Effect of a Superposed Alternating Field on Apparent Magnetic Permeability and Hysteresis Loss

Abstract

It is often assumed that when an alternating magnetizing force is superposed on a constant magnetizing force, the permeability of the magnetic material is increased and the hysteresis loss decreased owing to the shaking-up action of the alternating field on the magnetic particles. It is shown that these effects are only apparent and that the true permeability and hysteresis remain approximately the same as under constant field conditions. What are actually measured are the midpoints of displaced minor hysteresis loops. From the known laws of variation of these minor loops the apparent magnetization curves and hysteresis loops under a superposed alternating field can be calculated. The method of doing this is illustrated. Under these assumptions a superposed alternating field actually increases the hysteresis loss instead of suppressing it as has often been assumed, the resulting hysteresis including losses due to displaced minor loops besides those due to an increased major loop. These conclusions are supported by magnetization curves obtained with 1 percent Si steel punchings which are shown to agree with the calculated curves. Some consideration is given to the case when the fields are at right angles but at present it is not susceptible to quantitative calculations.