Servicing the commutator

Servicing the commutator

A commutator in good condition should be smooth and free from pitting or signs of the segments burning. Clean with a rag moistened in petrol. If necessary, polish using a fine glasspaper whilst rotating the armature. Never use emerycloth.

If the commutator is badly worn it may be skimmed using a centre lathe. Use a high rotational speed and take a light cut using a sharp tool. Remove only enough metal to clean up the commutator face and then polish with fine glasspaper. To undercut the insulation between the segments, it is suggested that a hacksaw blade is ground on its sides until it is the thickness of the insulation and carefully work between each pair of segments until the insulation has been cut to a depth of .0394 inch below the surface.

A thorough visual inspection may determine the cause of armature failure. Breaks in armature windings cause burnt commutator segments. Short-circuited windings are discoloured by overheating, with badly burnt commutator segments.

Testing field coils

When tested with an ohmmeter the reading should be 8+.1—.3 ohms. Failing an ohmmeter use a 12-volt supply and connect it in series with an ammeter across the field terminal and the yoke or body. The meter should read approximately 2 amps. If there is no reading the field coil winding has a break in it. If the reading is much more than 2 amps or the ohmmeter records at much less than 8 ohms it shows that field coil insulation has broken down. Renewal of the field coils is a specialist operation best left to a service station.