A Limited Prediction of Maximum Energy Product from the Magnetization Hysteresis Loop

Abstract

Recently, as the demand for devices using magnetic energy of permanent magnets such as electric cars and wind power generators has increased, researches have been actively carried out to develop new high-efficiency and low-cost permanent magnets. The figure-of-merit of permanent magnets is the energy product which is the twice the energy stored in the stray field of the magnet. It can be obtained from the volume integral of the square of the stray field outside the magnet or from the volume integral of the dot product between the demagnetizing field, Hd and the magnetic field, B inside the magnet itself, and thus the “BH” indicates the energy product. However, this energy product of magnets is often used without a clear understanding of its definition, even though it is a basic concept that is widely known to relevant industry workers and researchers. Basically, the energy product BH corresponds to the energy stored in the stray field produced by the magnet itself. Consequently, it should be measured from Hd and B at the remanent state, i.e., without any external field. Since the demagnetizing field depends on the shape of magnet, the energy product varies with the shape and thus, its maximum value (BHmax) can be obtained from the remanet state in the various shape of magnet. However, it has been found occasionally that the BH and BHmax are obtained from the BH loop, even from the MH loop. Here, we will discuss how to measure the energy product and how to predict correctly its maximum value (BHmax) from the loop.