Magnet technical Glossary
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.
Anisotropic Magnet: A magnet having a preferred direction
of magnetic orientation, so that the magnetic characteristics are
optimum in one preferred direction.
Closed Circuit: This
exists when the flux path external to a permanent magnet is confined
within high permeability materials that compose the magnet circuit.
Coercive Force, Hc: The demagnetizing
force, measured in Oersteds, necessary to reduce observed induction,
B, to Zero after the magnet has previously been brought to saturation.
Curie Temperature, Tc: The temperature
at which the parallel alignment of elementary magnetic moments
completely disappears, and the material is no longer able to hold
magnetization.
Demagnetization Curve: The second quadrant of the
hysteresis loop, generally describing the behavior of magnetic characteristics
in actual use, Also known as the B-H 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 pemeability
is very much larger than 1 (from 60 to several thousand times 1),and
which exhibits hysteresis phenomena.
Flux, O :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 .
Fringing Fields: Leakage flux
particularly associated with edge effects in a magnetic circuit.
Gauss: Lines of magnetic flux
per square centimeter, cgs unit of flux density, equivalent to lines
per square inch in the English system, and webers per square meter
or Tesla in the SI system.
Hysteresis Loop: A closed curve obtained for
a material by plotting corresponding values of magnetic induction
, B , (on the abscissa) against magnetizing force, H (on the ordinate).
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.
Intrinsic Coercive force, Hci:
Measured in Oersteds in the cgs system, this is a measure of the
material's inherent ability to resist demagnetization. It is the
demagnetization force correponding to zero intrinsic induction in
the magnetic material after saturation. Practical consequences of
high Hci values are seen in greater temperature stability for a
given class of material, and greater stability in
dynamic operating conditions.
Irreversible Loss: Defined as the partial demagnetization
of a magnet caused by external fields of other factors. These losses
are only
recoverable by remagnetization . Magnets can be stabilized to prevent
the variation of performance caused by irreversible losses.
Isotropic Magnet: A magnet material
whose magnetic properties are the
same in any direction, and which can therefore be magnetized in
any direction without loss of magnetic characteristics.
Keeper: A piece of soft iron that is placed
on or between the poles of a magnet, decreasing the reluctance of
the air gap and thereby reducing
the flux leakage from the magnet.
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.
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Length of Air-Gap, Lg: The Length of the path of the central
flux line in the air-gap
.
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
Coeffcient.
Magnetic Circuit: An assembly consisting of some or all
of the following: permanent magnets, ferromagnetic conduction elements,
air gaps,
electrical currents.
Magnetic Flux,
Ø: The total magnetic induction over a given area.
When the magnetic induction, B, is uniformly disributed over an area
A, Ø=BA. The general equation is Ø=B. dA.
Magnetizing
Force, H: The magnetomotive force per unit length at any
point in a magnetic circuits. Measured in oersteds in the cgs system.
Magnetomotive Force, F: Analogous
to voltage in electrical circuits, this is the magnetic potential
difference between any two points.
Maximum Energy Product, BHmax: The point on the
Demagnetization Curve where the product of B and H is a maximum and
the required volume of
magnet material required to project a given energy into its surroundings
is a minimum. Measured in Mega Gauss Oersteds, MGOe.
North
Pole: That pole of a magnet which, when freely suspended,
would pointto 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 speciications.
Oersted, Oe: A cgs unit of
measure used to describe magnetizing force. The English system equivalent
is Ampere Turns per Inch, and the SI system's is Ampere Turns per
Meter.
Orientation Direction:
The direction in which an anisotropic manet should be magnetized in
order to achieve optimum magnetic properties. Also known
as the "axis"," easy axis", or"angle of inclination".
Paramagnetic
Material: A material having a permeability slightly greater
than 1.
Permeance: The
inverse of reluctance, analogous to coductance in electrical circuits.
Permeance Coefficient, Pc:
Ratio of the magnetic induction, Bd, to its self demagnetizing force,
Hd. Pc=Bd/ Hd. his is also known as the "load line" or
operating point of the magnet, and is useful inestimating the flux
output 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, and Lg is the length of an air
gap that the magnet is subjected to. Pc is therefore a function of
the geometry of the magnetic circuit.
Pole
Pieces: Ferromagnetic materials placed on magnetic poles
used to shape and alter the effect of lines of flux.
Relative Permeability , µr: The ratio of permeability
of a medium to that of a vacuum:µr
=µ/µ0. In the cgs system,µ0
=1 in a vacuum by definition. The permeability of air is also for
all practical purposes equal to 1 in the cgs system.
Reluctance, R: Analogous to resistance in an electrical
circuit, reluctance is related to the magnetomotive force, F , and
the magnetic flux, Ø by the equation R=FØ, (paralleling
Ohm's Law)where F is the magnetomotive force, and Ø is the
magnetic flux, (in cgs units).
Remenance,
Bd: The magnetic induction which emains in a magnetic circuit
after the removal of an applied magnetizing force. If there is an
air gap
in the circuit, the remanence will be less than the residual induction,
Br.
Residual Induction, Br: This
is the point at which the hysteresis loop crosses the B axis at zero
magnetizing force, and represents the maximum
flux output from the given magnet material. By definition, this point
occurs at zero air gap, and therefore cannot be seen in practical
use of magnet materials.
Return Path:
Conduction elements in a magnetic circuit which provide
a low reluctance path for the magnetic flux.
Reversible Temperature Coefficient: A measure of
the reversible changes in flux caused by temperature variations.
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.
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