What is the voltage drop in the carbon and graphite brushes?

The voltage drop across carbon and graphite brushes in electrical machines like motors and generators is a parameter that can vary with the design, operating conditions, and materials used for the brushes. However, a typical value often cited for the voltage drop across a single brush is around 1 to 2 volts under normal operating conditions. This value is not fixed and can fluctuate based on factors like brush pressure, speed of operation, current density, and the specific electrical and thermal properties of the carbon or graphite material being used. It’s important to note that this voltage drop occurs due to the resistance offered by the brushes as the electric current passes from the rotating commutator (or slip rings in the case of an alternator) to the stationary external circuit or vice versa. Manufacturers aim to minimize this voltage drop to improve efficiency but must balance this against other considerations like brush wear, heat generation, and overall system reliability.