Increased efficiency of electrical steel by targeted residual stress

by Dr Tobias Neuwirth (FRM-II, TUM)

Monday 22 Apr 2024, 14:30 → 15:30 Europe/Berlin

PH HS 3 (Physics Department)

In the transportation sector, battery electric vehicles (BEVs) are an option to reduce the consumption of fossil fuels, which helps to slow down climate change. The battery capacity, as well as the energy efficiency and density of electric drives are key factors impacting the driving range and therefore utility of BEVs. In this work, we focus on the efficiency of electric drives. The typically used drive types in BEVs require careful guidance of the magnetic flux.
Conventionally, cutouts in the electrical steel comprising the magnetic core of the drive guide the magnetic flux. As part of an interdisciplinary project, we studied the replacement of cutouts with embossed areas. The embossing introduces residual stress, which locally reduces the magnetic permeability and guides the flux while maintaining mechanical strength. By optimizing the embossing parameters, the achievable rotational speed of the electric drive increases and the stray fields are reduced, subsequently increasing efficiency.
Neutron grating interferometry (nGI) is a unique technique able to study the local magnetic properties by visualizing magnetic flux distribution in electrical steel. Knowledge about the flux distribution is required for optimizing the embossing parameters. 
With nGI, we proved that residual stress generates magnetic barriers, able to guide the magnetic flux. The embossing parameters chosen significantly influence the resulting barrier. A sequence of small embossing points shows the best compromise between guidance of the magnetic flux and deformation of the electrical steel.
Further, we mapped the orientation and size of magnetic domains for the first time using nGI employing different models. In addition, we analyzed the local magnetic hysteresis dependent on residual stress. We found that the connection to standard hysteresis measurements is not trivial.