With the development of large power semiconductor devices, the technology of Pulse-Width Modulation (PWM) has been widely used in the field of locomotive traction. Insulation of the inverter-fed traction motor faces the continuous square wave pulses, it is found that a number of inverter-fed motors failed prematurely. Many research reports show that dielectric overheating and space charge and PD are the main factors which lead to premature failure of inverter-fed motors. In order to compare the similarities and differences, the insulation of inverter-fed motors is aged under AC and continuous high-voltage square pulse. The differences between AC aging and square wave pulse aging are studied based on change curves of tan δ and PD pattern. Thermally stimulated current (TSC) proves that square wave pulse bring more defects than AC to the insulation materials. Under square wave pulse condition, not only damage action of partial discharges is intensified with voltage, frequency and temperature increasing, but also damage action of space charges increase.
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