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Converting three-phase into single/split phase

  1. Feb 25, 2011 #1
    I recently acquired a 12kw three-phase Onan propane generator, like new, given to me basically.

    Is there a way to convert three-phase into single phase for use in a normal residential home, without swapping-out the gen head?

    My instincts tell me the answer is going to be, "step down transformer" or converter of some sort. But I was hoping there may be another way or either hoping to have it confirmed that I am on the right track with the transformer (sic) train of thought, pardon the pun.

    Viewing available diagrams, I see no way to do it via a simple wiring conversion.

    Any suggestions or advice is greatly appreciated in advance men. Thank you.
  2. jcsd
  3. Feb 25, 2011 #2


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    You could convert one of the phases to a single phase output with a transformer but this would not give a balanced load for your generator.

    Or, you could use a 3 phase transformer to give 3 phase output at a suitable voltage. But then you would have to wire your house to use this power in a balanced way.

    Or, you could use a 3 phase rectifier system to produce DC and generate AC with a suitable converter. This would be expensive and probably inefficient.

    What would make most sense is to advertise the machine and sell it. Then go and buy something that is suitable. Sounds like a great machine and you should get a good price for it.
  4. Feb 25, 2011 #3
    Well ... thank you. That's not what I wanted to read but it's pretty much what I expected to read.

    I really do not have any 3 phase motors attached to anything laying around that would benefit from me being able to crank this bad boy up from time to time.

    And I've already got enough batteries being charged by solar panels and a trickle charger to keep them fully juiced for our needs. I'd have to invest in quite a few more (deep cycle batteries) to really make using this thing worth my while in terms of storing energy, and even then it would not payoff because of the cost of LP gas to run the gennie.

    Anyone looking for a good deal on an Onan 12 kw gennie with only 1100 total hrs on it, auto start, plus it comes with one heck of a fine transfer switch made for a large commercial building? It is a thing of beauty. lol ... looks like it is now for sale at a bargain basement price.

    I'll put it on eBay and see what happens. Hate looking a gift horse in the mouth but I sure wish this would have been a single phase diesel Onan.

    vk6kro, I appreciate it very much. Thank you for taking the time to respond.
  5. Feb 26, 2011 #4


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    If a friend gave it to you, then you need to check with the friend to verify that it is OK to sell it. Just a courtesy.

    That is an industrial machine. I imagine it as being about the size of a truck engine and weighing about as much, so you need to make it "pickup only".

    What voltage is it, and how many wires come out of it?

    Also, wait for a day or two. Someone here might come up with a good use for it.
  6. Feb 26, 2011 #5
    Good idea - I just called him and he said fine, he's happy to have it out of his garage. It took four of us to load it into the back of a pickup truck. You are correct about the size and weight, roughly the size of a truck engine. Looks like four wires coming out of it. Three black and one white. 12.5 kw gen head.

    It is 120/208v and I've got the transfer switch with it too which is about a $650.00 extra. It came from a nursing home where he worked and they gave it to him to get it off the property after upgrading to a big diesel genny that was single phase and four times bigger because the place had grown since the old genny was installed new in '88. The clock shows 1044 hrs of total run time. It would come on automatically by timer, once each week, and run for a few minutes before shutting off.

    The upgrade had something to do with their new oxygen set up for their patients as well.

    Here's a picture. I doubt it will sell very fast. He says he had it on eBay for a long time and it never sold although he received a lot of calls about it, none were from close enough to pick it up and no one wanted to pay shipping.


    I've got the covers for it to that encloses the sides so it is weather proof so to speak. I wish there was something I could do with it but I am going to approach a couple of shops I know that might be interested in trading it for some welding work or engine work I need done for some other projects. I need a stack of good aluminum frames built for some solar panels I am going to build out of some cells I recently came into possession of, so I may get lucky and work a barter deal.

    It looks better in person than it does in the picture.
    Last edited: Feb 26, 2011
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