Using 230v transformer on 120v?


Searched but wasn't able to find info on this topic.

I am looking to build one of the Pass amp variations that uses relatively low power supply voltage. Something like 15 - 18 vac from the transformer.

I've located a surplus torrid that has a 230 volt primary and 30 volt secondary windings.

My question is, what happens if I run this transformer on 120 volt?

Can the transformer still deliver close to the rated current at half the voltage? Or will it burn up trying to do so?

I'm trying to figure out if I can simply buy big surplus transformers rated at 230 or 240 vac and have them be fine running at half voltage and yet still be able to provide a low impedance, big power reserve if needed?

If they do need to be de-rated, how much de-rating would be required?

Thanks in advance for any thoughts/advice you're willing to provide!
The reason I asked about current de-rating is because I'm guessing the windings for a 230 volt primary will have lighter gauge winding wire than a comparable 120 volt winding that is rated for same current. Hence it might get hotter when trying to supply the same current.

That said, the primary winding should also have a higher impedance so maybe it won't matter.

These were my thoughts and why I asked.
Current depends on copper cross area or wire diameter (or cross sectional area), not on voltages applied. Use it worry-less.

But if the primary winding wire is smaller diameter because it was designed for 230 volt, wouldn't it get hotter trying to supply the rated current using 120 volts?

My thought being if the voltage is halved, the current going through the primary (and secondary windings) doubles if the same VA rating is maintained. Hence the windings might overhead and fail.

That's why I asked about de-rating.

And there is also the core saturating and other aspects that I know nothing about. I figured it was worth asking so I can learn more about this stuff.

Thanks to all who have answered thus far!
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“because it was designed for 230 volt, wouldn't it get hotter trying to supply the rated current using 120 volts?”

VA rating would normally stay the same, but since you don’t know the wire used on the primary, it is best keep the same current limit. This means for your example, drop the rated VA by half.

If you knew for certain the wire rating of the primary, this might be different.

Saturation when using a lower voltage? No, not going to happen. If you keep the current below the rated value it won’t overheat. It should actually be cooler.

Btw: Using cap input power supply? Drop the current draw by a quarter to a third.
The transformer I bought is rated at 230 volt primary, 30 volt secondary @ 16.67 amp x2, 1000 VA.

Is there any downside to using a 230 or 240 volt designed transformer?

So just to be clear, I should really think of this as a 500 VA unit? Is there a way to limit the transformer current (besides using MOV) to ensure it doesn't overdraw the primary (thinking of in rush current draw) or will it limit itself?

I figured this might also help others because it could open the door to more surplus transformer options.

Thanks to each of you for your help!
Yes, as others have said, the VA rating is 1/2.
The other things to consider are the size and weight.
For example, from one manufacture of toroids:
1) a 400 VA, 30 V sec transformer weighs 9 lbs and measures 5.1" dia. x 3.1" high
2) a 200 VA, 30 V sec transformer weighs 6 lbs and measures 4.9" dia. x 2.5" high
So the 230 V transformer is 50% heavier and about 30% larger by volume than the 115 V for the same useful VA. Might not be an issue but that's for you to decide.

Edit: Yes the 400 VA unit should be 60 V, but the physical paramters are the same as the 30 V.
And you limit the primary current by limiting the load on the secondary, not by restricitng the primary.
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The amount of Iron will help in cooling so as long as you yteat it as a 500VA device you will be well within limits. As it is not supposed to be a portable or even transportable device you can rest easy. Most transformers over here in 230V land are dual primary so you are unlucky to get a single primary TX.
Disagree. Regulation depends on direct DC resistance of the secondary winding(s) plus any resistance in the primary winding itself (DCR) and including switches, plugs, fuses, etc. reflected by the square of turns ratio. Currents and voltages don't participate in the equation.
I have to disagree with this disagreement :^) Regulation also depends on the voltage - it's the ratio of the voltage lost to resistances divided by the unloaded voltage.
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