Can a Variac be modified to output higher voltages?

Question

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?

Answer 1

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.