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GLOSSARY OF TERMS |
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Air Gap- Basically the "EXTERNAL" distance from one pole of the magnet to the other though a non magnetic material (usually air).
There are two parts to the magnetic circuit: the internal path from one pole to the other physically through the magnet. This is where the magnet develops the Magneto Motive Force (mmf) which
causes flux to flow from one pole to the other "outside" the magnet.
It is the external path which the flux travels on from one pole to the other (usually through air) which is called the air gap. In practical terms it is the portion of the magnetic circuit which is of interest to us, for this is where the properties of the magnet can be used.
Ampere Turn- The product of the current, in amperes, flowing in a coil, and the number of turns of wire in that coil. It is a measure of electro magnet strength and is not a significant factor in permanent magnet design.
Anisotropic Magnet- A magnet which can be more easily magnetized on one direction than another. Generally this results from techniques used by manufacturers during the production of magnet materials, and it increases the magnetic strength of the material. Samarium Cobalt and Neodymium Iron magnets are all Anisotropic. Ceramic and Alnico magnets are available as either anisotropic or isotropic.
B/H Curve- The result of plotting the value of the magnetic field (H) that is applied against the resultant flux density (B) achieved. This curve describes the qualities of any magnetic material. This is covered in more detail under Hysteresis Loop.
Coercive Force- There are two kinds of Coercive Force--normal (Hc) and intrinsic (Hci). However, unless you are a "purist" you really don't need to know the difference. (If you really want to know, give us a call. ) For now let's just say "coercivity" is a measure of how hard it is to demagnetize a magnet--and not surprisingly ... of how hard it is to Magnetize it. It is measured in Oersteds. Coercive force is to the magnet system what Voltage is to an electrical system.
Curie Temperature (Tc)- The temperature at which a magnet loses all of its magnetic properties.
Demagnetization Curve- This is the second quadrant of the Hysteresis Loop and it graphically shows what happens to the magnet after it is magnetized and during the operation of the magnet in your application.
Demagnetization Force- A magnetizing force --typically in the direction "OPPOSITE" to the force used to magnetize it in the first place. In operation magnets are subjected to various "demagnetizing" influences--including temperature--which tend to demagnetize it. Proper design anticipates the extent of these forces.
Electromagnet- A magnet consisting of a solenoid with an iron core, which has a magnetic field existing --unlike a permanent magnet--only during the time of current flow through the solenoid.
NOTE 2: The demagnetization curve is usually plotted to the left of the vertical axis (negative values of Hd) and the energy product curve to the right.
Ferromagnetic Material- A material that either is a source of magnetic flux (hard ferromagnetic) or a conductor of magnetic flux (soft ferromagnetic). Any ferromagnetic material must have some component of iron, nickel, or cobalt. Hard materials are used as permanent magnets while soft materials are used as pole pieces or the cores in electromagnets.
Flux- See Magnetic Flux.
Flux Density (B)- See Magnetic Flux Density.
Flux meter- An instrument that measures the total magnetic flux in a given area, measured in Maxwells (C.G.S.) or Webers
(S.I.).
Gauss- The (C.G.S.) unit for magnetic induction (flux density), measured in flux lines per square centimeter. See Induction, Magnetic and Magnetic Flux Density.
Gauss meter- An instrument that measures the flux density of a magnetic field, usually measured in Gauss (C.G.S.).
Gilbert- The C.G.S. unit for magnetomotive force.
Hysteresis Loop- A plot of magnetizing force versus resultant magnetization (also called a B/H curve) of the material as it is successively magnetized to saturation , demagnetized, magnetized in the opposite direction and finally remagnetized. With continued recycles, this plot will be a closed loop which completely describes the characteristics of the magnetic material. The size and shape of this "loop" is important for both hard and soft materials.
With soft materials, which are generally used in alternating circuits, the area inside this "loop" should be as thin as possible (it is a measure of energy loss). But with hard materials
the "fatter" the loop, the stronger the magnet will be.
The first quadrant of the loop (that is +X and +Y) is called the magnetization curve. It is of interest because it shows how much magnetizing force must be applied to saturate a magnet. The second quadrant (+X and -Y) is called the Demagnetization Curve.
Irreversible Losses- A permanent reduction in magnetic flux due to exposure to high permanent temperatures, external fields, or other demagnetizing factors. These losses are usually recoverable by
remagnetization.
Isotropic Material- A material that can be magnetized along any axis or direction (a magnetically unoriented material). See also Anisotropic Magnet.
Keeper- A soft iron piece temporarily added between the poles of a magnetic circuit to protect it from demagnetizing influences. Also called a shunt. No longer used with the current high coercive force materials in use today.
Kilogauss- (Kilo equals 1,000). One Kilogauss is equal to 1,000 Maxwells per square centimeter.
Leakage Flux (F)- The portion of the flux which does not pass through the working air gap or other useful parts of the magnetic circuit.
Magnetic Induction- The magnetic flux induced in a material by an applied magnetizing field. Usually measured in gauss removed.
Residual Induction (Br)- The magnetic induction remaining in a saturated magnetic material after the magnetizing field has been removed.
MGO- Mega (million) Gauss Oersteds. Unit of measure typically used in stating the maximum energy product for a given material. See Maximum Energy Product.
Magnetic Circuit- Consists of all elements, including air gaps and non-magnetic materials that the magnetic flux from a magnet travels on, starting from the north pole of the magnet to the south pole. See Open and Closed Magnetic Circuit.
Magnetic Field Strength (H)- See
Magnetizing
Force.
Magnetic Flux- Often called "lines of force" or "lines of flux" which exists
between the poles of a magnet.. Measured in Maxwells (C.G.S.) or Webers
(S.I.).
Magnetic Flux Density (B)- Lines of flux per unit area, usually measured in
Gauss (C.G.S.). One line of flux per square centimeter is one Maxwell.
Magnetomotive Force (F or mmf)- That which tends to produce a magnetic field.
Commonly produced by a current flowing through a coil of wire. Measured in
Gilberts (C.G.S.) or Ampere Turns (S.I.).
Maximum Energy Product (BHmax)- The maximum product of (Bd) (Hd) taken from points on the second quadrant demagnetization curve for a given magnetic material. Usually measured in Mega Gauss-Oersted (MGO). NOTE: The value of (Hd) is taken from the demagnetization curve. Its value is dependent on the value of (Bd).
Maximum Operating Temperature- It is the highest temperature that a magnet can be exposed to without a significant permanent decrease in magnetic strength.
Magnetic Pole- An area where the lines of flux are concentrated.
Magnetic Line of Force- An imaginary line in a magnetic field which, at every point, has the direction of the magnetic flux at that point.
Magnetization Curve- The first quadrant portion of the hysteresis loop (B/H)
Curve for a magnetic material.
Magnetizing Force (H)- The magnetomotive force per unit of magnet length, measured in Oersteds (C.G.S.) or ampere-turns per meter (S.I).
Maxwell- The C.G.S. unit for total magnetic flux, measured in flux lines per square centimeter.
North Pole- The north pole of a magnet is the one attracted to the magnetic north pole of the earth. This north-seeking pole is identified by the letter N. By accepted convention, the lines of flux travel from the north pole to the south pole.
-O-
Oersted (Oe)- The C.G.S. unit for magnetizing force.
Open Magnetic Circuit- A circuit where the magnetic flux is conducted partially or completely through air. See Magnetic Circuit.
Orientation- Used to describe the direction of magnetization of a material.
See also Isotropic and
Anisotropic
Magnets.
Paramagnetic Materials- Materials that are not attracted to magnetic fields (wood, plastic, aluminum, etc.)
Permeance (P)- A measure of relative ease with which flux passes through a given material or space. It is calculated by dividing magnetic flux by magnetomotive force. Permeance is the reciprocal of reluctance.
Permeability (mu)-The ratio of the magnetic induction of a material to the magnetizing force producing it (B/H). NOTE: The permeability of air is 1.
Permeance Coefficient (Pc) (Bd) (Hd)- Numerically equal to the circuits (Bd) divided by the circuits (Hd). The value may be found by noting where the measured value of (Bd) intersects the normal demagnetization curve. This value is solely dependent on the geometry of the magnetic circuit. See Load Line.
Pole- See Magnetic Pole.
Pole Piece- Usually a soft iron piece attached to the pole of a magnet. It is typically used to control, direct, intensify, or isolate the lines of magnetic flux. A pole piece can also be used to protect a magnet. Typically used in pairs.
Reluctance (R)- A measure of the relative resistance of a material to the passage of flux. It is calculated by dividing magnetomotive force by magnetic flux. Reluctance is the reciprocal of permeance.
Saturation- The state where an increase in magnetizing force produces no further increase in magnetic induction in a magnetic material.
Shunt- A soft iron piece temporarily added between the pole of a magnetic circuit to protect it from demagnetizing influences. Also called a keeper.
South Pole- The south pole of a magnet is the one attracted to the south pole of the earth. This south-seeking pole is identified by the letter S. By accepted convention, the lines of flux travel from the north pole to the south pole.
Stabilization- The process of exposing a magnet or a magnetic assembly to elevated temperatures or external magnetic fields to demagnetize it to a predetermined level. Once done the magnet will suffer no future degradation when exposed to that level of demagnetizing influence.
Temperature Coefficient- A factor that is used to calculate the decrease in magnetic flux corresponding to an increase in operating temperature. The loss in magnetic flux is recovered when the operating temperature is decreased.
Tesla- The S.I. unit for magnetic induction (flux density). One Tesla equals
10,000 Gauss.
Weber- The S.I. unit for total magnetic flux.
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