# Using a 3 phase transformer with single phase input

#### strantor

Joined Oct 3, 2010
6,811
Any reason I can't do this?
I assume I need to derate it by $$1-\frac{1}{\sqrt{3}}$$ and disregard (cap off) the 3rd primary and secondary legs?

#### Kermit2

Joined Feb 5, 2010
4,162
If the extraneous windings are not conducting current then they are not generating heat. You may not have to de-rate it at all.

#### strantor

Joined Oct 3, 2010
6,811
If the extraneous windings are not conducting current then they are not generating heat. You may not have to de-rate it at all.
I like when I try to do things the wrong way and there are unintended benefits.

#### strantor

Joined Oct 3, 2010
6,811
P.S. (for future posters) I just realized I might have left room for misunderstanding; I'm looking for single phase transformation - single in, single out. I'm not trying to generate 3 phase from single phase.

#### tcmtech

Joined Nov 4, 2013
2,867
When running a three phase transformer as a single phase unit you can use the two outermost winding sets in parallel which will give you near full rated capacity.

If you run all three primaries in parallel the center winding set needs to be wired out of phase with the the other two but most transformers will go into core saturation if you do it thus the recommendation to just run with the outer two sets instead.

Secondaries of all three can be configured however you want though.

#### strantor

Joined Oct 3, 2010
6,811
When running a three phase transformer as a single phase unit you can use the two outermost winding sets in parallel which will give you near full rated capacity. If you run all three primaries in parallel the center winding set needs to be wired out of phase with the the other two but most transformers will go into core saturation if you do it thus the recommendation to just run with the outer two sets instead.

Secondaries of all three can be configured however you want though.
That all makes perfect sense. I trust you say this from experience; sounds like something you would try.
Thanks for the input; In my mind's eye I saw myself using the one outside winding and the middle one, no reason why. But using the two outer gives better heat dispersion.
I figured as much about the center winding being out of phase if using all 3, and about the core saturation, which is why I was planning on just using two, but I was off the mark about which two to use. You may have saved my transformer!
Thanks again.

#### tcmtech

Joined Nov 4, 2013
2,867
Done it a number of times and and only ever had one transformer that did work with all three primary winding sets powered. Everything else had high idle current hot windings at no load and lots of core saturation hum.

Take one primary out of service and they quiet down and cool off every time.