
Calculating Permeance CoefficientsPermeance Coefficients (Pc) are important in material selection to ensure optimal design. The Permeance Coefficient of a magnet is also referred to as the "operating slope", load line or B/H ("B over H") of the magnet. When no other permanent or soft magnetic material is nearby, that is when the magnet is in "open circuit", the Pc can be calculated solely from the dimensions of the magnet.Calculations based upon simplified equations of geometry (from Evershed) and approximations of the effective length of the material result in the ballistic demagnetizing factor, N_{B}. The Pc is calculated from N_{B} by the formula: Pc = 1 (1/N_{B}). The calculated value represents the Pc at the neutral crosssection of the magnet. A quite "readable" explanation of the calculation of Pc based on the ballistic demagnetizing factor is available in Rollin J. Parker's book Advances in Permanent Magnetism, pages 149154. (See the Reference Book list). Starting in the 1960's and as a result of the development of ferrite magnets with a "straight line" normal curve ("square loop" intrinsic hysteresigraph curve), R.I. Joseph, E. Schlomann and others advanced a calculation based on uniform magnetization throughout the magnet volume. The demagnetizing factor from these calculations is called N_{M}, the magnetometirc demagnetizing factor. In the 1990s, additional calculations were reported by Chen, Pardo, Sanchez, and others from the University of Madrid, Spain, which incorporated the materials susceptibility. Demagnetizing factors incorporating susceptibility are called N_{F}, the fluxmetric demagnetizing factor. For permanent magnet materials, the susceptibility is only slightly above zero, and N_{F} is very close to N_{M}. While FEA software utilizes values of N_{M}, laboratory measurements on fullsize magnets fall closest to the those from the ballistic demagnetizing factor, (N_{B}. Most, if not all, FEA programs now use N_{M} and those are the calculations provided herein. Please select which Permeance Coefficient calculation best suits your application by referring to the diagrams below.
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