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I have a company that makes servo AC voltage regulators and some other transformer-based products, and today some of my customers asked me to give him a quotation for an 800 kW Variac, the biggest one I have made was nearly 30 kW.

At first, I thought he was joking, but I realized he was not. He wanted to test something using this huge Variac.

I can make a Variac like this by combining several high-power smaller ones in parallel, but it will be cumbersome and big.

I would use an 800 kW inverter but it couldn't give an output more than input. Does anyone have a suggestion?

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    \$\begingroup\$ It might be worth investigating tap-changers and the tech that they use. \$\endgroup\$
    – Andy aka
    Commented Jan 15 at 18:05

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You could consider a matched motor sync alternator pair, with controllable field winding. Mechanical but flexible.

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    \$\begingroup\$ Thanks for the interesting idea, but I think it will be more complicated and expensive, and more susceptible to malfunctions. \$\endgroup\$ Commented Jan 16 at 5:24
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    \$\begingroup\$ Absolutely agree it may not suit, but I have actually seen a 10MW version of this setup for testing of very high power UPS systems, hence why I mentioned. \$\endgroup\$
    – colintd
    Commented Jan 16 at 10:12
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A suitably sized 3Ph VFD could do the trick. Whatever supply they have available if they want something this big they can spend the money to get the right 3Ph voltage. Near Megawatt inverters exist that are capable of controlling their output voltages at a fixed frequency. Coupled with properly rated transformers on the output and perhaps filtering equipment you can get a "clean enough" 3ph AC waveform capable of delivering 800kW.

I used to simulate unreliable AC mains with a 60kW inverter feeding a Xformer and filters to act as clean mains source. We would droop the output voltage down while maintaining 60Hz output to simulate droop, sometimes down to ~5Vac. In principle it would be the same thing "just" sized for the higher power requirement.

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    \$\begingroup\$ Adding a transformer to the inverter output to boost the voltage to 450VAC is very expensive, I think the cost (and weight) of this method is near doing the whole thing using only the Variac. \$\endgroup\$ Commented Jan 15 at 19:58
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    \$\begingroup\$ I think you'll find the transformer is the "cheaper" of the components I hinted at. \$\endgroup\$ Commented Jan 15 at 20:05

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