Endurance testing of rotating machines insulation systems: Do sinusoidal and square voltage waveforms provide comparable results?

According to International Electrotechnical Commission (IEC) technical specification (TS) 60034-18-42, to ensure the insulation system reliability of inverter-fed form wound motors, endurance tests should be performed on formettes to compare new insulation systems with proven ones. IEC TS 60034-18-42 makes reference to sinusoidal testing, as it assumes that sinusoidal and square voltage waveforms will provide comparable breakdown times if they have the same frequency and peak-to-peak voltage values. Given this premises, it becomes crucial to assess this assumption and, in case, provide the limit values for square voltage waveform parameters (i.e., rise time) that can provide results that are comparable with those obtained using sinusoidal waveforms. Therefore, the effects of square voltages waveform parameters on life tests are discussed in this paper. The tests have been performed on magnet wires, as their insulation is similar to turn insulation of form-wound machines. Partial discharge (PD) data were recorded during the tests resorting to a ultra-high frequency system and are used to explain the influence of square voltage waveform on life test results. Based on the experimental results obtained in this study, when performing endurance tests on inverter-fed motor insulation systems, square voltages waveforms having appropriate parameters should be selected to get objective endurance test results.