A Method for Measuring Normal and Shear Stiffness of Laminate Stacks of Electric Motors

Abstract

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.

Comparative analysis of stiffness measurement methods for lamination stacks in electric machines

“…Due to these reasons it is not optimal for performing cyclic load experiments. The second type of procedure, which was used to perform the quasi static measurements in this research, involves direct compression and displacement measurement of the specimen, and is described in detail in [2].…”

“…For axial compression and shear loads (Figure 1 b and c) the resulting stiffness values are highly nonlinear. [2] The causes for this nonlinearity are the cut surfaces of the sheet edges, the nonlinear stiffness of the varnish, the surface roughness, and the imperfect planar shape of the individual sheets. [3].…”