Effect of voltage and frequency cycling on the lifetime of enameled wires for low voltage inverter-fed motors

Electrical insulation system (EIS) of motors, for example in electric or hybrid road vehicles, experiences many cycles of varying stress of different kinds (thermal, electrical, mechanical). The standard methodology to qualify an EIS is through constant stress accelerated testing. The variable nature of the stress imposed in real situations may be an additional factor to be considered, yet is omitted in a constant stress test. Cycling of low voltage machines has not been a focus of many studies so far. A campaign of accelerated ageing tests of constant stresses and cycling is done on twisted pairs of PI (polyimide) enameled wire. The tests were achieved in an automated test bench that allowed periodic changes of voltage and frequency. High acceleration was assured by setting the ageing voltage over the Partial Discharge Inception Voltage (PDIV). Equivalent static and cycling test results are compared and no evident difference in lifetime is found so far for the tested materials.