Erosion can be the result of improper film on the commutator or a wear condition such as threading. ...
The condition of the commutator film directly affects friction and erosion and thus brush life. In order to achieve good brush life, the commutator must have good film.
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When electric current is passed between the carbon and copper in the presence of water vapor, a microscopic layer of copper carbon composite or film, is formed. Good film is chocolate brown or burnished bronze to dark brown or black and uniform in color. It is not bright copper or burnt black copper. Consult a commutator color and appearance picture chart to determine the condition of the commutator. There is a condition known as false filming in which brush graphite deposits become cooked on the commutator resulting in an appearance similar to dark film. Oil can also leave a coating which resembles film. If this film can be easily wiped away, it's not the desired good commutator film!
Commutator filming is a continuous process. That is, the film is continuously being formed and stripped away. A good film is only 200 nano inch thick (0.000,000,2 inch or 0.000,005,08 mm). Thus the conditions required to build good film must always be present. Changes in current, humidity, etc. will affect the commutator film.