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Commutator Surface Chart

Commutator Surface Chart


Over the entire commutator surface is one of the normal conditions often seen on a well functioning machine. Film tone is dependent on the brush grade.


Good condition. Film can be light to dark in colour but the important feature is that it is uniform and even. Normally, a good film will have a slightly polished appearance.


This non-uniform filming condition is quite common. The possible causes are: commutator roundness, brush contact pressure, unequal magnetic fields and chemical vapours all contribute to this type of film development.



Repeating light and dark filming patterns related to the number of armature coils per slot. This pattern is related to machine design and winding condition. A brush grade adjustment could help improve this condition.


of only the film is not detrimental to the commutator. Brush and commutator life are not at risk in this condition. This type of filming can be related to low electrical load on brush, high humidity, oil and aggressive fumes.


Bright spots in the film suggest poor contact or overloading. the resultant under-brush sparking tends to remove the patina and will eventually groove the commutator over a period of time.



is the erosion of the trailing edge of the commutator bar. Maladjusted electrical symmetry of the brush holders or a poor commutation. If not corrected, this condition can cause severe commutator damage or a flashover.


results in commutator erosion of every second, third, or fourth bar depending on the winding design of the armature. Brush bounce, Improper brush material, brush design or electrical adjustment of the machine can cause this condition. This condition severely damages the commutator and reduces brush life.


A streaky film with no commutator wear, tracks can vary in width and colour. Caused by atmospheric conditions (humidity, oil vapour or other gases) or insufficient load. If not corrected, it can develop into commutator damage.



Results in bars being eroded in a pattern related to 1/2 the number of brush arms, progressing into a pattern equal to the no. of brush arms. This condition is caused by an electrical disturbance such as an unbalanced armature, failed equalizers or a poor riser connection.


Is the uniform circumferential wear of the width of the brush, which is exhibited on the commutator. Excessive abrasive dust in the atmosphere, an abrasive brush and/or extreme light spring pressure (below 100g/cm²) can cause this condition. Proper brush applications and filtering the air on force ventilated motors can reduce the commutator wear.


Occurs when high energy transfers copper in a molten state. These particles become coated by contaminants from the surrounding environment or the brush treatment and do not oxidize properly to form the film on the comm. surface. These particles accumulate at the edge of the bar, eventually shorting across the insulating mica. Or, badly undercut segment edges.