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Glossary
1. Magnetizing Force, H
The magnetomotive force per unit length at any point in a magnetic circuit. This is measured in Oe(oersteds, CGS unit system) or A/m(amperes per meter, SI unit system). 1Oe=1/(4π)x103A/m
2. Induction, B
This is the magnetic flux per unit area of section in the applied magnetic direction of flux. This is measured in Gauss(Gs, CGS unit) or teslas(T, SI unit).
3. Flux
Magnetic flux is the condition existing in a medium subjected to a magnetizing force. It is the total magnetic induction over a given area. This value is quantified by E.M.F. (electromotive force). This measurement of force is a Maxwell（CGS unit）or webers(Wb, SI unit).
4. Residual Induction, Br
This represents the maximum flux output from a given magnet material measured at the point where the Hysteresis Loop crosses the B axis at zero magnetizing force.
5. Coercive Force, Hc
The demagnetizing force necessary to reduce observed induction B to zero after the magnet has been brought to saturation. Coercive force is measured in Oersteds or A/m.
6. Intrinsic Coercive Force
This is a measure of the resistance of the magnet material to a demagnetizing force. Permanent magnets with high intrinsic coercivity values are usually classified as 'hard' permanent magnets. Intrinsic coercive force indicates magnetic stability at high temperatures.
7. Maximum Energy Product, BH max.
There is a point at the Hysteresis Loop at which the product of magnetizing force H and induction B reaches a maximum. This maximum value is called the Maximum Energy Product and is measured in MGOe or J/m3.
8.Oersted, Oe
A unit measure of magnetizing force (CGS unit). One unit of Oe is defined as the field strength value induced by an infinite long linear conductor bearing one unity ampere electric current that locates 0.2 centimeters away from the measured position.
9. Gauss
Lines of magnetic flux per square centimeter. Gauss is measured in CGS units, equal to Maxwell lines and Webers per square meter or Tesla in the Si system.
10. Hysteresis Loop
A closed curve calculated by plotting corresponding values of magnetic induction B on the abscissa against magnetizing force H.
11. Demagnetization Curve
The second/left quadrant of the hysteresis loop, generally describing the behavior of magnetic characteristics in actual use. Also known as the B-H curve.
12. Knee of the Demagnetization Curve
The point at which the B-H curve ceases to be linear. If the operating point of the magnet falls below the knee, the magnet will not be able to recover full magnetic potential without the application of a magnetizing force.
13. Load Line
A line drawn from the origin of the Demagnetization Curve with a slope. The intersection of the -B/H curve and slope represents the operating point of the magnet. Also see Permeance Coefficient, Pc
14. Permeance Coefficient, Pc
Ratio of the magnetic induction to self demagnetizing force. This is also known as the 'load line' or operating point of the magnet.
15.Curie Temperature, Tc
The temperature at which a material loses its permanent magnetic properties completely and is no longer able to hold magnetism.
16. Reversible Temperature Coefficient
A measure of the reversible changes in flux caused by temperature variations.
17. Irreversible Loss
This is the partial demagnetization of a magnet material when introduced to external factors such as high/low temperatures and demagnetizing fields. Losses can only by rectified by remagnetization.
18. Anisotropic Magnet
A magnet which has a preferred direction of orientation so that the magnetic characteristics are optimum in one preferred direction.
19. Isotropic Magnet
A magnet which does not have a preferred direction of orientation and therefore can be magnetized in any direction without the loss of magnetic characteristics.
20. Ferromagnetic Material
A material whose permeability is very much larger than one common material, and which exhibits hysteresis magnetizing and demagnetizing characteristics. The greater the flux carrying potential, the bigger the value. i.e. one to several thousands.
21. Saturation
This is the condition whereby a magnet or ferromagnetic material has reached a maximum value and an increase in the appliance of magnetizing force produces no increase in induction. i.e. saturation flux densities for steels range from 16,000 to 20,000 Gauss.
22. Magnetic Circuit
An assembly consisting of some or all of the following: permanent magnets, ferromagnetic conduction elements, air gaps, electrical currents.
23. Air Gap
A non-magnetic discontinuity in a magnetic circuit (i.e. the distance between two magnetic poles). This gap often includes other materials such as brass, aluminium or paint.
24. Closed Circuit
This exists when the flux path external to the permanent magnet is confined within high permeability materials which contain the magnet circuit.
25. Magnetomotive Force, F
This is the potential magnetic difference between any two points.
26. Reluctance, R
Reluctance is the resistance in a magnetic circuit and is related to the magnetomotive force F and magnetic flux (R =F/ magnetic flux).
27. Length of Air Gap, Lg
Indicates the length of the central flux path across an air gap.
28. Leakage Flux
This is the loss of magnetic flux which occurs through leakage caused by saturation or air gaps introduced into the magnetic circuit. This induces a loss of efficiency in the circuit which cannot be recovered.
29.Fringing Fields
Leakage flux particularly associated with edge effects and leakage patterns in a magnetic circuit.
30. Return Path
A magnetic circuit which provides a low reluctance path for the magnetic flux.
31. Remenence
Remenence is the magnetic induction which remains in a magnetic circuit after the removal of an applied magnetizing force. If there is an air gap in the circuit, the remenence will be less than the residual induction Br.
Approximately Field and Holding Force Calculation.
Field - Space Flux Density
For some simpl, e shaped permanent magnets, Biot-Savart law can be applied to calculate the flux density along the symmetric axis of the magnet. By equating the permanent magnet to solenoids or current sheet with the same shape, only the dimensions and remenence Br of the magnet are required. These equations are useful for approximate calculation of permanent magnets.
As illustrated in below Fig.1-3, a cylindrical magnet is magnetized axially, with radius r and length l. The flux density at a distance d from the surface, along its axis, is given by:
Fig.1-3：Sketch of a cylindrical magnet.
Figure 2-3 is a rectangular magnet that is magnetized along its length, with thickness 2t, width 2w and length l. The flux density at a distance d from the surface, along its magnetized symmetric axis, is given by:
Fig.2-3：Sketch of a rectangular magnet.
As shown in Fig.3-3 of a cylindrical tube (ring) magnet, by subtracting the flux density of a cylinder with inside diameter of 2r from that of a cylinder with outside diameter of 2R, we have the flux density along the axis:
Fig.3-3：Sketch of a cylindrical tube (ring) magnet.
Holding Force
For the simple shaped permanent magnets, the basic force formula for a magnet attracted on an infinitly large iron body is:
Br (measured in kGs) and the lm (measured in cm) are the remenence and height of the magnet, and Ag(measured in cm2) is the area of attracting surface. The unit of holding force F is g(Gram). |
