A new method to control common mode currents focusing on common mode current paths produced in motor drive systems

A control method is studied which provides countermeasures for common mode currents, micro-surges and shaft currents appearing in motor drive systems. Flow of common mode currents differs between motor drive systems because common mode current paths are varied by the installation of the systems. First, EMI noise sources producing common mode currents are analyzed using simulations. Experimental results are compared with analyzed results, and the cause and the paths of common mode currents are clarified. It is found that common mode currents occur due to multi-series resonance phenomena with multi-noise sources produced by voltage fluctuations due to switching operations and micro-surges appearing at terminals of the AC reactor and motor. Circuit parameters for common mode current paths must be researched in order to suppress multi-series resonance phenomena. A simple method is proposed which is characterized by field of adjustment circuit parameters to suitable values to prevent electrical shock by remote control via the web. Effectiveness of the proposed method is verified through simulations and experiments. Further, the common mode currents leaking into the ground as shaft currents are studied. As a result, a method is proposed that controls both the micro-surge occurring at motor terminals and the shaft current using multi-layer power printed circuit technique. In the method, the second layer structure is important because the layer has the function to bypass and damp micro-surges. A decision method for parameters of the circuit composed of the second layer is studied from the standpoint of bypassing and attenuating the micro-surge component through theoretical and experimental analyses. Finally, suppression effects of the proposed method on the macro-surge and the shaft current are verified through experiments. Moreover, it is found through experiments that the proposed method satisfactorily attenuates radiated EMI noise appearing on motors.