| Air gap, is a low permeability
gap in the flux path of a magnetic circuit. Often air, but inclusive
of other materials such as paint, aluminum, etc.
Anisotropy, Literally means having different
properties depending on the inspected direction. Magnets which
are anisotropic, or have an easy axis of magnetization, have
their anisotropy developed by two methods: Shape and Magnetocrystalline.
As Area of the air gap, or the cross sectional area of the air
gap perpendicular to the flux path, is 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.
Am Area of the magnet, is 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.
Br Magnetic induction, is 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.
Bd Remnant induction, is 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.)
Bd/Hd, Slope of the operating line, is the
ratio of the remnant induction, Bd, to a demagnetizing force,
Hd. It is also referred to as the permeance coefficient, shear
line, load line and unit permeance.
BdHd Energy product, indicates 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.
(BH) max Maximum energy product, is the maximum
product of (BdHd) which can be obtained on the demagnetization
curve.
Bis, (or J) Saturation intrinsic induction,
is the maximum intrinsic induction possible in a material.
Bg, Magnetic induction in the air gap, is the
average value of magnetic induction over the area of the air
gap, A; or it is the magnetic induction measured at a specific
point within the air gap; measured in gauss.
Bi (or J) Intrinsic induction, is the contribution of the magnetic
material 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 in gauss; B = magnetic
induction in gauss; H = field strength in oersteds .
Bm, Recoil induction, is the magnetic induction
that remains in a magnetic material after magnetizing and conditioning
for final use; measured in gauss.
Bo, Magnetic induction, at the point of the maximum
energy product (BH)max; measured in gauss.
Br, Residual induction (or flux density), is
the magnetic induction corresponding to zero magnetizing force
in a magnetic material after saturation in a closed circuit;
measured in gauss.
A closed circuit condition exists when the
external flux path of a permanent magnet is confined with high
permeability material.
Curie Temperature, The transition temperature
above which the alloy loses its magnetic properties. This is
not the maximum serviceable temperature, which is usually much
lower.
The demagnetization curve is the second (or
fourth) quadrant of a major hysteresis loop. Points on this
curve are designated by the coordinates Bd and Hd.
Domains, Areas in a magnetic alloy which have
the same orientation. The magnetic domains are regions where
the atomic moments of atoms cooperate and allow for a common
magnetic moment. It is the domains which are rotated and manipulated
by an external magnetizing field to create a useful magnet which
has a net magnetic moment. In un-magnetized material the domains
are un-oriented and cancel each other out. In this condition
there is no net external field.
Eddy currents, are circulating electrical currents
that are induced in electrically conductive elements when exposed
to changing magnetic fields, creating an opposing force to the
magnetic flux. Eddy currents can be harnessed to perform useful
work (such as dampening of movement), or may be unwanted consequences
of certain designs, which should be accounted for of minimized.
Electromagnet, is a magnet, consisting of solenoid
with an iron core, which has a magnetic field existing only
during the time of current flow through the coil.
f Reluctance factor, accounts for the apparent
magnetic circuit reluctance. This factor is required due to
the treatment of H, and H, as constants.
F Leakage factor, accounts for flux leakage from
the magnetic circuit. It is the ratio between the magnetic flux
at the magnet neutral section and the average flux present in
the air gap. F=(B mA m)/(B, A g).
F Magneto motive force, (magnetic potential
difference), is the line integral of the field strength, H,
between any two points, p1 and p2.
p2
F=? H dl
p1
F = magneto motive force in gilberts
H = field strength in oersteds
dl = an element of length between the two points, in centimeters.
Ferromagnetic material, is a material whose permeability
is very much larger than 1 (from 60 to several thousands times
1), and which exhibits hysteresis phenomena.
Flux, is the condition existing in a medium
subjected to a magnetizing force. This quantity is characterized
by the fact that an electromotive force is induced in a conductor
surrounding the flux at any time the flux changes in magnitude.
The cgs unit of flux is the Maxwell.
A flux meter is an instrument that measures the
change of flux linkage with a search coil.
Fringing fields are leakage flux particularly
associated with edge effects in a magnetic circuit.
The gauss is the unit of magnetic induction, B,
in the cgs electromagnetic system. One gauss is equal to one
maxwell per square centimeter.
A gauss meter is 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.
The gilbert is the unit of magneto motive force,
F, in the cgs electromagnetic system.
H Magnetic field strength, (magnetizing or
demagnetizing force), is 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.
Hc Coercive force of a material, is equal to
the demagnetizing force required to reduce residual induction,
Br to zero in a magnetic field after magnetizing to saturation;
measured in oersteds.
Hci Intrinsic coercive force of a material,
< indicates its resistance to demagnetization. It is equal
to the demagnetizing force which reduces the intrinsic induction,
Bi, in the material to zero after magnetizing to saturation;
measured in oersteds.
Hd, is that value of H corresponding to the
remnant induction, Bd; on the demagnetization curve, measured
in oersteds.
Hmv, is that value of H corresponding to the recoil
induction, B,; measured in oersteds.
Ho, is the magnetic field strength at the point
of the maximum energy product (BH)max; measured in oersteds.
Hs, Net effective magnetizing force, is the
magnetizing force required in the material, to magnetize to
saturation measured in oersteds.
A hysteresis loop is a closed curve obtained for
a material by plotting (usually to rectangular coordinates)
corresponding values of magnetic induction, B, for ordinates
and magnetizing force, H, for abscissa when the material is
passing through a complete cycle between definite limits of
either magnetizing force, H, or magnetic induction. B.
Irreversible losses are defined as partial
demagnetization of the magnet, caused by exposure to high or
low temperatures external fields or other factors. These losses
are recoverable by remagnetization. Magnets can be stabilized
against irreversible losses by partial demagnetization induced
by temperature cycles or by external magnetic fields.
J, see Bi Intrinsic induction.
Js, see Bis, Saturation intrinsic induction.
A keeper is a piece (or pieces) of soft iron
that is 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.
Keepers, A keeper is a high permeability material,
typically mild steel, which is installed on a magnet or magnetic
assembly to reduce the reluctance of the magnetic circuit. This
reduces the overall leakage fields generated by the magnet or
magnetic assembly. Keepers are typically installed to help the
magnet or magnetic assembly resist demagnetization during handling,
transportation, or storage. Keepers are typically found on Alnico
magnets and Alnico magnetic assemblies.
Knee of the demagnetization curve is the point
at which the B-H curve ceases to be linear. All magnet materials,
even if their second quadrant curves are straight line at room
temperature, develop a knee at some temperature. Alnico 5 exhibits
a knee at room temperature. If the operating point of a magnet
falls below the knee, small changes in H produce large changes
in B, and the magnet will not be able to recover its original
flux output without re-magnetization.
Leakage flux is flux, ?, whose path is outside
the useful or intended magnetic circuit; measured in maxwells.
lg Length of the air gap, is the length of
the path of the central flux line of the air gap; measured in
centimeters.
lm Length of the magnet, is the total length of
magnet material traversed in one complete revolution of the
centerline of the magnetic circuit; measured in centimeters.
lm/D Dimension ratio, is the ratio of the length
of a magnet to its diameter, or the diameter of a circle of
equivalent cross-sectional area. For simple geometries, such
as bars and rods, the dimension ratio is related to the slope
of the operating line of the magnet, BdHa.
Load line is a line drawn from the origin of
the demagnetization curve with a slope of B/H, the intersection
of which with the B-H curve represents the operating point of
the magnet. Also see permeance coefficient.
Magnetic Assemblies, A combination of materials,
magnetic and non-magnetic, which form a particular solution.
Incorporates a permanent magnet as the flux generator and usually
relies on mild steel to conduct the flux to the workface. Allows
for better means of mounting-tapped holes, threads, press fits,
etc.
Magnetic circuit, an assembly consisting of some
or all of the following: permanent magnets, ferromagnetic conduction
elements, air gaps, and electrical currents.
Magnetic Length, The physical length of the
magnet dimension which corresponds to the direction the magnet
is magnetized. This may or may not be the magnet's orientation
direction.
The major hysteresis loop of a material is
the closed loop obtained when the material is cycled between
positive and negative saturation.
The maxwell is the unit of magnetic flux in the
cgs electromagnetic system. One maxwell is one line of magnetic
flux.
The neutral section of a permanent magnet is
defined by a plane passing through the magnet perpendicular
to its central flux line at the point of maximum flux.
North pole, is the pole of a magnet which, when
freely suspended, would point to the north magnetic pole of
the earth. The definition of polarity can be a confusing issue,
and it is often the best to clarify by using ¡°north seeking
pole¡± instead of ¡°north pole¡± in specifications.
The oersted is the unit of magnetic field strength,
H, in the cgs electromagnetic system. One oersted equals a magneto
motive force of one gilbert per centimeter of flux path.
An open circuit condition exists when a magnetized
magnet is by itself with no external flux path of high permeability
material.
The operating line for a given permanent magnet
circuit is a straight line passing through the origin of the
demagnetization curve with a slope of negative Bd/Hd. (Also
known as permeance coefficient line.)
The operating point of a permanent magnet is that
point on a demagnetization curve defined by the coordinates
(BdHd) or that point within the demagnetization curve defined
by the coordinates (BmHm).
Orientation direction, is the direction in
which an anisotropic magnet should be magnetized in order to
achieve optimum magnetic properties. Also known as the ¡°axis,¡±
¡°easy axis,¡± or ¡°angle of inclination.¡±
An oriented (anisotropic) material is one that
has better magnetic properties in a given direction.
Paramagnetic material, is a material having
a permeability slightly greater than 1.
A permeameter is an instrument that can measure,
and often record, the magnetic characteristics of a specimen.
P Permeance, is the reciprocal of the reluctance,
R, measured in maxwells per gilbert.
Pole pieces, are ferromagnetic materials placed
on magnetic poles used to shape and alter the effect of lines
of flux.
R Reluctance, is somewhat analogous to electrical
resistance. It is the quantity that determines the magnetic
flux, ?, resulting from a given magneto motive force, F.
Where: R=F/?
R=reluctance, in gilberts per maxwell
F=magneto motive force, I gilberts
?=flux, in maxwells.
Return path, are conduction elements in a magnetic
circuit, which provide a low reluctance path for the magnetic
flux.
Saturation, A condition where the increase in
applied external field yields no increase in induction. When
this condition is met, all of the elementary magnetic moments
have the same alignment. This condition is important in permanent
magnet alloys and in Ferromagnetic alloys. Magnet alloys must
always be magnetized to saturation. The magnet may not be used
at this level, but before conditioning and stabilization the
magnet must always first be magnetized to saturation. Usually
saturation should not be exceeded in Ferromagnetic alloys which
comprise the yoke or return path elements of a magnetic circuit.
If ferromagnetic elements are saturat ed there will be flux
leakage in the system and a redesign should be considered.
A search coil is a coiled conductor, usually
of known area and number of turns, that is used with a flux
meter to measure the change of flux linkage with the coil.
Sintered, A sintered magnet is comprised of a
compacted powder which is then subjected to a heat treat operation
where the full density and magnetic orientation is achieved.
Stabilization, is exposure of a magnet to demagnetizing
influences expected to be encountered in use in order to prevent
irreversible losses during actual operation. Demagnetizing influences
can be caused by high or low temperatures, or by external magnetic
fields.
Tc, Curie temperature, is the transition temperature
above which a material loses its magnet properties.
T max Maximum service temperature, is the maximum
temperature to which the magnet may be exposed with no significant
long-range instability or structural changes.
Reversible temperature coefficients are changes
in flux which occur with temperature change. These are spontaneously
regained when the temperature is returned to its original point.Magnetic
saturation of a material exists when an increase in magnetizing
force produces no increase in intrinsic induction.
The temperature coefficient is 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.
An unoriented (isotropic) material has equal magnetic
properties in all directions.
Vg Air gap volume, is the useful volume of
air or nonmagnetic material between magnetic poles; measured
in cubic centimeters.
Weber, is the practical unit of magnetic flux.
It is the amount of magnetic flux which, when linked at a uniform
rate with a single-turn electric current during an interval
of 1 second, will induce in this circuit an electromotive of
force of 1 volt.
¦Ì permeability, is the general term used to
express various relationships between magnetic induction, B,
and the field strength, H.
¦Ìre recoil permeability, is the average slope
of the recoil hysteresis loop. Also known as the minor loop.
D magnetic flux, is a contrived but measurable
concept that has evolved in an attempt to describe the ¡°flow¡±
of a magnetic field. Mathematically, it is the surface integral
of the normal component of the magnetic induction, B, over an
area, A.
D = ??B ? dA
where:
D = magnetic flux, in maxwells
B = magnetic induction, in gauss
dA = an element of area, in square centimeters
When the magnetic induction, B, is uniformly distributed and
is normal to the area, A, the flux, ? = BA.
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