Common mode current paths and their modeling in PWM inverter-fed drives

This paper investigates the paths of common mode currents in a PWM drive system. On the grounds of experimental results, a high frequency model of this system has been established. Separate analysis of the drive components and their contributions to common mode current flows have been done. On this basis, the authors have ascertained that heat sink-to-DC link capacitance is essential for creating the grounding currents paths. The large value of this capacitance constrains the CM currents to flow on a motor side of the system and reduces the conducted emissions measured on a supply side of the inverter (e.g. by means of LISN). In the case of a small value of heat sink-to-DC link capacitance a larger part of the CM currents can flow via mains, and the inductance of this path contributes to a creation of a low frequency part of the CM currents. In spite of lumped simplifications the model allows to predict the mechanisms of creations of oscillations of the CM currents in a real system with a good accuracy.