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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.

 

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.


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 corresponding 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 operation conditions.

 

Maximum Energy product (BH)max: 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 surrounding is a minimum. Measured in Mega Gauss Oersteds (MGOe).


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.


Isotropic: Structural equal concerning the directions of space, which means for magnets that none of the directions of space is preferred against others. Isotropic magnets may be magnetized in all directions with the identical magnetic features.


Anisotropic: Structural unequal concerning the directions of space, which means for magnets that they are exposed during the production process to a high magnetic field which forces the ”elementary magnets“ to align themselves in the direction given by the field. Later on, when magnetized along the preferred axis, one obtains much better magnetic values than for other directions of space. Anisotropic Magnets have a preferred direction of magnetic orientation, so that the magnetic characteristics are optimum in one preferred direction.


Max. Working Temperature: The magnets must work under the max working temperature. If the magnets work under exceeding this temperature, the magnet will be demagnetized a lot.


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.


Irreversible Loss: Defined as the partial demagnetization of a magnet caused by external fields or other factors. These losses are only recoverable by demagnetization. Magnets can be stabilized to prevent the variation of performance caused by irreversible losses.


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.


Closed Circuit: This exists when the flux path external to a permanent magnet is confined within high permeability materials that compose the magnet circuit.


Eddy Currents: Circulating electrical currents that are induced in electrically conductive elements when exposed to charging 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.


FluxThe 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.


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.


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 temperatures, develop a knee at some temperature. AlNiCo5 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.


Leakages Flux: That portion of the magnetic flux that is lost through leakage in the magnetic circuit due to saturation or air-gap, and is therefore unable to be used.


Length of Air-Gap (Lg): The length of the path of the central flux line in the air-gap.


Magnetic Circuit: An assembly consisting of some or all of the following: permanent magnets, ferromagnetic conduction elements, air gaps, electrical currents.


Magnetic Flux: The total magnetic induction over a given area. When the magnetic induction, B, is uniformly distributed over an area A, ?=BA. The general equation is ?=∫∫B × dA.


Magnetizing Force (H): The magnetomotive force per unit length at any point in a magnetic circuit. Measured in oersteds in the cgs system.



Orientation Direction: The direction in which an anisotropic magnet should be magnetized in order to achieve optimum magnetic properties. It is also known as the “axis”, “easy axis”, or “angle of inclination”.


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. 


Temperature

Temperature effects fall into three categories:

Reversible losses

Irreversible but recoverable losses

Irreversible and unrecoverable losses


Reversible losses:

These are losses that are recovered when the magnet returns to its original temperature. Reversible losses cannot be eliminated by magnet stabilization. Reversible losses are described by the Reversible Temperature Coefficients (Tc), shown in following table, Tc is expressed as % per degree Centigrade. These figures vary for specific grades of each material but are representative of the class of material as a whole. Because the temperature coefficients of Br and Hc are significantly different, the demagnetization curve develops a “ knee” at elevated temperatures.

Material

Tc of Br (%)

Tc of Hc (%)

NdFeB

-0.12

-0.60

SmCo

-0.04

-0.3

AlNiCo

-0.02

+0.01

Ceramic

-0.20

+0.30


Irreversible but recoverable losses

These losses are defined as partial demagnetization of the magnet from exposure to high or low temperatures. These losses are only recoverable by remagnetization, and are not recovered when the temperature returns to its original value. These losses occur when the operating point of the magnet falls below the knee of the demagnetization curve. An efficient permanent magnet design should have a magnetic circuit in which the magnet operates at a permeance coefficient above the knee of the demagnetization curve at expected elevated temperatures. This will prevent performance variations at elevated temperatures.

 

Irreversible and unrecoverable losses

Metallurgical changes occur in magnets exposed to very high temperatures and are not recoverable by remagnetization.