an important aspect for their design. This paper discusses insulator computation models and their applications to 34.5 kV and 765 kV nonceramic insulators. Possible model simplifications are reviewed and discussed. Various 765 kV insulator designs and tower types are covered; the magnitude and distribution of E are reviewed. Verification tests are also described

Despite worldwide efforts and many recent advances, electrical machine modeling using finite elements can be computationally very expensive, especially if three-dimensional models are required. This contribution is concerned with speeding up the process. Some examples of line start permanent magnet machines and induction motors are provided.

The transient characteristics of an experimental test rig consisting of a fixed solid iron stator and moving rotor, when subjected to a step input voltage, are measured. A three-dimensional finite-element scheme that takes into account the nonlinear magnetic properties, eddy-current effects, and the dynamic motion of the rotor was used to simulate the response of the rig. The measurements were used as a benchmark against which the simulation results were compared.

A new parameterization technique which allows rapid alteration of an initial template 2D or 3D finite element mesh is described. An advantage of this method is that once a template mesh has been set up, new devices similar to that of the template can be modeled easily by altering the contents of a simple form or file. These alterations can be easily carried out by a casual user or by an optimization program.

The treatment of 3D machine problems using the finite element method to model the field is examined. Methods for coupling to external circuits are reviewed and it is shown that symmetry can be taken care of by the suitable scaling of partitions of the system matrix. 3D filament wound coils and massive conductor circuits are discussed.

Germ cells arise during early C. elegans embryogenesis from an invariant sequence of asymmetric divisions that separate germ cell precursors from somatic precursors. We show that maternal-effect lethal mutations in the gene pos-1 cause germ cell precursors to inappropriately adopt somatic cell fates. During early embryogenesis, pos-1 mRNA and POS-1 protein are present predominantly in the germ precursors. POS-1 is a novel protein with two copies of a CCCH finger motif previously described in the germline proteins PIE-1 and MEX-1 in C. elegans, and in the mammalian TIS11/Nup475/TTP protein. However, mutations in pos-1 cause several defects in the development of the germline blastomeres that are distinct from those caused by mutations in pie-1 or mex-1. The earliest defect detected in pos-1 mutants is the failure to express APX-1 protein from maternally provided apx-1 mRNA, suggesting that POS-1 may have an important role in regulating the expression of maternal mRNAs in germline blastomeres.

The paper considers the determination, from a knowledge of the perturbation of a constant incident field, the susceptibility of regions where the susceptibility is 0.01 to 10 ppm different from the majority of the space of interest. This is pertinent to magnetic resonance imaging (MRI), and is an inverse problem. Two forward methods of calculating the B field are considered. First, the normal finite element method and secondly, the method of considering the induced polarity in the region of enhanced susceptibility, and calculating the difference field. The accuracies of the methods are explored and the induced polarity method is chosen for the inverse calculation. An iterative technique for inverse calculation is described utilising knowledge of the geometry of the problem, which is available from an MR plot. The numerical work is validated using experimental data from an MR imaging machine.

Taylor's frozen turbulence approximation relates spatial statistics to temporal statistics. Lumley's two-term approximation gives corrections for the effect of fluctuating convection velocity. Such corrections are derived for every turbulence statistic. The statistic may be a tensor of any rank, and may be a correlation or structure function or spectrum. The practicality of other approximations, such as assuming the convectiog velocity to be a Gaussian random variable, is considered as are the inaccuracies of approximations for the case of the dissipation range of spectra. Extensions to cases of multiple-position and space-averaging sensors are noted. Local homogeneity and stationarity and the statistical independence of large-scale and small-scale turbulence quantities are necessary approximations. Local isotropy is shown to be the simplest case of a requirement for local symmetry. Specific expressions are given for isotropic tensors of rank up to fourth rank. Specific expressions are given for the inertial range, for which the correction is shown to vanish to second order for the triple-velocity correlation and structure function. Previous correction formulas are verified, with the exception of the correction for the stress cospectrum, which is shown to have been erroneous. The incompressibility condition is used to simplify the corrections. Fluctuating convection causes erroneous measured compressibility.

Work on two aspects of iron-cored compulsators is reported. Firstly, detailed modelling and analysis of a passively compensated form of machine is considered. A coupled-circuit model is presented which allows current variation in all windings to be found. This model gives rapid numerical solutions and also permits algebraic solutions to be obtained. Comparison with experimental results is shown to be good, provided that linear conditions prevail. To account for iron saturation, a detailed finite element model (which includes externally connected circuits) is presented. This model is shown to give good predictions of performance in all conditions. The second area of reported work concerns a passively compensated machine with a three-phase stator and compensating windings. Results for two stator connections (120/spl deg/ and 60/spl deg/) are given. It is shown that the 120/spl deg/ connection gives a 'long' pulse output of approximately 1.5 times the AC cycle time before loading. The 60/spl deg/ connection gives a shorter pulse, but of larger amplitude. >

Frequency-dependent effects in railway traction power systems arise from the impedance of substation and locomotive line filters and the traction line. Harmonic noise from traction drives and substations can excite resonances and produce overcurrent or overvoltage conditions at critical points in the network. In this paper, the harmonic feeding impedances of a 3 kV DC traction system seen from the rectifier substation, locomotive drive converter and pantograph terminals are presented. Several substation and locomotive filters are considered with a frequency-dependent traction line. Resonances attributed to the substation filter, locomotive filter and traction line are separate and distinct, the line introducing poles and zeros in the audio frequency (AF) range which vary in frequency and magnitude with locomotive position. >

Measurements of the performance of a 5-kVA induction machine are compared with predictions carried out using finite elements and semianalytic techniques. Both transient and steady-state performance results are compared and sufficient dimensions are provided that others may use the experimental results as a benchmark.

The Commission on Powder Diffraction of the IUCr has undertaken a round robin of Rietveld refinement with the aims of: (i) evaluating a cross section of currently used software; (ii) examining the range and effect of various strategies of refinement; (iii) assessing the precision and accuracy (spread) of the derived parameters; (iv) comparing and contrasting various instruments and methods of data collection. These aims were addressed by circulating to 51 participants upon request: (i) two constant-wavelength X-ray and neutron powder diffraction patterns collected on PbSO4 for refinement; (ii) a sample of phase-pure monoclinic ZrO2 for both data collection and refinement. In the latter case, the raw data were requested to be returned for reanalysis with a `standard' version of Rietveld software and an `optimal' refinement protocol. A total of 23 respondents provided 18 X-ray and 20 neutron refinements of the PbSO4 crystal structure from the 'standard' data sets using 12 different Rietveld analysis programs. These results constitute Part I of the round robin and have been described previously [Hill (1992). J. Appl. Cryst. 25, 589–610]. The 28 contributors to the m-ZrO2 section of the survey were based in 12 countries and collected 27 X-ray and 14 neutron data sets, using 20 different X-ray and 11 different neutron powder diffraction instruments. The conventional X-ray instruments included 13 reflection (flat-plate) and eight transmission (capillary or thin-film) machines and used three different radiations (Co, Cu and Mo). Two additional flat-plate data sets were collected with synchrotron X-rays. The neutron data were collected on 12 constant-wavelength and two time-of-flight instruments, the former utilizing wavelengths between 1.0 and 1.9 Å. The data sets yielded 27 X-ray and 15 neutron refinements of the m-ZrO2 crystal structure. The conditions used for data collection varied widely for both types of radiation: wavelengths ranged from 0.7 to 1.9 Å, step widths from 0.01 to 0.12°2θ, step counting times from 0.1 to 46 s for X-rays and up to 30 min for neutrons, data-collection time from 4 min to 3 d, maximum step intensities from 350 to 99000 counts, minimum d spacings from 0.53 to 1.17 Å and numbers of unique reflections from 71 to 912 (not including the time-of-flight neutron data). Variations in resolution between instruments were especially marked in the case of the neutron data but were less pronounced for the X-ray machines; the two instruments situated at synchrotron X-ray sources displayed the narrowest peak widths. The peak-to-background ratios varied markedly; in descending order of peak-to-background ratio were single-wavelength X-rays (conventional and synchrotron sources, using incident-beam monochromators), two-wavelength X-rays in parafocusing (reflection) mode, two-wavelength X-rays in transmission mode and constant-wavelength neutrons. Refinement conditions were also markedly inconsistent, with the total number of refined parameters varying from 20 to 46. The major factors associated with lower accuracy of the derived crystal structure parameters were: (i) the use of insufficiently flexible peak-shape and/or background functions; (ii) omission of the high-angle data from the refinement, especially the data with d spacings below about 1 Å; (iii) use of an insufficiently wide range of diffraction angles on either side of the peak (i.e. peak truncation), especially for the reflection profiles with substantial Lorentzian (or Cauchy) character; (iv) poor instrumental resolution and/or a peak-to-background ratio less than about 50; (v) low pattern intensity (i.e. maximum step intensity less than about 2000 counts), especially at small d spacings; (vi) an observations-to-parameters ratio of less than about five. The X-ray- and neutron-data Zr-atom coordinates are distributed over a relatively narrow and similar range of values about the weighted mean values, viz 0.014 to 0.028 Å and 0.009 to 0.014 Å, respectively. On the other hand, while the values of the O-atom coordinates derived from the neutron data are determined with about the same accuracy as those of the Zr-atom ones, viz 0.006 to 0.017 Å about the mean, the corresponding values derived from the X-ray data are distributed over a very much wider range, viz 0.091 to 0.193 Å, no doubt due to the lower scattering power of the O atom. The atomic displacement (`thermal') parameters are reasonably determined with X-rays when flat-plate reflection-geometry instruments are used but transmission geometry produces very poor parameters ranging from large negative to large positive values; the poor quality of the latter results is due to the strong correlation between displacement and absorption effects and the generally smaller number of reflections included in the data. All but the lowest-resolution neutron data support a sensible anisotropic displacement ellipsoid for the atoms. The precision and accuracy of the population of crystal structural parameters produced from the participants' refinements were almost always substantially improved by reanalysis of the data using a `standard' program and an `optimal' refinement protocol. The mean probable errors, taken as the mean deviations of the individual estimates of the parameters from the weighted mean value, show that about two-thirds of the variation in the m-ZrO2 parameters is due to differences in the instrumental and data-collection conditions. The remaining one-third of the variation is due to differences in the software and/or the refinement strategy used. On average, the mean probable errors of the Rietveld parameters are larger than their derived estimated standard deviations by a factor of around two for coordinates, about five for the displacement parameters and around 16 for unit-cell dimensions. Of the X-ray instruments, flat-plate reflection-geometry ones provided the best crystal structure parameters for the sample of m-ZrO2 distributed in this study, but the quality was degraded when the data were cut off at d spacings larger than about 1 Å. The X-ray transmission geometries produced the poorest atomic parameters because of the generally poorer peak-to-background ratio and the limited range of data available (with resultant lower observations-to-parameters ratio). The results obtained with neutron data were of roughly equivalent quality to those obtained from X-rays in the case of the Zr atom, but neutrons were markedly superior for the determination of the O-atom coordinates and displacement parameters, as expected. The time-of-flight neutron and synchrotron X-ray results were not significantly different from those obtained in the conventional neutron and better-quality conventional X-ray analyses.

Many devices can only be accurately modelled using a combined circuit and finite element model (FEM), This paper describes the coupling of massive conductor circuits modelled using 3D finite elements and the A/spl psi/ scheme with an external circuit model. The massive conductor circuits by their nature lead to the fact that the magnetic scalar region becomes multiply connected. The cuts which are introduced to deal with the multiply connected magnetic scaler regions lead naturally to the current and voltage variables required to couple the field model and lumped circuit model. >

Frequency-dependent distributed transmission line self and mutual impedances and admittances for a single-track, power-rail electrified railway are obtained by experiment and modeling. Impedance results obtained by numerical calculation from an electromagnetic minimum energy field solution using the finite-element method are compared with analytic impedance models based on the Carson-Pollaczek equations for a stratified, weakly conducting ground and with practical experimental measurements made on a short length of rail track. The running rail self conductances and the power rail to running rail mutual capacitances are also evaluated by the finite-element method and compared with experimental measurements. A physical interpretation of the results is given in terms of conductive and induced ground currents caused by finite ground conductivity and relative permittivity. >

A novel scheme for automatically generating cuts in the magnetic scalar region of a finite element mesh is presented. Cuts are generated allowing multiply connected eddy current problems to be solved. The scheme has no topological limitations; it has been tested for knotted conductors, interlinked conductors, and hollow conductors. The scheme is fully automatic, requiring just a standard well-formed finite element mesh and appropriate boundary conditions. It has been implemented and tested in the finite element package MEGA for the A psi formulation. >

In a multiple-track railroad, the conductors form mutually-coupled ground return circuits. Accurate electrical circuit analysis, required for signaling, traction, and electromagnetic compatibility analysis, is difficult to achieve because of the variability in the equivalent electrical components from their material properties and environmental conditions. The paper describes an application of the finite element method to the electromagnetic analysis of rail track. After validation by benchmark tests using single rail, single- and three-rail track, the technique is used to predict crosstalk between adjacent tracks of a two-track electric railroad. >

A method is described for the determination of ground conductivity as a continuous function of depth and frequency for applications along spatially linear structures such as railway tracks. The technique involves measurements of mutual resistance using a modified dipole array excited with AC currents up to audio frequency. After representation of the experimental data, by analytic functions, the ground conductivity-depth variation is obtained as a degenerate hypergeometric function. The determined ground conductivity is utilized to model the self and mutual conductance of and between the running rails in a single-track railway. The result is verified by experimental measurement. >

Simulation algorithms to model the voltage distribution characteristics in AC autotransformer-fed railroads with single- and double-end supply are described. The theory is based on the solution of algebraic equations for a single train, moving along a track section represented as a coupled transmission line. The simulation was implemented on a PC with continuous displays of the catenary and rail voltages, and an historical record of the train voltage for movement at constant speed along the section. The simulator enables rapid parametric analyses to be made in order to optimize the power feeding arrangement, and sample displays of catenary, rail, and train voltages are given for single train operation under constant current and constant power. >

The Commission on Powder Diffraction of the International Union of Crystallography has undertaken an intercomparison of Rietveld refinements performed with two `standard' PbSO4 powder diffraction patterns: a conventional (two-wavelength) X-ray pattern collected on a Bragg–Brentano diffractometer with Cu Kα radiation and a constant-wavelength neutron pattern collected on the D1A diffractometer at the Institut Laue–Langevin. The aims of this project were: (i) to evaluate a cross section of currently used Rietveld refinement software; (ii) to examine the range and effect of various strategies of Rietveld refinement; (iii) to assess the precision and accuracy (spread) of the parameters derived by Rietveld analysis. 23 participants provided 18 refinements with the X-ray data and 20 refinements with the neutron data, using 11 different Rietveld-analysis programs. Analysis of the submitted results shows that refinement strategies play a large part in determining the detailed outcome of a Rietveld refinement. The wide variation in the values of the agreement indices obtained in these studies of the same data sets highlights the need for standardization both of the refinement procedures and of the type of data included in the algorithms used for assessing the fit. The major factors limiting the accuracy of the derived PbSO4 crystal structure parameters were: (i) use of insufficiently flexible peak shape and/or background functions; (ii) elimination of the high-angle diffraction data from the refinement; (iii) inclusion of an insufficiently wide range of diffraction angles on either side of the centroid of each peak during the step intensity calculation; and, additionally for X-rays, (iv) simultaneous release of the O-atom site-occupancy and displacement parameters. Rietveld analysis of the PbSO4 X-ray powder diffraction data provided atomic coordinates and isotropic displacement parameters for the Pb and S atoms that are precise (i.e. have small e.s.d.s) and are in reasonable agreement with the values derived from a single crystal study (viz the spread of coordinates is over the range 0.007–0.042 Å). On the other hand, the `light' O-atom parameters show relatively poor precision and have a disconcertingly wide spread of values about the weighted mean (viz 0.12–0.19 Å for the coordinates). Despite the much lower intrinsic resolution of the neutron data (i.e. peak widths some four times those of the X-ray data), the coordinates and anisotropic displacement parameters obtained for the Pb and O atoms are very precise and have a relatively narrow distribution about the single-crystal results, namely 0.004–0.020 Å for the coordinates. The range of coordinates determined from the neutron data for the relatively `light' S atom is correspondingly larger, namely 0.024–0.043 Å, about equivalent to that obtained from the X-ray data. In general, and as expected, the e.s.d.s from the Rietveld analyses are substantially smaller than the observed inter-refinement variation of the unit-cell dimensions, atomic coordinates and isotropic displacement parameters by factors of up to, respectively, 17, 5 and 22 for X-rays, and 25, 3 and 5 for neutrons. This investigation indicates that results of possibly high precision but low accuracy are not uncommon in Rietveld analysis. The disparity between individual refinements can be expected to increase further when, unlike here, the analyses are undertaken using data sets collected under diverse experimental conditions.

Nonlinearity in the internal self-impedance of railroad rail can give rise to power frequency harmonics in traction systems. Limiting case analytical models of rail impedance, based on linear complex permeability and complete saturation, are compared with measurements for currents to 800 A and frequencies to 10 kHz. The results show that for accurate representation an effective permeability of the material must be defined by prior modeling of the eddy current flux distribution within the rail iron. The usual assumption of rail, internal self-impedance invariance with current is shown to be unrealistic. >

A computer program, PEAKS ® , has been written to provide a quantitative determination of the relative amounts of up to 11 crystalline phases commonly found in samples of leadacid battery materials. The necessary observed data are the heights (or areas) of a set of pre-selected X-ray powder diffraction peaks collected with Cu Kα radiation. The program is designed for use on an IBM (or compatible) personal computer and has been written in the “C” language to facilitate data input and output and to simplify user interaction with the software. The method is rapid, accurate, and does not require an expert knowledge of either diffraction or crystallography. The amount of amorphous material present in the sample may also be determined by the addition of a known proportion of an internal standard material (CeO 2 ).

A theoretical approach to teaching the principles of electrodynamic levitation is described. Two theories are used to obtain analytical solutions to predict the performance of a simple MAGLEY arrangement. The first theory, based on Maxwell's equations and vector force relationships, is for a simplified model allowing for a finite-thickness track conductor. The second theory is for a thin plate track and is a straightforward calculation of the lift and drag forces for a particular electromagnetic repulsion geometry, involving solution by one-dimensional integral equations. The magnetic flux source travels at a constant velocity and is derived from a sinusoidally distributed current sheet which in practice can be derived from windings or permanent magnets. Edge and end effects are neglected in both cases, which is equivalent to assuming infinite iron above the excitation surface and below the conducting plate. The simplified theory is illustrated quantitatively by a laboratory experiment in which a tray of permanent magnets is suspended over a rotating conducting plane. Gap magnetic field and drag force are measured as functions of velocity. The theoretical approach allows the following aspects to be investigated: the magnetomotive force and physical dimensions to produce suitable field magnitudes and measurable forces; the effect of harmonics by practical measurement of field profiles; and the effect of skin depth and conducting plate dimensions. >

The incorporation of digital error-control coding into railway track circuits represents a new approach for track-train data transmission for automatic train control. Methods based on error probability analysis are presented for assessing the safety and reliability of synchronizable error control coding in this application. The results for a sample coding scheme have been verified by experimentation using a calibrated source injecting audio-frequency Gaussian noise into a practical track circuit. Safety and reliability must be traded off, although the safety can be increased by incorporating dictionary checking in the code-receiver algorithm and the reliability can be enhanced by allowing single synchronization error correction. The technique demonstrates the possibility of designing a track-circuit data transmission system to given target safety and reliability levels of the same order of magnitude as the known failure rates of existing equipment. >

A new application using synchronizable error-control coding for train detection and track-train data transmission with audio frequency railway track circuits is described, with algorithms for quasi-optimal code construction and performance assessment. A new coding format is presented for frequency-shift-keying-modulated track circuits with which a fixed-duration-message telegram is transmitted. Only two-track-circuit separation frequencies are required, and although there exists a maximum rate of transmission for a given track-circuit parameter set, the data transmission capability is in general limited only by the telegram length. The problem addressed is that of assigning codes within the message to satisfy target safety and reliability criteria. The proposed telegram-type code comprises a synchronization sequence followed by message data bits arranged as three blocks and terminated by error-control bits with a final parity check. Data encoding is by conventional Hamming codes. Several hundred messages are available for typical track-circuit parameter sets. >

An RF coil, made by twisting a conventional wire loop, was tuned to form an extended sensor to provide an effective method of showing the track of a catheter. A twisted-pair coil can be made small enough in diameter to pass through needles in common clinical use. The coil has a very small field of view and in transmit/receive operation has minimal effect on the magnetization of the surrounding tissue.

A new electrical machine which starts directly on line and does not require power electronics has been developed. Compared with the standard line start induction motor it has about 30% less losses and a similar starting performance. The new machine has a conventional stator and a two part rotor.

The use of iron powder material for the pole faces of iron cored magnetic resonance imaging magnets is shown in this paper to reduce substantially the formation of eddy currents, which cause a delay in the build-up of applied gradient fields. These delays can result in errors in the computed image.

High efficiency at full load as well as at partial load and high torque density are the challenging characteristics required by the propulsion system for a solar aircraft. The proposed solution is a multi-disc axial flux PM motor with a mechanical structure made of plastic materials: (a) the multi-disc geometry allows high torque density by increasing the number of air-gap surfaces; (b) the PMs and the (almost) ironless magnetic circuit, cutting the no load losses, allow very high efficiency over the complete working range; (c) the motor structure made of plastic materials, with high thermal and mechanical characteristics, allows further increases in the torque density. The paper presents the motor design and performances.

The paper is concerned with the analysis of linear induction motors. It compares the use of an early simple space transient analysis with finite element modelling using the A/spl psi/ method for 3D problems and the Minkowski transformation for moving conductor regions. Special attention was paid to the effect of the geometry of the trailing edge of the machine. >

Practical results are presented for the three axes forces produced on the primary of a linear brushless DC machine which is supplied from a three-phase delta-modulated inverter. Conditions of both lateral alignment and lateral displacement are considered. Finite element analysis using both two and three dimensional modelling is compared with the practical results. It is shown that a modified two dimensional model is adequate, where it can be used, in the aligned position and that the full three dimensional method gives good results when the machine is axially misaligned. >

The stratospheric unmanned aircraft for the proposed European Union Helinet telecommunications infrastructure will use inverter/brushless AC motor drives for the propellers. One form of machine being considered is of multi-disc axial flux slotless construction. This has no iron losses due to the fundamental field but, since the windings are in the air-gaps, eddy current losses are present in the strands of the coil conductors. These losses are calculated, using a finely meshed finite element model and a time-stepped approach. The winding factors of the three types of winding without coil end-region overlaps are determined. Design comparisons are drawn between axial flux and conventional radial flux designs.

A novel brushless ac permanent-magnet motor is described. This is intended for use as a highly efficient machine for a propeller-driven stratospheric aircraft. The machine is lightweight because of the use of plastics for the structural elements. A form of modeling is described in which the parameters for a decoupled equivalent circuit are calculated by three-dimensional finite-element analysis of a module of the machine. Matlab is then used to simulate the equivalent circuit action. It is shown that the fundamental component of the current waveforms agrees closely with a simple analysis approach.

In accordance with the theory of contact exchange, it is hypothesized that the presence of negative charge in microbial exopolysaccharides increases the rate of cation transport. These typically acidic materials may provide a fast-track for the diffusion of nutrient cations through the polymer layer for uptake at the organism cell surface. We have measured the diffusion coefficient of a model cation, Mn 2+, through xanthan, de-acetylated xanthan, scleroglucan and chitosan using spatially resolved NMR relaxometry. The concentration of Mn 2+ in solution was measured by recording the change in the spin-spin ( T 2 ) relaxation time of water 1 H over time in compartments either side of a polymer layer. This approach provides a sensitive, in situ, non-invasive method of measuring the rate of diffusion of paramagnetic cations through hydrophilic polysaccharides. The negatively-charged polysaccharides, xanthan and de-acetylated xanthan, permitted a significantly faster rate (2–2.5×) of cation transport compared to the uncharged polymer, scleroglucan. The positively-charged polysaccharide chitosan reduced the rate of Mn 2+ diffusion to around half the value obtained for scleroglucan. These results suggest that the presence and nature of fixed charges on the polysaccharide molecule affects the rate of cation transport in accordance with the theory of contact exchange. The presence of negative charge on microbial exopolysaccharides may thus improve the availability of nutrient cations at the organism cell surface.

Presents the use of a 3D eddy current FE procedure for the analysis and design of two different induction machine structures: a radial and an axial machine. In the first case, attention is devoted to the simulation of locked rotor conditions and a linear 3D time harmonic eddy current FE analysis has been employed. The obtained results have been compared to 2D analysis and experimental data. The axial flux machine is analyzed under fixed speed conditions and a 3D time‐stepping and velocity eddy current problems are solved to evaluate machine performances. Different design configurations are analyzed in order to define the best solution.

Standstill forces on tubular induction machines are calculated by two methods: a simple one-dimensional technique and a two-dimensional axisymmetric finite element analysis. The end effect force produced by the finite length of the machine is calculated. The work is confirmed by tests taken on a practical machine. >

A method for solving problems involving eddy currents flowing in thin surface layers in magnetically nonlinear iron is described. It is shown how an extension of the method of D. Rodger and N. Atkinson (1988) for modeling eddy currents in thin sheets can be used to model low skin depth problems using sheet elements which cover the surface of the eddy current regions. The resulting equations can be nonlinear if the permeability is dependent on fields. The volume of the iron need not be modeled using this method. Only the fundamental harmonic of the AC supply is considered and hysteresis effects are ignored. Nevertheless, the method has been shown to be useful in situations where a full volume, nonlinear transient solution would be computationally too expensive. Some experimental results are compared with simulations from the MEGA program package. >

A scheme for modelling the current distribution in massive voltage-forced or current-forced conductors is presented. The method is based on a 3-D finite-elements model which use the A psi V field representation. The scheme has been verified by simulating a toroidal choke and comparing the predicted AC losses with experimental measurements. The method has been shown to give reasonable results when applied to the toroidal choke problem, considering that the double helix was idealized as a set of circular conductors. Selectively increasing the order of representation has been shown to be a good technique for modelling skin depth. One disadvantage of the scheme is the number of variables per node, five on the conductor to nonconductor interface. This leads to a relatively dense matrix which is costly to solve. >

Recent developments in the Rietveld method for the analysis of powder diffraction data have seen the method evolve from its original purpose of crystal structure refinement to nclude the determination of phase abundance in polycrysalline mixtures and the estimation of crystal size and strain parameters. However, the Rietveld method is not easy to use and may deter many powder diffractionists, who are not inerested in structure refinement per se, from using the method in its non-structural applications.In order to overcome the difficulties in using the Rietveld method, a program, QPDA (for Quantitative Powder Diffraction Analysis), has been written that sets the conditions necessary for a single or multi-phase refinement, runs the Rietveld program and extracts phase abundance and size/strain information from the refined parameters. The program comprises a user-friendly, default-driven system of subroutines, written initially in VAX Fortran, and operates from a database of inorganic materials frequently encountered in a wide range of minerals and materials science industries.Equations (3) and (5) should be corrected to read as follows