Project developing electrically conductive lubricants to protect electric motors from discharges in the bearings
A joint research project, funded by the German Federal Ministry for Education and Research, is investigating the development of ionic-fluid-enhanced electrically conductive lubricants to protect electric motors from the surface damage that can result from electrical discharges in the bearings (electrical discharge machining, or EDM). The initiative was launched to prepare for future vehicles which will require higher voltages than current models.
At present, 12 volts provide all conventional automotive electric systems—from lights and radios to air conditioners—with sufficient power. Within the next few years, the figure may rise to 48 volts to support a growing number of functions. The voltage levels of electric and hybrid vehicles are even higher: these vehicles can require as much as 400 volts. Higher voltage levels result in stronger alternating electric fields in alternators and electric motors, explained Dr. Gerd Dornhöfer, a Bosch scientist taking part in the “SchmiRmaL” project (Switchable intelligent tribological systems with minimal friction losses and maximum lifespan).
This can cause electrical discharge in the ball bearings of motors and alternators (bearing currents). When this occurs, resulting sparks can melt tiny areas of the metal’s surface. This, in turn, leads to uneven raceways. As a result of this, the ball bearings first begin to make noise, and then to malfunction too soon.
The purpose of the new lubricants is to prevent that damage from happening.
When a current is generated between the shaft and housing of an electric motor, the lubricant coating in the bearing acts as an insulator. As rotation speed increases, the lubricant greases in the ball bearings separate the bearings from the raceway. This is comparable to hydroplaning on wet roads. Unlike on roads, however, this phenomenon is desirable in ball bearings, as it minimizes the friction generated by the bearings as well as the surface damage.
However, this can also lead the bearings to recharge when the lubricant film is intact, similar to a capacitor. When the built-up voltage is sufficient, it can penetrate the insulating lubricant grease. This energy suffices to briefly melt a tiny area of metal on the bearing’s surface. If this happens repeatedly, tiny imperfections—electrical pitting—eventually appear on the bearing.
In the future, the energy of these discharges may become greater as the power density and voltages of automotive electric systems increase.
The SchmiRmal project’s strategy focuses on developing new lubricants which remain conductive even at higher voltage levels. As a result, these lubricants do not act as insulators; voltage levels no longer build up, nor does potentially destructive electrostatic discharge.
This can be achieved in several ways. One could, for instance, add fine metal particles to the grease to conduct the current. But this would mean that the lubricant grease would also act as an abrasive, and of course we want to avoid this.—Dr. Gerd Dornhöfer
The SchmiRmaL project is pursuing the use of ionic fluids—which conduct electricity—as additives to the lubricant. The initial material was a commercially available industrial lubricant. By using the right ionic fluids combined with conductive carbon, its resistance can be reduced by a factor of ten million,, Dr. Dornhöfer said. This is enough to prevent the unwanted electrical discharges.
While the new grease is black, it otherwise largely resembles its predecessor. The project team currently is investigating all of the grease’s characteristics. It must be heat resistant and have good cold flow properties; the additive should not compromise the grease’s corrosion protection properties. The new grease myst also not pose a hazard to human health or the environment. Findings have so far been promising.
The project is set to run until April 2015.
The new lubricants can also increase the service life and reliability of machine elements that experience high levels of strain, especially roller and plain bearings and transmission components. Moreover, performance can be improved for motors of the same size, or maintained if motors are smaller. At the same time, the lubricants contribute to reducing energy consumption and to increasing efficiency.
The project participants and areas of contribution include:
- Klüber Lubrication SE & Co. KG, a leading lubricant manufacturer
- IoLiTec-Ionic Liquids Technologies GmbH (Heilbronn), ionic liquid developer;
- Schaeffler Technologies GmbH & Co. KG, rolling bearings;
- Inprotec AG, abrasion-resistant coatings;
- SCHUNK GmbH & Co. KG , valve durability;
- Fraunhofer Institute for Algorithms and Scientific Computing SCAI (Sankt Augustin), forecasting the potential environmental impact of new ionic liquids;
- Fraunhofer Institute for Mechanics of Materials IWM (Freiburg im Breisgau), investigating the potential lubricating effect of ionic liquids; and
- Bosch, application and testing under real-world conditions.
“Bearing currents in modern AC drive systems”, Vector, April 2014
Heino, Toni (2014) ”Bearing Currents and Their Mitigation in Frequency Converter-driven Induction Motors”