How to calculate current carrying capacity of Carbon Brushes

How to calculate current carrying capacity of Carbon Brushes:

For many years inventors were trying to transmit current from a stationary body to a rotating body. It was the main obstacle in rotating machinery. Finally, they found out a way. Initially, it was done by bundling copper wires and fixed than with a stationary body and keep contact with the rotating body. In this way, the copper bundles looked like a paintbrush. And the word brush came from there.

With the development of knowledge and technology and after years of experiment finally, a well-structured process invented. Now in almost all modern technology different grades of carbon blocks are used as the transmitting medium. Though they don’t look like the brush as a tradition, they are well known as carbon brush.


With the help of brush holder and spring, the remain touched with the rotating body and transmit current.


Current carrying capacity is normally expressed as amps per square cm or amps per square inch. Current density is simply the value of current that is passing through per unit area of carbon brush.

Learn More:   Electrical Substation (SS)

There are some points that affect the current density of carbon brush such as temperature, vibration, nature of operation continuous or intermittent, speed etc.

Current density in a carbon brush using in DC motor can be calculated by the following equation:

How to calculate current carrying capacity of Carbon Brushes
In Brush circuit contact area from the Total number of brushes, half brushes are used to carry in the current and half brushes are used to carry out the current.

For bottom angled brush the contact area is not just width X thickness but the contact area should be taken into account.

A carbon brush should not be operating at low current density. A low-density operation is more dangerous than over density operation.


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