Air Contaminants That Affect Carbon Brushes, Motors, & Generators

A carbon brush is the part of the motor that conducts electrical current between the stationary wires (stator) and the rotating wires (rotor) of a motor or generator. A brush riding on a commutator develops a film, or patina, over the commutator. The film is mainly copper oxide that has formed on the commutator surface because of a reaction among copper, graphite, oxygen and other free particles within a range of temperatures. This film has a friction component and voltage rating (measured in the form of contact drop through the film). There are a variety of contaminants that may be found in the air that may cause problems with how the film forms on the collector and how it affects contact drop, operating temperature, brush wear, and slip ring or commutator wear.