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Why does Ohm's law not apply to transformers?

  1. Jan 11, 2016 #1
    How can a transformer increase the voltage yet lower the current? Are the equations P=VI and V=RI contradictory?

    Thanks in advance and sorry if I made any english mistakes.
     
  2. jcsd
  3. Jan 11, 2016 #2
    Hi, this discussion might help you with this.
     
  4. Jan 11, 2016 #3

    cnh1995

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    Not at all. You'll need to study transformer magnetics first. Primary current "due to" the primary voltage source is called magnetizing current and it establishes the flux in the core. When we say VpIp=VsIs, Ip is the reflected secondary current, an extra current drawn from supply to totally cancel out the demagnetizing action of secondary mmf and maintain constant flux in the core(the one produced by magnetizing current). Magnetizing current is very very small compared to Ip, hence is neglected in explanations about the transformer. For power transfer, Ip is responsible. Get any good electrical machinery book and you'll see all these things explained in detail.
     
  5. Jan 11, 2016 #4

    sophiecentaur

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    Not when you realise that the effect of a transformer is to modify (transform) the Resistance of the load to a different value, as seen by the supply. In order to understand this you have to get the causes and effects in the right order. The produce VI will be unchanged but the R of the load will determine the current through it for a given secondary Volts. The Load Current times the Secondary Volts is the Power dissipated and so is the Supply Current times the Primary Volts.
    We all know that the Voltage is altered by the ratio of the numbers of turns in primary and secondary but also the Load resistance is transformed by the square of the turns ratio. So a mains transformer will supply high current at a low voltage (to a low resistance load) but demand a low current from the mains voltage supply (presenting itself as a high resistance).
     
  6. Jan 11, 2016 #5

    russ_watters

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    Staff: Mentor

    Increase the voltage vs what? Was the circuit run without a transformer before the transformer was installed? What was the current then?

    The equations work fine, the problem is that you are not being strict/specific in your analysis.
     
  7. Jan 11, 2016 #6
    I think I got it, thanks a lot!
     
  8. Jan 11, 2016 #7

    CWatters

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    It doesn't lower the current. It's best to think of the current in the secondary as determined by the load not the transformer. No load => no secondary current Is. If the load is a resistor of value R then Is = Vs/R.

    If the transformer increases the voltage (in the secondary compared to primary) then the current in the primary Ip will be greater than the secondary Is. So it looks like the transformer is reducing the current in the secondary compared to the primary but in reality it's the other way around. The secondary current is dictating the primary current. This is due to the law of Conservation of Energy. If the transformer is lossless (aka "Ideal") then the power going into the primary must equal that drawn by the load on the secondary.

    Conservation of energy...
    Pp=Ps
    but P=IV so..

    IpVp=IsVs
    rearrange
    Ip = Is* (Vs/Vp)

    So if Vs > Vp then Ip > Is
     
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