Using 3 phase step down transformer as 1 phase step up???

Thread Starter


Joined Apr 29, 2010
Hi all,
Im new here :) glad to join you.
Bit of a question.
Im after a transformer to step 240v up to 400v around the 3-4 KVA.
Now saw a 3 phase transformer going for a good price and wondered if i could use it?

It was disighed as 3x 400v in and 3x80 out...

Below is the layout with the way im supposing i can connect it to get the 240v im after. Can this be done?? Would resestances in the secondary windings affect the 240v series output?.. or anything weard that i dont know about? ( Im expecting the three primary winding to be identical and the same with the secondaries... as I would think it be crazy if there was differances between the phases..

Also the original KVA rating is about 3280 KVA. I take it this figger would be total and not per phase?... So connected the way i have showen would be 3280KVA?

Thanks :)


Joined Mar 6, 2009
There may be issues with the resultant transformer core magnetic flux density levels which could show up as excessive core saturation. Three phase transformer cores are designed from the perspective that the magnetic circuit is excited by a balanced 3-phase voltage supply - not a single phase supply.

What is the specific transformer construction type you are looking to buy? Is it 3 limb or 5 limb core type.

There would definitely be no problem with three separate single phase 400:80V transformers wired according to your plan.


Joined Jul 23, 2009
As a long-time magnetics designer, including many 3-phase transformers, I would say that you should have no problem. Yes, the secondary winding resistance will come into play, but if the unit is a 3 to 4 KVA part, it should be low enough to not cause huge loses. However, it is easy to measure. Put a known current thru the entire 3-connected secondary, of about 1 amp DC to about 4 amps DC. Using a DVM, measure the voltage across the entire secondary--make sure you put the voltmeter leads right at the transformer secondary terminals or wires. Compute the DCR as Vsec/ISec. This is a much more accurate way than using a DMM's ohmmeter as they are not good for low ohms readings of much less than about 2 or 3 ohms. This method will give you much better accuracy or the DCR and then you can compute how it will affect your load circuit.
Kamran Kazem

Thread Starter


Joined Apr 29, 2010
Thanks guys,

t_n_k, idealy im looking for a 1 phase 3 wire autotransfomer 230v to 400/430v, (sorry i dont understand what 3 limb or 5 limb core type is?). Will be using to build a phase converter. what to include phase voltage sencing with zero fired scr's switching in capasitor banks for automated phase voltage balancing, but have yet to sort that out :). But I expect to find some good resorces here.

kkazem, would you concure that core saturation could be a potentional problem with running the 3x 80v secondaries in series for 240v input? I have know way of testing transformer before sale as the seller is not local to me :(.

Could someone explane to me, (just for my understanding), the details of how and why single phase supply could lead to core saturation through the 80v winding arranged in series? as apposed to balanced 3-phase voltage running through them seperatly? Am i wrong to assume that the 240v is equaly split into 80v between each of the 3 windings? Or is it something like each of the windings causing a slight phase shift in relation to the other 80v windings? Would this even happen? and if so I could only see this getting messy when trying to bring the out of phase 400v from each of the, (now), secondary windings back to gether through their parallel configeration?? Or would non of this actualy occor?

Thanks for your time :)
Hope im not throwing to many crazy questions out there.
i want to make two transformer which is step and step down both. i need 220 v input and 440 v output and the 440 v input and 220 v output.actually i will be use this transformer in line distribution