Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Patent
1995-05-19
1996-10-15
O'Shea, Sandra L.
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
324225, G01N 2772, G01B 730
Patent
active
055657745
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to means for the characterisation of magnetic materials by measurement of their magnetic characteristics. The invention is particularly, but not exclusively, suitable for the characterisation of high energy permanent magnets.
Permanent magnets are steadily becoming more commonplace in both domestic and industrial applications and magnetic performance is being improved by the use of new materials such as rare earth/iron materials, for example neodymium-iron-boron. Such new materials have extremely high values of intrinsic coercivity, to the extent that they cannot be characterized by traditional techniques. In order to measure the complete hysteresis loop of such materials, from which the magnetic characteristics may be deduced, an external field sufficient to produce a flux density up to 15 Teslas or greater may be required; this requirement will undoubtedly increase further as new generations of magnetic materials, for example samarium-iron-nitride, are developed.
DESCRIPTION OF THE PRIOR ART
Existing instruments for measurement of magnetic characteristics include the permeameter and the vibrating sample magnetometer. In the permeameter, the magnetic sample is placed in a closed magnetic circuit between the poles of an electromagnet the current supply to which is slowly ramped up and down as the working point of the magnetic sample is monitored. This instrument exhibits a zero self-demagnetising factor and negligible adverse eddy current effects but has a maximum effective applied field capacity to generate a flux density of only about 2 Teslas, due to limitation imposed by the iron yoke of the electromagnet. Even with the use of cobalt-iron pole pieces to focus the magnetic flux, this is still inadequate for high-performance 3-dimensional transition metal alloy magnets, and restricts its use to the measurement of part only of the second quadrant characteristics on samples which have been pre-magnetised in a much stronger field.
The vibrating sample magnetometer may use a superconducting solenoid to generate the applied field, whereby flux densities of up to about 20 Teslas can be obtained. However, the capital and running costs of such instruments militate against their use in any but research applications and their operating times are in any event too slow for many industrial purposes.
There is therefore a need for an instrument that will provide a quick-response assessment of magnetic characteristics and which also takes account of magnetic geometry, at capital and running costs which would enable it to be used in industry as well as in research. A pulsed field magnetometry system has been proposed to meet such a need (see IEEE Transactions on Magnetics, 24 (1988), no. 2,970-973), wherein a pulsed magnetic field is applied to the sample magnet, the applied field strength and the magnetic flux density in the magnet being measured by sensing coils, but hitherto such systems have not provided basis for a practical instrument for measuring magnetic characteristics because of the errors arising from the strong eddy currents induced in the sample magnet as a result of the time-varying magnetic field produced during the application of the pulsed field. The adverse effects of eddy currents could be avoided by delivering a magnetising pulse with a slow rise time, but this would increase the capital expense.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a pulsed field magnetometry system in which the errors arising from eddy currents are substantially eliminated, the system being realisable at a cost which makes it feasible for production purposes. A further object is to provide a system in which the presence of eddy currents may be detected and/or measured.
According to one aspect of the invention, apparatus for the measurement of magnetic characteristics of a sample magnetic material comprises means to generate a pulsed magnetic field for application to the sample, means for determining the applied magnetic field strength an
REFERENCES:
patent: 4647856 (1987-03-01), Melgui et al.
patent: 4843316 (1989-06-01), Hesterman
"A Pulsed Field Magnetometer For The Charaterization Of Hard Materials", Grossinger et al, IEEE Transactions on Magnetics, vol. 24, No. 2 Mar. 1988 .
Dudding John
Grossinger Roland
Jewell Geraint
Hirst Magnetic Instruments Limited
O'Shea Sandra L.
Phillips Roger
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