New tools to determine the vulnerability of stator windings to voltage surges from IFDs

It is well known that repetitive voltage surges from modern inverter fed drives (IFD) may age the insulation of motor stator windings, even on motors rated 440 V, leading to premature failure. Insulation aging seems to be correlated to the partial discharge (PD) activity of surges with very fast risetimes and high magnitudes. The magnitude and the risetime of the surges for a particular motor are determined by a host of factors, including power cable length, cable insulation, grounding, IFD type, and motor surge impedance, Hence, the nature of the surges can not be estimated without direct measurement. This paper describes a portable instrument which can accurately measure the characteristics of a surge, even those with a risetime as fast as 50 ns. Measurements with this instrument can be carried out both economically and with minimal training. Also discussed is a new device which permits the easy measurement of PD during 100 ns risetime surges. The latter can be used to determine the discharge inception voltage of stators under actual steep fronted surge conditions