Magnetic and eddy current effects in an open-loop pulsed hysteresis graph system for magnetization of rare-earth magnets

A companion paper by Bretchko and Ludwig describes an open-loop pulsed hysteresis graph system that is capable of magnetizing ceramic and rare-earth permanent magnets such as neodymium, samarium cobalt, and Alnico in cylindrical and rectangular shapes. The prototype system relies on an air-core excitation coil with an inner diameter of 3 cm and a length of 10 cm. A pulsed power supply provides a transient current pulse of up to 9000 A into the air-core coil. The pulse duration is typically about 15-20 ms. In addition, a pair of Helmholtz coils records the applied magnetic field strength, whereas a local coil, tightly wound around the permanent magnet, records the magnetic flux density in the sample. Both fields are electronically acquired, digitized, and processed in a computer to obtain the hysteresis graph response of the sample. The processing allows compensation for the presence of the demagnetization field due to the open-loop system configuration. In this paper, we present an analytical approach to take into account the effect of permeability in cylindrical samples. Furthermore, we propose a numerical approach that can characterize conductive materials such as Alnico, whose eddy-current influence significantly affects the open-loop recording of the B-H curve.