Isolation Transformer - oil leaking  - Page 1
Forum section: electronics / testgear

author: Gtx21
author_rank: Contributor
timestamp: August 21, 2018, 04:44:09 am
content: I just purchased a new Isolation transformer for bench workI opened it up to verify the Neutral and Ground connections and found the legs of the actual transformer to be heavily covered in oil, dripping down from the wire wrappings.How much oil, if any, is acceptable to be leaking out of a new transformer?See yellow circles in this pic. It’s a bit hard to get he oil to shine in the photo: :

author: james_s
author_rank: Super Contributor
timestamp: August 21, 2018, 05:00:03 am
content: There's no oil in a transformer like this, either you're seeing varnish used the keep the windings from vibrating and humming or oil got spilled into it.

author: Circlotron
author_rank: Super Contributor
timestamp: August 21, 2018, 05:01:45 am
content: Looks more like dry varnish than wet oil.When small transformers are made they are heated up in an oven then dipped in this stuff similar to varnish then baked in an oven for a time. After it has been dipped some of the varnish runs out and it looks like this is what you can see. The varnish stuff provided extra insulation and reduced the chances of the transformer buzzing.Edit -> beat me to it. As above. ^^

author: Brumby
author_rank: Unknown
timestamp: August 21, 2018, 05:04:07 am
content: There shouldn't be any "oil" with a transformer like that. Transformers with oil will have a sealed chamber and you won't see the windings.Does this deposit seem hard - like dried varnish? Because that's what I would say it is. Some has simply trickled down from where it was applied.If so, it's good to go - but if it really is something oily, then I would be checking further.Edit:Doublesnap!

author: tautech
author_rank: Super Contributor
timestamp: August 21, 2018, 05:11:42 am
content: Probably wax.Not an issue.

author: Gtx21
author_rank: Contributor
timestamp: August 21, 2018, 08:57:18 am
content: Hmmm. Thanks all.It has a slimy texture to it. I just assumed it was oil based on the light reflection and the darkening of the winding paper.It does leave a residue on my finger, though isn’t actively flowing anymore. It did clearly flow out of the transformer and down though. I didn’t know that only the large transformers used oil. TIL.

author: tautech
author_rank: Super Contributor
timestamp: August 21, 2018, 09:02:53 am
content: Quote from: Gtx21 on August 21, 2018, 08:57:18 amThat's the clue......wax from the transformer paper.It's been a bit too warm.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 21, 2018, 11:15:41 am
content: Its varnish to lock the nuts in place, act as a cheap thread locker ...

author: james_s
author_rank: Super Contributor
timestamp: August 21, 2018, 04:25:14 pm
content: Oil filled transformers are pretty obvious, they are housed in a metal drum or box that is literally filled with oil, submerging the transformer. The oil serves as insulation and to transfer heat to the housing. The smallest ones I've seen were about the size of a 5 gallon bucket.

author: Old Printer
author_rank: Frequent Contributor
timestamp: August 21, 2018, 04:37:21 pm
content: One of these is on my short list of things to get. Could you tell us where you got it and maybe a product number & some specs? Thanks

author: Gtx21
author_rank: Contributor
timestamp: August 21, 2018, 09:18:46 pm
content: Quote from: Old Printer on August 21, 2018, 04:37:21 pmThis is the one I purchasedhttps://www.amazon.com/Valuepro-GRP-1200-Transformer-Isolation-Benchtop/dp/B00R5CDG8O/, a Jameco unbranded generic.It can do 4A (so it says) on secondary side and is fused at 7A on the secondary side. There are 4 outlets, 2 are not “isolated” and are wired straight from the wall input; 2 are “isolated”. There’s a 15A fuse on the wall input and a 7 A fuse on the secondary output. There is also a toggle on/off switch for the “Isolated” outlets on the front.All of the grounds on all of the outlets are shorted together, with the chassis, to earth ground.The neutral and ground on the isolated ports are in fact isolated from each other.There are some connections internally that aren’t as tight as I would like them. They use spade connectors for all connections. There is no active cooling. Just some vents. I haven’t measured the output voltage yet to see how 1:1 it is.It was very one of the cheapest I could find, so took a swing at it.

author: james_s
author_rank: Super Contributor
timestamp: August 21, 2018, 10:14:24 pm
content: You shouldn't need any active cooling with a basic iron transformer. As long as it stays within a reasonable temperature rise, typically 70C or so above ambient it will be fine.

author: Circlotron
author_rank: Super Contributor
timestamp: August 21, 2018, 11:20:38 pm
content: Quote from: james_s on August 21, 2018, 04:25:14 pmOld style car ignition coils are oil filled too.

author: james_s
author_rank: Super Contributor
timestamp: August 21, 2018, 11:56:45 pm
content: Good point, that makes those the smallest oil filled transformers I've seen. Either way they are fairly obvious, a metal can with liquid sloshing inside.

author: bsfeechannel
author_rank: Super Contributor
timestamp: August 22, 2018, 12:46:15 am
content: It's almost certainly varnish. The insulation in this transformer is paper. Paper attracts moisture. To prevent that, you pot it in varnish. They must have used air drying varnish. The most common for general applications. Some of it gets trapped in voids and gaps of the bobbin, the windings, and the laminations, and then leaks if not completely cured.

author: tautech
author_rank: Super Contributor
timestamp: August 22, 2018, 12:58:14 am
content: Quote from: bsfeechannel on August 22, 2018, 12:46:15 amNot always.See here:https://ludens.cl/Electron/trafos/trafos.htmlIn antique transformers, the most usual insulating material was paper, impregnated with something like beeswax, tar or the like. This impregnation had several purposes: Mainly, it would seal the pores of the paper, making it a really good insulator, while without the impregnation it would only insulate as well as the same thickness of air! But in addition, it kept moisture out, it helped stick the thin wires in place during winding, and it improved the thermal conductivity of the completed winding assembly.

author: james_s
author_rank: Super Contributor
timestamp: August 22, 2018, 04:35:49 am
content: I do have some waxed transformers, but they're 1970s vintage, I haven't seen anything modern made like that.

author: tautech
author_rank: Super Contributor
timestamp: August 22, 2018, 05:13:44 am
content: Quote from: james_s on August 22, 2018, 04:35:49 amConsider for a 1:1 isolation transformer only the bobbin and inter-winding insulation need care taken with and even old style and low cost materials would today be adequate. Oiled or waxed paper or card would be fine.Quite different for say a compact SMPS transformer of modern requirements.