Open-loop pulsed hysteresis graph system for the magnetization of rare-earth magnets

An open-loop pulsed hysteresis graph system can magnetize and characterize ceramic and rare-earth permanent magnets such as neodymium, samarium cobalt, and alnico of 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 on the order of 15 to 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 digital computer to obtain the hysteresis graph response of the sample. The processing takes into account the demagnetization field due to the open-loop system configuration. This paper presents the system concept in terms of the hardware and software realizations. An integral part of the data processing is calculation of the demagnetization factors that are needed to correct for the open-loop magnetization arrangement. Furthermore, a finite-element postprocessing procedure corrects for the influence of eddy currents in highly conductive samples. In tests of the prototype system with ceramic and neodymium permanent magnet samples, the pulsed field predictions compare favorably against reference data and dc hysteresis graph measurements of samples of low conductivity. Even for high-conductivity samples such as alnico, good agreement can be achieved through suitable post processing steps.