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Commutator Wear

Author: E.E. Kimberly

As the commutator bars move under the brushes of a generator carrying load, minute sparks are formed and hasten the erosion of the two surfaces. If brushes are properly fitted to a new commutator of a properly designed generator, the minute sparks will gradually become invisible to the eye and will all but disappear. The commutator should then assume a chocolate-colored polish and should operate for months without appreciable loss of metal.

Unless the mica insulation between bars is undercut below the commutator surface, the unequal wear of mica and copper will cause the brushes finally to rest largely on the mica edges and there will be acute arcing and erosion of the copper. The commutator is then said to have high mica. It is important, therefore, to use a brush of the proper abrasive properties . to wear away the mica at the rate at which the copper is vaporized. Only brushes recommended by the generator manufacturer or a carbon-brush specialist should be used.

When high mica has developed, the commutator should be turned to a true surface and the brushes should be refitted. Undercutting the mica from 3/64 to 1/16 in. is often to be recommended. Slight roughness may be removed with sand-paper. Emery cloth should never be used on a commutator, because emery is a conductor of electricity and, if allowed to lodge between bars, may cause the short-circuiting of one or more coils.

Last Update: 2011-02-23