Effects of insulation ageing on the conductive heat transfer from the winding body into machine periphery/stator core pack

With the proliferation of electrical machines into functions previously undertaken by mechanical systems, the demand for compact, high energy and torque density, high performance electrical machines has increased. The thermal behaviour of an electrical machine and its degradation over time is a critical factor, particularly if high reliability is a requirement. A designer needs good knowledge of the heat transfer characteristics of the machine to improve its output capabilities, reliability and life span, among other factors. During a typical machine's lifetime it will be subjected to many thermal cycles and various electrical and mechanical stresses. These result in a reduction in the physical properties of the insulation system, comprising wire insulation, varnish or potting compound and slot liner. This paper evaluates how this ageing of material properties affects the conductive heat transfer from the winding body to the stator core pack. A set of six single coil motorettes with insulation components rated at 200°C or more (class N or R) are constructed for the testing, and are subjected to multifactor ageing stresses: thermal, humidity, vibration and electrical. After each stress cycle, the insulation resistance, capacitance and dissipation factor is measured, and at the end of the cycle, the conductive heat transfer thermal resistance is measured. Following thermal conditioning and initial two seven-day cycles of thermal ageing at 240°C, representative of approximately 20% insulation life, it was found that the mean equivalent thermal resistance between the winding body and stator back iron had increased 22% from 0.230 to 0.295 °C/W. This suggests that heat extraction from the winding body into the machine periphery deteriorates as the insulation system ages.