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    Back to Back Transformers

    Im currently building an isolation transformer from 2 240v-12v transformers, wired with the 2 12V secondaries joined to give 240V out, which I am getting.

    The Issue I have is that I'm getting 100Hz out from 50Hz in.
    Can anyone tell me why that may be?

    #2
    Re: Back to Back Transformers

    Either you're measuring it wrong or the source transformer has a rectifier output... ?

    Comment


      #3
      Re: Back to Back Transformers

      Both the transformers are from old 12V battery chargers both of which had rectifiers in them aswell.
      If the transfomers had rectifacation would they still need a seperate rectifier.
      I'm just measuring with my multimeter and going off what its reading which is 100Hz

      Comment


        #4
        Re: Back to Back Transformers

        closest i could find without using diodes is this .but your set up isnt like it ... maybe try moving the transformers further away from each other ...

        Comment


          #5
          Re: Back to Back Transformers

          Show us how you have them all connected up.

          You need to connect the 12V winding directly together, no diode, nothing but just the 12VAC winding of each transformer.
          240VAC into first transformer 240VAC winding, 12VAC winding of first transformer to 12VAC winding of second transformer, 240VAC winding of second transformer for 240VAC Isolated output.
          Last edited by budm; 08-20-2020, 04:11 PM.
          Never stop learning
          Basic LCD TV and Monitor troubleshooting guides.
          http://www.badcaps.net/forum/showthr...956#post305956

          Voltage Regulator (LDO) testing:
          http://www.badcaps.net/forum/showthr...999#post300999

          Inverter testing using old CFL:
          http://www.badcaps.net/forum/showthr...er+testing+cfl

          Tear down pictures : Hit the ">" Show Albums and stories" on the left side
          http://s807.photobucket.com/user/budm/library/

          TV Factory reset codes listing:
          http://www.badcaps.net/forum/showthread.php?t=24809

          Comment


            #6
            Re: Back to Back Transformers

            Thats how I've got them setup.
            I've attached an image of how there wired. The transformer in the bottom of the image is the 240V in, with a fuse before it, then the two 12V's are connected together then the output is the transfomer at the top.This goes into a power meter display then into a DPST switch.
            Attached Files

            Comment


              #7
              Re: Back to Back Transformers

              So direct connection without any diodes anywhere, you are seeing 240VAC 100Hz at the output?
              Never stop learning
              Basic LCD TV and Monitor troubleshooting guides.
              http://www.badcaps.net/forum/showthr...956#post305956

              Voltage Regulator (LDO) testing:
              http://www.badcaps.net/forum/showthr...999#post300999

              Inverter testing using old CFL:
              http://www.badcaps.net/forum/showthr...er+testing+cfl

              Tear down pictures : Hit the ">" Show Albums and stories" on the left side
              http://s807.photobucket.com/user/budm/library/

              TV Factory reset codes listing:
              http://www.badcaps.net/forum/showthread.php?t=24809

              Comment


                #8
                Re: Back to Back Transformers

                you may be getting transfer between the transformers because they are oriented in line with each other.
                magnetic flux through the air or even the metal baseplate.
                do you have a scope?

                Comment


                  #9
                  Re: Back to Back Transformers

                  The coupling is a nice theory but doesn't really hold water, you need a phase shift of some sort to get doubling and coupling is in phase or at least very close to be in phase. Still doesn't explain...

                  What about the frequency of the 12V between the two? What about after the measurement jig before the first transformer?

                  I have to say that there's some measurement issue or some other noise source like the measurement jig... Scope would indeed be nice.

                  Comment


                    #10
                    Re: Back to Back Transformers

                    PZEM-021 energy/power meter uses a capacitive-dropper for power and likely causing some ringing on the AC waveform output of the second transformer when it has no load on it. Are you sure you've got the right winding, I see some steel lamination cut off and extra windings on it and 247V label?
                    Connect a light bulb load on the output and I'd guess the frequency reading would mysteriously half.

                    Comment


                      #11
                      Re: Back to Back Transformers

                      Yes with direct connection and no diodes my multimeter is showing 100Hz

                      Yes I have a scope

                      I only get 50Hz on the 12V windings

                      Even if I disconnect the energy/power meter I still get 100Hz on the output Transformer.

                      The output transfomer does have more than one tap but I still get 100Hz on both of them.

                      Comment


                        #12
                        Re: Back to Back Transformers

                        You do have the right phasing or you will not get the right voltage or current

                        On the primary and secondary winding

                        I done this before and you have to get it right for it to work correctly
                        Last edited by sam_sam_sam; 08-21-2020, 03:54 AM.
                        9 PC LCD Monitor
                        6 LCD Flat Screen TV
                        30 Desk Top Switching Power Supply
                        10 Battery Charger Switching Power Supply for Power Tool
                        6 18v Lithium Battery Power Boards for Tool Battery Packs
                        1 XBox 360 Switching Power Supply and M Board
                        25 Servo Drives 220/460 3 Phase
                        6 De-soldering Station Switching Power Supply 1 Power Supply
                        1 Dell Mother Board
                        15 Computer Power Supply
                        1 HP Printer Supply & Control Board * lighting finished it *


                        These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

                        1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
                        2 Hem Saw Computer Stack Board

                        All of these had CAPs POOF
                        All of the mosfet that are taken out by bad caps

                        Comment


                          #13
                          Re: Back to Back Transformers

                          What do you mean by having the right phasing ?

                          Comment


                            #14
                            Re: Back to Back Transformers

                            Have you noticed that on some transformers have a dot on them on one corner one side then on the side of the transformer there is another dot on the opposite corner

                            This is the phasing of the transformer when you hook up two in series you hook it one way when you hook it up in parallel you hook up another way

                            This also depends on the transformer you are using

                            Most transformers are not marked by the way

                            If you google how to determine the phasing you should be able to figure it out
                            9 PC LCD Monitor
                            6 LCD Flat Screen TV
                            30 Desk Top Switching Power Supply
                            10 Battery Charger Switching Power Supply for Power Tool
                            6 18v Lithium Battery Power Boards for Tool Battery Packs
                            1 XBox 360 Switching Power Supply and M Board
                            25 Servo Drives 220/460 3 Phase
                            6 De-soldering Station Switching Power Supply 1 Power Supply
                            1 Dell Mother Board
                            15 Computer Power Supply
                            1 HP Printer Supply & Control Board * lighting finished it *


                            These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

                            1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
                            2 Hem Saw Computer Stack Board

                            All of these had CAPs POOF
                            All of the mosfet that are taken out by bad caps

                            Comment


                              #15
                              Re: Back to Back Transformers

                              Again this does not make sense. Phasing at most will invert or not invert the sinusoid of the AC current, but the downstream equipment should not care what phase it's running at. You can try swapping the connection of the 12V/low voltage wires to put things in phase.

                              This must be some noise problem that your meter is picking up though I'd say the 100Hz is a second harmonic that the meter is picking up. The only thing that sticks out at the moment is that your secondary transformer is using two windings hooked up in parallel or this should be the case. A mismatch may cause some weird behavior. What if you use the two transformers the other way around -- use the low impedance of line power to override the parallel winding mismatches?

                              Comment


                                #16
                                Re: Back to Back Transformers

                                Do you mean that the output transformer's 240V side has dual windings in parallel, as I am only connected to one of them.

                                Comment


                                  #17
                                  Re: Back to Back Transformers

                                  yeah you have a mess there, can't exactly tell how you're connected. Might be some noise being introduced there too, dunno...

                                  Comment


                                    #18
                                    Re: Back to Back Transformers

                                    Originally posted by eccerr0r View Post
                                    Again this does not make sense. Phasing at most will invert or not invert the sinusoid of the AC current, but the downstream equipment should not care what phase it's running at. You can try swapping the connection of the 12V/low voltage wires to put things in phase.

                                    This must be some noise problem that your meter is picking up though I'd say the 100Hz is a second harmonic that the meter is picking up. The only thing that sticks out at the moment is that your secondary transformer is using two windings hooked up in parallel or this should be the case. A mismatch may cause some weird behavior. What if you use the two transformers the other way around -- use the low impedance of line power to override the parallel winding mismatches?

                                    No what I am talking about is if you have it out of phase then you will not get any voltage or current or you are shorting both transformers out
                                    9 PC LCD Monitor
                                    6 LCD Flat Screen TV
                                    30 Desk Top Switching Power Supply
                                    10 Battery Charger Switching Power Supply for Power Tool
                                    6 18v Lithium Battery Power Boards for Tool Battery Packs
                                    1 XBox 360 Switching Power Supply and M Board
                                    25 Servo Drives 220/460 3 Phase
                                    6 De-soldering Station Switching Power Supply 1 Power Supply
                                    1 Dell Mother Board
                                    15 Computer Power Supply
                                    1 HP Printer Supply & Control Board * lighting finished it *


                                    These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

                                    1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
                                    2 Hem Saw Computer Stack Board

                                    All of these had CAPs POOF
                                    All of the mosfet that are taken out by bad caps

                                    Comment


                                      #19
                                      Re: Back to Back Transformers

                                      The windings are all in parallel and the signal (AC line) is symmetric/sinusoidal, so you don't have to worry about it.

                                      If one was planning to make a voltage doubler out of these transformers, they'd need to be in series, and this would make a difference.

                                      However 0° or 180° phase difference, the peaks and valleys would all appear at the same time, cancelling each other out. Need a phase shift to double the frequency somehow...

                                      Comment


                                        #20
                                        Re: Back to Back Transformers

                                        This schematic diagram might help. If OP can verify.

                                        Phasing is important for the dual-primary tranny but more so if they were autotransformer connected which is what sam is talking about.
                                        Adding a small load will get rid of the frequency doubling OP is seeing.

                                        I want an earth ground connection and don't see it.
                                        Attached Files

                                        Comment

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