Wide bandgap inverter duty bearings : development and lifetime prediction of conductive grease bearings with integrated graphene nanoparticles.


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As society pushes toward a sustainable, electrified future, the superior qualities of wide bandgap (WBG) devices will enable the next generation of power electronics that allow for greater system efficiencies and reduced form factor when compared to conventional silicon devices. However, the integration of WBG devices into motor drive systems exacerbates the existing application issues surrounding pulse width modulated (PWM) variable frequency drives (VFDs) that lead to system degradation and costly premature failure. A significant application issue of WBG devices in motor drive systems is the premature failure of motor bearings due to an increase in vibration caused by electric discharge machining (EDM) resulting from inverter-induced, high frequency bearing currents. This dissertation presents a novel method to reduce bearing currents and mitigate EDM damage by increasing the conductivity of motor bearings via conductive bearing lubricant. A survey of commercially available conductive greases is conducted, and the possibility of increasing lubricant conductivity through the integration of graphene nanoparticles is explored. Novel conductive greases are developed from base materials, so that the effects of grease formulations can be precisely controlled and studied. Through extensive experimental validation, statistical analysis, and rheological characterization, the most promising conductive grease formulations are selected, and through the accelerated aging of conductive grease bearings, their lifetimes are predicted. The work presented in this dissertation includes key contributions to the development of WBG inverter duty bearings and conductive bearing grease, which contributes significant value by extending the bearing lifetimes in PWM inverter driven induction motor applications and preventing costly premature bearing failure.