Experimental Measurement and Comparison of Common Mode Voltage, Shaft Voltage and the Bearing Current in Two-level and Multilevel Inverter Fed Induction Motor
Keywords:
CM voltage, shaft voltage, bearing current, SVM scheme, 2-level inverter, multilevel inverter, induction motor.Abstract
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). 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. The flow of bearing current leads to premature bearing failures and the EMI affects
the communication and measuring systems / circuits. The influence of different parameters of a variable speed drive system on the phenomena of inverter-induced bearing currents has been studied earlier under exactly the same conditions on inverter-operated A.C motor. Detailed modeling may not always be possible with practical Applications in the field, where many parameters might be unknown. Therefore, this paper presents experimental methods of measurement of the
common mode voltage,shaft voltage and bearing current for a modified 3- phase squirrel cage induction motor (IM) connected to an neutral point clamped (NPC) inverter bridge. Experiments have been carried out on 2-level and multi-level inverter fed IM drives using space vector modulation (SVM) scheme. Microcontroller was used to generate SVM pulses
along with other associated electronic interface circuits to operate the inverter bridges. Necessary converter circuits were fabricated and tested for giving the proper DC voltage supply voltage to the inverter bridge. Standard current probe, LISN and high frequency 4-channel Digital Signal Oscilloscope (DSO) with differential probes and were used to measure the shaft
voltage, bearing current& other parameters. 4 Channel Mixed Signal Oscilloscope (MSO) was used to record the digital signals from the µcontroller. As per Federal Communications Commission (FCC) and Special Committee on Radio Interference (CISPR) standard, graphs were plotted showing Frequency vs Common mode voltage,shaft voltage in dBμV and the bearing current in dBμA using the signal analysis software
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