Types of Carbon
There are five main groups which differ in use of raw material and manufacturing process:
1. Hard carbon brushes
The most important raw materials are amorphous carbons like retort coke and petroleum coke, bonded with pitch or resin, the raw materials are pressed and kilned at temperatures up to 1200º C.
The kilning process carbonises the binder and a relatively hard carbon is produced with much better polishing qualities than electrographite grades. Therefore, hard carbon brushes are mostly used on small machines with flush mica which require strongly polishing brushes. The permissible electrical and thermal load for hard carbon brushes is rather low and their elasticity limits their use to machines with peripheral speeds of up to 20 m/sec.
2. Carbon-graphite brushes
Electrographite, the most important raw material, and the binder pitch or resin are pressed and heat-treated similarly to hard carbon. The principle field of application for carbon graphite brushes is, therefore, intermediate between those of electrographitic and hard carbon brushes. Their polishing properties are better; their electrical carrying capacity is lower than in the case of electrographitic brushes. Carbon graphite brushes are also characterized by outstanding commutating abilities as a result of their high contact drop. They are used on machines where electrographitic brushes cannot be used due to their low polishing ability, and where hard carbon brushes are unsuitable because of their lower commutating properties. Carbon graphite brushes are mainly used on F.H.P. motors with mica undercut and on small thyristor fed DC machines.
3. Resin-bonded brushes
In principle, the raw material is graphite, bonded with resin, which is pressed and heat treated in a special process. The advantage of special graphite brushes is their high contact drop and high internal resistance. They also have good oxidation resistance. These properties are very valuable for machines with high commutating requirements. It is to its disadvantage that the high contact resistance causes high electrical loss due to circulating current. This might lead to destruction of the resin bond. Therefore, the main field of application for special graphite brushes covers all machines with high commutating requirements, but with relatively low brush current. These are three-phase commutator motors and small machines with mica undercut. Due to their low elasticity, special graphite brushes are normally used only up to 35 m/sec.
4. Electrographite brushes
The raw materials petroleum coke or retort coke are subjected to processes similar to those described under 1 to 3 above and then further heat-treated at temperatures up to 2500ºC. This converts the carbon material into a micro- crystalline form of graphite, called electrographite, a material having good electrical and thermal conductivity. Its elasticity and refractoriness are improved and its polishing properties are generally lower than hard carbon or graphite brushes. Depending on the composition of the basic materials, brushes having widely variable characteristics can be produced. Electrographite brushes, therefore, cover a wide range from the robust traction brush to very elastic grades having outstanding commutating abilities. Because of all these characteristics, electrographite grades are found in all classes of service, though they are mainly used on large commutator machines with high electrical, thermal and mechanical requirements.
5. Metal graphite brushes
Raw materials are metal graphite mixtures which are processed on a powdered metal basis. The metal most frequently used is electrolytic copper. For certain applications also silver, tin, lead and other metal powders are used. The behaviour of metal graphite brushes is intermediate between graphite brushes and metal brushes. Depending on the proportion of graphite and metal, either the good frictional qualities of graphite or the high conductivity of metal predominates. Contact and internal resistance are relatively small. From this point of view, this group is the opposite of the special graphite class. Consequently, the main field of application cover all types of machines with high brush loads, with or without high commutating requirements, such as slip rings and low voltage machines. Owing to their low elasticity, metal graphite brushes can only be used at peripheral speeds up to approximately 30 m/sec.