Common wear patterns on commutators
Looking at the wear patterns on a commutator is a fairly simple way of analysing the motor performance. When a motor is operating normally, the brush current will vaporise some of the copper molecules and deposit them as a copper oxide film on the commutator, this film is usually uniform tan to charcoal brown in colour. If you see faint charcoal streaks this may also be an indication of normal commutation. If the film appears to be heavy this isn't always something to worry about, especially if it is even and no areas are unusually dark or worn. What you need to be specifically aware of are the three conditions outlined below.
Slot bar marking or burning
If either the first or last conductor in the slot passes under the brush before or after entering the field of commutation you may see alternating light and dark bars on the commutator. In certain situations this is made worse, electrical overload, an incorrect neutral setting, contamination or an improper interpole strength can be factors. Resurfacing the commutator will be required if the trailing edges of the darker bars looks to be etched or burnt.
Most commonly found on motors using long-life brushes, a dark streak appearing along the brush path is an indication that metal is migrating from the commutator to the brush. Things to look for are insufficient brush tension (this should be checked as part of your routine maintenance), too light a load, or incorrect brush grades (too porous) can be responsible. Additionally streaking can also be caused by contamination from harsh chemicals such as chlorine, hydrochloric acid or silicon vapours. As always early intervention and correction of these factors can stop the condition progressing to the more severe Threading.
The transfer of excessive quantities of metal to the brushes during commutation is referred to as threading. Threading is present when the brush paths start to resemble the threads on a bolt. Threading may be a result of inadequate brush spring tension or too light a load. If ignored or undetected it is a condition that will result in rapid brush wear, if detected early enough the commutator can be machined in place to restore the finish.
Grooving is typically caused by atmospheric abrasive dusts, brush grades that are too abrasive can also result in grooving. With grooving the grooves tend to wear smoothly across the width of the brushes, with material from the commutator being worn away rather than being transferred to the brushes. As it progresses, the brushes are pinched by the sloped walls and tension is diverted from the brush face. The result is increased electrical resistance, with more heat being generated at both the brushes and the commutator. If grooving is not rectified is can lead to arcing between brush holders (flashover) which can seriously damage the brush holders, commutator or both.
When grooving is first detected, the first step is to check for and eradicate air born abrasive dust contamination, this may involve the use of filters or ducting to supply "clean" air. Which ever remedy is employed is it is vital to supply an adequate airflow. If the grooving progresses too far, you will be left with no option other than to replace the commutator and rewind the armature.