Backfeeding transformer for shop power

[Supa-Heat]

I fully understand this could be extremely dangerous if I don’t do it right and I am not going to do anything until I’m 100% certain what I’m doing.

My local liquidation auction has tons of industrial transformers ending in the next few days. Very few very low bids on them. They are mostly 15-75kva 1ph 480-240/120 stepdown xformers. Some 3ph and other misc. I’m setting up my new shop soon and while I don’t need it yet, I love having options. Would it be possible to backfeed a transformer to produce 480v from 240? My shop only has 240 1ph. I’d love to get 3 ph someday but that’s a future project. My current understanding is that what I’m wanting to do is possible and safe. I understand that in theory I will get a 3-4% drop from expected on secondary voltage at no load and 5-6% at full load. I’m also unsure how/where my neutral would be produced. Any thoughts?


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[billg]

You can't get three phase from single phase with a transformer.

[Supa-Heat]

You can't get three phase from single phase with a transformer.

I know, I’m referring to single. Although there’s not really anything I’d ever need 480 1ph for... I think some well pumps use it.


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[VTP99]

I'm currently looking for a buck/ boost R3 transformer.
480volt primary 240 volt secondary 10kva 1phase.
I wonder if they have one like that ?
Yes with this type of transformer you can wire them either way. You can not change the phase.

[Supa-Heat]

I'm currently looking for a buck/ boost R3 transformer.
480volt primary 240 volt secondary 10kva 1phase.
I wonder if they have one like that ?
Yes with this type of transformer you can wire them either way. You can not change the phase.

Smallest they have is a 15kva 480P 240s 1ph. Would that work for you?


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[VTP99]

If you know the make and model I could look up the physical size. Existing one fits in a outdoor enclosure.

[Supa-Heat]

If you know the make and model I could look up the physical size. Existing one fits in a outdoor enclosure.

Looks about the size of a suitcase.





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[VTP99]

Any R3 enclosure types. Those are indoor I need outdoor.

[HVAC_Marc]

You can't get three phase from single phase with a transformer.

yes you can. there's electronics involved and they are $$$$$$$ Another member here helped my friend out with finding a single phase to 3 phase transformer. They cost more than the rotary converters that are designed for the same higher loads.

[billg]

Than it's not just a transformer, is it?
It would be like hooking up a drive powered off single phase to power a 3 phase transformer, or something to that effect.
Making a 3 phase converter is simple and cheap anyway, so why bother.

yes you can. there's electronics involved and they are $$$$$$$ Another member here helped my friend out with finding a single phase to 3 phase transformer. They cost more than the rotary converters that are designed for the same higher loads.

[HVAC_Marc]

Than it's not just a transformer, is it?
It would be like hooking up a drive powered off single phase to power a 3 phase transformer, or something to that effect.

I forget exactly who it was who referenced it for me. Maybe kangaroogod. Anyway... It *looked* like a giant transformer in a box. I assume there was a a separate control board that made a phase offset for the 3rd leg. I do remember is was LARGE DOLLARS vs. a rotary 3 phase converter.

A drive changes frequency so i dont think it'll really apply to this discussion.

[Supa-Heat]

Than it's not just a transformer, is it?
It would be like hooking up a drive powered off single phase to power a 3 phase transformer, or something to that effect.
Making a 3 phase converter is simple and cheap anyway, so why bother.

Tell me more about this simple and cheap 3 phase converter... all options I’ve seen are pricy and intensive.


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[HVAC_Marc]

Tell me more about this simple and cheap 3 phase converter... all options I’ve seen are pricy and intensive.


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There isnt one. The transformer by itself was like 9k. The rotary converter was around 3-4k.

[billg]

All you need is a 3 phase motor about twice the size of the largest motor you plan to run, a hard start kit from an a/c to start it, momentarily start-stop switch, a relay, contactor and some capacitors (I save the old good pcb oil ones). All of which you can get used.

Tell me more about this simple and cheap 3 phase converter... all options I’ve seen are pricy and intensive.


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[HVAC_Marc]

and dont forget- you have to start the converter every time you want to start the unit you want to use.

OR, like my friend...


Let it run 24/7/365

[billg]

It's just a matter of pushing two buttons instead of one, or just let it run while you're working.

[HVAC_Marc]

my friend sold his 3 phase soft serve machine and bought a water cooled unit instead

[norcal01]

Please note that the NEC only permits backfeeding a transformer if the manufacturers instructions permit it, see NEC art. 450.11(B) which was added in the 2014 edition.

[coastalcon]

I have a fish processing customer that only has a 208 3ph service to their plant. I have had to install 3 transformers this year to supply new 480 V equipment and to supply shipping containers. Any 480-208 3 ph step down transformer has worked. I have had one unit that would not produce more than 430 v no matter the tap connections and I had another 150 kva xformer that would not go lower than 500 volts.

I also have fed single ph 240 to 3ph VFD's and they power 3ph motors just fine. I suppose you could feed the output of the VFD into a 3ph transformer to supply a 480 motor and it would function fine???

[coastalcon]

Oh and before anyone says you can't do this by code. All of these projects were done according to PE stamped design and mfg. instructions and signed off by AHJ...