Experimental measurement of shaft voltage and bearing current in an inverter fed three phase induction motor drive

With Pulse Width Modulation (PWM) inverter driven induction motors (IM) both internally and externally sourced bearing currents may exists. Currents flowing through induction motor bearings have the potential of creating premature bearing failures. IM powered by using PWM voltage source inverters (VSI) will generate high frequency noise voltage with respect to ground (gnd). Due to fast switching action of the inverter devices these high frequency noise voltage transients induce capacitive coupling to the gnd and hence the capacitive currents. These currents from the rotor will flow through the bearing to the gnd. The various paths of capacitive Current are generally through magnet wire insulation, stator slot liners, motor air gap, bearing grease and stator slot top sticks or wedges. With PWM inverter driven IM, the high frequency pulses bridge the stator winding star point and rotor by capacitive action to induce a shaft voltage. The generated induced voltage causes a current flow to the gnd through bearing due to which electrical discharge machining (EDM) takes place in the bearing. This EDM produces fluting in the inner race of the bearing. This paper presents experimental methods of measurement of the shaft voltage and bearing current for modified 3-phase squirrel cage IM connected to an inverter bridge. Experiments have been carried out on 3-level inverter fed IM drives in space vector modulation (SVM) scheme. PIC Microcontroller was used to generate SVM pulses along with other associated electronic interface circuits to operate the 3-level inverter bridge. Necessary converter circuits were fabricated and tested for giving the proper DC voltage to the inverter bridge. Standard current probe, LISN, high frequency 4-channel Mixed Signal Oscilloscope (MSO) with differential probes were used to measure the shaft voltage, bearing current and other parameters. For comparing the results with Federal Communications Commission (FCC) and Special Committee on Radio Interference (CISPR) standard, graphs were plotted showing Frequency vs shaft voltage in dBμV and the bearing current in dBμA using signal analysis software for the 3-level (SVM) and compared with the 2-level (SPWM) inverter published results.