Conductive Bearing Grease for VFD EDM Damage Reduction
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Solution Overview
Problem
Electrical energy discharges in bearings, particularly in motor systems driven by variable frequency drives, cause damaging voltage build-ups and premature failure due to high dv/dt issues with Si-based and SiC-based power electronics, which existing mitigation techniques fail to adequately address.
Innovation Solution
A conductive grease formulation incorporating graphene nanoplatelets is used to reduce electric discharge energy between bearing races, lowering the threshold voltage of pulse discharges and thereby decreasing energy release and preventing damage, while maintaining suitable grease composition for long bearing life at operational temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional bearing grease is used, then the bearing provides adequate lubrication, but electrical discharge energy exceeds damaging thresholds causing EDM pitting
Solution Approach 1:
The patent changes the electrical conductivity parameter of the bearing grease by incorporating graphene nanoplatelets. This transforms the grease from electrically insulating to conductive, enabling it to dissipate electrical charges and reduce discharge energy below damaging thresholds while maintaining lubrication properties
Solution Approach 2:
The patent creates a composite grease material by combining conventional lubricating grease with graphene nanoplatelets. This composite provides both lubrication properties from the base grease and electrical conductivity from the graphene, simultaneously addressing both lubrication and electrical discharge protection needs
2Productivity
If SiC-based power electronics are used, then switching frequency and efficiency improve, but dv/dt rates increase causing higher CM voltage and bearing currents
Solution Approach 1:
The conductive grease acts as an intermediary between the bearing races, providing a controlled electrical path that dissipates bearing currents generated by high dv/dt switching. This mediator protects the bearing surfaces while allowing the high-frequency SiC switching to operate
3Object-affected harmful factors
If bearing conductivity is increased to reduce voltage build-up, then electric discharge energy decreases, but bearing lubrication performance may deteriorate
Solution Approach 1:
The composite grease formulation combines lubricating base oil with small amounts of conductive graphene nanoplatelets. This composition achieves adequate electrical conductivity for charge dissipation while maintaining the lubrication properties of the base oil, preventing both voltage build-up and lubrication deterioration
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The conductive grease effectively lowers electric discharge energy below damaging thresholds, extending bearing life and preventing EDM pitting, even under high dv/dt conditions from Si and SiC inverters, with experimental validation showing significant reduction in shaft voltage and bearing current.
Implementation Method 1
The higher conductivity of the inventive grease results in lowering of the threshold voltage of pulse discharges
Implementation Method 2
Electrical controllers for motors and other electrical machinery can cause an electrical voltage to build up on the motor shaft due to electrostatic energy stored in motor capacitances
Data Source
AI summary
The present disclosure provides a formulated conductive grease for an associated bearing that can be used in a drive system, such as a variable frequency drive system for motors having a formulation sufficient to reduce the voltage build up that causes damaging electric discharge machining (“EDM”) on rotational bearing supported surfaces, such as inner and outer races, while still maintaining a grease composition suitable for bearing long life at operational temperatures. The higher conductivity of the inventive grease results in lowering of the threshold voltage of pulse discharges, which consequently decreases the energy release in each pulse and the resulting bearing temperatures, avoiding bearing damage. The disclosure further provides a bearing lifetime prediction model given typical discharge conditions from both Si and SiC inverters using a bearing state of health (SOH) metric.