|
Item |
Description |
|
Air Gap |
A low permeability gap in the
flux path of a magnetic circuit. Often air, but inclusive of
other materials such as paint, aluminum, etc. |
|
Area of
the air gap, Ag |
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. |
|
Anisotropic |
Material that have a
"preferred" magnetization direction. These materials
are typically manufactured in the influence of
strong magnetic fields, and can only be magnetized
through the preferred axis. |
|
Area of
the magnet, Am |
The cross sectional
area of the magnet perpendicular to the central flux
lne, measured in sq. cm. at any point along its
length. In design, Am is usually considered the area
at neutral section of the magnet. |
|
Br,
Magnetic induction |
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 the
resultant intrinsic induction. Magnetic induction is
the flux per unit area normal to the direction of
the magnetic path. |
|
Bd,
Remanent 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
remanent induction, Bd, to a demagnetizing force, Hd.
It is also referred to as the permeance coefficient,
shear lien, load line and unit permeance. |
|
Bd/Hd
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. |
|
Bg
magnetic induction in the air gap |
Is the average value
of magnetic induction over tha area of the air gap,
Ag; or it is the magnetic induction measured at a
specific point within the air gap; measured in gauss |
|
BH curve |
See Demagnetization
Curve |
|
(BH)max,
Maximum energy products |
is the maximum
product of (BdHd) which can be obtained on the
demagnetization curve. |
|
Bi or Bj
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. |
|
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,
shown in data as "Br". |
|
Closed
circuit condition |
A condition that
exists when the external flux path of a permanent
magnet is confined with high permeability material. |
|
Coercive
Force, Hcb |
The demagnetizing
force, measured in Oersteds, necessary to reduce
observed induction, B, to zero after the magnet has
previously been brought to saturation. |
|
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. This curve is also referred
to as the Second Quadrant Curve, BH Curve, or
abbreviated to "curve". |
|
Eddy
Currents |
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 damping of movement),
or may be unwanted consequences of certain designs
which should be accounted for or minimized. |
|
Ferromagnetic material |
A material whose
permeability is very much larger than 1 (from 60 to
several thousand times 1), and which exhibits
hysteresis phenomena. |
|
Flux, ¦µ |
The condition
existing in a medium subjected ot magnetizing force.
This quantity is characterized by the face 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. |
|
Fluxmeter |
Is an instrument
that measures the change of flux linkage with a
search coil. |
|
Gauss |
Unit of magnetic
induction, B, in the cgs electromagnetic system. One
gauss is equal to one maxwell per square centimeter. |
|
Gaussmeter |
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. |
|
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 go saturation; measured in
oersteds. |
|
Hd |
Is that value of H
corresponding to the remanent induction, Bd;
measured in oersteds. |
|
Hk
Coercive force-k |
The value of Hcj at
0.9Br. This value gives an indication of the "squareness"
of the intrinsic curve. The more square the
intrinsic curve, the closer the material is to being
ideal. Hk values that approach the Hci values are
considered extremely good materials. |
|
Hysteresis Loop |
The 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. |
|
Induction, B |
The magnetic flux
per unit area of a section normal to the direction
of flux. Measured in gauss, in the cgs system of
units. |
|
Irreversible Losses |
Partial
demagnetization of the magnet, caused by exposure o
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. |
|
Isotropic |
A magnetic material
that has the "same" properties in all directions.
Such a material may be magnetized in any direction
since it does not have a "preferred" alignment
direction. |
|
Knee of
the Demagnetization Curve |
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
remagnetization. |
|
Leakage
flux |
that portion of the
magnetic flux that is lost through leakage in the
magnetic circuit due to saturation or air-gaps, and
is therefore unable to be used. |
|
Load
line |
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
flux, ¦µ |
The total magnetic
induction over a given area. When the magnetic
induction, B, is uniformly distributed over an area
A,
¦µ=BA. The general
equation is ¦µ=¡Ò¡ÒB¡¤¡Ò¡ÒdA. |
|
Magnetic
Saturation |
Of a material exists
when an increase in magnetizing force, H, does not
cause an increase in the intrinsic magnetic
induction, B, of the material. |
|
Maxwell |
Unit of magnetic
flux in the cgs electromagnetic system. One maxwell
is one line of magnetic flux. |
|
North
pole |
That ole 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
best to clarify by using "north seeking pole"
instead of "north pole" in specifications. |
|
Oersted,
Oe |
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 |
Exists when a
magnetized magnet is by itself with no external flux
path of high permeability material. |
|
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/H. (Also known as
permeance coefficient line.) |
|
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 |
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". |
|
Permeance |
The inverse of
reluctance, analogous to conductance in electrical
circuits. |
|
Permeance Coefficient, Pc |
Ratio of the
magnetic induction, Bd, to its self demagnetizing
force, Hd. Pc=Bd/Hd. this is also known as the "load
line" or operating point of the magnet, and is
useful in estimating the flux output of the magnet
in various conditions. As a first order
approximation, Bd/Hd=Lm/Lg, where Lm is the length
of the magnet, anad Lg is the length of an air gap
that the magnet is subjected to. Pc is there fore a
function of the geometry of the magnetic circuit. |
|
µ
permeability |
Is the general term
used to express various relationships between
magnetic induction, B, and the field strength, H. |
|
µre
recoil permeability |
The average slope of
the recoil hysteresis loop. Also known as a minor
loop. |
|
Pole
pieces |
Ferromagnetic
materials placed on magnetic poles used to shape and
alter the effect of lines of flux. |
|
Permeameter |
Is an instrument
that can measure, and often record, the magnetic
characteristics of a specimen. |
|
Permeance coefficient line |
see operating line. |
|
Polarity |
The characteristic
of a particular pole at a particular location of a
permanent magnet. Differentiates the North from the
South Pole. |
|
Relative
Permeability, µr |
The ratio
permeability of a medium to that of vacuum;
µr=µ/µo.
In the cgs system,
µo=1
in a vacuum by definition. The permeability of air
is also for all practical purposed equal to 1 in the
cgs system. |
|
R
Reluctance |
Is somewhat
analogous to electrical resistance. It is the
quantity that determines the magnetic flux,
¦µ reslting from
given magnetomotive force, F where: R=F/¦µ.
R=reluctance, in gilberts er maxwell, F=magnetomotive
force, in gilberts, ¦µ=flux, in maxwell. |
|
recoil
induction, Bm |
The magnetic
induction that remains in a magnetic material after
magnetizing and conditioning for final use; measured
in gauss |
|
Reversible temperature coefficient |
Are changes in flux,
which occur with temperature change. These are
spontaneously regained when the temperature is
returned to its original point. |
|
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. |
|
Saturation |
The condition under
which all elementary magnetic moments have become
oriented in one direction. A ferromagnetic material
is saturated when an increase in the applied
magnetizing force produces no increase in induction.
saturation flux densities for steels are in the
range of 16,000 to 20,000 Gauss. |
|
Stabilization |
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
magnetic properties. |
|
Temperature coefficient of Br |
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. |
|
Tw,
Maximum Operating Temperature |
The maximum
operating temperature, also known as maximum service
temperature, is the temperature at which the magnet
may be exposed to continuously with no significant
long-range instability or structural changes. Note
that this temperature is a function of the operating
point of the magnet, and not an absolute value. |
|
Vg Air
gap volume |
Is the useful volume
of air or non-magnetic material between magnetic
pole; measured in cubic centimeters. |
