Armature cleaning DC Dynamo

I have just dismanteld an old lister dynamo 17amps at 50/70
volts, and the armature is very dirty and covered in crap, what is the
best way to clean an old armature as not to damage it or the
insulation, when i worked for ABB many years ago we used to steam clean
them and then bake them in an oven but i fear this would damage it to
much , after i get it clean i intend to use insulating varnish to re
seal it.
Thanks in advance
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How about wiping it over with a thinners or similar, if you have a compressor blow the muck out with low pressure air and leave to dry. If its old wont the varnish damage what maybe shellac. Your local rewind company may dip it and bake it for you.
Martin P
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If the commutator is basically sound by just has dirt on it, then a wipe with a non-fluffy rag dipped in something non-oily and not too strongly solvent would be OK.
If the commutator needs skimming themn get it done properly, you can't go and buy another....
The commutator is normally a darkish brown colour, only modern starters have copper loaded brushes which give bright copper comm's.
If it ain't broke, don't fix it! :-))
Peter -- Peter & Rita Forbes Email: Web:
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Peter A Forbes
After extended use, the comm can be grooved by the carbon & arcing. I'd check it for continuity with battery & bulb between each segment & then earth. You should get continuity between each segment. If in doubt, use a meter - they should of course all read about the same. If one is lossy, check where the wire is crimped or soldered into the ends of the comm segments.
Heresy, I know, but I've always filed mine back to round with a decent, chalk-loaded, safe edged file. Place the armature on the partially open vice jaws & hold it with one hand if light. If massive it'll sit there obligingly while you get on with the filing. The trick is to move the file in an arc to trace the diameter of the commutator. Keep rotating the armature, taking ONE cut at the comm at each turn.
Personally, I'd not put an armature in a lathe & turn the comm because of the danger of flicking a segment out - and then you are knackered just like the comm! It will take you a while, but careful hand work is the key to so many difficult operations. Clean & rechalk the file frequently.
As to the armature windings, remember that if its old it is probably painted with shellac (murdered beetle wing cases!) which is dissolved in alcohol, so you won't want to clean it with meths, you see ...
If it was me, I'd dry brush & Hoover as much off as I could, but if its oily crap, I'd wash it in petrol & at once pat it dry with a cloth. Immediately, I'd place it in a warm place to drive off fumes. Leave it there for a couple of days to ensure dryness, then paint the exposed insulated wire with a decent oil paint - I like to use red or orange personally, but clear varnish will do just fine.
Finally, you'll need to undercut the mica between the segments & this can be achieved with a broken all-hard hacksaw blade, suitably ground to shape. Larger comms may need a power hacksaw blade.
One tip - don't use emery paper on a comm, it gets stuck in the copper & gnaws the brushes. 400 grit+ wet & dry used wet will be ok but make sure you wipe it dry straightaway.
Repeat as necessary with the field coils.
J. Kim Siddorn,
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Kim Siddorn
Anything with carborundum is a no-no as it is conductive....
Glass paper is the thing.
Peter -- Peter A Forbes Prepair Ltd, Luton, UK
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Prepair Ltd
Thanks guys, the com is in very good condition with no visable grooves in it so this will just need a quick clean, I think i will brush clean it with some white spirit and dry off and then drown it in with the insulating varnish I have , it is low VOC stuff not very agressive so should be OK.
thanks again bob
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And cut a scrap of angle iron to the length of the com. and use it as a guide to help prevent the blade sliding across the copper.
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Tony Brooks
I set an armature up in a lathe, and mounted a section of ground hacksaw blade in the toolpost on it's side at centre height. All I had to do then was wind in the cross-slide to the required depth and move the saddle along the bed to make the cut indexing the armature for each segment.
It did a very professional job, and no risk of slipping.
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