AC motor question

[jerryk]

I have a motor out of a fairly recent Dewalt shop vac that has failed.  Symptoms are that it starts up at max rpm but then slowly winds down.  Brushes make a lot of spark that eventually wraps the commutator as it winds down.  My attempt to repair it includes regrinding the brushes square and returning the commutator on a lathe.  Testing the winding resistances, I could not detect any anomalies that I know of.  I don't have any idea how to test the commutator itself.  I always thought the windings on these motors were bullet proof.

Before toss the whole shop vac is there any thing that I'm missing or doing wrong or is it a hopeless case?  FYI - New motors cost near the price of the shop vac.

Jerry

Link to motor in action.
https://youtu.be/zMhZJSxdHrM

[johansen]

I have never been able to fix a motor with arcing that bad.

But have also never found a motor that slowed down slowly. Usually what happens is the commutator heats up to the point thr bars loosen up  and its all downhill from there

[Swake]

That is some serious sparking, haha
If you know for sure the carbon brushes are good and the commutator is clean then:

Measure resistance from one block to an adjacent block on the commutator. You're measuring 2 windings. They should all measure the same resistance, if not it is for the bin. It is not the absolute value that is of interest, but more the same value. If you find adjacent blocks that are open, then it is for the bin.
You could do the same test for the individual windings. Winding are connected on opposite blocks about 180° apart. All windings have to measure the same resistance.

Third test is to measure resistance from winding (blocks on the commutator) to the body of the rotor and the shaft. If there is a short somewhere it is for the bin.