Isolation transformer primary/secondary?

[whoaru99]

Picked up these two transfomers, with a bit of work I may add, alleged to be isolation transformers. 30lbs each, the leads are 12ga.

There is no connection between the two windings, nor to the frame.

Applying 120V in nets 117V out. Same regardless if the red or black wires are connected as primary.

Is there a specific criteria to be considered isolation transformers, aside from just two separate windings?

Is there a proper primary of the two windings? If so, how to identify it?

 

[NAD80]

Is there a identifier as to part number / manufacturer?. Measure the resistance of each winding. If each winding measures the same, then they might be a isolation xfmer. A normal xfmer has a higher primary resistance, lower secondary resistance. The difference in the colors are just to show each winding.

 

[dB happy]

The primary and secondary probably have the same number of windings. There's always some loss in the cores, a couple volts is normal. I made an isolation transformer a long time ago by chopping up a couple microwave transformers, and putting 2 primaries on one core. I chose one that had a couple more turns for the secondary. I got 118.9v in 121.1v out with no load. With a 10a load, the secondary dropped to 116v.

 

[asilker]

Interesting.

 

[arts]

Traditionally (but who knows anymore) black was always the primary,red was the secondary,and was the highest voltage winding if multiple secondaries were used.

Based on your description,that is a serious pair of transformers

 

[gadget73]

Thats what I was going to say, black is typically primary. There may be some hipot insulation test that is used to officially class them as an isolation transformer but thats largely for certification purposes. If you aren't building commercial gear around it and selling it to someone who might sue you its likely just fine for bench use.

Either way they sound like serious lumps of iron. 30 lbs and 12awg windings? Should be able to handle a lot of current.

 

[whoaru99]

Good point. Now that I think about them as any other transformer you are probably correct the black leads would be the considered the primary leads.

Trying to figure approximate rating based on size and weight comparing to other transformers. There seems to be fair correlation up to 1kVA.

 

[arts]

Based on weight and lead size,my guess would be somewhere in the 750-1000va range.

Most isolation xmfr's have an electrostatic shield between primary and secondary,which is sometimes brought out by a lead and grounded to one of the frame bolts.I have also seen others where a solid lead is brought out and spot welded to an outer lamination.

 

[whoaru99]

I had fleeting thought of using them in series of the secondaries to create some sort of big amp with +/-165V rails, but that seems like a lot of work.

Perhaps instead I'll make a 60-0-60 type of isolated, balanced mains supply. Wire the two primaries in series and run that from 120V, with the secondaries in series I should then get 60-0-60 balanced power.

 

[gadget73]

If those were sitting here, I'd probably add a fully isolated power strip to the bench just for occasions where you need such a thing.

 

[Alan0354]

Isolation transformer is isolation transformer. I don't know whether it makes any difference using either the black or red as primary. I would just pick the one the wound inside as primary, but is it even important?

For us before, we worry about isolation voltage. We bus power up to over 10KV and power a lot of electronics, so we always hi-pot the primary and secondary to make sure it will withstand the HV. But this does not apply to you. So I said you are good to go.

 

[whoaru99]

Not sure where the 117V came from.

While playing around today I got 126V (unloaded) out of each one with 122V input.

 

[mbz]

What is output voltage when loaded, say a 200W appliance...

 

[whoaru99]

Dunno, haven't tried that yet; maybe a bit later.

 

[mbz]

While investigating torridal step down transformers I picked up the following,

- transformer output voltage applies at rated power/VA. No load voltages will be
higher, secondary voltage will drop as load increses.
- allow about 3-4% for iron and other lossed, so loaded 120V in 117V out sounds ok.

 

[whoaru99]

Closest I could find without going much larger (larger I will try in time) was 150W.

With 150W load, 121.5V in produced 125.8V out.

 

[mbz]

I got 118.9v in 121.1v out with no load. With a 10a load, the secondary dropped to 116v.

From dB happy above

With 150W load, 121.5V in produced 125.8V out.

Yeah, I can understand your concern, however the transformers appear to be isolation transformers, 120V in, 120V out at rated power/VA.
From the spec's I've read, a no load/full load difference of 4-5% is OK.

 

[gadget73]

does it do any different if you reverse which is used as the primary?

 

[pdm4606]

I'd dim bulb them to see what happens. Can't go too wrong. For sure you'll see something happen without doing too much harm.

 

[whoaru99]

does it do any different if you reverse which is used as the primary?

I'm back to the 121V in, 117V out with the red leads used as primary. However, I don't really trust the measurements or the method because I've gotten two different results.

I need to terminate the leads with some connectors I can actually use rather than continuing with iffy test clip connections. Probably going to lop off the Molex(?) connectors and crimp on some spades. That way I can connect to barrier terminal strip for sure contact.