Brushes in graphite
Graphite for electrical use is produced through a mixing process of various powders determined by its final use. Synthetic graphite is the basic compound, common to all categories of graphite. It is produced industrially under a process of purification where all volatile materials (sands, pyrite) in the feedstock are driven off. Successively, synthetic graphite is grinded, sifted and mixed with other compounds such as tar, resins, metallic powders that give to the mixture specific characteristics. The mix are then compacted in small blocks, ready to be process according specific use (size, shunts). Brushes and current collection strips supplied by Cetec are classified as below:
1. ELECTRO-GRAPHITE QUALITY
Electro-graphite is mostly used on CC motors of any electric power . Brushes in electro-graphite can be used from 8 to 12 A/cm2 with temporary maximum limit of 20-25 A/cm2. They fit for peripheral speed ≤ 50m/sec. Electro-graphite has good commutation properties. Electro-graphite brushes have a medium voltage drop and a medium/low friction coefficient.
2. GRAPHITE QUALITY
Graphitic brushes are mostly used on steel rings of asynchronous machines or synchronous with high peripheral speed machines (turbo-alternators) Graphitic brushes are able to cushion efficiently impacts and mechanical vibrations due to the fact that they are soft quality brushes. They bear a current density from 10 to 13 A/cm2 and peripheral speed up to 100m/sec.
3. CARBO-GRAPHITE QUALITY
They are brushes produced with a mix of natural graphite powder and carbon powder. With high hardness benefits, those brushes have good commutation properties also with medium voltage drops and significant temperature resistance. They are mostly used on power tools’motors or motors with low peripheral speed. They bear a current density from 8 to 16 A/cm2 and peripheral speed up to 25m/sec.
4. METAL-GRAPHITE QUALITY
For this category, metallic powders are added in the preparation of the mixture. Production process requests to cook the mixture under controlled atmosphere and adequate temperature to confer solidity and molecule cohesion. Brushes in metal-graphite have a higher conductivity, a low electrical resistance and a low friction coefficient. They are mostly used on electrical machines with continuous current and low voltage, on bronze rings of asynchronous motors, industrial current collectors, asynchronous machines motors with low peripheral speed. In permanent current flow, they bear a current density from 12 to 30 A/cm2 and can sustain current up to 90A/cm2. They can tolerate a peripheral speed up to 35m/sec. This category of metal graphite includes the silver-impregnated quality. This quality has an excellent electrical conductivity and therefore is used on machines and installation that require a minimum current drop. For instance, silver-impregnated graphite are requested for dynamo-tachimetric, in transfer systems (signals and measures).
5. BAKELITE-GRAPHITE QUALITY
The bakelite-graphite quality is obtained by mixing natural or synthetic graphite with synthetic resins. Those types of brushes are obtained by molding. They are brushes with high mechanical and electrical resistance. They can work with low current density. For this reason, they are installed on motors with variable speed, on current convertors, auxiliary exciters. The current density limit is variable, depending on the type of the resin. Generally those qualities have a density between 8 and 13 A/cm2. In any case, the peripheral speed can not exceed the maximum of 40m/sec.