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Resistance and frequency

  1. Nov 17, 2014 #1
    Hi everyone,
    I am trying to simulate a circuit containing resistors, inductors and capacitors. When i calculate the Impedance of the circuit i find it to be for example 150+100j for a frequency of 5Hz. When i run the same simulation for a frequency of 15 Hz i find it to be 162+134j. I understand that inductive impedance increases with frequency but what about resistance? It cannot be independent since it changes too! Is there any possible relationship formula connecting these two parameters?
    Thank you
     
  2. jcsd
  3. Nov 17, 2014 #2

    phinds

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    Ohmic resistance doesn't change until you get up to frequencies where the skin effect becomes significant; it's the impedance that is changing.
     
  4. Nov 17, 2014 #3
    But i run the simulation on very low frequencies. Is it possible that skin effect takes place in such spectrum?
     
  5. Nov 17, 2014 #4

    donpacino

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    what is the circuit layout?
     
  6. Nov 17, 2014 #5

    phinds

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    No, but again, it doesn't have to. You are not seeing any change in the ohmic resistance, you are seeing a change in the impedance. Resistance does not change with frequency until you get to very high frequency. Impedance of a reactive circuit changes with any frequency change.
     
  7. Nov 17, 2014 #6
    Plinds you are correct but let me type my question differently. I know the formula for changing the imaginary part of the impedance and it comes from 2 x pi x f x L. But how does the real part change? Is there a formula for that? And lets say that the real part is the total resistance of the circuit, including the inductor series resistance. How can someone describe the change in that parameter?
    Thanks for the response btw
     
  8. Nov 17, 2014 #7

    sophiecentaur

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    I think you are making an unwarranted assumption that the Resistive Component must stay the same for all circuits. If you have an R in series with a C, the R will stay the same with frequency but a more complicated circuit will not necessarily pan out that way - as you have shown. Believe the results and not your intuition. :)

    PS I think the comments about skin effect are not relevant here and just likely to confuse a chap. Unless you actually model the skin effect in the simulation (straightforward circuit analysis) you can't include it in the calculations.
     
  9. Nov 17, 2014 #8
    Thank you for the answer sophiecentaur. The fact is i understand that the combination of resistances and other elements in a circuit has different results. Lets say i have an RL circuit containing only a resistance and an inductor. When i simulate that with LTSpice the total impedance is changing with frequency. My question is if i can have the impedance data for 10 Hz using field measurement, could i possibly calculate the impedance at 50 Hz using some kind of relationship formula between frequency and impedance or resistance?
     
  10. Nov 17, 2014 #9

    donpacino

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    lets take a look at a simple circuit, a capacito

    At DC the resistance of that circuit is inf, or open. at very high frequency the circuit is zero, or short.
     
  11. Nov 17, 2014 #10
    Ok agree with that.
     
  12. Nov 17, 2014 #11
    If a very good conductor has a large enough cross section, skin effect ( in a relative sense) can be significant at low frequencies too.
     
  13. Nov 17, 2014 #12

    donpacino

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    Hes running this in LTSPICE

    I am assuming he is using ideal components, in which case you would not see skin effect at all.
    op can you confirm?
     
  14. Nov 17, 2014 #13
    Than it is just a matter of reactance change.
     
  15. Nov 17, 2014 #14

    sophiecentaur

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    Using a simulation is all very well but it does nothing more than save you the effort of building the circuit. It doesn't actually show the relationships between the circuit components and the input signal values. If you want this "some kind of relationship formula etc. etc " you need to go through the proper analysis. That is the relationship you are after. There is no quick way into that sort of knowledge. I often get a bit grumpy about simulation software but you prove my point about this completely. Very few simulations (of anything) yield a good understanding of the way things work; they usually introduce as many pitfalls as good learning experiences when taken on their own.
     
  16. Nov 17, 2014 #15

    phinds

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    Which is exactly what I've been telling him since post #2
     
  17. Nov 17, 2014 #16

    sophiecentaur

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    Actually, a similar problem would arise if you just made up a reasonably complicated network of resistances and then asked for a 'simple relationship' between one of the resistances ( a variable one) and the overall resistance.
     
  18. Nov 18, 2014 #17
    Ok because maybe my question got a little complicated in the way. I know that impedance changes with frequency, i am also familiar with skin effect and especially i am not lazy enough to expect from couple simulations to extract a relationship between two variables. I have build at least 15 different circuits and ran over 200 simulations for different frequencies so i have a pretty big database! My original problem is that i want to apply a low frequency, low voltage signal in a 240 V, 50 Hz distribution power system, and by measuring the impedance in 5 Hz to calculate the impedance in 50 Hz. I know how to do this for the imaginary part of the impedance and it is 2*pi*f*L in a lagginc circuit. But how can i calculate the real part too? That is my question.
     
  19. Nov 18, 2014 #18
    Simply put, in general case you can't do that. Just from equivalent input impedance Z1=R1 + jX1 at some frequency ω1 you can't determine what would be equivalent impedance Z2=R2 + jX2 of the network at some other frequency ω1.
     
  20. Nov 18, 2014 #19
    Yes that what i figured out after a lot of simulation. the equations on different circuit configurations simply don't match.. Maybe there is indeed a palindrome that i cannot figure out but for now i have to declare that it is not achievable to predict it! Thanks anw
     
  21. Nov 18, 2014 #20

    donpacino

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    To understand why this doesn't work, look at the frequency response of a complicated circuit.
     
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