re-purposing yellow "site" transformers

[throbscottle]

May not mean anything to non-uk people!
So anyway, looking through eBay listings I keep seeing site transformers (if you don't know, they are used on building sites etc to provide 110v cte power for tools), sometimes very cheap, and keep wondering how easy or hard it would be to take one apart and re-wind it for something.
Anyone have experience with these?
(cte = centre tap earth)

 

[JimB]

The only 110v yellow transformer that I have ever opened up was completely potted in resin.

I have just opened my 240 to 240v yellow isolating transformer and looked in there, that is also completely potted.

I guess rewinding one would be impossible.

JimB

 

[spec]

The only 110v yellow transformer that I have ever opened up was completely potted in resin.

I have just opened my 240 to 240v yellow isolating transformer and looked in there, that is also completely potted.

I guess rewinding one would be impossible.

JimB

Hmm, that is a shame.

spec

 

[Diver300]

All the ones that I have seen have been completely potted (and yellow and heavy, obviously).

 

[throbscottle]

It makes sense really, thinking about what they're for and the life they have

 

[throbscottle]

All the same, it's worth remembering it's actually 55-0-55v, not just 110

 

[Nigel Goodwin]

All the same, it's worth remembering it's actually 55-0-55v, not just 110

Are they?, I can't say I've ever looked.

 

[schmitt trigger]

They are either potted, or they use this ultra heavy varnish that it is all but impossible to remove.

Power transformers use these tough compounds not only to protect the steel core, but to reduce the "humming".

 

[Nigel Goodwin]

They are either potted, or they use this ultra heavy varnish that it is all but impossible to remove.

Power transformers use these tough compounds not only to protect the steel core, but to reduce the "humming".

In the case of these 'site' transformers, it's to give the transformer a chance to survive given the abuse the idiot builders give them

 

[throbscottle]

Are they?, I can't say I've ever looked.

Hence the CTE designation

 

[Nigel Goodwin]

Hence the CTE designation

I've never even noticed the designation

However, my only experience of them has been repairing them - after brutal treatment at the hands of builders - and that only consists of refitting ripped off sockets, or ripped out wires.

 

[dr pepper]

I have a couple too, at least one is full of potting goop, thats why they are so heavy, and yes they are 55 - earth pin - 55, so the biggest shock you can get is just 55v.

I have a couple of 110v trannies salvaged out of control panels too, but you have to be brave to rewind them ,that entails removing all the laminates, I have done that once, dont want to do it again.

 

[throbscottle]

It's getting all the laminates in again that's the hard part, especially when you've ever so slightly squeezed the bobbin by winding it...
I've had some success getting potting compound off a PCB, hard part was getting it started down to a point where I could pry bits off. I wonder if the application of heat would make it more amenable to digging into...

 

[spec]

Epoxys are very hard at temperatures below their 'glass transition point' (GTP). Above that temperature the epoxy changes to a putty like substance.

Depending on the epoxy type, the GTP can range from 100 Deg C to 350 Deg C. If you are lucky enough to have a low GTP epoxy you can boil the transformer in water and then pick/pull the epoxy off. For higher temperatures you need a liquid with a higher boiling point, like engine oil, or put the transformer in an oven.

Once the transformer has been cooked, you can use a normal heat gun to warm any stubborn epoxy.

That is the theory anyway.

spec

 

[spec]

It's getting all the laminates in again that's the hard part, especially when you've ever so slightly squeezed the bobbin by winding it...
I've had some success getting potting compound off a PCB, hard part was getting it started down to a point where I could pry bits off. I wonder if the application of heat would make it more amenable to digging into...

I know what you mean. I have rewound quite a few laminated transformers along the way and also ended up being unable to re-insert the E laminations and had to file the bobbin in the horizontal plane to get them in. In the vertical plane you just leave a few lams out.

After a while I learned to make a tight fitting square piece of wood and bang that into the bobbin before messing with with the windings.

spec

 

[throbscottle]

I'm so tempted now to get one just to try the experiment. Don't know what I'd want a 500va minimum transformer for though???

 

[Nigel Goodwin]

Epoxys are very hard at temperatures below their 'glass transition point' (GTP). Above that temperature the epoxy changes to a putty like substance.

Depending on the epoxy type, the GTP can be 100 Deg C to 350 Deg C. If you are lucky enough to have a low GTP epoxy you can boil the transformer in water and then pick/pull the epoxy off. For higher temperatures you need a liquid with a higher boiling point, like engine oil, or put the transformer in an oven.

Once the transformer has been cooked, you can use a normal heat gun to warm any stubborn epoxy.

That is the theory anyway.

Must admit I've never heard of that, or any suggestion of any such thing - assuming it's true?, then it won't be a low temperature, as the transformer is likely to be specified to run higher than that anyway.

 

[spec]

Must admit I've never heard of that, or any suggestion of any such thing - assuming it's true?, then it won't be a low temperature, as the transformer is likely to be specified to run higher than that anyway.

 

[spec]

I don't think a transformer would be designed to run all that hot.

The softening of epoxy at a high temperature is a fairly well known phenomena. It is one of the shortcomings of epoxy from a mechanical point of view.

At work we did a lot of potting and the potting shop had heated baths of acetone or similar (cant remember) to remove the potting compound. It took about 4 to 24 hours to do the job depending on the size of the unit. They used to moan like hell if we asked them to de-pot a big item though.

Not all potting compounds are as strong as Araldite. Some of the aerated types a much less tenacious. On the other hand some potting compounds like Scotchcast (I think) are like granite and there is no way you could remove them by heating.

I have removed small amounts of potting compound with heat, but nothing on the scale of a site transformer.

spec

 

[spec]

I'm so tempted now to get one just to try the experiment. Don't know what I'd want a 500va minimum transformer for though???

Perhaps start with something smaller but it would be interesting to know if you could de-pot a site transformer.

I could think of many uses for a 500VA transformer.

spec