Brushes throwing excessive sparks 1

[TwstLok]

Hello all. Amongst many other duties, I'm responsible for repairing all power tools for our company. I basically learned by cracking them open and using my head. THis has worked well for several years...until recently.
After working on a Milwaukee portable bandsaw, I fired it up to test it. I noticed that it now made a very...raspy, scratchy, almost screaching sound but not high pitched. I also noticed that the brushes were giving off an incredible amount of sparks.
This has happened with 3 tools that I've worked on lately. Two are the same make/model. The 3rd is a saw an employee owns that is doing the same thing.
His saw runs but it sounds aweful and burns through brushes like crazy.
Can we figure this out please? I have a fairly good grasp of how the motors work. When I disassemble them, I always clean off the comutator etc and this has NEVER caused a problem.
Thank you!

 

[hacksaw]

going back to an earlier day, the commutator may need dressing and cleaning as you are doing, but brush composition, alignment, and spring tension can also be a factor. a clamp on amp meter might help sort out what is going on.

some of the modern tools, reversal rotates the brush assembly so you have yet another alighment to worry with.

 

[ccjersey]

if the brushes are from the tool manufacturer, they should be fine and usually have the new spring on them already. I have heard to never use emery cloth on a commutator, only "sandpaper", don't know that I have ever seen a problem from the emery cloth though. The commutator doesn't have to be shiny copper to do its job though.

Have you checked the height of the "mica" between the commutator bars? I usually take a hooked sharpened tool and scrape the mica down some. Not real precise, but the slots are so narrow it is hard to do anything better.


I think something big has failed like armature or possibly stator. If the armature winding has shorted to the laminations, an ohmeter will pick that up, but turn to turn shorts won't show up this way. A starter/generator shop or electric motor shop may have a "growler" to detect the turn to turn shorts.

It may be time for some new tools, maybe you can put two old ones together and make one! Odds are they will both have the same problem though.

Jim

 

[itsmoked]

Twist;
The mica cc jersey is talkin about is of course the stuff between the commutating bars. If it is sticking up above the commutating bars one tiny iota the brushes are launched off of the mica and land somewhere later on the bars.. this is verrrrry bad. Much heap big sparky. So shops often have a jig with a dremel tool like apparatus, that lets you run a straight pass down the mica strips. Lowers the mica, squares up the C. bars.

Excessive load will also cause arcing. This is where an ammeter would help. If you can compare unloaded good tool amps with the unloaded bad tool this would show up pronto. Can't go by nameplate as that is some "nebulous" loaded rating. If a bearing was going out, say on your band saw, it may be hard to turn the blade by hand.

 

[bklauba]

Both the fast brush wear and the sound are symptoms of a partial short 'tween the rotor windings/commutator and the rotor shaft. This signal is now going thru the bearings to ground. The screeching sound is the bearings passing current, making intermittent contact thru the grease. Have you megged 'tween the comm and the shaft? The short might well be carbon dust, debris, etc. Sometimes this does not show up with an ohmmeter, or the ohmmeter will indicate 100,000's or megs of ohms, which would seem to be tolerable. But the ohmmeter's voltage is only 3 - 5 VDC, which is not good enuff to check the integrity of the insulation. Rule of thumb is at least operating voltage, up to twice.

If you don't have a megger (the bare-bones versions are about $ 200 new), you could do the following:

1. (With the rotor out of the tool),ground the rotor shaft, thru both an ammeter and a fuse (like 1 amp).

2. Wrap a bare wire around the comm, and carefully feed the bare wire the operating voltage. (Do this on a wooden bench, not a metal one!)

If the fuse blows, you have a bad short to ground. If it doesn't, but the meter reads anything significant (like more than a few milliamps), you have a mild short to ground (a condition that perhaps can be tolerated, but often is a precursor to it getting worse).

You should pressure wash with soapy water, then rinse in water, and bake out the rotor for a few hours at least at 300 degrees F. (Megger will tell you if you got the water out.) If the rotor still passes current, using either test method, after wash & bake (sometimes a 2nd bake is needed), then the rotor should be replaced.

If the rotor passes the test, and if the comm looks, both surface wise and 'tween the slots, reassemble with new bearings and brushes. You can run the ohmmeter 'tween the comm-bars to check for continuity, and growl the rotor (itsmoked suggestion).

If needed, turn the comm, drag the slots, and rechamfer the comm-bars, so you would re-establish a good riding surface for the brushes. (If you haven't done this B4, see if a small elec motor tech will show you how. Some simple, but special tools are normally req'd, plus a rotor fixture or small lathe.)

If it megs well after all this (or, if you have no meg, do the test outlined above again), new bearings and brushes and you should be there.

This is essentially what a motor svc shop would do.

Of course, a new rotor might be cheaper than doing all this. Often the pricing of the replacement part is not based upon cost, but rather upon the cost of reconditioning the old rotor.

BK

 

[TwstLok]

BK,
Thanks for that! Ya, these are cheap armatures out of power tools. I've priced new ones, like $40 each. Certainly not worth my time at all, not to mention the several hours I've already devoted to them.
It's simply the principle of the matter. I just want to know what the problem is. Just replacing them with new ones wont help my frustration and curiosity!
You were the first person to mention a "megger". What is that? Is that another term for a growler?
All of you guys have been quite helpful with your quick suggestions, glad I found this site for sure.

 

[Skogsgurra]

Google "Megger" to find out more.

It is a 500 or 1000 V (or any "high voltage") MEGohm metER (hence the name) and also a trade name for the company that makes (some of) them. Should be used whenever there is a suspicion that isolation is bad (but careful with circuits with electronics in them).

 

[itsmoked]

baklauba: Nice response! That's a cut and past to my motors file 4shure.

 

[bklauba]

In this "toilet-paper-world", just chucking the bad part makes more sense. Little motors are often not worth doing anything besides replacing parts. (It's different with motors req'ing brushes about the size of a brick!)

Be careful when you "clean" a comm. Wiping anything off the comm-bars might just wipe it into the slots, 'tween the comm-bars. Now you have a short.

Sometimes I use a low-lint rag, or a small sponge, and isopropyl alcohol to clean a small comm, then shine them up with a Scotch (green) pad, called BPH by some folks, wiping parallel to the comm-bars, off the end of the comm, so that (hopefully) the debris and copper dust leaves with the wipe direction. Avoid letting the cleaning agent just run 'tween the slots; its likely got grit dissolved in it.

Hope this helped.

BK

 

[TwstLok]

BK,
That was an interesting one about grit getting between the comm bars and causing a short.
I hadn't thought of that one but like I said, I'm not an electric motor guy.
But what you described is a very good possibility.
Lots of great suggestions on here.
Thanks

 

[TwstLok]

BK,
Ok, so I got to tinkering with it again. I cleaned the commutator. All clean between each bar. I have total continuity between all bars on the commutator. I didn't think that was supposed to be the case but I guess so.

Also, between the commutator and the shaft, I have no continuity or no shorts to that shaft.
Bear in mind, this is a very small armature out of a hand tool.

I read something that said the brush shouldn't be any wider than one bar of the commutator? Maybe that depends on what kind of armature it is? The brushes (stock) that I removed cover an entire bar and a little of the bars to either side.

So, I think I'm going to find a local electric motor replace place and pay the guy to let me sit there and watch him test this thing.

Oh, I keep forgetting to ask, what exactly is the growler providing? When you turn it on, is that activating an electromagnet in the growler and grabbing the armature?

Anyway, still kinda stuck (obviously).

I read the part about wrapping a wire around the comm with a 1A fuse inline and hook it up to operating voltage. Well, that voltage is 110Vac! Not sure if you were thinking of a 12v cordless drill or something.

Anyway, do my findings to this point make sense with the continuity?
Thanks
Sean

 

[TwstLok]

Ok, just called a couple local places. Looks like I wont be watching anyone do anything for $120/hr. However, one of the gentlemen were nice enough to listen for a moment. Seems my continuity between all bars on the commutator is a good thing, as is the isolation between the bars and the shaft. Sounds like the comm is ok?
He said it sounds like a wire in the armature is shorted.
Are any of you electrical motor repair guys in the S.F. Bay Area and have a shop?
I mean, I'm going to just replace the armatures in question with fresh ones and I'm sure the tools will be happy again. However, that wont cure my curiosity. If I can find out exactly what it is, I can be reassured that it is NOT something that I caused. I doubt I caused it only because I've done the exact same thing with hundreds of tools with good results.
Anyway, I'm at a loss here. At the risk of my wife not being happy for a while, I think I'm going to purchase a growler and see if I can go from there.

Sean