Dimensioning of permanent magnets

The properties of magnetic materials are depicted by their demagnetization curve which illustrates the fall in flux density B in the saturated magnet as an increasing opposing field is applied.

At the top of these diagrams there is a scale for the dimension ratio L/D (length of magnet/diameter of magnet) for open, cylindrical magnets.

If this scale is joined to the origin then the intersection with the demagnetization curve gives approximately the mean value of the flow density and field strength in a magnet with this geometry. In the case of block magnets, the cross-sectional area is converted into a circular area with the same content. It can be seen that when the geometry is predefined, it is the selection of the material that makes a decisive contribution to optimization. High-rebalance Alnico materials are dimensioned with an L/D ratio >3, whereas highly coercive Ox materials have a flat geometry with L/D < 1.