Fatigue Aspects of Electrical Steels : Development and validation of a new method to estimate the fatigue aspects of thin electrical steel sheets in an IPM rotor stack

Sammanfattning

Electrical steels are commonly used in the rotor in electrical motor applications. The rotors contain a series of punched electrical steel sheets laminated together, forming the rotor geometry. In the rotor, permanent magnets are mounted and together with the stator, which creates the magnetic field, they produce a torque. For these electrical motors, there is demands on soft magnetic properties, but also on the mechanical properties. One of the mechanical properties of importance is fatigue, where the electrical motor should not experience any catastrophic failure caused by fatigue, during its operation span. There is a general understanding of the fatigue aspects of electrical steels in the rotors of electrical motors. As of today, fatigue testing of thin steel sheets has been conducted, but there is very limited testing done on specimen with small cross-sections that represent the bridge locations of the rotor laminate. The purpose of this thesis work is to investigate deeper on how the fatigue aspects of electrical steels depend on the small cross-section, punched edge effect and few but large grains. A new test specimen was developed to evaluate fatigue properties using the staircase method together with the S-N curve. These tests are of interest as they portray the fatigue aspects and mechanism of thin small cross-section “bridges” in the rotor stack laminates. Detailed microscopy of the material and fracture surfaces were performed to explain fatigue mechanisms and behaviour. This thesis work is highly relevant to the dimensioning of thin bridges in the rotors for high-speed electrical motors, as the development of electrical motors with higher rotational speeds are becoming more relevant.