Characterization of the Local Incremental Permeability of a Ferromagnetic Plate Based on a Four Needles Technique

The performances of electrical machines depend highly on the behavior of ferromagnetic materials. In some applications, these materials operate under dc polarization, i.e., when the magnetic field oscillates around a dc bias. In that condition, it is required to know the incremental permeability, which characterizes the magnetic behavior of the material around the operating point. In this paper, a non-destructive approach, involving a combination of experiment and finite-element (FE) technique, is presented in order to determine the incremental permeability. The proposed sensor is based on the four-needle method. With this sensor, Bowler et al. have proposed a method to determine the initial permeability of homogeneous metal plates based on an analytical model. Here, we propose to use the same kind of sensor to determine the incremental permeability. The measurement process is analyzed using a FE model. It is shown that the analytical approach reaches its limits if the permeability of the plate and its thickness become too high. A combination between the measurements and an FE model is introduced to overcome this difficulty to determine the incremental permeability. The study of two magnetic steel samples illustrates the interest of this method.