Effective permeability



Effective magnetic permeability, µe or µeff - a term used in analysis of magnetic performance of gapped cores. For a non-homogeneous core (e.g. air gapped or composed of powder particles) this would be the value of permeability of a hypothetical homogeneous material which would exhibit the same permeability.

Effect of air gap
Magnetic permeability of a magnetic material is defined as the slope of the B-H curve or (or B-H loop). With increasing air gap the slope is reduced, and changes caused by non-linearity of the material (due to variations in flux density, temperature, bias, time, etc.) are reduced.

With the gap present, higher magnetotomotive force (excitation) is required to reach the same flux density. Similar behaviour could be obtained if the magnetic circuit was made not from a gapped core, but from non-gapped core made from material with lower permeability. A value of permeability required to obtain equivalent B-H performance is therefore the value of effective permeability.

Equations and calculations
It is possible to analytically calculate the value of relative effective permeability for simple magnetic circuits, with a uniform gap.

There are several assumptions:
 * the cross section area of the magnetic circuit is constant at every point of the circuit, and is the same for the core and for the gap
 * the length of the air gap is much shorter than the total path lenght of the magnetic core
 * the magnetisation is uniform and fringing effect is neglected
 * permeability of the core material is much greater than the permeability of air gap

The equation is derived by using the concept of magnetic reluctance and with the assumptions listed above. All values of permeability (input and output) are given as relative permeability (so the value of "1" means permeability of air gap). The length of the core and the gap must be given in the same units. For instance, if the core length is given in millimetres, then also the air gap length must be given in millimitres. But the equation holds for any other length units: inches/inches, metres/metres, etc.

Equations can also be derived for branched or non-uniform magnetic circuits, but these are obviously configuration-dependent and must be calculated for each specific structure.

Composite materials


The value of effective permeability is important for composite materials, which may contain significant volumetric percentage of non-magnetic material. The small particles (as in powder cores) have rather high permeability, but the bulk of the core made out of such material exhibits effective permeability whose value is tailored for specific applications.

For instance, Ferrotron 119 used for flux concentrators in induction heating has a maximum relative permeability of 8.0 (despite being made from ferromagnetic particles), because it is designed to work at high frequency (up to 5 MHz) and high excitation (20 kA/m).

However, because such a magnetic core does not have a concentrated air gap then the simple equation given above cannot be used. Depending on the complexity of given material the calculations can become very difficult to solve or formulate.

Hence, from application of view the end users of composite cores can rely on the effective permeability values given by the manufacturers of the materials or magnetic cores. If the product is a magnetic core, then the AL value (inductance per turn) is often more useful than effective permeability. .

Nevertheless, in order to easier distinguish the type of material from which a given core is made the name of the material often refers to the value of effective permeability, for instance Ferroxcube uses notation Sendust 75, where 75 is the value of effective relative permeability at room temperature.