More Power For Electric Drives With Hybrid Bearings

Electric drives, for example in buses, trams and trains or in the automotive industry, are real power packs today. The components installed in them must withstand extreme temperatures, reach high speeds and enable strong accelerations. External influences such as storms, rain, mud or snow should not harm them. Reliable lubrication must also be ensured so that the electric drives work for a long time. In view of such a requirement profile, rolling bearings with standard components in electrical applications are increasingly reaching their limits.

Arch enemy current continuity

This applies all the more, since conventional bearings can cause electrical continuity due to operational reasons. This undesirable flow of energy takes place in the contact zone between the rolling elements and the inner and outer ring raceways. The result: electrical discharge machining. It damages both the metallic bearing components (such as the base and counter body) and the lubricant in the rolling contact.

Picture gallery

Picture gallery with 7 pictures

Practical experience from SKF has shown that spark discharge repeatedly heats up and cokes the lubricant too much (first picture in picture gallery). This reduces the lubricating performance and the service life of the lubricant.

In addition, the rolling elements and raceways of the rings of conventional steel bearings are covered over time by microcraters, which are accompanied by a structural change in the zone near the surface. The second picture in the picture gallery shows a steel ball strewn with microcraters, the surface of which appears gray and matt compared to an undamaged ball (right).

As a consequence of the continuity of the current, ripples may form on an inner ring raceway (third picture in the picture gallery). This increases wear, causes vibrations and is particularly noticeable through increased bearing running noise.

Downside of higher power density

To improve the efficiency and dynamic performance of speed-controlled drives, IGBTs (bipolar transistors with insulated gate electrodes) are used today. These fast switching power semiconductor components enable the desired pulse width modulated output voltage form.

However, there is also a disadvantageous effect: In addition to the conventional, low-frequency voltages and currents that flow in the motor during mains operation, parasitic, high-frequency currents also occur. These result, among other things, from the phase voltages at the converter output, which occur as a series of rectangular pulses. The sum of the three phase voltages is not zero, which creates a so-called "common mode voltage".

In addition, the voltage signals change not only with a high switching frequency (= frequent pulses), but also very quickly within an extremely short period of time (= partial pulses). So-called “dV / dt currents” arise due to the high rise rates of the voltage signals. Both phenomena lead to high-frequency bearing currents - and these can significantly shorten the life of "conventional" bearings.

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Content of the article:

• Page 1: More power for electric drives with hybrid bearings
• Page 2: Material mix as a solution

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