A commutator wears every time the brushes pass over it. Most can be skimmed in a lathe once or twice; eventually there is not enough copper left, or the damage is the kind skimming does not fix. These are the signs that it is time for a new one.
Replace it when you see
Copper worn down to the mica
The mica between the segments is undercut slightly below the copper. Once the copper has worn down to the mica level the brushes bounce, arc and wear fast. If the brush surface is already thin, another skim will leave too little copper for the next set of brushes.
Burnt or blackened bars
One or two dark bars, evenly spaced, usually mean an open or shorted winding rather than a commutator fault, and a new commutator will not cure it on its own. All the bars burnt, or a band of heavy pitting, is heat damage to the commutator and it should be replaced.
Raised, loose or sunken segments
Run a fingernail round the surface. Any bar that stands proud, rocks or sits low has lost its grip in the moulding or the mica has broken down. This cannot be machined out and will destroy brushes quickly.
Flat spots and ovality
A flat spot where the brushes have sat while the machine was parked, or an out-of-round surface you can see with a dial gauge, can sometimes be skimmed. If skimming would take the diameter below the maker's minimum, replace.
Cracked or failed moulding
Cracks in the insulating body at the ends, or a commutator that has shifted on the shaft, mean the part is finished.
Brush symptoms that point at the commutator
- New brushes wearing out in weeks
- Heavy sparking at the brushes under load
- Black dust packed into the slots soon after cleaning
- A starter that spins but is slow or weak with a good battery and good cables
Skim, replace, or new armature?
| Situation | Usual answer |
|---|---|
| Light scoring, plenty of copper, windings test good | Skim and undercut |
| Copper at or near the mica, bars loose, moulding cracked | New commutator on the existing armature |
| Commutator worn and windings burnt or open | Rewind with a new commutator, or a new armature if available |
| Obsolete unit, no armature available | New commutator, made to measure if needed |
Next step
If yours is on the replace list, measure it and send us the numbers, or pick the size from the standard range. If you are not sure, email us a photo and we will tell you what we see.