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Carbon brushes in railway applications

This current transmission technology is over a century old, but in no way obsolete, even where the latest traction vehicles are frequently equipped with brushless motors. Carbon brushes establish electrical contact to the rotating component of a machine via sliding rings or collectors. Railway applications in particular place an extremely high demand on carbon brush sliding contacts. These include brief overloading, large current peaks, longer idling, weak load operation, aggressive gases and vapours, high ambient temperatures, oil mists and high ambient dust and ash content. Air humidity also makes conditions more difficult.

In addition to block brushes, some other carbon brush versions have established themselves over the years. They adapt carbon brush sliding contacts to suit special electrical and mechanical conditions and thus improve operating characteristics. The most important basic versions include:

Twin and triple carbon brushes, where contacting conditions are improved by two or three brush sections of equal size set behind each other in a tangential plane. Machines running in reversing mode thus operate with greater stability.

Split carbon brushes with a spreading top are a special form of twin brush in which both brush sections have their upper surface inclined towards the middle of the brush. During operation characterised by vibration, increased friction between the carbon brush and the wall of the holder (damping) caused by spreading improves contact with the commutator.

Sandwich-type carbon brushes are designed for situations where commutation is difficult, to avoid spark formation, bar scorching or heavy wear.

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