Study Of Impulse Magnetizing NdFeB Permanent Ring Magnets By Finite Element Analysis

The impulse magnetization for NdFeB permanent ring magnets including radialhomopolar and twelve-pole magnetization is studied in this paper using finite-elementanalytic software ANSYS combining the experiment. The factors are systematicallydiscussed containing the initial voltage, the distribution of magnetizing field, the corematerial, the number of coil, the twist way and yoke based on the finite-elementanalysis and the experimental results.The results show that the decrease of initial voltage can reduce the peakmagnetizing current, and at the same time, the increasing capacitance can also prolongcontinuous time of magnetizing current. Therefore, the adverse effect of eddy currentcan be greatly reduced through decreasing the initial voltage and increasing thecapacitance. On the conditions that the saturation magnetic field intensity ishomogeneous, initial voltage should be selected a minimum value. For example, it isproper that the initial voltage is selected to be 1550v for radial homopolarmagnetization and 1000v for the twelve-pole magnetization. The core should work inthe saturation area to ensure the uniformity of magnetized ring magnet. In order todecrease power loss, the initial voltage and magnetizing current should be minimumvalues. Using silicon steel with high resistivity as the core benefits achieving fastdissipation of flux lines by decreasing the effect of eddy current. On the contrary,using the iron with high permeability as the yoke can get the good magneticconductibility.During the radial homopolar magnetization, it would preferred to use the coilwith more layers and less tunas on one layer on the condition of the same total coil turns. However, in order to reduce the voltage and the current, the turns on one layershould fit the voltage of saturation magnetization of the core. In the experiment ofradial homopolar magnetization, the coil with 4 turns and 8 layers can fulfil therequirement of saturation magnetization with small voltage and current. When the coilhas the same diameter with the ring magnet, producing little inductance and eddycurrent are reduced and the core can be easily magnetized due to the permeation to thecenter of the magnetic filed.During the twelve-pole magnetization, the thicker copper wire produces thebigger current and the homogeneous distribution of flux lines, which makes the coreenter the saturation area easily. However, it is difficult to achieve the coil from thethicker copper wire in practice. The copper wire withφ1mm is the best selection inthe experiments of twelve-pole magnetization coil. Using yoke can reduce magneticcharacteristic, for example, the surface magnetic field strength, but increase the widthof peak value, which can satisfy the requirement of rectangular waveform for themotor applications. In the experiments, the waveform with a yoke is 1.67 times widerthan that without yoke at the same surface field strength. The ring magnet should bein the middle of the core in the axial direction where the magnetic field is strong andsymmetrical. The calculated result shows that the core is proper with the height1.2-1.5 times higher than that of ring magnet.This paper is studying the rules in practical magnetization using theory analysismethod, and it benefits giving the guidance for the practical magnetization of NdFeBpermanent ring magnets.