Reverse-feeding a small step-down transformer.

As a result of renovations at work I've come into possession of a few step-down transformers, 120 VAC in, 24 VAC out, rated at 5 amps out. Just for shits and giggles, I'm curious about reverse-feeding it via the variac and using it as a step-up transformer. In theory, I should be able to get 600 VAC if I feed live into what is ordinarily the low-voltage (secondary) winding. Even more, probably, since my variac can put out 143 volts at max.

Anybody ever played around with transformers like this? I tested it a little but limited the input to 35 volts, at which point the output was about 160 volts and the transformer was humming noticeably. I was expecting ~175 volts, but I suspect these units have compensated windings. Factoring in that assumption, the predicted voltage is indeed close to 160. I didn't try to load the output, I just tested the voltage.

Potential problems I see:

- Excessive inrush current
- Increased magnetizing/idle current due to lower turns/volt on the secondary
- Dielectric breakdown / arcing (read: POP!) on the primary due to high voltage. I think most magnet wire, at least the kind that gets connected to mains, is rated to 600V or more but of course, hard to tell without taking the transformer apart.
- Dielectric breakdown on the secondary since it would be taking 120 volts (up to 140 if fed from the variac) instead of 24.
- Massive overcurrent on the input, followed by flames, when the output shorts out as described above.

I probably won't try to run this up to full voltage since I think it will just explode and catch on fire, which is a waste of a transformer. Waste of a house, too, if that burns down. If anyone has experience back-feeding little transformers like this to get high volts, I'd be interested in hearing your stories, especially anything that went wrong or blew up.

You've pretty much covered the issues when doing it in reverse. It's also worth noting the winding resistance will be higher on the secondary.

I've never used a power transformer in reverse before, but I personally wouldn't try it. The way that the transformer is designed will work against you in reverse. Best to get one that is designed for step up.

Not gonna work.

Try it anyway. You'll find that even with no load, the current will begin to increase to catastrophic levels when increasing the voltage much beyond the rated voltage. Has something to do with core saturation I think? You can make it work if you increase the frequency, but that's hairy business.

I've done it carefully but with small devices.
Don't exceed the rated secondary voltage input
or current AND remember ------
PEAK voltage = RMS voltage AC x ROOTA2...
HMike

I've used step-downs in reverse before for various applications. Just don't exceed the normal operating conditions, that's all. Cores are rated to a VA, don't exceed that, the wire gauge current limits, and the insulation voltage limits, and all is well.

In a bind I've used two step down xformers back-to-back as an isolation transformer. Again, just mind the VA ratings.

I did it with a step down from a heat pump. But my circuit sucked. I had two of the transformers. I ran the first one from a 12volt dc phone cord wall jack, and it successfully stepped it up to 120v. But the next one only went to 220v because the current was so small that the first transformer dropped to 37v. When I plugged the secondary into the wall, it smoked, so it can't take it.

I imagine using a ballast on the mains/input side of it would prevent the worst if the transformer did decide to pop. 2.5 to 3 amps or so would probably do it. The step-up ratio of these units is about 4.3, so that would allow 500+ volts at ~500mA for short durations, assuming the insulation handles it. Not sure what you could do with that but it would rectify to ~700 volts DC which I'm sure would be good for charging up caps or something.

USAbro said:

I ran the first [transformer] from a 12volt dc phone cord wall jack, and it successfully stepped it up to 120v.

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You mean AC. Transformers do not work with DC.

Marco Polo said:

I imagine using a ballast on the mains/input side of it would prevent the worst if the transformer did decide to pop. 2.5 to 3 amps or so would probably do it.

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You're misunderstanding how ballasting works. A ballast is just increased reactance in series with the load. Phase relationships aside, reactance acts as a resistance. This means the full voltage is at the load with minimal current, but as the current increases, the ballast tends to drop the extra voltage. Since increasing the voltage into a transformer winding above its rating will result in a higher current, the ballast becomes more active (or, REactive, technically) and drops voltage, much like a simple resistor would. That defeats the purpose of sending a higher voltage to a winding.

Cyparagon said:

You're misunderstanding how ballasting works. A ballast is just increased reactance in series with the load. Phase relationships aside, reactance acts as a resistance. This means the full voltage is at the load with minimal current, but as the current increases, the ballast tends to drop the extra voltage. Since increasing the voltage into a transformer winding above its rating will result in a higher current, the ballast becomes more active (or, REactive, technically) and drops voltage, much like a simple resistor would. That defeats the purpose of sending a higher voltage to a winding.

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And so the result of this would be that as load increases, the output voltage would fall more and more. I figured something like that would happen. I think that, at most, if I did anything with this it would be to charge capacitors, or something involving very low current. I wouldn't want to actually run up against the current limit of the ballast. The ballast would just be there to limit fault current if one of the transformer windings decides to pop. I'm not going to be running motors or anything current-heavy off of it.

Realistically, will probably not bother with it anyway because I can probably find something more useful to do, but it's winter and cold, and that makes me bored sometimes.