In its broadest sense, the permeance coefficient is a figure of merit for a magnet, or magnetic circuit, which indicates the ease with which flux travels from the North Pole to the South Pole (of course, magnetic flux does not actually travel, or flow. However, it is conceptually beneficial to describe the system in this manner).
After magnetization, a magnet, or magnetic circuit, will operate at some point on the demagnetization curve for that magnet. Certain shapes of magnetic circuit will operate lower down on the demagnetization curve than others. The permeance coefficient, calculated by using values based purely on the geometrical parameters of the magnetic circuit, allows the magnetic design engineer to determine this operating point of the magnet on the demagnetization curve.
A good example to indicate the qualitative difference between magnets of low and high permeance coefficient would be to consider a long, pencil shaped magnet (magnetized through its length) and a flat, coin shaped magnet magnetized through its thickness. In the case of the pencil shaped magnet, it is clear that the distance the flux has to travel from North to South poles is roughly the same if the flux has to travel back through the magnet itself, or if the flux has to travel outside of the magnet through the surrounding air. As we know, magnetic flux (like all physical systems) takes the easiest path presented to it (the path of least reluctance, in magnetic terms). Therefore, in the pencil shaped magnet, the vast majority of the magnetic flux will travel to the South Pole of the magnet by flowing outside of the magnet itself. Because of this, the amount of self demagnetization the magnet sees, (Hd), is very low. In such a situation, the magnet operates very close to Br, high up on the demagnetization curve.
Considering the flat, coin shaped magnet, one can imagine a line of magnetic flux emerging from the centre of the North Pole surface. This line of magnetic flux will again seek out the path of least reluctance. However, in a coin shaped magnet, the path to the opposite pole through the surrounding air is quite long. In fact, in some cases the magnetic flux would prefer to travel back through the magnet itself, (against its direction of magnetization), in order to get to the South Pole. In this instance, the amount of self demagnetization that the magnet sees can be quite significant. In this case, Hd has quite a high value and the magnet will operate much further down the demagnetization curve, closer to Hc.
The permeance coefficient is mathematically equal to Bd/Hd. Calculation of the permeance coefficient allows the magnetics design engineer to determine the operating point of a magnet by construction of a load line from the origin of the BH curve at a gradient equal to the permeance coefficient. The intersection of this load line and the demagnetization curve (at the point (Hd, Bd)), is the operating point of the magnet.
