Why do brush gear for DC machines utilize brushes made from carbon?

Brush gear for DC machines utilizes brushes made from carbon primarily because carbon reduces the friction at the commutator. In a DC machine, the brushes make contact with the rotating commutator, which is integral to the operation of the motor or generator.

When the brushes are made of carbon, they have a lower coefficient of friction compared to metals. This characteristic is essential because it minimizes wear and tear on both the brushes and the commutator, leading to longer operational life and reduced maintenance needs. Lower friction also helps in ensuring smoother contact and better electrical performance, which is crucial in achieving efficient operation.

While other materials might be less expensive or more durable, those factors are secondary to the primary concern of minimizing friction. Additionally, although carbon does have good electrical conductivity, it is not as efficient in this regard as copper or other metals, making this aspect not the main reason for carbon's usage in brush gear. The combination of low friction, adequate conductivity, and durability makes carbon an ideal choice for brushes in DC machines.