Experimental investigation and measurement of common mode voltage in a 5-level inverter fed adjustable speed 3-phase Induction Motor drive

The Induction Motor (IM) is cheap and robust when compared to the other electrical motors. Hence it is one of the best choices to run the vehicles by IM. Since the vehicles have to start from rest it requires variable speed drives and one of the convenient method of speed control of IM is by the use of converter-inverter system. It is known that by using the inverter to generate three phase AC supply from a single DC source it introduces common mode (CM) voltage at the stator star point of the IM with respect to general ground (gnd)[1]. In addition high frequency switching noise pulses will be induced due to the fast switching of the inverter devices at the IM supply lines. This CM voltage and high frequency switching noise pulses are harmful and will be responsible for the flow of current through the bearing to the general gnd and also creates electromagnetic interference (EMI) problems respectively [1]. The flow of bearing current leads to premature bearing failures [2] and the EMI affects the communication and measuring systems / circuits. Hence the investigation and measurement of CM voltage is relevant to inverter fed IM drive for the Vehicle power propulsion. This paper discusses the investigation and Experimental measurement of the CM voltage and high frequency switching noise voltage under space vector modulation (SVM) scheme for the 5 level Neutral Point Clamped (NPC) inverter fed three phase IM drive with 500V DC to inverter input voltage. For easy comparison of results with FCC/ CISPR standards the experimental results are also plotted in frequency vs dBμV. In addition the experimental results of 5-level inverter are compared with that of already available [3] 3-level and 2-level inverter fed IM drive circuit results.