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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 of different types of defects has been studied through several groups of breakdown test. Furthermore, fine 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.