IMPROVE PREDICTABILITY OF WIND TURBINE MAINTENANCE 2

Market Classifi cation
Almost 10% fan slip rings in the market are fiber brush slip rings,
which need little maintenance. Most slip rings (about 60%) are made of composition metal brush and generally are power feed contact sintered from metal and graphite. Even if the metal brush slip rings have a working life of 75 million revolutions as claimed by its manufacturer, they may require frequent maintenance. Single wire precious metal brushes (usually made of gold) are adopted for the other 30% fan slip rings. Both the composite metal brush slip ring and the single wire brush require more maintenance than that of fiber brush slip ring.
The cause lies in: composition electric brush is designed to firstly wear electric brush in order to avoid abrasion of slip ring and so long electric brush will furthest prolong replace period. Such material encounters three challenges on large power and/ or high reliability slip ring application:
1)  The running abrasive dust has electrical conductivity, abrasive property and powder waste, which will be periodically cleaned;
2)  The brush material is very sensitive to humidity; if relative humidity is less than15% or more than 85%, it will cause unequal abrasion;
3)  The composition electric brush occupies large space when used for signal class circuit.
Comparable to composition electric brush design, the single brush metal electric brush slip ring has some advantages, less abrasive dust and higher capability of signal circuit, however the single wire still has disadvantages:
1)  The single brush has limited current-carrying capability for less dimension;
2)  For example, most contacts between golds need lubrication and it is very difficult to keep complete lubrication all along.
Nowadays in the market, there are graphite/ metal electric brush slip ring for drive circuit and gold single-wire electric brush slip ring for signal circuit. Such “Mixed Using” can reach acceptable result for application of low revolution requirement of slip ring assembly, however the brush mixed with “Graphite/ metal” and “Gold contacting gold” in the same assembly is not reliable for fan running. Graphite/metal abrasive dust pollutes gold signal contact, produces abrasive property metal powder and especially when contact lubricant causing early invalidity of signal circuit happens.
The fiber brush is designed for compact fibrous “Brush” bound from several metal fibers. In general, such fiber and single wire can be designed equal noble metal and the slip ring for brush running is plated noble metal. The noble metal is used to prevent oxide and coverage layer on the contact and ensure tiny contact power. Less contact power drops abrasion speed and the abrasive dust produced from electric brush can be almost neglected. What’s more, several metal fibers can provide good electrical conductivity and very high current density and engineers can put fiber brush for drive circuit and signal circuit.