Carbon is mostly available at the earth in the form of graphite. This contains about 90% Carbon. The armorphous carbon is found in the form of coal coke, charcoal, petroleum etc. The manufacturing process of electrical carbon products consists of grinding of the raw carbon materials, With the binding agent, mix the powered carbon (coal tar) moulding of requisite components and baking them. Copper or bronze powder is mixed with carbon moulding compound to increase the conductivity. Carbon has very high resistivity 4600 x 10-8 ohm-m in graphite form. It has negative temperature co-efficient of resistance, pressure sensitive resistance, and low surface friction. The main application of carbon is brush for DC motors. Some important properties of carbon are listed below:
Type of Brushes | Commutator brushes at velocity of 25 m /sec current density of 20 A/sq. cm and pressure of 0.8 kg/sq. cm | ||||
Kind of Brush | Hardness kg/sq.mm | Resistivity x ohm-m | Contact voltage drop / brush pair V | Maximum wear in 20 hrs mm | Max. Friction, co-efficient |
Graphite | 8 - 22 | 800 - 2200 | 0.6 – 1.8 | 0.4 - 0.5 | 0.3 |
Electro- graphitised | 3- 50 | 6- 50 | 1.1- 2.5 | 0.4 - 0.6 | 0.23 - 0.3 |
Carbon graphite | 1 - 42 | 40 - 50 | 1.5 – 2.5 | 0.1 | 0.3 |
Copper graphite | 4 - 25 | 0.0 - 15 | 0.1 – 2.2 | 0.15 - 3 8 | 0.2 - 0.26 |
