Stator insulation of inverter-fed motor experiences continuous impulses with significant harmonic and transient. The insulation aging mechanisms are quite different under the effects of continuous impulse and under sine wave. Contrast aging tests are carried out on twisted pairs simulating inter-turn insulation and insulation to ground of inverter-fed traction motor under AC and square wave pulse conditions. The variations of tan δ concerning different aging factors with aging time and measuring voltage are investigated. The mechanism of a form of voltage and the turn-to-turn insulation to insulation dielectric loss and development are analyzed. The experimental results show that when measuring voltage is pulse voltage and the discharge occurring within the interturn insulation

partial discharge （PD） and space charge result in sharp increase of insulation defects. When no discharge into space charge repeatedly and extract insulation caused insulation injury. PD results in insulation heat under AC voltage

and space charge results in insulation heat under square wave pulse condition. Slot discharge occurs due to the special insulation structure of insulation to ground. Space charge is likely to be the key factor resulting in aging of insulation materials under continuous impulse.