I am trying to repair a corded sawzall suffering from excessive arcing, with blindingly bright green sparks circling around most of the commutator when it is at full power.


Both the commutator and the brushes were badly pitted and streaked when I opened the saw.

We replaced the brushes which barely helped. Next, I re-surfaced the commutator with a file and very fine sandpaper while turning it on a drill press. This made the tool run much better and reduced the arcing somewhat, but the arcing was still excessive immediately causes streaking and damage to the commutator. I cleaned everything with compressed air and made sure there was no gunk between armatures that could cause a short. Nevertheless the arcing persists. What could be going on?

 

Usually indicates shorted armature coil.

 

The really BAD NEWS is that my experience agrees that MAX is correct. Shorted turns on the armature coil.

 

Well shoot thanks. We’ll probably just scrap it. Is there a way to test if the armature coils are shorted in the future?

 

The normal way is on the bench with testing the armature with a 'Growler' tester.

 

The alternative method to check armatures requires an ohm meter with very good resolution on the very low resistance range. It also requires a reliable and repeatable means to contact segments on exactly opposite sides of the commutator.

 

Another option is to live with it until the motor totally fails. If the shorted segment burns open, then the motor will have two positions where it will not start. THAT can be very inconvenient, but it may allow completing a day's work.

 

made sure there was no gunk between armatures that could cause a short. Nevertheless the arcing persists. What could be going on?

Modern armatures do not normally need undercutting like they once did, The majority now use mica between bars and it wears with the comm itself, so you may not need it.
Check if sawsall can supply a new armature?

 

https://documents.milwaukeetool.com/54-40-2700.pdf

 

As I revisited the photo in post #1 I realized that I recently recycled a high speed brushed motor with a similar display. But in that case it was a loose commutator segment. and with a motor claimed to run 11,000 RPM no repair I could do easily would work.

 

I took a closer look at the armature and I found an open wire, as well as lots of scuffed insulation that could cause shorts. I don’t think we’re fixing this one. Thank you all!

 

Now the challenge will be to determine how to prevent the damage to the replacement armature. Usualy motor interiors are fairly well protected.