Up to the middle of nineteenth century magnetism was considered a physical phenomenon separated by the electric ones. But the processes that rules the magnetism are strongly tied up to the electric phenomenon by mean of the Maxwell equations. For a best understanding of magnets behaviour we bring here some technical terms: they are presented in a precise form, also comprehensible to who draws near for the first time to this matter.
Distance from a non magnetic material laid between the poles of a magnetic circuit: it can be seen as a obstacle to the magnetic flux flow, so, if no useful to the device, it should be minimized. To increase the magnetic induction and its homogeneity, it is necessary to minimize the air gap.
It is the preferred direction along which a magnet can be oriented some magnets have high anisotropy (anisotropic magnets), some others not (isotropic). The anisotropic orientation can arise from the production process but in many cases it is due to the material structure. In anisotropic magnets the best properties can be obtained along the anisotropy direction.
See Magnetic Induction
Chemical elements of 2nd group (alkaline earthy). The most important material for magnetism is barite: it is added as barium carbonate to iron oxide to produce barium BaFe12O19(barium ferrite).
It is the maximum energy product in a permanent magnet: more in detail, it is the maximum of the BH values in 2nd quadrant of hysteresis loop. BH product represents the energy density per volume unit: in general we can state that increasing the (B • H)max value we get a decrease of the magnet volume useful to obtain the same performance.
Specific magnetizing and demagnetizing procedure by which it is possible to minimize permanent magnet tolerances. For specific applications it is needed to tune the magnetic flux in more strict range than usual (ex. In specific electric motors, sensors, electromechanical relays).
It is the value of magnetizing field H during whose action the induction B of a ferromagnetic material previously magnetized to saturation, was set to 0. The coercive strengths jHc (intrinsic) and bHc (extrinsic) can by found: this distinction is technically important for magnets with high coercive strength. You can have the coercive strength jHc from the J hysteresis loop, while Hc is got from B hysteresis loop. The SI unity of coercive strength is given in KA/m or in Oersted.
The temperature at which a ferromagnetic material becomes paramagnetic and looses its magnetic properties. Name derived from M.me Curie, physicist and chemist of early ’900.
Decrease of magnetization condition (that is of B) in a magnet which also means decrease of its performances, by mean of strength field H opposed to its magnetization direction: to get a complete demagnetization it is required a oscillating strength field. A partial or total demagnetization can also happen at very high (or low for ferrite magnets) temperature: it will be a partial and irreversible demagnetization when temperature is between max working temperature and Curie temperature, total if Curie temperature is passed Max working temperature is linked to material properties, to magnet and magnetic circuit geometry while Curie temperature is characteristic of the material itself.
Is the part of hysteresis loop in 2nd quadrant, where H value is negative. The main properties deducible from this curve are Br (remanence), Hc (coercive force) and maximum Energy product. The DIN 17410 definition consists in 2 numbers, where the first one is the minimum value of energy product, the second the minimum coercive force divided by 10, in SI unit system. For ex. 28/26 means: (B • H)max> 28 kj/m3 , jHc> 26 • 10 – 260 KA/m.
Density weight in Kg/m3, in g/cm3 or Kg/dm3 (1 g/cm3 = 1 Kg/dm3 =103 Kg/m3 )
It is a factor mainly depending on magnet geometry and expresses the possibility to get good performance from the magnet geometry or magnetic circuit. If the working point of magnet is linked with the origin of coordinates system B-H, a straight line is obtained. The demagnetisation factor N is linked to such line and it is a non-dimensional quantity, assuming values between 0 (closed magnetic circuit) and 1 (magnetic circuit completely open). It’s tied up to the permeance from the relation: P=1−1/N
Materials with permeability much higher than 1. Their behaviour can be described as joined action of magnetic domains, each with its magnetic elementary moment. Permanent magnet as permeability slighly higher than 1, while soft ferromagnetic materials have relative permeability much higher than 1 (from 10^2 to 10^6).
Flux density of the magnetic induction B. In case B is crossing uniformly surface A, it’s equal to the ratio between the flux and the cross section area A.
Measuring instrument of the magnetic flux: the measure is obtained by integrating electronically or digitally the voltage at the measuring coil.
Part of the total magnetic flux that passes through a useful gap of a magnetic circuit; the remaining part of the total flux is said “leakage flux”.
Flusso utile
Parte del flusso magnetico totale che attraversa il traferro utile del circuito magnetico analizzato. La parte restante del flusso viene detta flusso disperso.
Forza coercitiva
È il valore di intensità di campo magnetizzante H che consente di annullare la densità di flusso B nel ciclo di isteresi. Si distinguono le forze coercitive intrinseca, HcJ, ed estrinseca, HcB. Questa distinzione è tecnicamente importante per tutti i magneti con grande forza coercitiva. La forza coercitiva HcJ si rileva dal ciclo di isteresi di J, il valore di HcB dal ciclo di isteresi di B. L’unità di forza coercitiva viene data in KA/m, oppure in A/m, o in Oersted.
Also called magnetometer. Instrument to measure the flux density: it usually works with the Hall effect in semiconductor of the sensor. It directly shows the density of magnetic flux, without movement of the measuring probe.
Gaussmetro
Strumento di misura della densità di flusso B: normalmente sfrutta l’effetto Hall dei semiconduttori; indica direttamente, senza movimento della sonda di misura, la densità di flusso magnetico.
Intensity of the magnetic field: this property establishes how a material is magnetically strained. H tightly depends on the current circulating into a winding, set around or in proximity of the material submitted to H.
H
Intensità del campo magnetico: questa grandezza stabilisce come viene sollecitato magneticamente un materiale. H dipende strettamente dalla corrente circolante in un avvolgimento in prossimità del materiale sottoposto ad una determinata intensità di magnetizzazione H.
Barium ferrite, strontium ferrite or lead, with chemical formula MeO • 6Fe2O3 where MeO represents a metallic oxide. All the permanent magnets in hard ferrite are hexagonal for ex. BaO • 6Fe2O3 or SrO • 6Fe2O3.
It can be produced by tracing the induction B versus magnetic strength field H, first with H positive (magnetization loop-1st quadrant of hysteresis loop), then switching to H negative (2nd quadrant loop, in demagnetization loop). It can be performed for B or J.
Losses of the magnet properties to high temperature, not recoverable by returning to the initial temperature (normally this is the room temperature). For ferrite the remanence decays at low temperatures.
Flux of the magnetic induction B through a surface A. If B is homogenous across A ihen is equal to B*A, otherwise it is the mathematical integral of B on surface A. Unity of measure in SI system: 1 Wb (Weber) = 1 Vs (Volt*second).
It’s the property that shows the state of magnetization of a magnetic material: its main original definition is founded on the effect that an
induction field equal to 1 Wb/m2 creates on a metallic conductor crossed by current. Unity Tesla (T=Wb/m2). B = μo H + j.
Also called intrinsic induction. Contribution of the material to the flux density: J = B — μo H.
Surface of permanent magnet from which the magnetic flux goes out or enters: it can be Norh or South.
To magnetize a magnet, an external strength field some times higher its coercive strength is employed. The duration of the magnetization
is generally very brief, from few microseconds to some seconds.
Measurement unity of the magnetic flux in the GCS system (see also the magnetic flux) and it is equal to 10−8 Tesla m2.
Measurement unit of magnetic strength field H in GCS. It comes from the name of Danish Physicist Hans Christian Oersted.
The direction of magnetization along which the magnet reaches the best values in terms of Br, Hc and BHmax (see also anisotropy) and it is due to the magnetic preorientation or structure of the material. Often in magnets with circular symmetry it is axial. In magnets with rectangular section it is often the minimum thickness. In segments of arc is the diametrical direction (that is the parallel lines to the diameter) or radial.
It is the ratio between the magnetic induction B and the magnetic field H. It can be explained as a sort of magnetic “transmissibility” or “conductibility”. In vacuum it is a constant: µo = 1,256 H/ms (T / A/m). In diamagnetic materials µr <1, while in paramagnetic materials µr>1; in ferromagnetic material µr >> 1 (from 102 to 106). It is often used the relative permeability , defined as µr = µ/µ o = B/µo H.
Ratio betweeng the induction B and the product between o and H: in a magnetic circuit the higher is the permeance module, the closer is the magnetic circuit (no flux leakages).
Ratio between the height of a magnet and its diameter; it is a very important ratio especially in open circuit magnets (without soft ferromagnetic parts). In demagnetization curves the values h/D can be plotted, so that for every h/D value the induction can be computed at the working point. On this ratio depends the magnetic performances of a magnet of given size.
Permanent magnet permeability in second quadrant of hysteresis loop.
It’s the value of induction Br in a closed circuit magnetic material (no air gaps), when H is equal to 0. In SI is given in tesla (T), in millitesla
(mT); in cgs is given in Gauss(G).
Reversible o repeatable. A magnet behaviour is thermally reversible when it returns to its original properties after a heating or cooling process.
Condition in which the induction B increases in the material with an inclination equal to o, as consequence of an increase of H; after reaching the condition of saturation, the magnet can get the maximum values of magnetic properties.
Process of high pressure powder compacting at high temperature, with the purpose to get material compacted and homogenized. The temperatures of Sintering are, approximalety: for hard ferrite around 1200°C – 1250°C, for rare earths magnets around 1050°C – 1200°C.
To stabilize a magnet it is possible to raise it to a defined temperature, near to the maximum working temperature of the magnet or to dip it in an oscillating magnetic field and produce light demagnetization of the same magnet. This should prevent possible small demagnetisation, produced by inside and external factors.
Multiplying the values of induction B by the intensity of field H we get a range tied up to the energy density for volume unit. See also the (B • H)max value.
Chemical element of the II group (alkaline-earthy metals). it comes from the minerals stronzianites and celestine. The strontium is added in the form strontium carbonate instead of barium carbonate and confers to hard ferrite magnets a particularly elevated coercive strength.
It describes the dependence between magnetization and magnetic field. The relation with the Magnetization is: M = X • μt H e μr = X+1
In magnetic materials the variation of residual induction or coercive force may be related with temperature changes: it is a very important parameter as magnets are very sensitive to temperature value.
Measuring unit of the magnetic flux density , also called magnetic induction. 1 T = 1 Vs/m2 = or 10.000 Gausses. It comes from the name of the Serbian physicist Nicola Tesla (1856-1943).
Measuring unit of the magnetic flux 1 Wb = 1 V s = 108 Maxwell. Name derived from Mr. miby Wilhelm Weber
The most elevated temperature at which a magnet can be maintained without irreversible flux losses. The working temperature also depends on the magnetic circuit in which the magnet is employed: it produces more easily effects of demagnetization in case of low permeance, that is in case of open magnetic circuit. The most unfavourable case will be therefore a single magnet with very small L/D ratio (for instance 0.3 or smaller).
Point of the demagnetization curve that gives the values of the flux density B and of the coercive field H in the working point. The greater is the length of the magnet in the direction of magnetization, the closer is B to Br. In a closed magnetic circuit, the working point corresponds to the value of Br.