Tribological characterization of a ball bearing subjected to an electric field : Electric drivetrain tribology

Electric machines are widely used in for instance the automotive industry in electric vehicles and in wind turbines. The electrical machines have mechanical bearings as an integral part used to transmit power and load. In addition, the main function includes reducing friction between interacting surfaces. However, it is one of the most failing machine elements in these machines. To improve operational sustainability and reduce maintenance costs, understanding bearing failure mechanisms under electrical influence is important. One of the main reasons of bearing failure is linked to high frequency power switches typically used to enhance electric machines’ efficiency. The increase in switching rate induces more frequent common mode voltage fluctuations making the system vulnerable to bearing currents. A small voltage difference of a few ten volts can induce significant electric stress on the bearing depending on the lubricant film thickness and related tribological parameters. The electric charge build-up leads to electric current conduction (arc discharge which happens when the voltage exceeds the breakdown voltage) ultimately damaging the bearing. There are different mitigation strategies which are used to restrict bearing currents through grounding or using completely insulative bearings such as ceramic ones. However, at the moment, there are no satisfactory solutions and there is a need for efficient and economical solutions to the problem. On the other hand, various filters are used to reduce the amplitude and its frequency of bearing currents, but they only partly solve the problem. Similarly, the insulative surface coatings provides high electrical resistance but start acting as capacitors. At a sufficiently high voltage difference, the current passes through the system. Therefore, mitigation strategies are still being explored to improve system performance and service-life. To understand the bearing discharge activity and electrical breakdowns, an electrified