Impact of residual stress evoked by pyramidal embossing on the magnetic material properties of non-oriented electrical steel

Abstract

Targeted magnetic flux guidance in the rotor cross section of rotational electrical machines is crucial for the machine’s efficiency. Cutouts in the electrical steel sheets are integrated in the rotor sheets for magnetic flux guidance. These cutouts create thin structures in the rotor sheets which limit the maximum achievable rotational speed under centrifugal forces and the maximum energy density of the rotating electrical machine. In this paper, embossing-induced residual stress, employing the magneto-mechanical Villari effect, is studied as an innovative and alternative flux barrier design with negligible mechanical material deterioration. The overall objective is to replace cutouts by embossings, increasing the mechanical strength of the rotor. The identification of suitable embossing geometries, distributions and methodologies for the local introduction of residual stress is a major challenge. This paper examines finely distributed pyramidal embossings and their effect on the magnetic material behavior. The study is based on simulation and measurements of specimen with a single line of twenty embossing points performed with different punch forces. The magnetic material behavior is analyzed using neutron grating interferometry and a single sheet tester. Numerical examinations using finite element analysis and microhardness measurements provide a more detailed understanding of the interaction of residual stress distribution and magnetic material properties. The results reveal that residual stress induced by embossing affects magnetic material properties. Process parameters can be applied to adjust the magnetic material deterioration and the effect of magnetic flux guidance.

Characterization Methods along the Process Chain of Electrical Steel Sheet—From Best Practices to Advanced Characterization

“…However, recent publications have shown a possible application for this stress-induced deterioration of magnetic properties. According to several authors [ 73 , 74 , 75 ], embossings can be used instead of cut-outs to generate the necessary flux barriers in reluctance machines, whereby the mechanical strength of the rotor or stator could be increased. It has been shown that this deterioration of magnetic properties is based on a reduced domain size correlating with an elevated domain wall density, which, in turn, can be evaluated by neutron grating interferometry [ 75 , 76 ].…”

“…Gilch et al [ 75 ] used neutron grating interferometry to evaluate the magnetic domain structure next to embossings in electrical steel sheets having a thickness of 0.35 mm and a silicon content of 2.4 wt.%. A four-sided pyramidal geometry with a tip angle of 136.5° in combination with a flat die was used for embossing.…”

“… Exemplary neutron grating interferometry results of two Fe2.4wt.%Si samples being magnetized at 813 A/m and embossed along a line (position z = 0) with a pyramidal punch with a force of ( a ) 100 N and ( b ) 200 N. Reprinted with permission from the authors of [ 75 ], © 2021 Creative Commons CC BY. …”