Electric motors in automotive applications are subjected to high thermal and structural loads, while having strict requirements regarding dimensions, mass, and costs. The design of such motors requires sophisticated simulation models. The laminate stack in the rotor of such a motor is made of steel sheets and behaves transversally isotropic: the radial stiffness is equivalent to steel, and in the axial direction, it has a highly progressive nonlinear stiffness characteristic. The loading and unloading stiffness curves change from cycle to cycle when subjected to repetitive loads. In this paper, the usage of a single approximating curve to describe the longitudinal stiffness of the laminate stack is proposed. This approximation can be used in FEM models to reproduce the structural nonlinear behavior of such a laminate stack using a simpler approach than implementing the full loading and unloading curves in a material model, at a price of negligible loss of precision.

Structural simulations of electric motors require precise material models. Laminate stacks that are made of several identical steel sheets are particularly challenging to simulate using FEA. The structural stiffness of laminate stacks usually follows transversal isotropic behavior. Measuring a complete laminate stack used in passenger cars is challenging due to its size and the high testing load needed to reach real loads experienced while in operation. A new method capable of performing such measurements is presented in this article, with the help of equipment normally used for testing structures used in civil engineering. Two sets of exemplary results are presented utilizing this measurement procedure, that were performed on a real automotive rotor laminate stack: axial compression stiffness from a cyclic test, and shear stiffness at various axial preload levels. In the axial compression load case, the loading and unloading curves form a hysteresis, that changes in every test cycle. Shear stiffness shows high dependance on the axial compression preload. After loading and unloading the stack with shear loads, significant plastic deformations remain.