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AC Measurements

  1. Jan 22, 2009 #1
    If the current, voltage and power are measured at the terminals of a device containing only resistance and inductive reactance, how many values of R and X can be determined from just 1 A.C. measurement?

    (P.S. This is not homework but an elec engineer who has been stumped)
  2. jcsd
  3. Jan 22, 2009 #2


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    You mean one measurement of each voltage, current and power? What do you mean by how many values? As in: XL = 150 ohms, R = 150 ohms for instance?
  4. Jan 22, 2009 #3
    Yes. One measurement of each, how many values can you get like you have given examples
  5. Jan 22, 2009 #4


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    It may not be homework, but it's awfully homework/coursework-like. So many of the same rules apply. Tell us what you think, and we'll try to help guide you as YOU figure it out.

    As for some hints.... If you measure the voltage and current, then measuring the "power" gives you no new information. If you have a known source voltage and source impedance (like from an impedance analyzer instrument), then you can measure the complex impedance of the device at its input terminals.

    What you measure as a complex impedance can be built up infinitely many ways with resistors and inductors. You are measuring the total combined impedance. Oops, maybe I just gave the answer (bad Mentor).

    Why are you asking this question? What is the context?
  6. Jan 22, 2009 #5


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    Actually, it does. By measuring power I think the OP means measuring actual power, and NOT VA.
  7. Jan 22, 2009 #6
    Thanks for your replies ppl.
    The context is I was just browsing through an electric machines textbook on transformers (you may have seen my other post) and this question was at the end of the section. It had me puzzled thats all.
  8. Jan 22, 2009 #7
    I believe it refers to actual power also.
  9. Jan 22, 2009 #8
    My thinking is this...

    You have the actual power so you can obtain a resistive component value.
    From the voltage and current measurement you can obtain an impedance and using this value in conjunction with the resistive value already obtained you can obtain a reactive component.
    Thus, 2 components: 1 reactive and 1 resistive

    ?????? Agree ???????
  10. Jan 27, 2009 #9
    Since you are an electrical engineer you know what "Thevenin" means. If you measure the open circuit voltage at the terminals and you measure the short circuit current you can determine Z, the thevenin source impedance from Z=V(open circuit)/I(short circuit). That Z can be formed by many different circuit configurations of resistive and reactive components internal to the device.
    It is not clear to me what you mean by "Power" measurement. Power of what?
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