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OT: An EE brainteaser.........

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  • OT: An EE brainteaser.........

    Since we seem to be discussing matters of electrical nature recently..... here is a tester for the understanding of one type of component...... a transformer.

    OK

    Suppose you have a transformer with a single primary winding and a single secondary winding, each consisting of many turns of wire. Current in the primary produces a "magnetic flux" in the core, and the changing flux in the core induces a voltage in the secondary.... In fact, current in ANY winding produces a magnetic flux in the core.

    The "magnetic flux" in the core is created by current in the primary..... which draws a little current even if the secondary is open, conducting no current.

    If you apply suitable AC to the primary, and a load to the secondary, current will flow in secondary and the current in the primary windings will increase.

    Now...... A certain magnetic flux will exist in the core when the secondary has NO load.

    THE QUESTIONS:

    1) When the transformer is operating at its full design load, is the magnetic flux in the core larger, smaller, or the same as when there is no load?

    2) If the secondary is shorted, so that a very large current flows, is the flux in the core larger, smaller, or the same as when there is no load?
    1601

    Keep eye on ball.
    Hashim Khan

  • #2
    Flux is always highest under no load conditions.
    Free software for calculating bolt circles and similar: Click Here

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    • #3
      This would probably be a good time to mention the matter of "show your work".....

      Or at least to give your reasoning.
      1601

      Keep eye on ball.
      Hashim Khan

      Comment


      • #4
        No load, low current draw, high back emf requires maximum flux.

        Should mention losses include hysteresis in the iron, eddy currents in the iron, possible conduction between laminations, distance between laminations occupied by insulating material and a few others.
        Last edited by Evan; 09-18-2010, 11:07 PM.
        Free software for calculating bolt circles and similar: Click Here

        Comment


        • #5
          If the primary voltage and frequency are constant, the magnetic flux stays consant regardles of secondary load even dead short -that is, in a perdect transformer.

          In the real world I suppose there are small differences in total flux as the losses change over the transformer's load cycle.

          What do I win? Did I win?

          OK that's theory according to the book in Mr Wagner's electronc shop class 1958. But if the core flux stays contant why does magnetostrictive core noise (hum) increase with load? I could never figure that out.

          Here's another. Moving magnet/changing flux induces electron flow in a winding. So take a 0.010 feeler and go around the windings of an energized transformer and you will feel little or no buzzing drag as you would from an AC magnet. I can see how air core transformers works but a closed iron core transformer abolutely stumps me. And yes I know an iron core increases coupling between AC wndings by a factor of everal hundred or more.

          How does the linkage between iron core windings actually work so it explains the feeler behavior? Yeah, I know it works. I've been teaching and trouble-shooting transformers off and on for 50 years, even the water in pipes analogy, but once in a while somebody hands me this same poser and I have to fall back on "Well, it works.".Makes me feel dumb.
          Last edited by Forrest Addy; 09-19-2010, 09:05 AM.

          Comment


          • #6
            One thing I find interesting is that the parasitic current (probably not the correct term, but...) in the primary winding that appears during a no-load condtion seems to mostly disappear when the secondary is loaded.

            I had a 240 -> 120 transformer, about 3 Kva in size that drew about 1/2 amp with no secondary load. Loading the secondary to 10 amps would give me just 5 amps on the primary, the 1/2 amp draw seemed to "disappear".

            Comment


            • #7
              When I worked for the "Guvamint," we had office towers with as many as four 5 Mva transformers. At low load conditions, only one was supplying power and the others were on "standby." We determined that the three idle transformers were drawing 50 kw each just sitting there and we had a demand charge of $10.00/kw, plus our usage charge. I could not convince ANYBODY to al least turn off two of the four. The building I am referring to NEVER used more than one transformer, and only on the hottest days did the cooling fans on that unit come on for a few hours late in the day. Go figure!
              Duffy, Gatineau, Quebec

              Comment


              • #8
                Originally posted by Forrest Addy
                If the primary voltage and frequency are constant, the magnetic flux stays consant regardles of secondary load even dead short -that is, in a perdect transformer.

                In the real world I suppose there are small differences in total flux as the losses change over the transformer's load cycle.

                What do I win? Did I win?

                OK that's theory according to the book in Mr Wagner's electronc shop class 1958. But if the core flux stays contant why does magnetostrictive core noise (hum) increase with load? I could never figure that out.

                Here's another. Moving magnet/changing flux induces electron flow in a winding. So take a 0.010 feeler and go around the windings of an energized transformer and you will feel little or no buzzing drag as you would from an AC magnet. I can see how air core transformers works but a closed iron core transformer abolutely stumps me. And yes I know an iron core increases coupling between AC wndings by a factor of everal hundred or more.

                How does the linkage between iron core windings actually work so it explains the feeler behavior? Yeah, I know it works. I've been teaching and trouble-shooting transformers off and on for 50 years, even the water in pipes analogy, but once in a while somebody hands me this same poser and I have to fall back on "Well, it works.".Makes me feel dumb.
                Several points here.

                First, the basic question. The magnetic flux in the core is solely dependent on the net (vector) sum of the currents in the windings. The current in the primary is the first contributor. This current is at a mimimum when there is no load so the flux from this component is lower. As the load current increases, this primary current increases and the flux due to it also increases.

                But there is also the current in the secondary and it also has an effect on the same flux that is creating it. Here you must also consider the fact that the secondary current is going to be 90 degrees out of phase with the primary. This is because it is created by the RATE OF CHANGE in the primary's flux, not the magnitude of the flux itself. This rate of change is greatest when the primary's current is actually zero (the current is sinusoidal and it's rate of change is greatest when the current is crossing the zero axis and hence is at a zero value). So the secondary current has to be added to the primary’s current using vectors. The resultant is going to be another sinusoidal function with a phase angle somewhere between the two contributors. In magnitude it will be close to the magnitude of the current in the primary alone and the actual relationship may depend more on the losses and secondary effects than on the actual primary and secondary currents. In total, the overall flux will be at a minimum when there is no load and it will increase as the load increases. This relationship will be more or less linear.

                Second, the hum increases with load. Yes, this does happen. The transformers are constructed to contain the flux inside the cores. Any flux that in not contained is wasted as EM radiation (radio waves) so an efficient transformer contains all the flux. In reality, this is not perfect and some does escape. A big contributor to this escaped flux, escaped EM radiation, is the wire leads going into and out of the transformer. They are not actually part of the windings so the magnetic (EM) field around them CAN and DOES escape more easily. This is a principal contributor to the hum you typically hear around high power transformers. You will also hear it around switchgear. The steel cabinet can pick up the radiated EM field and vibrate (hum). This can happen even if there are no magnetic breakers in the cabinet. Same idea: it’s the wires themselves that radiate. Also, the core of the transforner itself can contribute to hum. If the laminations are not completely impregnated with the varnish to fix them, they can move and hence, hum. You will also hear it near high Voltage lines even with no transformers in the vicinity. The wires themselves can vibrate from their mutual fields.

                Finally, your feeler. An AC magnet is designed to have an external magnetic field. That is it’s purpose. This field is not only present at the poles, but in the space between the poles and in general it is present to some degree all around the magnet. So I would totally expect a probe or feeler to detect it. On the other hand, a transformer is designed to contain as much of the field/flux as possible. So, much less would be present in the space around it and it would be harder to detect. A more sensitive instrument would probably find some.
                Paul A.
                SE Texas

                Make it fit.
                You can't win and there is a penalty for trying!

                Comment


                • #9
                  Originally posted by Evan
                  No load, low current draw, high back emf requires maximum flux.

                  Should mention losses include hysteresis in the iron, eddy currents in the iron, possible conduction between laminations, distance between laminations occupied by insulating material and a few others.

                  The high back EMF does not require a maximum flux because there is little or no current flowing. It is not the magnitude of the flux that produces the EMF in the secondary, it is the RATE OF CHANGE of that flux that is proportional to the EMF produced. With no load to load down the EMF in the secondary, it can go quite high with a relatively low level of flux.
                  Paul A.
                  SE Texas

                  Make it fit.
                  You can't win and there is a penalty for trying!

                  Comment


                  • #10
                    I thought Faraday and Lentz already solved this problem...

                    Comment


                    • #11
                      The flux is always highest at no load for the reasons I have given.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        Originally posted by jdunmyer
                        One thing I find interesting is that the parasitic current (probably not the correct term, but...) in the primary winding that appears during a no-load condtion seems to mostly disappear when the secondary is loaded.

                        I had a 240 -> 120 transformer, about 3 Kva in size that drew about 1/2 amp with no secondary load. Loading the secondary to 10 amps would give me just 5 amps on the primary, the 1/2 amp draw seemed to "disappear".
                        If a current flows in the primary, the magnetic field is still exciting the molecules in the core & secondary & producing a secondary potential even if no secondary load, so it is doing 'work' seen as heat

                        john
                        John

                        I used to be indecisive. Now I'm not so sure , but I'm not a complete idiot - some bits are still missing

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                        • #13
                          Transformers - The Basics (Section 1)
                          http://sound.westhost.com/xfmr.htm'
                          Part 2 actually deals with when the flux is higher

                          Comment


                          • #14
                            If there is no load on the secondary, the primary winding resistance plus the core losses are what's responsible for producing the heat. Yes, there will be the full level of transformed voltage present on the secondary, and the losses incurred by the secondary are purely due to the winding resistance, and only when current flows in it. When the secondary is loaded, both windings will produce heat through losses.

                            Here's a question for those more knowledgeable that I on the subject- To start with, you have a magnetizing current in the core, and it can reach the saturation point of the iron. At the point this occurs, the inductance drops and the primary current increases, causing the winding to heat more. If you load the secondary, does this get you away from the saturation point to any extent? If what Evan is saying is true, and I believe it is, then by having some steady load on the secondary, you should be able to get away with a few less turns on the primary without driving the core into saturation. Of course the reverse should also be true- if you then unloaded the secondary completely, the core should be driven closer to saturation, maybe too close- does any of this make sense?
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                            • #15
                              I cant answer the question until I know if the transformer in question is an Autobot or a Decepticon.

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