Measurement of initial magnetization curve based on constant magnetic field excited by permanent magnet
Authors: Deng Dong-Ge  , Wu Xin-Jun  , Zuo Su  ,   School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
Published: December 2020

The initial magnetization curve is closely related to the stress in ferromagnetic material, thus it could be used to evaluate the stress in ferromagnetic member online. However, the initial magnetization curve measurement system recommended by the technical standard IEC 60404-4 is not suitable for online application. It is inevitable to use excitation coils to generate the excitation field and induction coils to obtain the magnetic flux density, however winding coils closely and uniformly online is not easy to operate. To obtain the initial magnetization curve easily, a calculation method for initial magnetization curve under constant magnetization based on time-space transformation is put forward in this paper. The theoretical correctness of this method is validated through simulation with the constant current coil magnetization. Considering the fact that the constant magnetic field could also be provided by permanent magnets and that magnetizing ferromagnetic members online by permanent magnets are convenient to achieve, in this paper, we put forward the measuring principle of initial magnetization curve based on a constant magnetic field excited by permanent magnets further and set up the corresponding measurement system. This system employs permanent magnetizers as the excitation magnetic source, and adopts symmetric magnetization methods to produce a constant magnetic field on a cylindrical rod-shaped member. The excited constant magnetic field changes along the axial position of the member. Under this exciting field, the axial and radial magnetic flux densities at different lift-offs from the surface of the member are measured by a testing probe including Hall chip array. Then, the axial and radial magnetic flux densities at the interface between the member and air are calculated based on the extrapolation method through utilizing polynomial function fitting and the Gauss's law for magnetism. Furthermore, the axial magnetic field strength within the member is calculated from the axial magnetic flux density at the interface according to the continuity of the tangential magnetic field strength. On the other hand, the induced magnetic flux density within the member is calculated from the radial magnetic flux density at the interface on the basis of the Gauss' law for magnetism, the basic equation of magnetization curve in Rayleigh region and the law of approach to saturation. Finally, the initial magnetization curve could be measured. System measurement results show that with no excitation coils nor induction coils, the initial magnetization curve of the cylindrical rod-shaped member can be easily obtained from the axial and radial magnetic flux densities at the interface of the member under the constant magnetic field excited by permanent magnetizers. The measurement error is less than 10%, and the standard deviation of the error is less than 0.01, which shows that the measurement repeatability is good. Therefore, this proposed system could provide a new approach to measuring the initial magnetization curve of cylindrical rod-shaped members online conveniently.