A simple, efficient and cost-effective approach to eliminate electrostatic discharge machining currents in wide-bandgap ineverter-fed motor drives

<p dir="ltr">The adoption of wide-bandgap (WBG) devices in motor drives, characterized by higher switching speeds and frequencies, significantly increases the risk of motor bearing degradation due to increased bearing voltages and currents. The bearing voltage is typically expressed as a ratio of the inverter common-mode (CM) voltage, denoted as bearing voltage ratio (BVR). However, recent studies show that the bearing voltage can be impacted by the motor neutral point voltage which can exceed the inverter CM voltage severalfold when high switching frequencies in WBG drives excite motor resonance, a condition less likely in conventional silicon drives due to their lower switching frequency limits. This paper provides experimental evidence that the BVR is more accurately presented as the ratio between the bearing voltage and neutral point voltage, rather than being defined in terms of the CM voltage. Since the bearing voltage is the primary source of bearing current generation, among which Electrostatic Discharge Machining (EDM) current is the major cause of bearing degradation, the root cause of EDM phenomenon and key factors influencing its occurrence and intensity are experimentally evaluated. A simple yet highly effective approach is proposed in this paper to eliminate the EDM currents, by increasing the equivalent high-frequency CM capacitance of the motor using a tiny lossless capacitor in the nano Farad range. The added capacitance is designed to optimally modify the machine antiresonance frequency, relative to the drive switching frequency, to reduce the bearing voltage below a critical threshold to eliminate the EDM current. The effectiveness of the proposed approach is experimentally verified using a 2.2kW three-phase induction motor fed by a wide bandgap two-level voltage-source inverter.</p>