Alternating current magnetic properties of cores made from pressed acicular steel particles

Abstract

This article describes results of magnetic testing of cores made from a composite material that yields powder metallurgically pressed compacts of high density suitable for a variety of ac magnetic applications. Small acicular steel particles are annealed, individually insulated, and uniaxially pressed using standard powder metallurgical techniques. The ac losses and permeability of the pressed compacts are strongly dependent upon particle geometry. The variations of the core loss among the different particle geometries was found to be a function of the cross sectional area of the acicular particles as well as the strain imparted to the particles during the pressing operation. The core loss, when measured at 60 Hz, exhibits a minimum at a certain cross sectional area. The shape of the curve is attributed to a decrease in eddy current loss with decreasing particle cross sectional area, while the hysteresis loss increases with decreasing particle cross sectional area. At a test frequency of 400 Hz, the eddy current component of core loss predominates, and the core loss decreases steadily with decreasing particle cross sectional area. Permeability was also found to depend upon particle geometry. The smaller the demagnetizing factor of the individual particles, the higher the permeability, while the increased strain in the smaller particles overwhelms the smaller demagnetizing factor resulting in a decrease in permeability. The net result is a maximum in permeability for the mid-sized particles.