Prediction of the impact of thermal cycling on machine lifetime based on accelerated life testing and finite element analysis

Electric machines are constructed from a variety of different materials which exhibit different mechanical and thermal expansion properties. Losses within the machine cause an increase in temperature, resulting in thermally induced internal stresses in insulation materials. Changes to the operating condition will result in different levels of loss being produced, and give rise to a thermal cycling profile to the machine and cyclic stress in the insulation. This paper studies the impact of these thermal cycles and resultant cyclic stress in the insulation on the lifetime of machines. Through the use of finite element analysis the internal stress levels, and corresponding stress cycle profiles are determined within the machine insulation layer, this data is then combined with experimental data, allowing stress-life (number of cycles) (SN)-curves to be generated for the machine type under test. The correlation between predicted insulation fatigue life with the measurements provides a physical insight of lifetime reduction due to thermal cycling.