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Permanent Magnet Guidelines

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Permanent Magnet Guidelines MMPA PMG-88 PERMANENT MAGNET GUIDELINES I. Basic physics of permanent magnet materials II. Design relationships, figures of merit and optimizing techniques III. Measuring IV. Magnetizing V. Stabilizing and handling VI. Specifications, standards and communications VII. Bibliography MAGNETIC MATERIALS PRODUCERS ASSOCIATION 8 SOUTH MICHIGAN AVENUE • SUITE 1000 • CHICAGO, ILLINOIS 60603 INTRODUCTION This guide is a supplement to our MMPA Standard No. 0100. It relates the information in the Standard to permanent magnet circuit problems. The guide is a bridge between unit property data and a permanent magnet component having a specific size and geometry in order to establish a magnetic field in a given magnetic circuit environment. The MMPA 0100 defines magnetic, thermal, physical and mechanical properties. The properties given are descriptive in nature and not intended as a basis of acceptance or rejection. Magnetic measure- ments are difficult to make and less accurate than corresponding electrical mea- surements. A considerable amount of detailed information must be exchanged between producer and user if magnetic quantities are to be compared at two locations. MMPA member companies feel that this publication will be helpful in allowing both user and producer to arrive at a realistic and meaningful specifica- tion framework. Acknowledgment The Magnetic Materials Producers Association acknowledges the out- standing contribution of Rollin J. Parker to this industry and designers and manufacturers of products usingpermanent magnet materials. Mr Parker the Technical Consultant to MMPA compiled and wrote this document. We also wish to thank the Standards and Engineering Com- mittee of MMPA which reviewed and edited this document. December 1987 3M July 1988 5M August 1996 1M December 1998 1 M CONTENTS The guide is divided into the following sections: Glossary of terms and conversion tables- A very important starting point since the whole basis of communication in the magnetic material industry involves measurement of defined unit properties. .l I. Basic physics of permanent magnet materials- An overview of the physics of permanent magnetism is felt to be an important part of learning about permanent magnets. .4 II. Design relationships, figures of merit and optimizing techniques- Relationships are developed that allow materials to be compared. The figures of merit are a guide to the best choice of unit properties for effectively using the energy conversion process in electromagnetic devices and systems. .7 III. Measuring . ...12 IV. Magnetizing . ~ , . .18 V. Stabilizing and handling. .22 VI. Specifications, standards and communications . .25 VII. Bibliography . .28 The sections on measuring, magnetizing and stabilizing contain the basic considerations that allow the user to choose the techniques most suitable for a specific permanent magnet component and operating environment. GLOSSARY OF TERMS As Area of the air gap, or the cross sectional area of the where: Bi = intrinsic induction in gauss; B = magnetic an gap perpendicular to the flux path, is the average induction in gauss; H = field strength in oersteds. cross sectional area of that portion of the air gap within which the application interaction occurs. Area is mea- B, Recoil induction, is the magnetic induction that sured in sq. cm. in a plane normal to the central flux line remains in a magnetic material after magnetizing and of the air gap. conditioning for final use; measured in gauss. A,,, Area of the magnet, is the cross sectional area of B, Magnetic induction, at the point of the maximum the magnet perpendicular to the central flux line, mea- enerajr product (BH),,,,; measured in gauss. sured in sq. cm. at any point along its length. In design, A,,, is usually considered the area at the neutral section of the magnet. B, Residual induction (or flux density), is the magnetic induction corresponding to zero magnetizing force in a B Magnetic induction, is the magnetic field induced by magnetic material after saturation in a closed circuit; a field strength, H, at a given point. It is the vector sum, measured in gauss. at each point within the substance, of the magnetic field strength and resultant intrinsic induction. Magnetic in- f Reluctance factor, accounts for the apparent mag- duction is the flux per unit area normal to the direction netic circuit reluctance. This factor is required due to of the magnetic path. the treatment of H, and H, as constants. B, Remanent induction, is any magnetic induction F Leakage factor, accounts for flux leakage from the that remains in a magnetic material after removal of an magnetic circuit. It is the ratio between the magnetic applied saturating magnetic field, H,. (Bd is the mag- flux at the magnet neutral section and the average flux netic induction at any point on the demagnetization present in the air gap. F = (B, A,,)/(B, Ag). curve: measured in gauss.) B,/H, Slope of the operating line, is the ratio of the F Magnetomotive force, (magnetic potential differ- remanent induction, Bd, to a demagnetizing force, H,. ence), is the line integral of the field strength, H, be- It is also referred to as the permeance coefficient, shear tween any two points, p1 and pz. line, load line and unit permeance. B,H, Energy product, indicates the energy that a mag- I;=;‘Hdl netic material can supply to an external magnetic circuit Pl when operating at any point on its demagnetization curve; measured in megagauss-oersteds. F = magnetomotive force in gilberts H = field strength in oersteds vw?lax Maximum energy product, is the maximum dl = an element of length between the two points, in product of (BtlHtt) which can be obtained on the demag- centimeters. netization curve. H Magnetic field strength, (magnetizing or demagne- Bi, (or J) Saturation intrinsic induction, is the maxi- tizing force), is the measure of the vector magnetic mum intrinsic induction possible in a material. quantity that determines the ability of an electric cur- rent, or a magnetic body, to induce a magnetic field at a B, Magnetic induction in the air gap, is the average given point; measured in oersteds. value of magnetic induction over the area of the air gap, A,; or it is the magnetic induction measured at a spe- cific point within the air gap; measured in gauss. H, Coercive force of a material, is equal to the demag- netizing force required to reduce residual induction, B,, Bi (or J) Intrinsic induction, is the contribution of the to zero in a magnetic field after magnetizing to satura- magnetic material to the total magnetic induction, B. It tion; measured in oersteds. is the vector difference between the magnetic induction in the material and the magnetic induction that would H,i Intrinsic coercive force of a material indicates its exist in a vacuum under the same field strength, H. This resistance to demagnetization. It is equal to the demag- relation is expressed by the equation: netizing force which reduces the intrinsic induction, Bit in the material to zero after magnetizing to saturation; Bi=B-H measured in oersteds. 1 H, is that value of H corresponding to the remanent pre recoil permeability, is the average slope of the re- induction, Bd; on the demagnetization curve, measured coil hysteresis loop. Also known as a minor loop. in oersteds. 4 magnetic flux, is a contrived but measurable concept H, is that value of H corresponding to the recoil induc- that has evolved in an attempt to describe the “flow” of a tion, B,; measured in oersteds. magnetic field. Mathematically, it is the surface integral of the normal component of the magnetic induction, B, H, is the magnetic field strength at the point of the over an area. A. maximum energy product (BH)maX; measured in oersteds. +=ijB*dA where: H, Net effective magnetizing force, is the magnetiz- C$ = magnetic flux, in maxwells ing force required in the material, to magnetize to satu- B = magnetic induction, in gauss ration measured in oersteds. dA = an element of area, in sauare centimeters When the magnetic induction, B, is uniformly distrib- J, see Bi Intrinsic induction. uted and is normal to the area, A, the flux, 4 = BA. J, see Bi, Saturation intrinsic induction. A closed circuit condition exists when the external Es Length of the air gap, is the length of the path of the flux path of a permanent magnet is confined with high central flux line of the air gap; measured in centimeters. permeability material. U,,., Length of the magnet, is the total length of magnet The demagnetization curve is the second (or fourth) material traversed in one complete revolution of the quadrant of a major hysteresis loop. Points on this curve center-line of the magnetic circuit; measured in centi- are designated by the coordinates Bd and Hd. meters. A fluxmeter is an instrument that measures the change P,.,,ID Dimension ratio, is the ratio of the length of a of flux linkage with a search coil. magnet to its diameter, or the diameter of a circle of equivalent cross-sectional area. For simple geometries, The gauss is the unit of magnetic induction, B, in the such as bars and rods, the dimension ratio is related to cgs electromagnetic system. One gauss is equal to one the slope of the operating line of the magnet, BdH,. maxwell per square centimeter. P Permeance, is the reciprocal of the reluctance, R, A gaussmeter is an instrument that measures the in- measured in maxwells per gilbert. stantaneous value of magnetic induction, B. Its prin- ciple of operation is usually based on one of the follow- R Reluctance, is somewhat analogous to electrical re- ing: the Hall-effect, nuclear magnetic resonance sistance. It is the quantity that determines the magnetic (NMR), or the rotating coil principle. flux, 4, resulting from a given magnetomotive force, F. The gilbert is the unit of magnetomotive force, F, in the R = F/qb cgs electromagnetic system.
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