Carbon brushes are essential components in DC motors and generators that make electrical contact with the rotating commutator. They serve critical functions that ensure reliable operation and manage the transfer of current.
Key Reasons for Using Carbon Brushes
The primary reasons for using carbon brushes on a commutator, as highlighted by sources like Testbook, revolve around managing wear and handling the current during the commutation process.
1. Reducing Wear and Tear
One significant advantage of using carbon is its ability to reduce the wear and tear of the commutator. Carbon is softer than the commutator segments (typically made of copper). This means that wear occurs primarily on the brushes rather than the more expensive and difficult-to-replace commutator. Brushes are designed to be easily replaceable, serving as a sacrificial element to protect the commutator's surface.
2. Facilitating Current Transfer During Commutation
Carbon brushes play a crucial role during commutation, the process where the current direction in the armature coils is reversed as they pass under the brushes.
According to the reference, when a brush is transitioning between commutator segments, it shorts out that particular coil connected to those segments. During this moment:
- The brush draws current from the rest of the coils.
- If a significant electromotive force (emf) were induced in the shorted coil at that exact moment, a huge current would flow, potentially causing arcing and damage.
Carbon, due to its specific properties like contact resistance and self-lubricating nature, helps manage this transition. It provides a relatively low-resistance path for normal current flow but its properties can help mitigate the effects of transient currents during the brief short-circuit period, although managing induced emf often requires additional design features or brush placement adjustments (like brush shifting or interpoles/compensating windings in larger machines). The material characteristics of carbon are well-suited for handling the sliding contact and current transfer requirements during this dynamic process.
Other Beneficial Properties of Carbon Brushes
Beyond the points mentioned in the reference, carbon brushes offer other advantages:
- Self-Lubricating: Graphite-based carbon has a layered structure that provides self-lubrication, reducing friction and heat generation at the contact point.
- Good Conductivity: Carbon is a good electrical conductor, allowing efficient transfer of current to or from the armature coils.
- Thermal Stability: Carbon can withstand relatively high temperatures generated by friction and current flow.
In summary, carbon brushes are chosen for commutators primarily to minimize wear on the commutator and to handle the complex task of current transfer and coil shorting during the commutation process effectively.
