On 9/7/2018 9:50 AM, Bob Engelhardt wrote:
Not "electronics", but probably in the S-E-R realm.

I have a really old electric demolition hammer with a bad motor
armature.Â* The fault is an open on one of the commutator segments.
Resulting in a lot of sparking when running & eventual erosion of the
brushes and/or adjacent commutator segment.

The tool is so old that Google doesn't find the model, let alone a
replacement part.Â* Rewinding the armature would be $125.Â* Not worth it
to me.

My intuition about the sparking is that the armature current is normally
continuous because the brushes overlap adjacent commutator segments, but
with one segment open, the current is interrupted and sparking results.

If so, my idea is to short the open segment to the adjacent one & allow
continuous current.Â* The phasing would be affected a little, but there
are 16 windings, so the effect should be small.Â* And a 1/16th (?) loss
of power, but that's already the case.

Does this make sense?

Thanks,
Bob
If the winding is open and the motor stops with the brushes on that
armature segment, the motor cannot start. I have a coupe of auction sale
purchases like that. Since you haven't described that condition, the
winding must have an internal short.

Have you disassembled the motor and cleaned the armature? There may be
something conductive between two armature segments.

Paul