Models of Magnetic Anisotropy for Nonoriented Silicon Steel Laminations of Electrical Machines

This dissertation deals with magnetic anisotropy and its impact on the flux density and loss distribution for non-oriented silicon steel laminations. Magnetic material models, such as anhysteretic reluctivity, Bergqvist’s vector Jiles-Atherton and energy-based hysteresis models, are considered in this work. The vector hysteresis models mentioned above are extended: first, to account for magnetic anisotropy observed in non-oriented silicon steel, and second, to predict hysteresis losses for the input excitation rotating in the plane of the sheet, as well as alternating in different directions. Non-oriented silicon steel lamination, commonly used in a rotating electrical machine’s magnetic core, shows a significant level of magnetic anisotropy. Numerical analysis tools often assume the laminated core is isotropic. Preliminary investigations reveal that the anisotropy alters the distribution of magnetic flux density in the core. Thus, some core parts get saturated, which may adversely affect the core losses. Hence, determining how the losses predicted by the anisotropic models differ from their isotropic counterparts is paramount. Firstly, an extension of the isotropic anhysteretic reluctivity model is proposed to account for magnetic anisotropy. After preliminary investigations, the ideas from the anisotropic reluctivity model are then incorporated into the advanced hysteresis models. Hysteresis models, such as Bergqvist's vector Jiles-Atherton and energy-based models, rely upon the anhysteretic properties for quantitatively describing the intrinsic anisotropy. Anhysteretic magnetic characteristics are identified from unidirectional alternating measurements in several directions to include intrinsic anisotropy in the models. Moreover, to reasonably predict the hysteresis losses, the model parameters are allowed to depend on the magnitude and polar direction of the flux density vector. Secondly, the anisotropic models are coupled with the finite element method. This coupling is then applied to simulate magnetic fields and associated losses in the commonly used measurement setups such as a toroidal inductor, round rotational single sheet tester, and a transformer-like device. The results of the finite element simulations are briefly compared and discussed. Furthermore, the material level validation of the proposed hysteresis models is in the peer-reviewed articles at the end of this thesis