Transformer Question

as long as you don't want too much isolation. What voltage breakdown do you get with the 2 primaries wouund one directly on the other?

NT

Kilovolts, probably. Test it for margin if you plan to float the load way off ground.

It will have more inter-winding capacitance than one of those extreme-expense isolation things.

that's at odds with my experience of hipot testing transformers. Enamel on enamel failed below 1kV. Unfortunately I don't remember what voltage it managed. Probably over 500v, but that's not a comfortable margin.

Yes. I don't know how much shock current that lets through.

NT

I made a twisted pair of #36 magnet wire and applied DC until it arced. 1400 volts.

Somebody should test a few ordinary dual-primary transformers for breakdown.

Winding-winding capacitance is easy to measure. Grounding the frame will reduce 3T capacitance.

microwave oven transformers are not designed to provide safety isolation, only functional isolation, that is why microwave ovens are grounded.

definitions:

so yeah, if you don't need safety isolation you can do that.

Microwave ovens typically have one end of the secondary grounded to the transformer core, so, if you hook 2 together hoping for two layers of functional isolation you need to not ground the transformer cores, else you only get one layer.

Microwave oven transformer cores are typically welded shut, so changing the windings is tricky (maybe you could open one with a 1mm thick cut-off wheel and re-close it using a MIG welder). but still you only get two layers of functional isolation, unless you do a complete replacement of the windings.

I got my 1500VA isolation transformer for $8 from the municipal recycling shop, this was in 2001, before the surge in copper prices.

Special copper wire?

He's probably thinking of triple-insulated wire, and I don't think you'll find that in any 50/60Hz isolation transformers. It's useful in SMPS transformers where you want to put the windings as close together as possible to link all the flux, but you don't care much about primary-secondary capacitance.

--sp

Nope, for a transformer the basic insulation in case of an earthed laminated core or the double insulation in case of no earth depends on the wire classification

Typical class F wire, which is approved for 155 degrees C.

See for example section 7 in this:

Or first table on this link:

During maximum power through the transformer or shorting of the secondary side, the winding gets hot, and must stay below 155 degrees to still have the insulation system in place

Cheers

Klaus

Relying on testing insulation is venturing on a slippery slope

Then you need to do 100% production testing and getting approval from UL or VDE is VERY difficult and time consuming

Cheers

Klaus

Insulation between primary and secondary is **not** dependent on the wire insulation. It's provided either by bobbin design (split bobbin) or by the UL/CSA approved (north america) tape between primary and secondary.

That's true whether the wire is class F or lower temperature rating.

--sp

On our instruments we used to do 100% Hipot with a 60 second soak time. Dead users are bad for business.

--sp

I assume the op wants to hack one isolation transformer for personal use. I don't think he intends to get UL or VDE certification.

If you are making a 1:1 transformer using a couple of MWT's ( MicroWave Tro nsformer ), You do not use the secondary windings. You can open a MWT usin g an ordinary hacksaw. Been there, done that. You use the primary winding from both transformers and remove te laminations that increase the leakage . inductance. A large hose clamp will work to hold the core pieces together, or you can kludge a nicer looking clamp with a couple pieces of steel and s ome fairly long bolts.

If you are making a low voltage transformer you can cut the secondary off , remove the flux short laminations and wind a new secondary. But I can nev er get a very nice secondary this way. _If you are wanting a honking big low voltage transformer you can use two ( or more ) MWT's with the original secondaries removed and wind a new secondary winding the secondary through both cores.

Dan

Don't most dual primary transformers have a layer of insulation between the two primaries?

Dan

I don't know. But they surely have to be insulated enough to withstand line transients. And of course the 120/240.

I've done the dual-primary trick many times, and I'm still here.

It's not unusual to see a 5% increase in applied voltage produce a 500% increase in I mag. Similarly, when a load is applied to the secondary the reverse happens, with I mag dropping by 500%.

..... Phil

Those articles require the disassembly ( grinding off weldings ) and rewinding of the transformer.

You are an utter IDIOT, Dan.

..... Phil

That is not adequate insulation for a mains isolation tranny, it could not pass safety standards anywhere.

..... Phil

YOU need to be WAY more careful with what you post !!!!!!!!!!!!!!!

W what you posted before was DANGEROUS advice - anyone reading it would think you meant to use the two transformers as they were.

You need to go back and retract that post NOW.

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.... Phil