# Can a Variac be modified to output higher voltages?

Since variable transformers can be had quite cheaply these days (under AUD 45 / USD 30 / EUR 27 today for a 500 watt one through eBay from China) they are cheap enough to experiment with.

I bought one of these and took it apart, and I see that one of the wires in the coil at the 220 V point has had its insulation stripped and a wire soldered to it, going to the mains input. I am guessing therefore that if I move this wire to a different point in the coil, I will be able to change the range of output voltages that the unit can produce.

If I want my Variac to output between 0 V and 415 V (just picking any voltage above the 265 V limit it has now) when fed with a 240 V input, am I correct in assuming that all I would need to do is move the mains input wire to roughly 60% along the coil? (240 / 415 = 57.8%).

What sort of issues would I have to consider when doing something like this? The only one I can think of is that the magnet wire used to wind the coil would need to have insulation thick enough to handle the increased voltage, however the variable transformer design suggests that this may not be so important. Since the arm likely connects two coil loops together anyway, the voltage between any two wires in the coil would be quite small. This might increase from say 1 V to 2V which I am guessing is unlikely to be a problem. The other wires all have standard 600 V insulation so it would seem to be an easy enough modification to do.

Have I missed anything? Would this kind of modification lead to other problems?

If I want my Variac to output between 0 V and 415 V when fed with a 240 V input, am I correct in assuming that all I would need to do is move the mains input wire to roughly 60% along the coil? (240 / 415 = 57.8%).

No. You are completely incorrect.

This is just another version of 'can I use my step-down transformer backwards to step up'?

Any transformer core of a given cross-section running at a certain frequency has a maximum 'volts per turn' it can produce before saturation. You can safely assume that your transformer's 240v tapping point is already at as low a turn count as it will safely go.

If you do tap at 60% of the 240v turn and connect the transformer to 240v, the fuse or breaker will go. If you replace either with something that will handle 10x the current, then your transformer will smoke briefly before failing. Transformers rely on the primary inductance to keep the magnetising current negligible. Once the core saturates, the inductance collapses, and the magnetising current becomes huge, invariably enough to overheat the windings.

The way using the smallest amount of material to get 415v from 240v is to get a 240v to 415v autotransformer. An alternative if you can't find one of those is to use a 240v to 175v step down transformer, and put the secondary in series with the primary supply. Depending on phasing, you will either get 65v or 415v. Once you've connected it to get the latter, you've built a home-made 240-415v autotransformer.

• Oh of course, I didn't think about the inductance limiting the input current at all! I don't need 415 V, I just figured that if one was going to modify the output voltage, a common one like 415 V would be as good a target as any. When you move the input voltage tap, does the transformer gradually use more current the further you move it, or does it operate normally until the saturation point and then suddenly draws a huge current? What is the difference between a 240/240 V autotransformer and a 240/415 V one? Does it just boil down to more iron in the core? Commented May 19, 2019 at 12:00
• There's no such thing as a 240v to 240v autotransformer, no point. How a transformer behaves as you approach saturation depends on the detail of the iron in the core. Some grades have a sharp knee, some have a gentle slope. As you move the input tap point down turn by turn, the magnetising current increases with each move. Eventually, you decide the current is too much for your specification. Microwave oven transformers run at magnetising current similar to load current, because the manufacturer can use less iron that way, they're fan blown, and the user pays for the extra power dissipated. Commented May 19, 2019 at 13:46
• Sorry when I said 240/240 V autotransformer, this was just a shorthand to refer to an autotransformer with a 240V input and a similar output range (say 0-265V), as opposed to the autotransformer you mentioned in your answer, which takes a 240 V input and can output 0 to around 415 V on the output, which I wrote as 240/415 V. I'm not sure what the correct abbreviated terms are for these so I just shortened it to 240/240 V, I wasn't suggesting an autotransformer with a fixed output voltage the same as the input because as you say that would indeed be pointless. Commented May 26, 2019 at 2:50