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

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    #16
    An option for the autotransformer mentioned by gar is either the 3 or 5 KVA unit below;
    (and vendor is approved by Wile E. Coyote )

    https://rexel-cdn.com/Products/Catal...F42CF6C#page=7

    https://www.mrosupply.com/electrical..._t253063_acme/
    https://www.mrosupply.com/electrical..._t253064_acme/

    With the 3 KVA unit you can get the full 3 KVA output of the generator on balanced 120V loads, a 240V-only load, or a 120V load on the phase that's directly connected to the generator. The derived phase at 180 degrees would support up to 1.5KVA.
    The 5 KVA unit would support 2.5KVA on the 180 deg phase but for $216 more. The inrush current would also be higher, so there's a higher risk of the inverter generator having a problem with that. But it could be a non-issue.

    Comment


      #17
      Originally posted by __dan View Post
      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.
      And don't forget that the incoming conductor that also connects to the output neutral should be supplied by the grounded conductor of the source circuit. (H4/X1 in my first diagram)
      Master Electrician
      Electrical Contractor
      Richmond, VA

      Comment


        #18
        Originally posted by synchro View Post
        An option for the autotransformer mentioned by gar is either the 3 or 5 KVA unit below;
        (and vendor is approved by Wile E. Coyote )

        https://rexel-cdn.com/Products/Catal...F42CF6C#page=7

        https://www.mrosupply.com/electrical..._t253063_acme/
        https://www.mrosupply.com/electrical..._t253064_acme/

        With the 3 KVA unit you can get the full 3 KVA output of the generator on balanced 120V loads, a 240V-only load, or a 120V load on the phase that's directly connected to the generator. The derived phase at 180 degrees would support up to 1.5KVA.
        The 5 KVA unit would support 2.5KVA on the 180 deg phase but for $216 more. The inrush current would also be higher, so there's a higher risk of the inverter generator having a problem with that. But it could be a non-issue.


        RIght on. Thanks guys, this makes sense. I think with the smaller power range, we can just minimize any load on the 180* 120V connection. Plus, I guess we can have low voltage on tap for whatever.....lol

        I would assume a buck/boost would be about the cheapest bargain barn solution here? I mean, really anything with a 120x240V primary should do.

        Comment


          #19
          Considering that Honda has a parallel capability, it is too bad it can't control the inverters to be 180 out of phase, rather than in phase.

          https://powerequipment.honda.com/gen...lel-capability
          Ron

          Comment


            #20
            Inverters are just the right choice on a generator. It would have been super easy for Honda to provide a 240V switch but I guess we can't get too fancy or the general public will find a way to destroy them. It would not change the IGBT pack one bit and just a simple processor adjustment but I guess I don't know what the alternator and DC bus voltage is on it.

            Here is something "I" would like to try. OK, the way most inverters work is if there is an AC wave present (being fed) at start up, they will synch to that. If they don't sense one, they just start their own. So, if you have one running, plug two together and start the second, you just synched them and double your amperage.

            But if I have them both running, and combine only the Neutral of each, I am just wondering if I can get 180*. I am guessing this won't work but would sure be nice. I think even with the small variance of an inverter, it would be too much and the phase relationship would be all over the map.


            To be honest though, the only thing in a home I can even think of that needs true split phase is a dryer. I realize feeding a load center with backup power is much easier when you can just plug in and go. However, I have been feeding my personal circuit at 120V on both legs for 20yrs. I know anything that is 240V will have a potential of 0V so its not like there is any risk.


            What WOULD be nice to see is one of the transformer companies build a special transformer for this. Delete the secondary entirely, and provide a beefed up 180* winding on the primary side so the entire KVA could be used on the 180* leg. The only other option to get full KVA from that 180* leg is double the size of the transformer. What I will probably do to protect the transformer is just use a 3KVA transformer and protect that 180* leg at 1.5KVA just so we don't roast any marshmellows.



            Comment


              #21
              190818-1604 EDT

              ron:

              I think the autotransformer is the better approach.

              Why? If I parallel two Honda 2200s I get 4400 capability at 120 V. Adding a 120 to 240 autotransformer makes 120-0-120 available. Adequately sized I can supply up to a maximum of 4400 on either 120 phase or 240 in any combination that does not exceed 4400 total load.

              No isolarion is required, and therefore with an autotransformer you can get the the 4400 capability in any combination with less transformer weight than an isolation transformer.

              In a home application there is a continuously varying load between the two phases. If you parallel the two phase circuits, then any combination of loading to 4400 is possible at 120 V, but no 240 V.

              .

              Comment


                #22
                Originally posted by fastline View Post

                However, I have been feeding my personal circuit at 120V on both legs for 20yrs. I know anything that is 240V will have a potential of 0V so its not like there is any risk.
                That would be fine as long as you don't have any MWBC's where the neutral would be overloaded using only one phase on all hot wires.

                Originally posted by fastline View Post

                What WOULD be nice to see is one of the transformer companies build a special transformer for this. Delete the secondary entirely, and provide a beefed up 180* winding on the primary side so the entire KVA could be used on the 180* leg. The only other option to get full KVA from that 180* leg is double the size of the transformer.
                Both half-windings of a 2:1 autotransformer like a 240V /120V flow the same amount of current, just in opposite directions to and from the center tap. So the gauge of each half-winding should be the same in a 2:1 autotransformer. And therefore the 240/120V center-tapped primary winding of a standard transformer (which would have half-windings of the same gauge) is very suitable as-is for use as an autotransformer.

                In general I agree with your comments and plans, just wanted to clarify a few points.

                Comment


                  #23
                  The auto transformer connection would only see the load of the current x the winding voltage, for either line or load. The transformer kVA rating need only be 1/2 the kVA of the load, for the 120 to 240 connection.

                  If it's a project for yourself and a surplus or Ebay unit is likely. the common buck boost autotransformer with 12 / 24 or 16 / 32 Volt secondary windings is what I have used (because I have a collection of takeouts and curio items in my basement).

                  You only need the primary 120 / 240 Volt winding and ignore the secondary. I am guessing it is very cheap amd easy to find on Ebay, because it already has a standard application for 280 to 240 buck boost. So many of them get put in and taken out, it's an Ebay item.

                  Something like this.

                  https://www.ebay.com/itm/GE-9T51B140...temCondition=4

                  Last edited by __dan; 08-19-19, 10:25 AM.
                  Lasciate ogne speranza, voi ch'intrate

                  Comment


                    #24
                    Originally posted by gar View Post
                    190818-1604 EDT

                    ron:

                    I think the autotransformer is the better approach.
                    I agree. The current offering from Honda only parallels the output, that's why I mentioned "it is too bad it can't control the inverters to be 180 out of phase, rather than in phase"

                    Ron

                    Comment


                      #25
                      190819-1141 EDT

                      ron:

                      The interesting aspect of the two parallel inverters with an autotransformer is that either phase can be loaded to the full output of the two inverters so long as the total load does not exceed the total capacity of the two inverters. However, it requires the additional weight of the autotransformer.

                      If the two inverters could be phased 180 apart, then the maximum load on one 120 V phase is limited to the ouput of only one inverter.

                      .

                      Comment


                        #26
                        Originally posted by __dan View Post
                        The auto transformer connection would only see the load of the current x the winding voltage, for either line or load. The transformer kVA rating need only be 1/2 the kVA of the load, for the 120 to 240 connection.

                        I think we might need to clarify this so everyone is on the same page and I can understand why you say this. Why would the KVA rating be ok at 1/2 of the load?

                        If you take a 2:1 primary (yes, we are ignoring the secondary), that is 2 coils on the primary for say 120x240 and say 3kva. Typical wiring is series wired for 240v, or parallel wired for 120, to get full kva. That means each coil at 120V is 1.5kva.

                        Now if you feed 120V to only one of the coils, you can obviously pull load from this same connection and completely by pass the transformer so you can get full power from the source. If you pull 240V, you can get 3kva, or full rating from the transformer. However, when pulling 120V from the non-powered 180* coil in the primary, you are at 120V and still limited by the ampacity of the coil, which will be 1.5kva.

                        However, considering the source is 3kva anyway, all you need to do is slightly bias the "load balance" to favor the stronger side. In a perfect world, you would have a perfect balance between legs which would still equal the 3kva.

                        I was just "dreaming" about parallel of inverters to get split phase simply to avoid a transformer. However, as mentioned above, that would mean each leg would be limited by the capacity of only one generator. I guess inrush on harder loads could be better handled with the transformer arrangement.
                        Last edited by fastline; 08-20-19, 06:26 PM.

                        Comment


                          #27
                          Originally posted by fastline View Post

                          I think we might need to clarify this so everyone is on the same page and I can understand why you say this. Why would the KVA rating be ok at 1/2 of the load?

                          .
                          I drew it out and i'm thinking it should work. If I have a 20 A line current at 120 V in. I could see how it splits as 10 A to half the winding that is powered and 10 A through the other winding that is in series with both the line side, the other half winding, and the load.

                          With the autotransformer connection, only half of the voltage needs the transformer, the other half is the line 120, in series with that other half winding, and the load.
                          Last edited by __dan; 08-21-19, 02:57 AM.
                          Lasciate ogne speranza, voi ch'intrate

                          Comment


                            #28
                            190821-0737 EDT

                            New possible idea on how to use an autotransformer and get the full power output of the sum of the 2 generators without the autotransformer having to pass that full power.

                            I have to believe that the second generator when added in parallel to the first generator knows how to synchronize with that first generator's output. If not, then a synchronizing patch cord is required. In either case what I am suggesting may work.

                            Do not connect the second generator in parallel with the first, but rather in series. Both generators will operate and supply 120 V loads with up to the maximum power of the individual generators. The outputs will not be locked in frequency and phase to each other. The common connection between the generators is not yet a neutral, but will become the neutral with the addition of my next item.

                            Add an autotransformer to the series connection of the two generators. Now the sync signal is provided to the second generator. You probably don't connect the second generator to the autotransformer until both generators are running, and possibly without some series resistance to begin. In this case the autotransformer is the synchronizing connection.

                            If a separate sync wire is required, then it may require a phase inverter.

                            If this power paralleling scheme will work, then full power of the two generators can go to either 120 circuit with an autotransformer 1/2 the size of the one in my previous post.

                            .

                            Comment


                              #29
                              Don't guess I follow on the series wiring on the generators. A digital inverter is going to sync differently than conventional. It needs to see full wave form to sync because it synthesizes the wave. Yes, you have to start one, then the other. You don't have to use a sync cord.

                              If you connect two generators, I envision N on Gen1 to L1 on Gen2. What are we making with that? They won't sync in that way.

                              However, there are sync ports to plug into if you "want to do it" that way. I guess I can force them to sync, but if you then connect as above, its a dead short, so......?

                              Drawing maybe?

                              Comment


                                #30
                                190821-1104 EDT

                                fastline:

                                The autotransformer is the means to sync the two generators when series connected. At least that is my hope. You should be able to think this thru and possibly try an experiment.

                                Connecting two generators in series will cause no problems with 120 V loads. A 240 V resistive load could be tried, and I would expect without synchronization that would go from +240 to 0 to -240 at a rate determined by the difference frequency of the two generators. Here I used + and - to indicate phase relationship.

                                Since you indicated a sync connection is possible you might try it and see what happens, it must be isolated from at least one generator output. If it forces both generators to be in phase, which is expected, then a simple series connection of the generators will produce 240. Power on each phase will be limited to that of its generator.

                                .

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