What happens if I apply voltage to a transformer that is different than it's rating?

[fbmstar]

Hey all, I've got a transformer with the option to supply the primary with 480V or 240V and the secondary output is 120V, all single phase. I'm in need of 240V single phase on the secondary.

What would happen if I wire the transformer with 240-120 configuration, but applied 480 to the 240 side?

I've asked a couple people whom I trust with these sorts of things, but they are both skeptical about what the current rating would be if this was done. it's a 1000VA transformer, and i need about 6 amps on the secondary. I was thinking based on ohms law, it was all linear, but they seem to think there's an exponential factor involved. Curious if this is a terrible idea or not. Appreciate the help!

-Brian

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[wmgeorge]

Strange as it seems it does work in reverse as I have done it. Since the wires are usually rated for 600 volts, nothing bad should happen.... but if smokes comes out.
VA is VA same as Watts kind of anyway, it comes out about 4 amps when I calculated quickly, You might look for a larger transformer or figure out a buck/boost wiring arrangement but VA is still VA.

[fbmstar]

thanks for the quick reply! I wasn't really planning on going in reverse. I do have 120V available, but not enough current. My supply options are 480V or 280V, and the servo driver needs 1000VA / 200V-240V+10%-15%. So, 170-264V needed and the 280 is a bit much

[Jim Dawson]

Sounds like you need a 480/240 1.5KVA transformer. Those are pretty common, should be a ton of them on EBay.

[skrubol]

It's not just the insulation rating in a transformer that comes in to play, the volts per turn matters as well. Doubling the volts per turn will likely cause significant unloaded loss in the transformer, causing it to get hot.
If it was much higher rated than you needed, you could use it as an autotransformer by leaving the secondary disconnected and pulling your 240v from one 480v line and the center tap. It wouldn't be isolated though, so would probably be pretty dangerous.

[wmgeorge]

It's not just the insulation rating in a transformer that comes in to play, the volts per turn matters as well. Doubling the volts per turn will likely cause significant unloaded loss in the transformer, causing it to get hot.
If it was much higher rated than you needed, you could use it as an autotransformer by leaving the secondary disconnected and pulling your 240v from one 480v line and the center tap. It wouldn't be isolated though, so would probably be pretty dangerous.

One line of 480 V 3 phase is 277 V to ground. I have seen and worked on 277 V HVAC equipment compressors and fan motors. We used 277 V lighting in the industrial plant. A transformer is just ratio, I have seen ham radio people take a pole pig transformer and feed 240 single phase in to generate 7200 volts HV for home brew 2 kW amplifiers. It works.