R.J. Hill-Cottingham

Twisted-pair RF coil suitable for locating the track of a catheter †

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. Magn Reson Med 41:636–638, 1999. 

3D finite element models and external circuits using the A/spl psi/ scheme with cuts

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. >

Automatic implementation of cuts in multiply connected magnetic scalar regions for 3D eddy current models

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. >

Calculation of AC losses in current forced conductors using 3D finite elements and the A psi V method

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. >

Active Axial-Magnetomotive Force Parallel-Airgap Serial Flux Magnetic Bearings

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.

Surface elements for modelling eddy currents in high permeability materials

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. >

Three dimensional force prediction in a model linear brushless DC motor

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. >

Numerical models of an induction machine

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.

Integrated finite element and circuit modelling of a brushless DC motor

The paper describes a modelling technique for a system incorporating a brushless DC motor and a six step inverter. The circuit equations are incorporated within a finite element model so that no parameter updating is required. The work is supported by experimental results from a practical system.

Prediction of thrust forces in tubular induction machines

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. >