Fatigue Propagation Characteristics of Insulation Crack Failure in Stator End Winding for Large Traction Motor Based on Magnetic–Thermal–Mechanical Coupling Model

Insulation is one of the most important and weakest parts of large motors, especially for the groundwall insulation in stator end windings that are often subject to high electromagnetic, thermal, and mechanical stress. Once the insulation failure occurs, it will threaten the normal and safe operation of the motor system. Most existing research mainly focuses on the electrical failure characteristics of motor insulation while ignoring the mechanical failure and coupled stress. In this article, the fatigue propagation and mechanical failure behavior of large motor stator insulation under coupled magnetic–thermal–mechanical stress are investigated by employing fracture theory and numerical methods. First, the uneven distributions of leakage magnetic flux density, winding loss, temperature rise, and dynamic electromagnetic force around the stator end region are analyzed and coupled by using the 3-D finite element (FE) method. Furthermore, the states of coupled stress in the insulation layer are determined by calculating transient electromagnetic field, thermal field, and structural field, where the weak points for concentration stress may be found. Second, the crack defect model mounted in the insulation layer is defined and simulated by using a semi-elliptical element and an arbitrary crack element, respectively. Also, the stress intensity factor (SIF) is introduced to quantitatively describe the fracture behavior and propagation degree of insulation crack failure. Third, the fatigue growth rate of crack under periodic coupling stress is calculated and analyzed according to the Paris equation. In the end, the influence of initial crack with different positions and directions on crack propagation is considered. The results in this study may be helpful to understand the fracture behavior and fatigue propagation characteristics of insulation crack failure under actual operation conditions and provide the theoretical reference for insulation damage evolution, insulation fault diagnosis, and residual life prediction of large motors.