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I've come across a variable transformer/variac which is designed to be used with 110V input. Our line supply here is 230V.

If I was careful not to exceed the VA rating of the transformer (920VA) would it be possible to safely use this with a 230V input? (I'd expect the output voltage to be twice of course, and I suppose replace the fuses with 1/2 current equivalents)

It's something like this:

enter image description here

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up vote 8 down vote accepted

No, the core will saturate and (if you're lucky) blow a fuse.

If your 230VAC happens to be (say) 400Hz, the answer might be different.

Wikipedia has a very nice animated image showing the effect of too low frequency. Too high voltage has the same effect. The value of (maximum primary voltage)/frequency is a constant for a given transformer design).

enter image description here

As you can see, the magnetizing current spikes up twice per cycle and the resistive heating losses increase with the current squared.

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Thank you! I'm glad I asked :) Just out of interest - what might happen if I wasn't lucky? – UpTheCreek Mar 5 '14 at 16:22
@UpTheCreek If there was only a fuse on the variac wiper (there really should be two fuses), then the winding could heat up to the point where the insulation was damaged, nasty fumes emitted and your variac could develop one or more permanent shorted turns (which means it would overheat every time the power was applied). You'd be "Up the Creek". – Spehro Pefhany Mar 5 '14 at 16:25
Heh, thanks :-) – UpTheCreek Mar 5 '14 at 16:49

While Sphero's answer rules if the device really is 110VAC input only, my experience with Variacs is that most of them are 240V friendly - to the point that they come packed with a dial that can be flipped to read correctly for 120VAC in or 240VAC in - so you might want to look "inside the case" and see if the Variac itself is indeed only 110VAC labeled, or of it's a 100-240VAC variac in a case that "makes it" 110VAC due to connections and external labelling.

We used a lot of them to control high voltage supply outputs by controlling the input voltage of the supply back in the 1980s, so I've got some practical experience with them.

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Interesting, I'll try and investigate. Thank you. – UpTheCreek Mar 5 '14 at 20:19

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