Destruction by charge current and its solution for PWM inverter feeding multiple motors through long cables

There are two significant problems exhibited in the PWM inverter feeding multiple motors through long cables with light loads. The first issue is the destruction of the inverter. In this case, the high-frequency charge current is identified to be the principal cause. The second issue concerns the tripping of the inverter with the over voltage of DC bus. The common-mode current, in this situation, feeds the power to the DC bus and causes this inverter tripping. This paper investigates into these problems through a case study in a textile industry. From the site measurement, the effects of the charge current on cable insulation and snubber capacitors are examined. To study the behavior of the charge current, a high-frequency model of the long cable is represented by an equivalent lump-circuit. Also, the mechanism of boosting the DC bus voltage is considered. It is revealed that the returning common-mode current at the inverter front-end, through the utility, can carry the power to charge the DC bus. A passive LC filter is finally designed and installed at the inverter's output to mitigate the charge current. On-site experiment with a 50 Hp inverter shows that the designed filter can effectively reduce the peak of the charge current to 1/10 and prevent the inverter from overheating. By bypassing the common mode current through a circulating path inside the inverter, the balance of power between the power from the common-mode current and the filter's resistive dissipation is obtained, and the over-voltage tripping of inverter can be avoided.