IGBT Pulse Width Modulation (PWM) inverter driven induction motors for adjustable speed applications have been widely used since their introduction. However, the use of PWM variable frequency traction motor can lead to premature failure of the winding insulation, which most often occurs due to breakdown of the interturn insulation. In order to investigate the mechanism and characteristics of dielectric breakdown in a new kind of polyimide film, which is used as primary insulation in PWM inverter-fed traction motor, an experimental activity has been carried out on specimen according to applications in locomotives. The effect o f different types of defects has been studied through several groups of breakdown test. Furthermore, line texture of the breakdown part based on energy spectrum analysis and stereoscan photograph is analyzed; finally, the characterization and mechanism of the dielectric breakdown are presented.

With the development of AC drive and power electronic elements, inverter supplies apply to W o n motors widely. At the same time, inverter mechanism brings on premature insulation aging in numbers of inverter fed motors all over the world. Insulation aging seems to be correlated with the partial discharges (PD) activity of surges with very fast rise time and high frequency. With the traditional method, it is hard to distinguish the PD signals from the surges. This paper introduces a new instrument, which can accurately measure the characteristics of PD during lOOns rise time surges. In addition, it can also be used to measure the partial discharge inception voltage (PDIV) and other characteristics of PD in inverter fed motors under local surge conditions. INSTRUCTIONSIn the past few years, rapid advances in power electronic components have lead to a new source of voltage surges [l]. Inverter Fed Drives (IFDs) of the PWM-type using IGBTs apply to traction motors widely. But these IFDs can create up to 20,000 fast risetime voltage surges per second, which lead to the electrical breakdown of the interturn insulation in motor windings.In lots of cases, the breakdown occurred at the first turn of windings of the motor [2-31.It is known that the reflected wave at the motor's terminal can lead to overshoot which is about twice to three times of normal condition, while the first turn of stator winding may take on about 90% of voltage under fast risetime voltage surges. Therefore, during the effect of fast cut-off surges, the first turn may take on over 20 times of voltage than normal magnitude. This overshoot is the main reason for breakdown of minor insulation in windings, especially of interturn insulation around the first turn of windings of variable frequency traction motor.It has been shown that the breakdown of minor insulation in windings is correlated to the partial discharge activity of surges with very fast risetimes and high magnitudes [4-61. But current partial discharge measuring systems are completely inappropriate for the measurement of partial discharges at PWM inverter fed motor, for the frequency spectra of the surges 6om the PWM inverters the frequency spectra of the pulses of partial discharges are found in the same measurement range and the surges are able to hide completely the signal of the partial discharges [7-91. In addition, only sine-shaped operation voltages are used in current PD measuring systems, which is very different from the IFDs [ll-131. This paper discusses a new PD measuring system, which has been developed to detect the partial discharge signals in traction motors fed by PWM inverter. That's a new tool to study the insulation aging caused by fast risetime surges. TEST CIRCUITThe fundamental circuit diagram of the experimental setup is presented in Figure 1. Unlike the PD measuring systems described earlier, this system is intended to be effectively and economically applied to monitor the partial discharges in inverter fed motors. Here, C, is the equivalent capacitance of the test s...

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