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120V to 120/240V. Transformer or other ideas?

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    120V to 120/240V. Transformer or other ideas?

    Going to be employing a Honda EU3000 generator as a backup charge source for an off-grid DC inverter system. I must provide the inverter with 120/240V, 180* power to make it happy. There is little sense in running a bigger generator that has 120/240V and it cuts into efficiency.


    the EU3000 is an inverter genny. I have a little transformer to step up to 240V when I need it but I have never seen a transformer like what I am looking for. Ideas? I was trying to think how I might consider a 120/240:240/480 transformer for the job, or possibly a pair of buck/boost and make the neutral?

    trying to avoid using two transformers going 120 to 240, then 240 back to 120/240 with center tap.

    #2
    What kind of current at 240v are you looking for?
    Master Electrician
    Electrical Contractor
    Richmond, VA

    Comment


      #3
      If I was having to use a transformer, I would probably just setup for worst case which might be 3kw on each leg so that would be 6kw at 240V. Obviously I cannot get that from the genny but it would prevent transformer damage if one one leg was loaded to max. I could probably back that out a touch as I know any such load would be minimal duty cycle.

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        #4
        120/240Pri x 120/240Sec

        Any maker can build what you need, but an off the shelf unit is available from Hammond. Others may have one, but this was easy to find.

        Click image for larger version

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        MTW Ω

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          #5
          Such a transformer is not uncommon:

          https://www.mrosupply.com/electrical...t353044s_acme/
          Ethan Brush - East West Electric. NY, WA. MA

          "You can't generalize"

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            #6
            Ha, I didn't even check the books assuming this was something special because I have never seen one. Thanks for the help guys!

            Whoops, I forgot something! Having not played with split phase secondary transformers much other than pole pigs, can I expect on a 3kva transformer to only be safe for 1.5kva per 120V half leg on the secondary side? Or would I be able to pull the full 3kva at 120V?
            Last edited by fastline; 08-09-19, 11:02 PM.

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              #7
              190809-1142 EDT

              fastline:

              All you need is one 120 to 120 isolation transformer, or a 120 to 240 autotransformer.

              With an appropriately sized transformer you can load either phase to the full generator capability, but total load must remain within the generator capability.

              .

              Comment


                #8
                Gar, as I understand, the inverter we need to connect to is going to need L1, L2, and N, and it will be checking that phasing of L1, L2 is 180*. Inputs from 3phase power will cause it to error out for example.

                I am not following how you will get full capacity from all legs? explain?

                Comment


                  #9
                  Originally posted by fastline View Post
                  Whoops, I forgot something! Having not played with split phase secondary transformers much other than pole pigs, can I expect on a 3kva transformer to only be safe for 1.5kva per 120V half leg on the secondary side? Or would I be able to pull the full 3kva at 120V?
                  1.5
                  Master Electrician
                  Electrical Contractor
                  Richmond, VA

                  Comment


                    #10
                    190810-0937 EDT

                    fastline:

                    If you have a 3 kVA 120 V generator, then max load on the generator is 3 kVA.

                    If I connect a 3kVA 120 V to 120 V isolation transformer to that generator, then I can load that transformer secondary to 3 kVA.

                    Next if I connect one lead of the transformer secondary to the appropriate (meaning phasing) primary lead, then I can produce 240 V.

                    Then I can do any loading to this combination that does not exceed 3 kVA loading to the generator. One loading possibility is 240 V at 3 kVA.

                    .

                    Comment


                      #11
                      I think SMA makes or made an autotransformer for use with a Sunny Island inverter that would do exactly what you want.

                      Comment


                        #12
                        Originally posted by gar View Post
                        190810-0937 EDT

                        fastline:

                        If you have a 3 kVA 120 V generator, then max load on the generator is 3 kVA.

                        If I connect a 3kVA 120 V to 120 V isolation transformer to that generator, then I can load that transformer secondary to 3 kVA.

                        Next if I connect one lead of the transformer secondary to the appropriate (meaning phasing) primary lead, then I can produce 240 V.

                        Then I can do any loading to this combination that does not exceed 3 kVA loading to the generator. One loading possibility is 240 V at 3 kVA.

                        .

                        You are going to have to draw that up. Remember I said the inverter needs to see 180* relation between 2 120V phases.

                        It sounds like you are making an autotransformer for the 240V, which I can see (and just lose isolation), but the 180* split 120?

                        Comment


                          #13
                          Originally posted by fastline View Post
                          You are going to have to draw that up. Remember I said the inverter needs to see 180* relation between 2 120V phases.

                          It sounds like you are making an autotransformer for the 240V, which I can see (and just lose isolation), but the 180* split 120?
                          Ignore that there are dual coils in series, and pretend that H1 to H4 is 120v and that X1 to X4 is 120v. The wire connected to H4/X1 should be brought out as the neutral between H1 and X4 on the HV side.







                          To help you see it differently. Again, pretend each winding is 120v.



                          Click image for larger version

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                          Last edited by LarryFine; 08-10-19, 05:19 PM.
                          Master Electrician
                          Electrical Contractor
                          Richmond, VA

                          Comment


                            #14
                            190810-1654 EDT

                            fastline:

                            You are listed as an engineer. What does that mean? Electrical, civil, chemical, or what?

                            I will assume electrical.

                            Thus, if you apply a sine wave to a transformer primary, and pick one terminal as your reference point, then relative to that reference point your secondary voltage will have a phase relationship of 0 or 180 degrees depending upon which of the secondary terminals is connected to that primary reference terminal.

                            You pick the one that gives you the 180 degree output, and thus get your 240 V with the said reference point of the primary being your neutral. Effectively you built an autotransformer from an isolation transformer.

                            Either building an autotransformer with one single continuous winding with a tap at the midpoint, or using a one to one isolation transformer with the correct phasing you get the same result.

                            .

                            Comment


                              #15
                              I have done it with a typical buck boost autotransformer, 120 / 240 primary, 18 / 36 secondary. Ignore the autotransformer secondary and feed 120 to one of the primary 120 V windings. From the two primary winding in series you will get 240 V out, but it is still an auto transformer connection and not a SDS.

                              Used it to feed a 240 V well pump from a 2 kW inverter generator. Transformer came from the collection in the basement. Voltage and running current were right where they should be, but I don't know if I would sell an off label use to a customer.
                              Lasciate ogne speranza, voi ch'intrate

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