High Frequency Life Testing of Stator Coil Insulation

This paper focuses on the development of insulation systems for high-speed and hence high-frequency electric motors with multi-MW ratings. With the growing demand for variable speed applications, an increasing number of medium and high voltage (4-13.8 kV) motors are being powered by adjustable speed drives (ASD's). One aspect that must be understood during the design of the motor is the impact that the voltage waveform from the ASD may have on the stator insulation. Both high fundamental frequency components (300- 600 Hz), switching frequency components (low kHz range) and fast rise time transient voltages (in the mus range) from the ASD's can increase the thermal and electrical stresses on the insulation resulting in a reduction of the service life. To study the effect of these ASD's on an epoxy-mica based insulation system, accelerated life tests were performed at 60 Hz, 418 Hz and 10 KHz. The main purpose of this study was to determine if the number of 'electrical cycles' to failure was constant for a given system over the range of voltages and frequencies tested and to explore if low frequency testing could be used as a predictor of insulation system life at higher frequencies. This paper will present the test setup and procedure, as well as the analysis and interpretation of the data. An examination of the resulting relationship between applied voltage, frequency and life of the insulation system will include a discussion of the ageing mechanisms at the various test voltages and frequencies.