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GLOSSORY OF TERMS
AREA OF THE AIR GAP (Ag) or the cross sectional area of the air gap perpendicular to the flux path - the average cross sectional area of that portion of the air gap within which the application interaction occurs. Area is measured in sq. cm. in a plane normal to the central flux line of the air gap.
AREA OF THE MAGNET (Am) - the cross sectional area of the magnet perpendicular to the central flux line, measured in sq. cm. at any point along its length. In design, Am is usually considered the area at the neutral section of the magnet.
AIR GAP VOLUME ( Vg) - the useful volume of air or non- magnetic material between magnetic poles; measured in cubic centimeters.
CLOSED CIRCUIT CONDITION - when the external flux path of a permanent magnet is confined, with no air gap, with high permeability material.
CURIE TEMPERATURE ( Tc) - the transition temperature above which a material loses its magnetic properties.
ENERGY PRODUCT (BdHd) - the energy that a magnetic material can supply to an external magnetic circuit when operating at any point on its demagnetization curve; measured in megagauss-oersteds (MGO or MGOe) or kilojule per meter (kJ/m).
ENERGY PRODUCT, MAXIMUM (BH)max - the maximum product of (BdHd) which can be obtained on the demagnetization curve.
FIELD STRENGTH, MAGNETIC (H) (magnetizing or demagnetizing force) - the measure of the vector magnetic quantity that determines the ability of an electric current, or a magnetic body, to induce a magnetic field at a given point; measured in Oersteds (Oe) or Ampere turns per meter (A/m).
FLUX, LEAKAGE - flux, ø, whose path is outside the useful or intended magnetic circuit: measured in maxwells or Webers.
flux, magnetic ( ø) - the surface integral of the magnetic induction, B, over an area, A.
ø = ∫ ∫ B • dA
where:
ø = magnetic flux, in maxwells
B = magnetic induction, in gauss
dA = an element of area, in square centimeters
FLUXMETER - an instrument that measures the change of flux linkage with a search coil; an integrating voltmeter.
FORCE, COERCIVE (Hc or Hcb) of a material - equal to the total demagnetizing force required to reduce B to zero.
FORCE, INTRINSIC COERCIVE (Hci or Hcj) - a material’s resistance to demagnetization. Equal to the total demagnetizing force required to reduce the intrinsic induction, Bi, in the material to zero.
force, Net effective magnetizing (Hs) - the magnetizing force required in the material, to magnetize to saturation.
GAUSS - the unit of magnetic induction, B, in the cgs electromagnetic system. One gauss is equal to one maxwell per square centimeter.
GAUSSMETER - an instrument that measures the instantaneous value of magnetic induction, B. Its principle of operation is usually based on one of the following: the Hall- effect, nuclear magnetic resonance (NMR), or the rotating coil principle.
GILBERT- the unit of magnetomotive force, F, in the cgs electromagnetic system.
HYSTERESIS LOOP - a graph showing the relationship between the induced magnetic flux density (B) and the applied magnetizing force (H). It is often referred to as the B-H loop.
INDUCTION, SATURATION INTRINSIC (Bis or Js) -the maximum intrinsic induction possible in a material.
INDUCTION IN THE AIR GAP, MAGNETIC (Bg) - the average value of magnetic induction over the area of the air gap, Ag; or it is the magnetic induction measured at a specific point within the air gap; measured in gauss.
INDUCTION, INTRINSIC (Bi or J), is the contribution of the magnetic material’s induction to the total magnetic induction, B. It is the vector difference between the magnetic induction in the material and the magnetic induction that would exist in a vacuum under the same field strength, H. This relation is expressed by the equation:
Bi = B –H
where: Bi = intrinsic induction; B = magnetic induction; H = field strength.
INDUCTION, RESIDUAL (or residual flux density) (Br) - the magnetic induction corresponding to zero magnetizing force in a magnetic material after saturation in a closed circuit; measured in gauss.
INDUCTION, MAGNETIC (B) - the magnetic field induced by a field strength, H, at a given point. It is the vector sum, at each point within the substance, of the magnetic field strength and resultant intrinsic induction. Magnetic induction is the flux per unit area normal to the direction of the magnetic path.
INDUCTION, REMANENT (Bd) - any magnetic induction that remains in a magnetic material after removal of an applied saturating magnetic field, Hs. (Bd is the magnetic induction at any point on the demagnetization curve; measured in gauss.)
Irreversible losses - partial demagnetization of the magnet, caused by exposure to high or low temperatures external fields or other factors. These losses are recoverable by re-magnetization. Magnets can be stabilized against irreversible losses by partial demagnetization induced by temperature cycles or by external magnetic fields
KEEPER - a piece (or pieces) of soft iron placed on or between the pole faces of a permanent magnet to decrease the reluctance of the air gap and thereby reduce the flux leakage from the magnet. It also makes the magnet less susceptible to demagnetizing influences.
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LEAKAGE FACTOR (f) - accounts for flux leakage from the magnetic circuit. The ratio between the magnetic flux at the magnet neutral section and the average flux present in the air gap.
f = (Bm Am)/(B Ag).
LENGTH OF THE AIR GAP (ℓg) - the length of the path of the central flux line of the air gap; measured in centimeters.
LENGTH OF THE MAGNET ( ℓm ) - the total length of magnet material traversed in one complete revolution of the center- line of the magnetic circuit; measured in centimeters.
MATERIAL, ISOTROPIC (NOT ORIENTED) - material with equal magnetic properties in all directions.
MATERIAL, ANISOTROPIC (ORIENTED) - material with better magnetic properties in a given direction.
MAXWELL - the unit of magnetic flux in the cgs electromagnetic system. One maxwell is one line of magnetic flux.
MAXIMUM SERVICE TEMPERATURE ( Tmax) - the maximum temperature to which the magnet may be exposed with no significant long range instability or structural changes.
Magnetomotive force - or magnetic potential difference (F) - the line integral of the field strength, H, between any two points, p1 and p2.
p2
F = ∫ H dl
p1
F = magnetomotive force in gilberts
H = field strength in oersteds
dl = an element of length between the two points, in
centimeters.
NEUTRAL SECTION - a plane passing through the magnet perpendicular to its central flux line at the point of maximum flux.
OERSTED - the unit of magnetic field strength, H, in the cgs electromagnetic system. One oersted equals a magnetomotive force of one gilbert per centimeter of flux path.
OPEN CIRCUIT CONDITION - when a magnetized magnet is by itself with no external flux path of high permeability material.
OPERATING LINE - a straight line passing through the origin of the demagnetization curve with a slope of negative Bd/Hd. (Also known as permeance coefficient line.)
OPERATING POINT - that point on a demagnetization curve defined by the coordinates (BdHd) or that point within the demagnetization curve defined by the coordinates (BmHm).
PERMEABILITY ( µ) - the general term used to express various relationships between magnetic induction, B, and the field strength, H.
PERMEABILITY, RECOIL ( µre) - the average slope of the recoil hysteresis loop. Also known as a minor loop.
PERMEAMETER - an instrument that can measure, and often record, the magnetic characteristics of a specimen material.
PERMEANCE ( P) - the reciprocal of the reluctance, R, measured in maxwells per gilbert.
RELUCTANCE FACTOR (r) - accounts for the apparent magnetic circuit reluctance. This factor is required due to the treatment of Hm and Hg as constants.
ReluctancE (R) - somewhat analogous to electrical resistance. It is the quantity that determines the magnetic flux, ø, resulting from a given magnetomotive force, F. where:
R = F/ø
R = reluctance
F = magnetomotive force
ø = flux
SATURATION, MAGNETIC - exists when an increase in magnetizing force produces no increase in intrinsic induction.
SEARCH COIL - a coiled conductor, usually of known area and number of turns, that is used with a fluxmeter to measure the change of flux linkage with the coil.
SLOPE OF THE OPERATING LINE (Bd/Hd) - the ratio of the remanent induction, Bd, to a demagnetizing force, Hd. It is also referred to as the permeance coefficient, shear line, load lineand unit permeance.
TEMPERATURE COEFFICIENT - a factor which describes the reversible change in a magnetic property with a change in temperature. The magnetic property spontaneously returns when the
temperature is cycled to its original point. It usually is expressed as the percentage change per unit of temperature.
TEMPERATURE COEFFICIENT, REVERSIBLE - changes in flux which occur with temperature change. These are spontaneously regained when the temperature is returned to its original point.
Ag Area of the air gap
Am Area of the magnet
B Magnetic induction
Br Remanent induction
B/H Slope of the operating line
BdHd Energy product
(BH)max Energy product, Maximum
Bis (or Js) Induction, saturation intrinsic
Bg Induction in the air gap, magnetic
Bi (or J) Induction, intrinsic
Br Induction, residual (or flux density)
r Reluctance factor
f Leakage factor
F Magnetomotive force, (magnetic potential difference)
H Magnetic field strength
Hc (or Hcb) Force, coercive
Hci (or Hcj) Force, intrinsic coercive
Hd Value of H corresponding to the remanent induction, Bd.
Hm Value of H corresponding to the recoil induction, Bm.
ℓg Length of the air gap
ℓm Length of the magnet
P (or Pc) Permeance
R Reluctance
Tc Curie temperature
Tmax Maximum service temperature
Vg Air gap volume
µ Permeability
µr Permeability, recoil
ø Flux, magnetic