Alternating and Rotational Losses up to Magnetic Saturation in Non-Oriented Steel Sheets

partially_open5sìpartially_openAppino, Carlo; Khan, Mahmood; de la Barrière, Olivier; Ragusa, Carlo; Fiorillo, FaustoAppino, Carlo; Khan, Mahmood; de la Barrière, Olivier; Ragusa, Carlo; Fiorillo, Faust

A simple compensation method for the accurate measurement of magnetic losses with a single strip tester

International audienceWe present a new method for the accurate characterization of soft magnetic sheets using a permeameter based on the precise compensation of the magnetomotive force (MMF) drop in the flux-closing yoke. It has been developed in order to overcome the systematic uncertainty affecting the value of the magnetic fieldstrength in single sheet testers when obtained, according to the standards, through the measurement of the magnetizing current. This phenomenon is more critical for high permeability materials, because of the reduced MMF drop across the sample. While additional sensors and auxiliary windings have been proposed in the literature, a novel approach is demonstrated here, based on the use of the permeameter upper half yoke as the MMF drop sensor and of an auxiliary winding on the lower half yoke, implementing compensation. This solution, dispensing one from dealing with the usually small signal levels of the conventional MMF drop sensors (e.g. Chattock coils), provides best results with the introduction of wedge-shaped magnetic poles, in order to accurately define the magnetic path length. The method is validated by measurements of power loss, apparent power, and hysteresis cycles on non-oriented and grain-oriented Fe-Si steel sheets, which are compared with local measurements performed on the same samples using H-coil and B-coil across a uniformly magnetized region

Hysteresis and nucleation field in two-dimensional magnetization process

We have investigated the vector nature of the magnetization process in disk-shaped Co-based amorphous samples, providing a joint description of spin rotation and domain nucleation processes. Thermo-magnetic treatments have been exploited to induce a macroscopic uniaxial anisotropy in the specimen plane. An alternating field Ha has been applied to the specimens along a direction forming with the easy axis an angle yHa between 0! and 90!: The two orthogonal components of the magnetization Mk and M? (along the easy axis and perpendicularly to it), and of the effective field Hk and H?; have been measured by means of a compensated winding and a many-turn H-coil placed on the sample surface. We have studied the evolution of the Mk vs. M? and Hk vs. H? loops as a function of yHa : In particular we have pointed out the condition leading to the nucleation of domains in a two-dimensional magnetization process, and obtained the nucleation field values HN for any yHa : These results have been interpreted proposing a model accounting for nucleation in the framework of the Stoner–Wohlfarth theory

Vector Model for Losses in Non-Oriented Steel sheets

The presence of two-dimensional (2D) magnetization processes in devices and electrical machines calls for the development of vector model for loss prediction. Starting from the Del Vecchio-Charap work [2], we developed a static model able to reproduce the 2D evolution of magnetization in soft non-oriented steel sheets: the most interesting case for applications. The material is assimilated to an ensemble of biaxial grains, each associated with a hysteron having two orthogonal easy axes. In each of them, the field driven irreversible switch of magnetization between the easy directions (Barkhausen jump: BJ) is governed by the value of local coercive field (about ten times smaller than the anisotropy fields); the ensemble of all these BJs accounts for the domain wall displacement. After the BJ, the local magnetization is brought to its energy minimum by the antagonism between anisotropy and Zeeman energy, so determining the reversible magnetization component. Domain wall reversible processes (bending) are not considered, whereas the role of macroscopic and internal demagnetizing fields is accounted for. The system average magnetization is obtained after integrating the outputs of single hysterons, each weighted by probability density functions suitably characterizing the material properties (i.e.: grain orientations, coercive and anisotropy fields). We have been able to reproduce the loss vs. polarization W(Jp) evolution in several non-oriented materials, subjected to alternating and rotating fields. In particular, under circular induction, the W drop at high Jp was always found, without introducing “ad hoc” fitting functions. It is remarkable that the outlined approach can be extended to systems magnetized in dynamic conditions. [1] C. Appino, C. Ragusa, and F. Fiorillo, “Can rotational magnetization be theoretically assessd?”, IJAEM 44 (20144), 355-370 [2] R.M. Del Vecchio, S. H. Charap, “Two dimensional hysteresis model”, IEEE Trans. Mag. 20 (1984), 1437-143

Vector magnetization processes in amorphous magnetic materials with uniaxial induced anisotropy

We report an investigation about the vector magnetization behavior of soft amorphous materials. Disk-shaped samples, with uniaxial-induced anisotropy, were submitted to a magnetizing field Ha; oscillating along different directions with respect to the easy axis. The measured effective field H and the magnetization display a general behavior versus Ha; pointing out the crucial role played by the demagnetizing field. Experiments revealed two regimes in the magnetization reversal. This phenomenology was interpreted in the framework of the N!eel phase theory applied to uniaxial systems. We showed that the two experimental regimes correspond to magnetization modes where two or one magnetic phases (magnetic domains) are present. We worked out the switching condition between the modes, which involve only magnitude and orientation of the applied field. Moreover, in the two-phase mode, a simple relationship connecting the magnetization direction to the applied field direction was found. The measured magnetization properties are then interpreted as the combined action of domain wall motion without hysteresis and coherent rotation inside domains. It is remarkable that agreement found is based on two parameters only: the demagnetizing coefficient and the anisotropy field of the specimens