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    Secondary half wave rectification

    Can secondary half way rectification over heat an industrial control transformer? And is so what conservative multiplier is typically used for de-rating? 480:120

    #2
    Originally posted by mbrooke View Post
    Can secondary half way rectification over heat an industrial control transformer? And is so what conservative multiplier is typically used for de-rating? 480:120
    I don't see why it would but I've never tried it. What do you want half wave for anyway?

    Comment


      #3
      191016-0815 EDT

      mbrooke:

      Yes. Don't run much DC in either the primary or secondary. This is why you must use special phase shift dimmers into a transformer.

      Unbalanced DC thru any coil on a transformer unbalances the hysteresis curve increasing saturation in one direction, and increasing magnetizing peak current in one direction. Thus, increasing RMS current. A full wave center tapped rectifier balances the two half wave rectified waveform DC components.

      Run experiments. You need an RMS meter that also simultaneously measures the DC component along with the AC component.

      Do temperature rise experiments.

      .
      Last edited by gar; 10-16-19, 08:33 AM.

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        #4
        You also lose 50% of your ampacity.
        Master Electrician
        Electrical Contractor
        Richmond, VA

        Comment


          #5
          Originally posted by gar View Post
          191016-0815 EDT

          mbrooke:

          Yes. Don't run much DC in either the primary or secondary. This is why you must use special phase shift dimmers into a transformer.

          Unbalanced DC thru any coil on a transformer unbalances the hysteresis curve increasing saturation in one direction, and increasing magnetizing peak current in one direction. Thus, increasing RMS current. A full wave center tapped rectifier balances the two half wave rectified waveform DC components.

          Run experiments. You need an RMS meter that also simultaneously measures the DC component along with the AC component.

          Do temperature rise experiments.

          .
          This is what I was thinking.

          I know when the diode is on the primary the thing gets HOT (had to unplug it)- tried that experiment long time ago. But I'd imagine the secondary would not be as bad... right?


          Comment


            #6
            191016-1021 EDT

            mbrooke:

            Generally I would judge just about as bad, possibly worse. It will depend on what the primary and secondary currents are, and how all circuit elements balance out.

            I think if the ampere-turns of DC are the same, that secondary loading maybe worse. The DC ampere-turns produces the flux unbalance, and if both primary and secondary have current flow, then power is dissipated in two places.

            .

            Comment


              #7
              Originally posted by mbrooke View Post

              This is what I was thinking.

              I know when the diode is on the primary the thing gets HOT (had to unplug it)- tried that experiment long time ago. But I'd imagine the secondary would not be as bad... right?

              On the primary, it will cause saturation. Don't do it.

              Comment


                #8
                Originally posted by gar View Post
                191016-1021 EDT

                mbrooke:

                Generally I would judge just about as bad, possibly worse. It will depend on what the primary and secondary currents are, and how all circuit elements balance out.

                I think if the ampere-turns of DC are the same, that secondary loading maybe worse. The DC ampere-turns produces the flux unbalance, and if both primary and secondary have current flow, then power is dissipated in two places.

                .
                50% va

                I know things like hair dryers and heater regulate power through half wave rectification. So at some power level it must be fine.

                Comment


                  #9
                  Originally posted by Besoeker3 View Post

                  On the primary, it will cause saturation. Don't do it.
                  I've tried it, and yes, it does saturate. Key is listening for the cracking noise of the paper and turning it off before the magic smoke lets out.

                  Comment


                    #10
                    The key difference is that a diode on the secondary cannot cause saturation in the same way as on the primary because the secondary voltage is dependent on the rate of change of the flux in the core. A load on the secondary can never increase the core flux, just the non-magnetizing current in the primary. You will see a drop in output voltage on the secondary before you see saturation of the core. It is still not a good idea because the DC component will definitely cause additional heating of the windings.

                    Comment


                      #11
                      191016-1342 EDT

                      mbrooke:

                      There is not just some level at which a problem occurs, it is a proportional thing. The greater the DC current the greater is the problem. But how sharply this occurs is a function of the shape of the hysteresis curve. Thus, the core material alloy is important. This is the basis of magnetic amplifiers.

                      .

                      Comment


                        #12
                        One problem with half wave is that all the power that needs to be delivered to the load is taken in very brief pulses that only happen once every cycle. So the peak current through the transformer is much higher than the output current of the rectifier/filter.

                        The better the filtering (assuming you are using a capacitor across the output), the shorter and larger the spikes are. So a 80 ma current may flow for 2 ms, and that allows the filter to supply a 10 mA load for a full 16 ms. In both cases the energy is the same 80*2 = 16*10.

                        With no filtering at all, the diode is allowed to conduct for a full half cycle, and the output current also only flows for one half cycle. So 10 mA out is only 10 mA in. That is probably your hair dryer.

                        An inductor at the input of the filter changes everything, and can get rid of all those nasty harmonics and spikes.


                        Comment


                          #13
                          191016-1452 EDT

                          steve66:

                          What you say is true, but it is only part of the story.

                          The DC flux bias is probably the most important factor, and is most easily observed with an unloaded secondary and a series diode in the primary.

                          All flux sources in a closed core add.

                          .

                          .

                          Comment


                            #14
                            GoldDigger's point is key. A half wave rectifier on the _primary_ blocks half of the magnetizing current causing huge saturation problems.

                            A half wave rectifier on the _secondary_ blocks half of the _load_ current. The magnetizing current is still AC. Thus DC saturation effects must be indirect, eg by increased voltage drop on one half of the primary AC cycle.

                            -Jon

                            Comment


                              #15
                              191016-1646 EDT

                              winnie:

                              No matter where the DC ampere-turns come from on the transformer, the same number of DC ampere-turns produce the same saturation effect. This assumes a single magnetic core path.

                              .

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