Application of grain-oriented electrical sheets in flux switching machines
Final Report Abstract
Grain-oriented metal sheets show substantially better magnetic properties compared to nonoriented metal sheets. These materials can reach higher maximum flux densities, while the magnetization losses remain significantly lower. Such materials increase the efficiency of transformers, and, therefore, should also considered in electric machines. Not only the lower magnetization losses, but also the current for an electric excitation can be smaller, due to the high permeability. Additionally, because of a high saturation polarization, the volume of the magnetic cores can be reduced or, alternatively, the power density increased. The optimized magnetic properties come along with strong anisotropic behavior. Transformers, the main application of this type of material, remain largely unaffected by the anisotropies. However, this is usually undesirable for electrical machines. In order to use these materials in electric motors, the preferred magnetic direction has to be aligned with the expected flux paths of the magnetic core. Due to the complex flux paths in electrical machines, these parts need to be separated into many segments, resulting in small mechanical gaps between the parts. Each of these gaps acts as an unwanted magnetic resistance. For testing purposes, a prototype of a Flux-Switching-Permanent-Magnet machine is being built, the stator of which consists of grain-oriented metal sheets. To design this machine, the anisotropic properties must first be measured. Therefore, a prototype of an anisometer is built, which allows the separate measurement of longitudinal and transverse magnetic fields. The results are interpreted using measurements made with an Epstein frame on samples in the rolling direction. So, the permeability can be set up as a nonlinear tensor, which leads to the effect of differing directions between the vectors of field strength and flux density. The magnetic permeability in tensor form is included in the finite element simulations of the software Comsol. In addition, a genereal approach to calculate magnetic problems using Magnetic Equivalent Circuits is developed, allowing an easy integration of the anisotropic materials. Fine meshed Magnetic Equivalent Circuits showed very promising results when considering the magnetic field results compared to finite element analysis. The first calculations for the prototype machine were carried out using the semi analytical Subdomain-Modeling technique. Based on the resulting parameters, a finite element model was set up. The model was used to vary the machine’s mechanical parameters, aiming to increase the mean torque and decrease the torque-ripple considering full anisotropic material properties. The prototype was then realized according to the final calculation results.
