Common mode oscillations in electrical drive systems

Purpose – The purpose of this paper is to present an analysis of common mode oscillations of several kilohertz in electrical drive systems. The analysed oscillations occur especially in electrical drive systems with active front end (AFE), common DC link and long motor cables and are independent of the well-known reflection phenomenon. Owing to the resulting overvoltages, the motor isolation lifetime may be significantly reduced. Design/methodology/approach – For the analysis of the described problem, all parts of the common mode system of an electrical drive system are carefully modelled. This leads to an analysis of the frequency behaviour of the common mode system. The excitation mechanisms are also analysed and simulation in the time domain is performed to show the resulting overvoltages. Finally, measurements confirm the findings. Findings – The investigations identified the reasons for the oscillations: the common mode system behaviour, including the common mode resonant behaviour of some special kinds of motor. Furthermore, the excitation mechanism is found to be dependent on the modulation schemes of the AFE and the inverters. Accordingly, a special remedy concerning the modulation is derived and compared to other known remedies. The results of the simulations show the good efficiency of the proposed remedy. Originality/value – The presented results describe important basics for the development of electrical drive systems. By taking these issues into consideration, many unpredictable failures can be avoided.