Non-oriented electrical steel sheets are applied as a core material in rotors and stators of electric machines in order to guide and magnify their magnetic flux density. Their contouring is often realized in a blanking process step, which results in plastic deformation of the cut edges and thus deteriorates the magnetic properties of the base material. This work evaluates the influence of the material’s grain size on its iron losses after the blanking process. Samples for the single sheet test were blanked at different cutting clearances (15 µm–70 µm) from sheets with identical chemical composition (3.2 wt.% Si) but varying average grain size (28 µm–210 µm) and thickness (0.25 mm and 0.5 mm). Additionally, in situ measurements of blanking force and punch travel were carried out. Results show that blanking-related iron losses either increase for 0.25 mm thick sheets or decrease for 0.5 mm thick sheets with increasing grain size. Although this is partly in contradiction to previous research, it can be explained by the interplay of dislocation annihilation and transgranular fracturing. The paper thus contributes to a deeper understanding of the blanking process of coarse-grained, thin electrical steel sheets.
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.
Der Eigenspannungszustand ist in vielen Fällen eine wesentliche Ursache für unerwartetes Versagen von umformtechnisch hergestellten Bauteilen im Betrieb.Die Überlagerung von Spannungszuständen in Folge der Betriebslasten mit dem Eigenspannungszustand im Bauteil ist mit den bisher verfügbaren Prognosemethoden nicht hinreichend genau abbildbar, so dass typischerweise versucht wird, den Eigenspannungszustand z. B. durch eine Wärmebehandlung zu minimieren.Allerdings haben Eigenspannungen per se auch das Potenzial, die mechanischen Eigenschaften von umformtechnisch hergestellten Bauteilen zu verbessern. Dieses bisher ungenutzte Potenzial ist das übergreifende Ziel des DFG-Schwerpunktprogramms 2013, aus dem heraus das vorliegende Sonderheft entstanden ist. EinleitungJedes durch Umformen hergestellte Bauteil weist einen durch die Fertigung bedingten Eigenspannungszustand auf. Laut Definition werden mechanische Spannungen als Eigenspannungen bezeichnet, wenn im Bauteil ein Temperaturgleichgewicht besteht und keine äußeren Kräfte oder Momente einwirken. Umformprozesse induzieren typischerweise Kräfte, die zu einer plastischen Verformung des Bauteils führen [1]. Nach dem Entlasten bzw. Öffnen der Werkzeuge verformt sich das Werkstück so lange, bis es im mechanischen Gleichgewicht ist und somit die Eigen-
In reluctance and permanent magnet synchronous machines, flux barriers are crucial for magnetic flux guidance. Designed as cutouts, flux barriers reduce the mechanical strength of the rotor construction. To operate these electric drives at higher rotational speed, an alternative flux barrier design is required. Since residual stress influences the magnetic properties of soft magnetic materials, this paper deals with embossing induced residual stress as flux barriers in non-oriented electrical steel with 2.4 wt% silicon and a sheet thickness of 0.35 mm. The investigated flux barriers were fabricated with a cylindrical or spherical punch at two different penetration depths and were compared to a flux barrier fabricated as cutout. A residual stress analysis using Finite Element Analysis helps understanding the mechanism of embossed flux barriers. Additionally, the influence of induced residual stress on the magnetic material behavior is measured using standardized single sheet tests and neutron grating interferometry measurements. This investigation aimed at a better understanding of the flux barrier design by local induction of residual stress.
In reluctance and permanent magnet synchronous machines, flux barriers are crucial for magnetic flux guidance. Designed as cutouts, flux barriers reduce the mechanical strength of the rotor construction. To operate these electric drives at higher rotational speed, an alternative flux barrier design is required. Since residual stress influences the magnetic properties of soft magnetic materials, this paper deals with embossing induced residual stress as flux barriers in nonoriented electrical steel with 2.4 wt% silicon and a sheet thickness of 0.35 mm. The investigated flux barriers were fabricated with a cylindrical or spherical punch at two different penetration depths and were compared to a flux barrier fabricated as cutout. A residual stress analysis using Finite Element Analysis helps understanding the mechanism of embossed flux barriers. Additionally, the influence of induced residual stress on the magnetic material behavior is measured using standardized single sheet tests and neutron grating interferometry measurements. This investigation aimed at a better understanding of the flux barrier design by local induction of residual stress.
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