New method for the optimum design of permanent magnets subjected to demagnetizing effects

Extensive design investigations with the presently most important permanent-magnet materials have disclosed that optimum operating conditions for all practical interesting demagnetizing conditions are confined to minor hysteresis loops originating from a portion of the main demagnetization curve between the point of maximum-energy product and a point below, which differs for various materials. It has also been revealed that the results of design methods, based on the assumption of one constant slope for all minor loops, may be rather sensitive to the choice of the constant value. Combining both results in the use of an average slope value computed for the confined optimum range, new design relations for permanent magnets subjected to demagnetizing effects by varying either external reluctance or externally applied magnetomotive force have been developed. They can be utilized to derive analytically or graphically general design charts for each permanent-magnet material, permitting the direct reading of the optimum per-unit values of the magnet dimensions, depending upon the degree of demagnetization, and the selection of the material which offers the smallest size or the most suitable shape for a considered application. Interrelations for the two types of demagnetization have been developed.