A good commutator will have a smooth polished surface dark chocolate brown in colour.
This is mostly the result of excessive sparking, The surface may be smoothened with fine sand paper on a curved block and polished with crocus cloth. Never use emery paper or cloth, as the fine emery particles may get to the commutator.
Ridge formation on commutator
Ridges are formed if the brush holders on different rocker arms are not properly staggered. Another possible cause in the case of large armatures is, if the armature does not oscillate to the extent of about 1/8 in. axially on either side of the core centre. The ridges may be filed off by a smooth file, or by means of specially shaped commutator grinding stones, whose curvature corresponds to that of the commutator.
If it is a generator, remove all carbon brushes and run it up, If it is a motor, run up to full speed, switch off the supply and quickly apply the grinding stone to the surface of the commutator, while the armature is slowing down. Quite a lot of metal can be removed this way if the work is properly done by a trained hand. After grinding is over, finish off with smooth sand paper and crocus cloth.
If the commutator surface is very bad, then the armature should be put on a lathe and skimmed, after checking the trueness of the shaft by a dial indicator. The cuts should be the lightest possible. The final cut should preferably be by a diamond tipped tool with fine cuts of the order of .0005 to .001 in. with a feed of 500 threads per in. Finally, mica should be undercut as usual.
Commutator is eccentric or having high bars and low bars
Excessive blackening or burning of a few commutator bars only shows presence of high or low bars, mostly caused by the bolts holding the commutator V-ring becoming slack. The bolts should be tightened up fully, after the commutator has run in for some time and is fully warm. After this is done, the commutator should be skimmed on a lathe. If only a portion of the commutator surface shows signs of severe sparking, it is a clear sign of the commutator being eccentric. No commutator should show a variation of more than 0.0254mm (.001 in.) when checked by a dial indicator, when mounted on its bearings on the motor.
Protecting mica between bars
All commutators should have the mica between the bars undercut to a depth of 0.8mm (1/32 in.) approximately. After grinding or turning of the commutator, undercutting should invariably be done either manually or by a machine. It is essential to round off the sharp edges of the bars, and remove all bars, as also to thoroughly clean the mica slots and blow off all dust. Undercutting should also be done periodically.
Flashover of commutator
Flashover across commutator is frequently caused by heavy accumulation of dust over the commutator the commutator. Such a flashover constitutes a dead short across the mains and before the fault is isolated by the circuit- surface or due to low-resistance leakage path either in the commutator risers, or between the bars or at the sides of breaker, the heat generated may be so much as to cause appreciable damage to the commutator surface and the surrounding area. The dust and dirt should be cleaned by washing and brushing with petrol; all carbonized matter, foreign particles and deposits should be scraped off. The inter-segment gaps should be kept clean; otherwise they may get filled up with dirt or carbon causing sparking and further carbonization.