Investigation of equivalent stator-winding thermal resistance during insulation system ageing

High performance electrical machine operation is limited by the losses generated in the machine and how well the heat developed by these losses is extracted. For conductive heat transfer from the main winding body, the insulation materials provide the main heat transfer pathway. As a machine ages through its lifetime, these materials will change their properties, and therefore these heat transfer characteristics will also alter. This paper describes the methodology and measurements on a set of motorettes undergoing an insulation ageing process to identify the change in heat transfer characteristics. The motorettes are subjected to a combination of thermal, electrical, mechanical and humidity stresses, before their thermal performance is measured. It has been found that the equivalent thermal resistance of the motorettes over the nominal lifetime of the insulation increases by over 100%, with the current carrying capability of the machine reducing by over 30% across the machine's lifetime. It is therefore recommended that this is taken into account during the initial thermal design of the machine to ensure it remains fit for purpose during its complete life.