The Generator and its Commutator

By Norman Nock

Originally published in the Austin-Healey Magazine, March 1983


The alternating current that the generator produces is no good for battery charging. To charge the battery we need direct current. In order to use the generator, then we must provide some means of reversing the connection to the external circuit of the generator, so as to make the current flow one way only in that circuit. This is the function of the commutator.


A commutator in good condition will be smooth and free from pits or burned spots. To remedy a badly worn commutator, mount the armature in a lathe and take a light cut with a very sharp tool. Undercut the insulators between the segment to a depth of 1/32”. I suggest having the armature checked and the commutators serviced by a qualified generator repair shop.


If upon first examination of the armature, it looks black or smells burnt, the windings are shorted or will be very soon.


If the armature outer body (laminations of the iron core) have touched the filed poles, the armature is faulty. A badly worn commutator end bush or a worn drive end housing, i.e. bearing loose in the housing usually causes this.


Excessive tightening of the fan belt is the usual cause of worn bush and housing.

Field coils can be retaped only if it is done carefully. The tape is available at generator repair shops. Remove the old tape slowly, a small section at a time. Retape this small section but DONOT OVER TIGHTEN with the new tape. If you make them to tight, you will not be able to reinstall the field coils onto their yokes.


Use only Lucas brushes so you are sure they are made of the correct material. If the wrong type material is used, the commutator will wear very quickly.


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