Magnet Technical Glossary
**A****ir 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 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:** The total magnetic induction over a given area. When the magnetic induction, B, is uniformly disributed 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 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 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.
**Reluctance, R:** Analogous to resistance in an electrical circuit, reluctance is related to the magnetomotive force, F , and the magnetic flux, by the equation R=F (paralleling Ohm Law)where F is the magnetomotive force, and 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. | |