How to size buck-boost transformer?

[Captainjack]

Here is my dilemma:

My incoming voltage single phase is 255v.
I am running a rotary phase converter and my output voltage is 255v.
My Fanuc machine wants to see 208v with 233 as maximum.
I am running a 60 amp single phase input circuit with the load amps usually about 20 to 30 amps.

The Grainger calculator says I need a 2 to 3 kva buck boost with 16-32 volt secondary, BUT the Federal Pacific and Acme selection tables say a 1kva will work to go from 255 to 239 volts at up to 66.67 amps.

If a buck boost has a 16-32 secondary, does that mean the incoming voltage will be reduced by 16 or 32 volts, or just one or the other depending on 120 or 240 input? The tables from the manufacturers don't show that you can reduce 255 to 223 (255-32), just 239.

I am confused! Please help!

[Warpspeed]

The transformer has to be large enough to drive the secondary load, volts times amps. The transformer neither knows nor cares that the secondary is connected in a buck or boost configuration.

If your secondary has to provide 16v at 67 amps then the transformer must be rated to supply (16 x 67) 1,072 VAs. A 1KVa transformer should work fine.

If your secondary has to provide 32 volts at 67 amps, then a 2KVA transformer will be required, because the output voltage has doubled.

I would start looking around for something in this range, maybe a 24 volt secondary at 65 amps, that would be a 1.6 KVA transformer.

[panaceabea]

The fellow from the local power company, Milltronics and Acme transformer use a slightly diff formula than the one on the Grainger sight, I believe there rating is at the 24 volts and needs to be 10 x more for 240 volt. That is how it was explained to me by the fellow at acme and the guy from FL power. I initialy used the formula at grainger , came up with 1 kva for my application and was very diapointed when I received a little transformer about the size of a baseball the next day on the UPS truck . It gave up a very nice puff of smoke as goo dripped to the floor.

I called both ACME and the manufacturer of my machine and both calculated that I needed 10-12 KVA , I purchased the 15kva from Acme and its worked beautifully. If you find a dealer that deals with acme the 15kva is just short of $550 , You can also find them on ebay for around $200 used , I bought a pallet of four a few months ago for $600