Question about VA rating at lower inputs

[Francois G]

I bought this transformer that was rated 700VA by the seller. The info I have is not much. It has a label as follows:
Input 0 — 115v — 230V
Output 0V — 720V, 0V — 6.3V, 0V — 5V

No amp ratings for any, but I’m more interested in feedback on how to use this beast.

If I use this with my US voltage at around 120 VAC connected to the 230V transformer inputs I expect to get around 375 Vac out (disregarding loaded vs. unloaded conditions). Correct? I realize that the low voltage output would be halved in voltage

Now my question: What would be the VA output under these conditions?

 

[scottjoplin]

The maximum secondary current will be governed by the winding gauge

 

[Osvaldo de Banfield]

As primary voltage decreases, decreases flux in the core and then, secondary(ies) voltage(s), power looses will also decreases, but the current may be slightly increased as the IR drop in the secondary winding is not altered. Also I²R in the winding maintains, so heating as a consequence of loading is the same.

 

[Francois G]

As primary voltage decreases, decreases flux in the core and then, secondary(ies) voltage(s), power looses will also decreases, but the current may be slightly increased as the IR drop in the secondary winding is not altered. Also I²R in the winding maintains, so heating as a consequence of loading is the same.

Dear Oswaldo, your opinion is very valued over many years of lurking in the background. For those of us, I just wanted to say thank you!

So, what does that explicitly mean in my case? Am I getting a 350 VA transformer with around 375Vac out?

 

[Vovk Z]

If I use this with my US voltage at around 120 VAC connected to the 230V transformer inputs I expect to get around 375 Vac out (disregarding loaded vs. unloaded conditions). Correct? I realize that the low voltage output would be halved in voltage

Now my question: What would be the VA output under these conditions?

Yes, it's correct. Rated current will be the same, so VA output will be proportionally decreased.

 

[PRR]

To a first approximation: you are only using half the volts you bought, so VA is half: 375VA.

Total heat will be lower at half voltage (less core loss). But core loss is small in modern iron. I'd provisionally allow almost 10% up-rounding: 410VA.

It is probably not important 375VA or 410VA in most audio uses.

It is important to know (as Osvaldo says) that taking half the voltage (both in and out) will not let you double the current.

 

[PRR]

And note your 6V and 5V windings will be no-good for tube heaters, being 3V and 2.5V. (Yes there are 2.5V tubes...) Series they look like 5.6V, and I would run a 12AX7 at _low_ current on 5.6V, but you didn't get hundreds of VA to run little 12AX7s.

 

[Francois G]

Thanks everyone. This was a sanity check of my expectations before I start experimenting. I will verify these numbers in practice, but I hope to use this transformer to build the HV supply for a KT88 PP amp.

 

[CBS240]

If the toroid is open through the center, you could add a turn or two to the series low voltage windings to get what you need. Doubtful that you will need a slow start circuit on the primary since it is operating far from full flux swing.

 

[Vovk Z]

Doubtful that you will need a slow start circuit on the primary since it is operating far from full flux swing.

Yes, at a halved voltage inrush current will be several times lower, may be even on order lower, or more.

 

[Osvaldo de Banfield]

Dear Oswaldo, your opinion is very valued over many years of lurking in the background. For those of us, I just wanted to say thank you!

I'm flattered for your comments. Electronics and electricity are my passion since very young. Father was a bank employed but since very young he transferred electricity knowledge to me, and gifted me electrical pieces that he catch from maintenance in the job. Now I'm Engineer and I continue having this passion.