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Thread: Backfeeding transformer for shop power

  1. #1
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    Backfeeding transformer for shop power

    I fully understand this could be extremely dangerous if I donít do it right and I am not going to do anything until Iím 100% certain what Iím 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. Iím setting up my new shop soon and while I donít 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. Iíd love to get 3 ph someday but thatís a future project. My current understanding is that what Iím 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. Iím also unsure how/where my neutral would be produced. Any thoughts?


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  2. #2
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    You can't get three phase from single phase with a transformer.
    It's not what you're capable of doing that defines you, it's what you do on a daily basis.

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  4. #3
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    Quote Originally Posted by billg View Post
    You can't get three phase from single phase with a transformer.
    I know, Iím referring to single. Although thereís not really anything Iíd ever need 480 1ph for... I think some well pumps use it.


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  5. #4
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    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.

  6. #5
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    Quote Originally Posted by VTP99 View Post
    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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  7. #6
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    If you know the make and model I could look up the physical size. Existing one fits in a outdoor enclosure.

  8. #7
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    Quote Originally Posted by VTP99 View Post
    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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    Any R3 enclosure types. Those are indoor I need outdoor.

  10. #9
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    Quote Originally Posted by billg View Post
    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.

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  12. #10
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    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.



    Quote Originally Posted by HVAC_Marc View Post
    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.

  13. #11
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    Quote Originally Posted by billg View Post
    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.

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  15. #12
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    Quote Originally Posted by billg View Post
    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 Iíve seen are pricy and intensive.


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  16. #13
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    Quote Originally Posted by Supa-Heat View Post
    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.

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  18. #14
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    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.


    Quote Originally Posted by Supa-Heat View Post
    Tell me more about this simple and cheap 3 phase converter... all options Iíve seen are pricy and intensive.


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  19. #15
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    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

  20. #16
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    It's just a matter of pushing two buttons instead of one, or just let it run while you're working.

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  22. #17
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    my friend sold his 3 phase soft serve machine and bought a water cooled unit instead

  23. #18
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    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.

  24. #19
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    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???

  25. #20
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    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...

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