Ageing in aircraft electromechanical chain: design of thermal cycling bench for winding elements

The study of the reliability of electric motors is of growing interest in the aircraft industry. The electrification of the electromechanical chain towards hydraulicless and bleedless systems, as well as perspectives of electric or hybrid propulsion, increase the need to have higher power density. This pushes the industry to raise the voltage levels inside the aircrafts, increasing the likelihood of failure within cables, connectors, windings, bus bars and printed circuit boards. High voltage phenomena such as partial discharges, space charges and electric arcs are investigated within the Highvolt project. In those component, actual mission profiles of motors (ON, OFF and load or speed change) can cause cyclic thermal gradients, producing defects in the electrical insulation systems (EIS). Indeed, the differences between the coefficients of thermal expansion (CTE) produce thermomechanical stresses at the interface between materials assembled together. Motorettes, which are random-wound assemblies with organic insulation materials aimed at reproducing the EIS of motors, are prone to such stresses. Markers like partial discharge inception voltage (PDIV), capacitance, dissipation factor and insulation resistance are expected to be affected by thermal cycling because of the degradation of the EIS (delaminations, defects, creep). Failure modes of such ageing are studied here. In order to carry out an ageing campaign on motorettes under active temperature cycling (ramps of a few °C per second), a preliminary experimental bench is being built. Models simulating the behaviour of motorette with thermal gradients have been implemented in a commercial multiphysics finite element software. This work will be presented alongside the initial results on accelerated ageing tests on active temperature cycling.