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Simple transformer power draw explanation please!

  1. Dec 7, 2007 #1
    I conducted a transformer efficiency experiment using a 240/110V double wound transformer, first with one heater connected in series then with two connected in parallell. What I dont understand is why when both (identical) heaters are connected current almost doubles on the primary side. Why is this? How can the primary side 'tell' if more power is needed. I am just a beginner at this so please explain in relatively layman terms please! All replies much appreciated.
  2. jcsd
  3. Dec 7, 2007 #2


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    Staff: Mentor

    There are two ways to think about it.

    Using just forumulas -- what is the formula for the secondary load impedance as reflected back to the primary side by the transformer?

    Using intuition -- As the load impedance value decreases, more AC current is drawn from the transformer secondary winding to support the secondary voltage. This is reflected as an increase in current required at the primary winding, in order to support the primary AC voltage. This model is most accurate when the output impedance of the AC source is low (as it is in the case of AC Mains connected transformers), and when the losses through the transformer (via eddy current losses and winding resistance losses) are small.

    Welcome to the PF, BTW.
  4. Dec 7, 2007 #3
    Thanks for the welcome and the prompt reply! I have not yet covered this reflection phenomenon, how does the primary winding draw more current and what is it about the coils that allows the primary to be affected by the secondary in this manner, and what are these reflection equations? Sorry to be so ignorant but my course is for electricians and everything is not explained too well. hope all all is well in your neck of the woods!
  5. Dec 7, 2007 #4
    Wait! So if the current is increasing on the primary side that must mean impedance has decreased by 1/2 somehow!? Why did that happen? Also power results for both sides almost double when 2 heaters are hooked up, I get that part.As you can see things are falling into place but Im sure I am still missing some vital ingredient.

  6. Dec 7, 2007 #5


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    Staff: Mentor

    The physical mechanism for this "feedback" of loading from the secondary to the primary is via the "back EMF" that is generated by the secondary current flowing in the secondary coil. Transformer action is basically like this (although it occurs more simulatneously than the following words will make it seem like)....

    The primary voltage source (the AC Mains in your example) impresses an AC voltage across the primary coil. If the secondary coil is not connected to any load, then the AC current flowing in the primary is the result of the AC voltage across the inductance (and parasitic resistance) of the primary coil alone. The AC voltage is generating an AC primary current, which generates an AC magnetic field in the magnetic material that the primary coil is wrapped around (the core). The higher the inductance of the primary, the lower the primary current, given a constant AC source voltage.

    But when a load is connected to the secondary coil, things change. The AC magnetic flux in the core induces an AC voltage in the secondary coil, which causes a secondary AC current to flow through the secondary load. But that secondary current flowing in the secondary coil generates a magnetic back-flux that opposes the forward flux coming from the primary coil. This generates a "back EMF" or reverse voltage at the primary coil, which is what causes a larger current to flow from the AC source to still support the full AC source voltage. It is this reverse magnetic flux from the secondary to the primary coil that provides the "feedback" mechanism that varies with how heavy the load is. The heavier the load, the more secondary current, so the more back magnetic flux in the core cancelling the forward flux from the primary coil, so the more current required to flow in the primary coil in order to stabilize everything with the primary and secondary voltages and currents obeying the simple transformer equations.

    Hope that helps. Things get more complicated when you include real parasitics of transformers (losses, leakage inductance, series resistance, etc.), but the above is the basics of transformer action. Check out this wikipedia.org page for more details and some drawings:

  7. Dec 7, 2007 #6
    You are an absolute gem, if you were my neighbour you would be getting mince pies! That clears the whole thing up nicely. I thought it must be something to do with the back e.m.f, like all this kind of learning the more times you hear it the clearer it becomes. Thank you! Much appreciated. Ben and Sophie.
  8. Dec 9, 2007 #7
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