On specimens choice for thermal lifetime assessment of low voltage electrical machines insulation

The insulation of a rotating electrical machine is a complex system. Predicting its lifetime is an extremely challenging task, as it involves elements of chemistry, material science and electrical engineering. This lifetime prediction/assessment, if performed under standard “in-service” operating conditions, can require months (and in some cases years) of testing. Therefore, accelerated lifetime tests, performed on suitable specimens, are conventionally run for fulfilling such task, as they reduce the total testing time. During these tests, the stress level applied to the examined system (i.e. insulation) is enhanced with respect to normal operating conditions, with the aim of reaching the end of life in a relatively short amount of time. It is thus of crucial importance selecting an appropriate specimen's typology, as the choice might strongly influence the tests' outcome. In this work, the accelerated thermal aging results for three different specimens' typologies, i.e. twisted pair, motorette and coil, are presented and apprised. The tests are carried out on a round enamelled magnet wire, to be employed for the winding of low voltage electrical machines. Among the three specimens, it is found that twisted pairs tend to largely over-estimate the thermal lifetime, with respect to both motorettes and coils.