Common Mode current modeling in motor-drive systems

Using semiconductor switches with higher switching frequency in motor-drive systems provides less power losses, higher efficiency, and more control on the voltage and current in the system [1]. However, the rapid changes in the switches states generate a common mode (CM) voltage at the output of the inverter that leads to a CM current. This CM current is the main source of bearing currents, shaft voltage, and electromagnetic interference with the surrounding systems [2]. Also, CM current is the origin of the radiation of long cables in the motor-drive systems. These problems emphasise the requirement to an accurate and fast-solving method to compute the CM current in motor-drive systems. Most of the methods given in the studies on the CM currents in motor-drive systems are either based on characterizing the parameters of the cable model using experiments [3, 4], or on the finite element analysis of the cable [5]. These methods are usually too time consuming, and they are not applicable without performing the measurements. Here, a simple and time-efficient method is presented to compute the CM current which is based hod given in [7]. The system under study is a motor-drive system consisting of a three-phase motor, a three-phase PWM-fed inverter, and a four-wire shielded cable. The focus is on computing the CM current in the motor-feeding cable, because this cable is the main path of the CM current to flow between the inverter and motor. In the proposed model the shield of the cable is modeled as discretized cylindrical conductors, and each conductor is treated as a line conductor. MTL equations given in [6] are applied to compute the CM current in these conductors and the neutral wire. Also, different lengths of the cable are considered to investigate the effect of the cable length on the CM currents. The computed CM currents by the MTL model are compared to a full-wave simulation of the system in CST/ cable studio to validate the model. The results show a good agreement in comparison to the results acquired by the full-wave simulation. Moreover, the MTL model is solved about 60 times faster than full-wave simulation for long cables.