Influence of Cable Insulation Structure on Terminal Over-voltage of Inverter-fed Motor

With the applications of AC adjusting speed drives, the premature insulation failure of PWM inverter-fed motor appeared. One of major reasons for motor insulation failure was sharp pulse over-voltage on the terminal of motor (or the end of cable) because of wave reflection on the end of long cable. The purpose of present work is mainly to investigate the influence of cable insulation structure on its wave impedance, wave speed and terminal over-voltage of motor. The distributed inductance and capacitance of six representative cables were calculated by the finite element method, and then the wave impedances and wave speeds of cables were gained. Based on these characteristic parameters, one cable with three circuit conductors and one concentric grounding conductor was sorted out, and suitably used for inverter drive system. The insulating structure of this cable was optimized with orthogonal design method. The analyzed results showed that the dielectric constant of insulating material is a most important factor to improve the characteristic parameter of cable. It is also suggested that the foamed or/and magnetic filling insulating material are used for the inverter-fed cable. When the insulating material with the foamed and magnetic filling material is applied, the over-voltage on the terminal of motor with the power of 75kW will decrease by 34%.