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PostPosted: Sat Oct 06, 2018 5:11 pm 
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I have recently acquired an electric saw which is in pretty good nick, despite being about 30 or 35 years old. In fact it appears to have been put on one side many years ago and abandoned. The reason isn't hard to understand; it sparks like crazy from the commutator. I don't mean just sparks, but a massive blue "flame" around the commutator when it is running.The brushes aren't sticking in the holders and seem to be a reasonable length (although notwithstanding that I have ordered some replacements) but the existing brushes are quite pitted at the ends. The bearings on the motor itself seem good with little play or run-out. Another little feature of this motor is that it periodically "freezes" and refuses to run at all (but then starts smoking at which point I power off instantly). I know that this saw has been used in the past to cut-up Tufnol (a really tough job for a little 830 watt saw) and that judging from the locations of dust in the tool it has been used sans dust extraction meaning that a lot of fine dust may have gotten inside the motor at some point (already blown out with compressed air). Is it possible that the coil has been "cooked" at some time causing the insulation to break down? How can I check this? Will it be repairable? Does anyone think that this motor wil need the coil rewinding?

This tool is badged Kango but I suspect that it was actually made in Germany or Switzerland by someone like Holz-Her or AEG (both long extinct) because the switch gear is all German - although there are no OEM names inside the casing anywhere at all.

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PostPosted: Sat Oct 06, 2018 6:14 pm 
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clean and re-lube - clean the com up with some ultra fine wire wool, you could try flatting the brushes and bedding them in again, the only big issue, was it dry stored ? if not the resin and the laminations maybe breaking down due to the damp/moisture

Motor rewinding can get very expensive as its a specialist job and often isn't cost effective



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PostPosted: Sat Oct 06, 2018 6:16 pm 
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carbon brushes can glaze over
clean the commutator with brasso or silver polish and clean the brushes with emery or a fine needle file
you will still get sparks till the brushes bed in again
the excessive heat may have soften the spring so stopping propper contact

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PostPosted: Sat Oct 06, 2018 6:38 pm 
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DON'T Use wire wool to clean the commutator, use fine emery cloth or similar (Wire wool snaps and you may end up shorting out a coil or two) The coils breaking down will not cause more sparks on the com, its more likely just a worn com or brushes.

Rewinding a motor can be cost effective, but it depends on the motor.

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PostPosted: Sat Oct 06, 2018 7:13 pm 
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Just to be completely pedantic, you shouldn't use emery on electrical gear either. That can also leave metallic particles with obvious results. Fine glass paper is the thing. You can get proper "brush seating stones", sometimes called "comm sticks, made by various companies including Martindale. They ain't that cheap though.

Assuming the brushes are OK, motor sounds like an armature winding shorted. Can (more rarely) be the field with a s/c coil as well. The field is in series with the armature, so it's being switched as well.



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PostPosted: Sat Oct 06, 2018 8:18 pm 
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Years ago I used to repair electric motors and don’t laugh but the preferred way to clean the commutator was to attach a piece of emery cloth to a wooden dowel and turn the motor on
Within seconds you’d have a nice clean commutator
Afterwards we then used an air line to remove any debris ,then we would strip out the field coil and remove any oxidation that had built up ,finally a new set of brushes
we were so confident we offered a 12 month warranty on out recon motors

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PostPosted: Sat Oct 06, 2018 10:36 pm 
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Thank you all. Yes, I know that having a motor rewound is costly, so if at all possible I want to avoid that.

Normally with old power tools I can breathe life back into them quite easily by replacing the cord, blowing out the switch, cleaning the electrical contacts (and sometimes fitting new spade connectors or bootlace furrules), replacing the brushes and ensuring that they slide smoothly in the brush holders and finally cleaning the commutator as Dave recommends with fine glass paper (followed with a good blow out with dry compressed air). This time it didn't work (OK, so in this instance I reground the ends of the brushes but they still have about 12mm left from an original 14mm or so - assuming that this is the AEG motor). What I got for my efforts was a spectacular display of pyrotechnics in which the end of the rotor (just inboard from the commutator) seemed to be bathed in light coloured flame. There is also a distinct smell of burning. Maybe time to do a complete rip down and see how good or bad the resin insulation is. At least I'll get the chance to check and repack the gearbox with grease (although it isn't noisy, so there probably isn't much wear in there, but I think it's still a good idea to replace 30-odd year old dried out grease)

Dave, is it possible to check for an armature coil short using a multi meter? Also, will it be necessary for me to to the strip-down or can it be done with the motor still assembled?

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PostPosted: Sat Oct 06, 2018 11:26 pm 
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All a multi meter will tell you is if you have or have not got continuity, you need an insulation resistance tester (Megger) to check if the insulation is breaking down.

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PostPosted: Sun Oct 07, 2018 12:22 pm 
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Well if you really want to get ultra pedantic you shouldn't clean the com at all but turn it on a com lathe


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PostPosted: Sun Oct 07, 2018 1:04 pm 
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Bob225 wrote:
Well if you really want to get ultra pedantic you shouldn't clean the com at all but turn it on a com lathe

We used to regularly skim motor comms if they needed it, and as you say it's the "right thing to do".
Limited number of times you can do that though, for motors operating in dirty and sometimes acidic environments where they just need a clean as there's no real wear.
We had spares for most stuff all ready to go on the motor store, and I've spent many an hour undercutting them after skimming with a Martindale cutter. But generally they'd just get a clean up with a stick in the motor if they needed it.
You'd have got a kick up the arris though if anyone had seen you with emery around them.

J&K you can test the motor with a multimeter, but as s-e says it generally doesn't tell you much.
There's a good description here
https://www.groschopp.com/how-to-check- ... -armature/

Other tests that can be done are with a "Martindale Growler" which will detect a shorted turn. I've never used one, but they're supposed to be good. A motor specialist may well have one.
There are also meters these days that will give direct readings of shorted turns and other problems.
Again Martindale.
http://www.martindaleco.com/pdfs/Electr ... trical.pdf


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PostPosted: Sun Oct 07, 2018 9:41 pm 
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Thank you once more, gents. Looks like it's a trip to the local motor rewind shop for a check-out before I can decide the little saw's fate, then

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PostPosted: Wed Oct 10, 2018 6:05 pm 
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The device to check the windings was called a growler, because of the sound it made, it was a coil on a laminated core which induced an alternating magnetic into the armature, there were two tests one was with a two pronged tool to measure the output from each winding, the other was a hack saw blade which one would hover over the armature with a shorted winding the attraction increased.

Image

In the days of dynamos there were the only sure way to test them, however for a motor normally you will see excessive sparks from commutator and hear a deep sound and it really is not necessary to test, in the main it is a replacement job.

I have under cut and skimmed armatures, it depends on the motor/dynamo and what the insulation is made of, and what brushes are made of, the mix carbon to copper, some don't need undercutting others do, normal tool was a hack saw blade either in a pad saw or insulation tape wrapped around it, and shaped on a grind stone, it was a very labour intensive job, as was re-winding, I have only re-wound 12 and 24 volt stuff, by hand, and successful, however only when we could not get new, first time I did it was in the Falklands where spares took 3 months to arrive. Just not worth it if new is available. 230 volt would be finer wire not sure you could re-wind.

Years ago for single phase and DC using a commutator motor allowed AC or DC and a lot of power from a small motor, typical is the electric hand drill. Induction motors have a very low start torque with single phase and are larger, however things have moved on, we see washing machines, fridge/freezers and other house hold stuff now with three phase motors and an inverter drive.

It has moved so fast I am now well behind as to what is available, even 15 years ago we were seeing the conveyors which typically had DC motors so you had some speed control, changing to three phase motors and inverters.

Although the inverter is expensive, the motor that uses an inverter is a lot cheaper, so as a pair the cost is coming down. At 830W I would be looking to see if I could use a drill to power it. 750W = 1 horse power and with squirrel cage motors they are large and hard to start under load.


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