re-purposing yellow "site" transformers

[tcmtech]

For the time and efforts involved I would just go and find an open wound transformer to play with. Old HID lighting ballasts are some of the cheapest to find and easiest to work with being most are designed like MOT's and have their laminations set up in easy to disassemble sections rather than interwoven laminations.

 

[dr pepper]

I have depotted a couple of things with a hot air gun, the compound turns to easily cut flexible rubber type stuff, allthough this might not work on all compounds.

The first one or 2 laminates get mangled pulling them off, and the trans will probably be ok without, to stop the bobbin from squishing just plane a piece of wood so that it fits inside tightly, you can also put a bolt through the centre of this to use as a drive on a drill.

I have scrapped some HID light fittings and kept the 400w ballast's, I just had a look at one, your right tcm the ballast looks as though the E's and the I's are all aligned like an mot, the mounting plate is crimped into place and holds everything together on this one.
Probably a good source of a quick tranny, and the cross sectional area on this one is hooge probably around 0.5m2.
Probably be good u p to 1 kc or so.

 

[throbscottle]

A very quick look search suggest old hid ballasts would be hard to obtain (and I had to look up what hid means). I see flourescent light ballasts in skips sometimes but I think they are rather smaller than the hid ones? Never really thought of re-winding one as a transformer.

 

[dr pepper]

Actually having looked at one of the tubes the ones I have are HQI, but they are similar.
The power rating is 400w, and the ballast looks more like its rated at a kw or so.
They are probably moving onto electronic ones these days, in fact they fit led's now.

 

[Nigel Goodwin]

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

Transformers can get VERY hot - I notice the video above isn't de-potting a transformer

 

[tomizett]

The only site transformer I've ever got hold of was potted in something crumbly, like some kind of cement. I must have got lucky there, because it was relatively easy to chisel away (it simply broke up into big slabs).
...so if you get hold of one for free, it's certainly worth popping the top off to have a look.

 

[spec]

Transformers can get VERY hot - I notice the video above isn't de-potting a transformer

Hmm, I think you need to define what very hot is and what transformer, but I do take your point.

It would be interesting to see how the potting compound in site transformers react to heat in practice.

spec

 

[spec]

The only site transformer I've ever got hold of was potted in something crumbly, like some kind of cement. I must have got lucky there, because it was relatively easy to chisel away (it simply broke up into big slabs).
...so if you get hold of one for free, it's certainly worth popping the top off to have a look.

Hy tomizett,

Yes there are different potting compounds- not all are like Araldite. Some potting compounds are relatively easy to remove.

spec

 

[spec]

Nigel,

This extract defines the temperate ranges for transformers:

Standard Ratings and Overload Capacity
Dry-type transformers are available in three standard temperature rises: 80C, 115C, or 150C. Liquid-filled transformers come in standard rises of 55C and 65C. These values are based on a maximum ambient temperature of 40C. That means, for example, that an 80C rise dry transformer will operate at an average winding temperature of 120C when at full-rated load, in a 40C ambient environment. (So-called hot spots within the transformer may be at a higher temperature than average.) Since most dry transformers use the same insulation on their windings (typically rated at 220C), irrespective of the design temperature rise, the 80C rise unit has more room for an occasional overload than a 150C rise unit, without damaging the insulation or affecting transformer life.

https://www.copper.org/environment/sustainable-energy/transformers/education/trans_efficiency.html

So it looks like a potting compound with a relatively high glass transition point may be required for site transformers, depending on their design, but this only means that a higher temperature would be required to remove the potting compound, a possibility already mentioned in post #14.

spec

 

[dr pepper]

Did someone post a vid?
I popped the lid off my 2kva yellow box trans, its a very hard yellow brown crud, I'd say it was epoxy, I wouldnt like to mess with it.
I spose you could load the trans with a load of bulbs and get it nice and hot then see.

 

[spec]

Did someone post a vid?
I popped the lid off my 2kva yellow box trans, its a very hard yellow brown crud, I'd say it was epoxy, I wouldnt like to mess with it.
I spose you could load the trans with a load of bulbs and get it nice and hot then see.

Hy dr pepper,

There is a couple of videos at post #18, but the first one show a relatively small module being de-potted rather than a site transformer and the potting compound softens at 100 Deg C, so boiling water is used.

I would suggest baking your site transformer at 200 Deg C in a domestic oven for three hours minimum- I'm sure your missus would not mind.

spec

 

[Nigel Goodwin]

but this only means that a higher temperature would be required to remove the potting compound, a possibility already mentioned in post #14.

Assuming it's not high enough to remove the varnish insulation from the windings

 

[tcmtech]

A very quick look search suggest old hid ballasts would be hard to obtain (and I had to look up what hid means). I see flourescent light ballasts in skips sometimes but I think they are rather smaller than the hid ones? Never really thought of re-winding one as a transformer.

HID ballasts, Mercury Vapor, Metal Halide, High Pressure Sodium and Low Pressure Sodium lighting, are all over the place.

Used ones are very easy to get a hold of. Just stop by any place that does metal recycling or street light maintenance and the odds are they have literally tons of old ones to pick from. Common ballast core sizes are anywhere from a few tens of watts to 1500 watts. 400 to 1000 watt being the most common.

 

[spec]

Assuming it's not high enough to remove the varnish insulation from the windings

Of course. The normal maximum temperature transformer wire is 210 Deg C, that is why I suggested a baking temperature of 200 Deg C.

spec

 

[throbscottle]

I think I'd go with a heat gun and just do small areas. Also allows for doing outdoors, I imagine it's a stinky process. I'll just keep looking out for one cheap enough for experimental purposes...

 

[spec]

I think I'd go with a heat gun and just do small areas. Also allows for doing outdoors, I imagine it's a stinky process. I'll just keep looking out for one cheap enough for experimental purposes...

Good idea

spec

 

[tcmtech]

If some of you guys are so determined to custom wind your own transformers that you would undertake the task of unpotting a potted commercial rough service transformer I may have to try selling some of my old HID ballast cores on eBay!
$5 for a 250 watt, $10 for a 400 watt and $20 for a 1000 watt plus shipping sounds like easy money to me!

It would sure beat the present $40 a ton I would get for them at the scrap yard now.

 

[throbscottle]

When you look at the prices of transformers, and you have a tight budget, if you want a slightly larger one for something then winding your own becomes something to seriously look at. Since most of us don't have an easy source of laminations etc, re-purposing starts to look like a good way to go.
So whilst un-potting is a bit of an extreme experiment, I think you may be onto a winner there tcmtech.

 

[tcmtech]

HMMM. I will have to think about that then! I have close to a 55-gallon drum worth of good condition ballast cores of assorted sizes and designs sitting around in my shop that would be perfect for guys doing custom transformer and inductor wind jobs.

 

[throbscottle]

I wish you luck! Just make sure you cover your postage and seller fees! Make sure you list them as more than just ballast cores