Methodology to Evaluate the Mechanical Stress in High Speed Electric Machines with Buried Magnets

Abstract

High speed electric machines are gaining in importance due to their high power density. The evaluation of the mechanical stress in the rotor is one crucial part of the design process for this type of machines. It is insufficient to evaluate just the static mechanical stress at maximum speed. The dynamic mechanical stress between the state of standstill and maximum speed needs to be considered as well to evaluate the high fatigue strength of the rotor.One criterion that is commonly used to analyze the dynamic load in a material is the Smith diagram. In this work, a new methodology is introduced to evaluate both, the dynamic and the static mechanical stress, in the rotor of an high speed electric machine. First, the von Mises stress criterion is used to examine the static mechanical stress in the rotor core. Then, the Smith diagram is used to evaluate the dynamic mechanical stress and the high fatigue resistant of the rotor.The methodology is then applied to a high speed electric machine with buried magnets in v-shape for analyzing the state of stress. Therefore, the state of stress is calculated using an FEM simulation model. It can be seen that high dynamic stress occurs at the bridges in the rotor. The stress alternates with up to σ alt = 300 MPa. The design is though limited due to the high static stress at the edge bridge of the magnet slot.