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Transformer currents in primary and secondary question

  1. May 27, 2012 #1
    I'm reviewing transformers and I have some confusion over the current

    s is for secondary and p is for primary

    I know that
    Vs = Vp(Ns/Np)
    Ip = Is(Ns/Np)

    So when you have a load across the secondary terminal you get your current through Vs/Rload.
    But does this mean for current in the primary coil, it has nothing to do with the inductance value or resistance (if I added a resistor before the inductor in the primary section)? V/Z = I does not work here?

    If this is true what would the current be if I left the secondary terminal open so no current will flow. What do i use to calculate current in the primary coil? Is it affected by the inductance value then?

    thanks in advance
  2. jcsd
  3. May 27, 2012 #2


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    Science Advisor

    Those formulas are for ideal transformers. There will be a current in the primary due to the load in the secondary, but there will also be a current due to the inductance of the primary winding.
    It is just a coil and it has to have some value of inductance and so it has a reactance which will allow a certain amount of current to flow from the supply.

    Cheap transformers are sometimes wound on inadequate iron formers and so don't have enough inductance or reactance to stop a relatively large current flowing even if there is no load.
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