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  Common Defects in Commutator

Introduction to Common Defects in Commutator

A good commutator will comprise a smooth polished surface dark chocolate brown in colour.

i) Commutator surface pitted and rough:

This is mainly the result of extreme sparking; the surface might be smoothened with fine sand paper on a curved block and polished along with crocus cloth. Never make use of emery paper or cloth, when the fine emery particles might get to the commutator.

ii) Ridge formation on commutator:

Ridges are created if the brush holders on dissimilar rocker arms are not appropriately staggered. Other possible cause in the matter of large armatures is, if the armature does not oscillate to expand of about 1/8 in. axially on either side of the core centre. The ridges might be filed off through a smooth file, or by means of particularly shaped commutator grinding stones, which curvature corresponds to that of the commutator. If it is a generator, eliminate all carbon brushes and run it up, if it is a motor, run up to full speed of it, and switch off the supply and fast apply the grinding stone to the surface of the commutator, when the armature is slowing down. Fairly a lot of metal can be eliminated this way if the work is correctly done by a trained hand. After grinding is over, finish off with smooth sand paper and crocus cloth.

If the commutator surface is extremely bad, then the armature should be put on a lathe and skimmed, after testing the trueness of the shaft through a dial indicator. The cuts should be the lightest feasible. The final cut should preferably be through a diamond tipped tool with fine cuts of the order of .0005 to .001 in. with a supply of 500 threads per in. At last, mica should be undercut as usual.

iii)   Commutator is eccentric or having high bars and low bars:

Extreme blackening or burning of some commutator bars only depicts existence of high or low bars, mainly caused through the bolts holding the commutator V-ring becoming slack. The bolts should be tightened up completely, and then the commutator has run in for some time and is completely warm. After this is completed, the commutator has to be skimmed on a lathe. If just a portion of the commutator surface depicts signs of harsh sparking, it is a clear sign of the commutator being eccentric. No commutator should depict a difference of more than 0.0254mm (.001 in.) while tested through a dial indicator, while mounted on its bearings on the motor.

iv)  Protecting mica between bars:

All commutators should comprise the mica among the bars undercut to a depth of 0.8mm (1/32 in.) about. After grinding / turning of the commutator, undercutting should always be done either by hand or through a machine. It is necessary to round off the sharp edges of the bars, and eliminate all bars, when also to carefully clean the mica slots and blow off all dust. Undercutting has to be as well done periodically.

v) Flashover of commutator:

Flashover across commutator is often caused through heavy accumulation of dust on the commutator surface or because of low-resistance leakage path either in the commutator risers, or among the bars or at the sides of the commutator. Such type of a flashover constitutes a dead short beyond the mains and before the fault is isolated through the circuit-breaker, the heat produced may be very much as to cause considerable damage to the surrounding area and commutator surface. The dust and dirt should be cleaned through washing and brushing along with petrol; all carbonized matter, foreign particles and deposits have to be scraped off. The inter-segment gaps should be maintained clean; if not they may get filled up along with dirt or carbon causing sparking and further carbonization.

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