cleaning rusty transformers and burnish shorting - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: April 26, 2020, 06:12:34 pm
content: so I have a slightly rusty MOT that i want to rewind as a low current power supply (<5V), the problem is some of the laminations are a bit rusty.I see people commonly sand them, etc.. but this shorts out the plates right? I see there is a weld across the plates anyway, but what effect do the minor shorts that would be assosiated with metal deformation during removal have on the transformer.. I assume its very small but is it definable?Is there a prefered way for cleaning them also (I thought maybe to use the expensive 'organic' rust remover (i.e. sold 20$ a gallon), like WD40 rust remover, rather then abrasives.

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 26, 2020, 06:35:41 pm
content: Yes, sanding is bad idea, unless you can re-coat them somehow. The individual laminations need to be insulated from one another to prevent eddy currents. When bolts are used to hold possible covers and brackets on the cores, or just hold the cores together, note that those bolts have shoulder bushings on them to insulate the bolt from the core. Since those bolts go through the core, in the field, it would be effectively a one turn shorted winding. The welds you mention are on the outside of the core and are there to prevent mechanical vibration, aka buzzing.As I’ve never re-used old E’s and I’s, I don’t know of the possible DIY methods to repurpose an old core. I would think a high temperature spray paint might work so long as it was non-conductive.Edit: I remembered seeing several write ups on diy transformers. First link seemed the most competent to me.https://ludens.cl/Electron/trafos/trafos.htmlhttp://classicweller.com/?page_id=71http://www.tronola.com/Rewinding_a_Power_Transformer.pdf

author: coppercone2
author_rank: Super Contributor
timestamp: April 26, 2020, 06:40:15 pm
content: i mean, its surface rust, but if you remove it chances are the metal might bend a little and make good electrical contact with the metal nearby (maybe). I don't know how much of micro bur it leaves, so it would stay shorted even if you paint it.My idea was to make a pool of rust cleaner and carefully dip the transformer in it, varying the height of the rust cleaner so it does not touch windings (and use the organic super gentle rust cleaner). Then when its clean, repaint it. And only use a brass brush to clean it.How much effect does the little short circuit between plates on the outside of the core have? I understand the flux is not dense there anyway.

author: Ian.M
author_rank: Super Contributor
timestamp: April 26, 2020, 07:11:35 pm
content: A phosphoric acid based gel rust remover would be safe to use on an assembled laminated transformer core, (as it will leave an insulating film of iron phosphate) provided you don't get it on winding or bobbins. Use a green pan scourer not a wire brush, and scrub inline with the laminations to avoid bending them.

author: coppercone2
author_rank: Super Contributor
timestamp: April 26, 2020, 07:27:43 pm
content: whats good for removing the goop? I think its epoxy, there is a 'stalagite' of it on one of the windings.. I think varnish would not do this.The rust kinda peenetrated under the clear coat, I am using the liquid organic rust remover in a shallow pool with the transformer set on small pieces of acrylic right above the windings, again the super gentle expensive one,I have a feeling it wont really work too well on the rust, but I don't know the integrity of the film on top of the rust.. maybe.. but if I have to remove it.. what should I use?Paint stripper goop (the new amine one they sell?, non MC).. and how to protect the windings from vapors (thought it should have reduced vapors compared to the Methylene Chloride one they used to sell).