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    Buck and Boost

    I need to boost 208 vac to 240vac what KVA Transformer do I need? The fla is 17,

    #2
    The manufacturers have online calculators that will size the units. I'm sure you can find it with a quick google. I know square d has one.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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      #3
      181911-1414 EDT

      Boggsc71:

      Quite obviously the quick answer is (240-208)*17/1000 = 0.544 . Other factors might modify this.

      .

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        #4
        Originally posted by gar View Post
        181911-1414 EDT

        Boggsc71:

        Quite obviously the quick answer is (240-208)*17/1000 = 0.544 . Other factors might modify this.

        .
        Thanks for the quick response.

        Comment


          #5
          Originally posted by gar View Post
          181911-1414 EDT

          Boggsc71:

          Quite obviously the quick answer is (240-208)*17/1000 = 0.544 . Other factors might modify this.

          .
          I'm confused. Why are the two voltages subtracted from one another? I would think it would be 240*17/1000 = 4.08 kVA, assuming the 17 Amperes is carried on the 240V side.

          Comment


            #6
            Originally posted by Carultch View Post
            I'm confused. Why are the two voltages subtracted from one another? I would think it would be 240*17/1000 = 4.08 kVA, assuming the 17 Amperes is carried on the 240V side.
            With an autotransformer, you essentially only need the kva for the voltage change.
            Ethan Brush - East West Electric. NY, WA. MA

            "You can't generalize"

            Comment


              #7
              Originally posted by electrofelon View Post
              With an autotransformer, you essentially only need the kva for the voltage change.
              That's not entirely accurate. The current through the common part of the winding is the difference between the input and output current but the current in the part of the winding that is used by only one side of the autotransformer is the full current for that side and needs to be sized for that current. In this case, the shared winding would carry 2.6A and the secondary only part of the winding would carry 17A. If the autotransformer was wound with a single size conductor it would have to be rated for 17A. Sometimes autotransformers are wound with two conductor sizes to take advantage of the reduced current in the shared winding.

              Comment


                #8
                Originally posted by pv_n00b View Post
                That's not entirely accurate. The current through the common part of the winding is the difference between the input and output current but the current in the part of the winding that is used by only one side of the autotransformer is the full current for that side and needs to be sized for that current. In this case, the shared winding would carry 2.6A and the secondary only part of the winding would carry 17A. If the autotransformer was wound with a single size conductor it would have to be rated for 17A. Sometimes autotransformers are wound with two conductor sizes to take advantage of the reduced current in the shared winding.
                So what do you think the kVA should be?
                Si hoc legere scis nimium eruditionis habes.

                Comment


                  #9
                  Originally posted by Carultch View Post
                  I'm confused. Why are the two voltages subtracted from one another? I would think it would be 240*17/1000 = 4.08 kVA, assuming the 17 Amperes is carried on the 240V side.
                  Multiply the delta V by the current.

                  Comment


                    #10
                    Originally posted by pv_n00b View Post
                    That's not entirely accurate. The current through the common part of the winding is the difference between the input and output current but the current in the part of the winding that is used by only one side of the autotransformer is the full current for that side and needs to be sized for that current. In this case, the shared winding would carry 2.6A and the secondary only part of the winding would carry 17A. If the autotransformer was wound with a single size conductor it would have to be rated for 17A. Sometimes autotransformers are wound with two conductor sizes to take advantage of the reduced current in the shared winding.
                    Most commonly for buck and boost applications under 600V, the transformer used has separate primary and secondary coils. These coils are then connected in an autotransformer configuration.

                    The secondary coil(s) are 12 or 16 or 24V, and often there are 2 coils which can be in series or parallel. These transformers have 600V insulation on both the primary and secondary side. In this case the transformer secondary will be rated to carry the full current needed to give the rated kVA at the secondary voltage. The secondary coil(s) will be wound with different size wire than the primary.

                    The other common autotransformer is the variable autotransformer. In this case there is only a single coil, generally made with a single size wire. The kVA rating will be at full output voltage, and at low voltage output the available kVA is less than the rating.

                    -Jon

                    Comment


                      #11
                      Originally posted by pv_n00b View Post
                      That's not entirely accurate. The current through the common part of the winding is the difference between the input and output current but the current in the part of the winding that is used by only one side of the autotransformer is the full current for that side and needs to be sized for that current. In this case, the shared winding would carry 2.6A and the secondary only part of the winding would carry 17A. If the autotransformer was wound with a single size conductor it would have to be rated for 17A. Sometimes autotransformers are wound with two conductor sizes to take advantage of the reduced current in the shared winding.
                      Originally posted by Besoeker View Post
                      So what do you think the kVA should be?
                      Originally posted by winnie View Post
                      Most commonly for buck and boost applications under 600V, the transformer used has separate primary and secondary coils. These coils are then connected in an autotransformer configuration.

                      The secondary coil(s) are 12 or 16 or 24V, and often there are 2 coils which can be in series or parallel. These transformers have 600V insulation on both the primary and secondary side. In this case the transformer secondary will be rated to carry the full current needed to give the rated kVA at the secondary voltage. The secondary coil(s) will be wound with different size wire than the primary.

                      The other common autotransformer is the variable autotransformer. In this case there is only a single coil, generally made with a single size wire. The kVA rating will be at full output voltage, and at low voltage output the available kVA is less than the rating.

                      -Jon
                      So, like I said: "essentially" delta v times amps
                      Ethan Brush - East West Electric. NY, WA. MA

                      "You can't generalize"

                      Comment


                        #12
                        Originally posted by electrofelon View Post
                        So, like I said: "essentially" delta v times amps
                        Um, It was I who said that, wasn't it?

                        Comment


                          #13
                          Originally posted by winnie View Post
                          Most commonly for buck and boost applications under 600V, the transformer used has separate primary and secondary coils.
                          I SLIGHTLY disagree with the terminology primary and secondary. In boost applications that terminology fits mostly, but in buck I don't think it does. There are, as you said, often 2 (usually identical) low voltage coils allowing a 12, 16, 24, 32, or 48 volt change (I've never run into the pair of 24V coils, but there is certainly no reason it wouldn't exist; all it offers is a 48V change.)

                          Of course, there is no problem in using a transformer with one primary (even if dual coils) and 1 or 2 secondaries as a buck-boost ...

                          George

                          Comment


                            #14
                            Originally posted by electrofelon View Post
                            So, like I said: "essentially" delta v times amps
                            That would be my take also.
                            I was questioning pv n00b's disagreement with that.
                            Si hoc legere scis nimium eruditionis habes.

                            Comment


                              #15
                              181013-1446 EDT

                              GeorgeB:

                              My definition of primary is the pair of terminals to which net positive average power is input. A secondary is any pair of terminals thru which net positive average power is delivered. Thus, in an autotransformer some winding may be part of both the primary and secondary.

                              Where this may not be useful is in a telephone transformer where simultaneously there can be uncorrelated power transferred in both directions.

                              For a distribution transformer supplying a home with solar panels the role of the primary can shift back and forth between the two physical coils based on the direction of average energy flow. In this case we have synchronous correlated signals.

                              .
                              Last edited by gar; 10-13-18, 03:12 PM.

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