Study of Magnetic Losses at Low Flux Densities in 35 Permalloy Sheet

Abstract

Energy losses in ferromagnetic materials subject to alternating fields have long been considered as due solely to hysteresis and eddy currents. However, at the low flux densities encountered in certain communication apparatus, a further loss is observed which has been variously termed ``residual,'' ``magnetic viscosity,'' or ``square law hysteresis.'' The search for an explanation of this loss has led to precise measurements of hysteresis loops with a vacuum ballistic galvanometer, and of a.c. losses with inductance bridges. From these results, it appears that that part of the a.c. effective resistance of a coil on a ferromagnetic core which is proportional to the coil current is strictly identified with the hysteresis loop area as measured by a ballistic galvanometer, or as indicated by harmonic generation in the coil. The hysteresis loop can now be constructed in detail as to size and skewness on the basis of a.c. bridge measurements. This conclusion was reached previously on a compressed iron powder core, and is now confirmed on an annealed laminated 35 permalloy core. Observed eddy current losses for this core exceed those calculated from classical theory by 20 percent. This excess is ascribed to the presence of low permeability surface layers on the sheet magnetic material. The a.c. residual loss per cycle (nominally independent of frequency, like hysteresis) is not observed by ballistic galvanometer measurements, although it indicates an energy loss some eight times the hysteresis loss for the smallest loop measured (Bm = 1.3 gauss). Analysis of the residual loss shows that it increases with frequency up to about 500 cycles, and remains constant at higher frequencies (to 10,000 cycles per second). Concurrently with the increase of residual loss, the permeability of the alloy is observed to decline with increasing frequency about 1 percent below the value predicted from eddy current shielding. This effect is most noticeable at frequencies below 1000 cycles.