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    What’s the voltage?

    What would be the voltage output of a small control transformer that is reverse fed? 480 applied to the 120 side. Roughly 1920, or does the 4:1 ratio hold up?

    Tom
    TBLO

    #2
    The output of a small transformer with 480V applied to the 120V side would be smoke

    Seriously: the 4:1 ratio starts to fall apart as the core gets saturated. You see more leakage flux and mutual coupling from primary to secondary, so the voltage ratio changes. On top of this the saturation of the core means more magnetizing current flows and there is more voltage drop in the primary. I would expect a short duration transient (until something fails or OCPD opens) with rather less than a 4:1 ratio.

    -Jon

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      #3
      Originally posted by ptonsparky View Post
      What would be the voltage output of a small control transformer that is reverse fed? 480 applied to the 120 side. Roughly 1920, or does the 4:1 ratio hold up?
      It would burn up if you tried it. So, don't.

      Comment


        #4
        Time for a basic theory refresher course?

        B = s(Vdt)/NA; thus, for a transformer designed for 120 V 60 Hz input, at about 20 degrees into the sine wave (say about 1 millisecond!), you have a short with the dc resistance of the 120 V winding. Happens every 1/2 cycle.

        Bmax for most core iron is around 1.8 to 2 Teslas. You go over that and it is called saturation as already mentioned.
        N = number of turns, A is core area (sqmeters), s(Vdt) is the integral of the time domain voltage waveform.

        Big enough breaker and as already said, you let the magic smoke out.

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          #5
          After a second or two the smoke would escape, unless a fuse or circuit breaker opened first.
          This presumes a standard 60 cycle supply.
          If the frequency was increased in line with the voltage, by use of a 240 cycle or higher frequency supply, then the transformer might well work and produce an approx. 1,920 volt output. The insulation of the transformer windings would be stressed to 4 times the design figure and might break down, again releasing the smoke. Or it might work at least for a while.
          This illustrates an advantage of higher frequencies in that transformers are smaller for a given rating.

          Comment


            #6
            Not to mention that the creepage distance on any line terminals would have been designed for 480V separation, which would be insufficient for 1920V. So even if the core survived for a few seconds, you would likely see a L-L flashover on the HV side.

            Flash-bang bomb...
            __________________________________________________ ____________________________
            Many people are shocked when they discover I am not a good electrician...

            I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

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              #7
              Originally posted by Besoeker3 View Post

              It would burn up if you tried it. So, don't.
              I believe he did try it and it didn't burn up.... Maybe I am incorrect
              They say I shot a man named Gray and took his wife to Italy
              She inherited a million bucks and when she died it came to me
              I can't help it if I'm lucky

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                #8
                This illustrates an advantage of higher frequencies in that transformers are smaller for a given rating.
                As any laptop/cellphone owner appreciates.

                Comment


                  #9
                  Originally posted by winnie View Post
                  The output of a small transformer with 480V applied to the 120V side would be smoke

                  Seriously: the 4:1 ratio starts to fall apart as the core gets saturated. You see more leakage flux and mutual coupling from primary to secondary, so the voltage ratio changes. On top of this the saturation of the core means more magnetizing current flows and there is more voltage drop in the primary. I would expect a short duration transient (until something fails or OCPD opens) with rather less than a 4:1 ratio.

                  -Jon
                  He already tried it, unintentionally, topic of another thread. It blew primary fuses immediately. After figuring out the problem and correcting it it still worked. An item connected to the secondary did fail though.
                  I live for today, I'm just a day behind.

                  Comment


                    #10
                    Originally posted by Dennis Alwon View Post

                    I believe he did try it and it didn't burn up.... Maybe I am incorrect
                    Yes, we tried, reference my 'What the...' the thread in Troubleshooting.

                    The fuses blew in a hurry but not before it destroyed an electronic thermostat so we had to have had some voltage increase. Just curious has to the possible spike.

                    Tom
                    TBLO

                    Comment


                      #11
                      In the 1960's, a friend Lee was building small (50-100W) 12V inverters for his own/others use. Now the issue was the available geranium [sic] power transistors had low gain; you needed several stages of gain after the oscillator to be able to drive them into saturation. A HS classmate & engineer Steve thought that too much trouble. He used a vibrator as the oscillator. This was in every car radio of the era, driving the transformer to make the HV the vacuum tubes needed. {If these terms are not familiar, ask your grandfather....}

                      Lee was outraged at this approach regarding it as well, rotten engineering. At the time he was designing larger inverters for his employer Lorain Products, as in the 50+100 KVA range. He had a deal with them to allow him to use the transformer shop for his projects if he bought the materials. [1] So before he cast Steve's transformers in clear epoxy, he wrote "RTNVTR" on the sides of the cores.

                      Steve's RTNVTR migrated to another high school classmate Phil, who I worked for decades ago. He took it to National Guard camp. While he was sleeping in (a specialty of his to this day....) two fellow soldiers borrowed it so one could shave. The conversation went like this:
                      • Dumb: Where will we get 12V?
                      • Dumber: Easy, we use a Jeep!
                      • Dumb: Ain't they 6V?
                      • Dumber: I know all about electricity.....We just hook two of them in series.
                      The minor issue was these were Korean War military Jeeps, as in 24V not 6....

                      The report was the inverter+shaver "...made a noise like a 707 taking off..." for several seconds, then the input fuse blew.

                      Surprisingly, that was the only damage to the inverter; the shaver however never worked again......



                      1] Hardly altruistic on their behalf. Lee was a smart cookie; he's the only person I ever met who understand and could explain ferroresonant transformer design. Partially from his tinkering, he held a half dozen patents assigned to LPC.


                      Comment


                        #12
                        Originally posted by ptonsparky View Post
                        What would be the voltage output of a small control transformer that is reverse fed? 480 applied to the 120 side. Roughly 1920, or does the 4:1 ratio hold up?
                        Not to take anything away from what the others have said, the short "theoretical" (i.e., in a perfect world with an ideal transformer) answer to your question is yes, the (now) secondary voltage would be approximately 1920 volts. Even if we (temporarily) disregard the influences of core saturation and the other things mentioned above, imposing 1920 volts on windings that are likely only rated for 600 volts should rather quickly cause leakage current from winding to winding. If none of the other things mentioned above had yet let out the smoke, that could do it all by itself.

                        While we are on that topic, I do not know whether the secondary side, 120V windings of this control transformer have an insulation system only rated for a nominal 120 volts. If that is the case, you could get winding-to-winding leakage current on the (now) primary side, by imposing 480 volts on those windings.


                        Charles E. Beck, P.E., Seattle
                        Comments based on 2017 NEC unless otherwise noted.

                        Comment


                          #13
                          One more small effect is that the windings of control transformers are usually "compensated", meaning in this case that the turns ratio is slightly less than 4:1 so that the 120V output voltage is met even with full load IR drop on primary and secondary. The result is that when operated in reverse the turns ratio gives a lower voltage than you would get from an uncompensated transformer of the same nominal voltage ratio. Small effect compared to the others mentioned, but needs to be considered when applying 120V to the 120V side.

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                            #14
                            Thanks for the information. We will watch the transformer for early failure and maybe take a voltage check prior to its next use.
                            Tom
                            TBLO

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