Impact of different power cables on the common-mode current in variable frequency drive systems

In industrial applications, variable frequency drives are employed extensively because of their flexibility and efficiency. Variable frequency drives use a rectifier and an inverter to drive an induction motor with variable speed. Inverters generate an unwanted common-mode voltage. Motor and power cables present in the industrial systems provide a capacitive and inductive coupling to the ground and to the protective earth which can cause a common-mode current to flow because of the generated common-mode voltage. This common-mode current can cause malfunctions such as the unwanted trigger of the protective devices and disturbances to nearby systems. Therefore, the optimization of the power cables with respect of the parasitic capacitive and inductive coupling is helpful to reduce the common-mode current. In this paper, a reference setup is used to evaluate the impact of different cable topologies on the common-mode current. The impact of the common-mode current is evaluated with a classical cable model to define the reference. The measured common-mode current is compared with two other cable topologies with reduced capacitive coupling and symmetrical structure between the phases and the protective earth. A simulation environment is proposed to evaluate the capacitive and inductive coupling in the proposed cables. A common-mode model is proposed to simulate the common-mode current in the system. The common-mode model is able to compute the common-mode current up to one megahertz.