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What is impedance?

  1. May 16, 2012 #1
    I wiki'd it and I am still having some trouble understanding it. Is it basically like resistance for AC circuits? Then why is resistance still mentioned when talking about AC?
     
  2. jcsd
  3. May 16, 2012 #2
    Simple resistance is the ratio of instantaneous voltage by current and is not a complex number. However impedance is a complex quantity and is related to the phase of current and voltages apart from its magnitudes.
     
  4. May 16, 2012 #3

    jim hardy

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    Keep your thinking simple and use language to your advantage. And you powers of observation, too.
    In simplest terms;

    Impedance is anything that impedes flow of current. It comes in two basic flavors, resistive and reactive.

    Resistance dissipates the energy carried by the current as heat. That's why resistors run warm and have a power rating.
    Reactance is usually associated with alternating current. It stores the energy and returns it to the source at a different time in the AC cycle. Thats why capacitors run pretty cool to the touch.

    Both flavors are measured in Ohms, which is the ratio (volts applied) / (amps the impedance let through).

    maybe that helps you take next step up the ladder of understanding ?

    old jim
     
    Last edited: May 16, 2012
  5. May 16, 2012 #4
    i kinda get it better now. So then why is a transformers impedance information so important? does it affect its effeciency?
     
  6. May 16, 2012 #5
    Resistance is given by volts/amps, the volts and amps are in phase. Reactance for capacitors and inductors is given by Volts/Amps but the volts and amps are out of phase by 90 degrees.
    Impedance is a combination of resistance and reactance. The volts and amps will (usually!!) be out of phase
     
  7. May 16, 2012 #6

    jim hardy

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    usually one wants to deliver power through a transformer. Knowing its impedance lets you estimate two things first off:

    1) what voltage drop will be incurred across it as load changes from light to heavy,
    2) in the unfortunate event of a short circuit across its output terminals how much current the transformer will allow to flow into that short.

    1 is used to provide reasonable constancy of voltage when load changes
    2 is used to select protective devices, usually circuit breakers or fuses, that are stout enough to withstand whatever the transformer can throw at them.. should they fail the fireball propagates where you REALLY don't want it to go.

    Very briefly - a 10% impedance transformer will drop ~10% voltage no load to full
    and will allow ~10X rated current into a fault.

    That's the basic principle.
    Keep your thinking simple.
    The arithmetic gets complex so you have to do it on paper.
    Build from the simple toward the complex.
     
  8. May 16, 2012 #7

    FOIWATER

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    the percent impedance of a transformer is important for a number of reasons, for safety purposes, the value of percent impedance determines how much fault current flows in transformer windings - it is expressed as impedance, because a transformer has both winding resistance and leakage reactance associated (referring to Jim's post).

    Practically, the percent impedance of a transformer is a percentage value, that is the ratio of voltage drop in the transformer due to winding resistance and leakage reactance to the rated voltage of the transformer.

    For further notes on leakage reactance - Ideally your coefficient of mutual coupling from primary winding to secondary winding would be one, but it cannot be so. Therefore, you have some flux which 'links' the two coils, and some which 'leaks' the primary, does not cross the secondary but rather collapses back onto the primary to cause voltage opposition to the source voltage. The opposition, does not cause resistance resulting in heat (which does still occur due to winding resistance) but rather the opposition is called reactive, and the opposition forms complexly with resistance to yield transformer impedance. It is this value then which is divided by rated voltage to give percent impedance.
     
    Last edited: May 16, 2012
  9. May 16, 2012 #8

    psparky

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    Let me grab Sophie's explanation here:

    Not just being picky here, but . . . . The word is Impedance. The word Resistance had other connotations and we mustn't confuse the two.
    We then say that the Impedance, Z is R+jX, where X is the Reactance. For a Capacitor, the Reactance is 1/ωC.

    You can't afford to neglect the use of Complex Numbers for this or you can't arrive at a phase shift.
    V=IZ gives you it.


    And actually, I think he meant to say that the reactance is -1/ωC. Without that minus sign you can't plug it correctly into R + jX.

    The reactance of a inductor will be ωL.

    The impedance of a capacitor is then (1/jωC).....same as (-j/ωC) If you put in formula above its actually R0 -(j/ωC). But since theres no resistance in an ideal cap...its just (1/jωC)

    Same idea with inductor.......except impedance will be JωL. (R0 + jωL)

    ω equals radians per second by the way.....or ω=2*pi*f....but you prob know that.

    AC or DC.....V=IR. Just in AC you will have phasors that are not sitting at zero degrees if caps or inductors are in the circuit. You will have to get handy with phasor addition and multiplication. You are already using phasors in DC.....just they are sitting at zero degrees so you don't realize it. And you actually use the 90 degree vectors in DC with the cap and inductor....just when ω=0...you end up with zero or infinity which also keeps you from seeing the vectors.

    If you are talking about the impedance of the whole circuit, you will have to add up all resistances and impedances like the formula implies.

    Don't be afraid of that "j" either. It simply means a vector with a magnitude of 1....sitting at 90 degrees. Think about it. As soon as you put a "5" in front of it for example....you now have a vector with a magnitude of 5 sitting at angle 90 once again. Put in the minus sign and your vector is now sitting at minus 90 degrees.

    Understanding power factor correction will help you understand things further as well.
    Be patient, fully understanding this stuff will take years and years....and even then you will still be learning....just like me and everyone else.
     
  10. May 16, 2012 #9

    jim hardy

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    Go Sparky !!!!

    indeed, the arithmetic gets complex. (i cant believe you guys let me get away with that awful pun)

    Operator j just rotates your phasor (vector) a quarter turn.
    So two operator j's rotate it a half turn,
    which reverses its direction,
    which is same as multiplying by negative 1,
    so operator j is equivalent to sqrt(-1) is the simple memory peg i used in early days.

    And it works - when you're doing this stuff on paper you can replace all the j2's with minus signs.

    Build the complex from the simple. Then you'll figure out the "why".
     
  11. May 17, 2012 #10

    psparky

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    I had to chuckle at this statement a little bit. Fully grasping impedance took me over 5 years of intense study and experience to learn....and I'm still learning it. Today's kids want to "wiki" in it 5 minutes. Sorry to pick on you my friend....but it is going to take a long, long time to get the big picture. There are no shortcuts.....only hours and hours and hours of studying if you want to go all the way. But stay in the game.....bells will gradually start to ring over time.
     
  12. May 17, 2012 #11
    Nothing wrong with using wikipedia to learn if you fact check as you go and understand to be cautious of who is writing the articles. Some wikipedia articles have more information and are better written than a lot of textbooks.
     
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