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In power transmission why inductance L increases with increased spacing

  1. Sep 10, 2016 #1
    In power transmission where single phase power is transmitted in 2 wire AC system.
    Now inductance is
    The equation for inductance is then:
    L = 2 X 10-7 In(De1/4/rx)

    Why does inductance increase with increase in distance between conductors? When the spacing is increased, the air gap increases between conductors, so reluctance increases, so flux linkages decrease, so inductance must decrease, as L = flux linkages / current.

    Why does inductance physically increase with increase in conductor spacing?
     
  2. jcsd
  3. Sep 10, 2016 #2

    jim hardy

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    i think you need to draw a picture.

    what's formula fpr inductance of a loop of wire ?
     
  4. Sep 10, 2016 #3
    Single+phase+Line+Inductance.jpg
    Formula for inductance is L = flux linkages / current.
    If distance is increased between the two conductors above, why does L increase? I think flux linkages would reduce because mutual flux would reduce.
    L = 2 * 10-7 ln ( D / r / 0.7788)

    I couldnt find equation for L in rectangular loop in the book.
     
  5. Sep 10, 2016 #4
    When you look at formula for inductance in a wire pair you usually see a caveat that one wire is assumed to be a source and the other a return. In that case flux cancels an amount. The closer the wires the more significantly flux cancels. That's why signal wires are often encased in a twisted pair, to reduce inductance.

    For two wires carrying current in the same direction flux does not cancel, but currents are shared between two conductors so the overall value is reduced in the manner of parallel inductance. Proximity of the conductors does not have the same effect. Flux couples rather than cancels as wires become closer together.
     
    Last edited: Sep 10, 2016
  6. Sep 10, 2016 #5
    Yes, even in the question in first post, the other conductor is return path for current in first conductor. So current in both conductors are opposite to each other.
    but if you look at the center, then the magnetic lines of force are in same director for both conductors. Like both lines of flux are going into the screen like a cross.

    so wouldnt the flux lines add each other instead of cancelling each other out?
     
  7. Sep 10, 2016 #6

    dlgoff

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  8. Sep 10, 2016 #7
    I'm sorry but how are they cancelling. The scenario that i asked is just as the first picture. The one with red and blue flux line. the currents in conductors are flowing in opp directions.

    And it says that the lines of force add in the region.
     
  9. Sep 10, 2016 #8

    dlgoff

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    You're right with this image. I misread it when uploading. Sorry.
     
  10. Sep 10, 2016 #9
    So the question remains same. Why does inductance increase with increase in spacing? In fact now if you look at image, it's clear the fluxes of the two lines are adding each other. So as distance between them increases, the flux reduces, so L must reduce.
    Why does L increase with spacing b/w conductors?
     
  11. Sep 10, 2016 #10

    jim hardy

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    Of course they add.


    Then why do you insist flux decreases as the wires separate?

    That transmission line has a source and a load so it is a loop that encloses a finite area.
    Anyplace in between those wires flux is being pushed, in your picture, into the screen by both wires.
    The more area the more flux.


    . tail1.jpg


    Try superposition....
    In your mind, double the distance between the wires. Within the area between them, each wire now pushes more not less flux into the screen.

    Formula for inductance of a flat loop is not so nice looking as for a solenoid and that's why most authors avoid it.

    https://www.eeweb.com/toolbox/rectangle-loop-inductance

    tail2.jpg


    old jim
     
  12. Sep 11, 2016 #11
    But if the distance between two wires is increased why would there be more flux at one point? I mean it's like saying that if i increase the air gap of induction motor there'd be more flux linkages.

    Flux equals mmf / reluctance. MMF depends directly on the current in the wire. Reluctance is the air gap reluctance. As distance increases the air gap would also increase between two conductors, so there should be less lines of flux. it's like a magnet will have strong force to objects near to it, than to objects away from it.

    I saw the formula but i dont understand actually how can it be. if that's the case that inductance increases with distance then each transmission line will have infinite inductance as there are many many transmission lines all around the world. And its possible that some conductor will carry current in opposite direction.

    I guess i'm missing some very basic point. Any insight?
     
  13. Sep 11, 2016 #12

    jim hardy

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    reluctance ?

    taill3.jpg

    In your drawing, each wire is surrounded by concentric circles of flux. The wider the separation of the wires the more of those circles you can fit between them.
    Is this where you found that drawing?
    http://www.skm-eleksys.com/2011/03/transmission-line-parameters-resistance.html
    I see the same one there.
    As they explain, Inductance is figured in (milli?)Henries per unit length , mile or km.

    Infinite? Only if length is infinite.
     
  14. Sep 11, 2016 #13
    Take another look at the magnetic circuit of the 2-wire electric circuit.

    The air gap you mention between the wires - that is not the air gap in a magnetic circuit on which you seem to be basing the reluctance, Jim has discussed reluctance..

    The magnetic circuit for a loop of wire is that area for the field flux. That area is the area encompased by the loop, and all of the area outside the loop. The area outside the loop is pretty much constant, so only the area within the loop affects the magnetic circuit. ( In the basic case at least )

    Decreasing the area - is moving the two wires together -
    Increasing the area - moving the two wires apart -

    --------------
    as an aside :
    A solenoid is just a bunch of 2-loop wire circuits piled on top of one another, with the flux produced from one loop added on to the other. One can look at one wire next to the other and see what happens. That is picture 2 in the dlgoff post. One can look one wire and the wire directly opposte - that is picture 1 in the same post.

    Try this post and follow the flux paths for a solenoid. - Replace with just one loop follow the logic.
    http://www.tpub.com/neets/book2/2b.htm
     
  15. Sep 11, 2016 #14
    If I try superposition, then the lines of flux one conductor pushes into the screen between the distance between the wires, well these lines of flux won't extend till infinity. I think there would less lines of flux as distance from conductor increases. Isn't that what happens in a magnet?

    A magnet pulls objects that are nearer but if you increase the distance between the magnetic material n magnet, the magnet may not be able to pull the material.
    Is is because lines of force/flux become sparse as distance between magnet n material increases.

    Similarly if I increase distance between conductors, if the flux lines increase, where did extra mmf come from for additional flux lines.

    If we take basic magnet, it has n s pole. The magnetic field is strongest near the pole n fades gradually.

    I think this is what i'm having trouble understanding. Why would there be more flux lines? The MMF is constant. As distance away from current carrying lines is increased there should be less lines.
    Isn't that what happens in case of magnet? Like if magnet is away from a nail, the magnet wont attract the nail. But if magnet is near to nail only then will magnet attract the nail.
    I understood the formula but actually where do the extra lines of flux come from? Where does the extra magnetic energy come from?
    Just like there's a fire, the heat is strongest near the fire.
    Like HV line, the ionization of air is maximum near the line.
     
    Last edited: Sep 11, 2016
  16. Sep 11, 2016 #15
    Yes that's right flux intensity falls off with distance, that's why mutual inductance also so falls off with distance. The problem is you are not understanding the manner in which the flux of two wires interacts as distance increases or decreases. For common paths, proximity couples flux increasing inductance to that of a single wire. For opposite paths proximity cancels flux reducing inductance below that of a single wire. You can use the idea of a loop as area goes to zero to get a feeling for it, but still that's the math for a loop not a linear path so it won't make a lot of sense with a zero area. That's something that can't happen realistically.
     
  17. Sep 11, 2016 #16
    That is what I thought that maybe the flux lines cancel each other n so as distance increases the cancelling element reduced.

    Buy if you look at dgloffs post. Then the flux lines add in case currents are in reverse direction.
     
  18. Sep 11, 2016 #17

    jim hardy

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    Why can't they extend to infinity ? They just become very faint.
    Look at Biot-Savart. Zero is the limit as r approaches infinity.

    Draw an end view. Each wire is surrounded by concentric circles. The wider the space between the wires the more of those circles traverse that space.
    Also realize that they're not really lines but a fluid-like continuum. We just represent the continuous field with lines whose closeness indicates intensity.

    Add "perceptibly"
     
    Last edited: Sep 11, 2016
  19. Sep 11, 2016 #18
    That's a good point. Forces created by some body such as gravity and magnetism never actually go to zero in space, they just become more and more disperse until they can be no longer be measured. The forces are still propagating through space albeit too small to be observed at some far off distance.
     
  20. Sep 12, 2016 #19
    So if they become faint then wouldn't their effect also be less? I mean isn't that what faint implies? Less strength.
    I looked up Biot savart's Law... Definition of BIOT-SAVART LAW. : a statement in electromagnetism: the magnetic intensity at any point due to a steady current in an infinitely long straight wire is directly proportional to the current and inversely proportional to the distance from point to wire.

    The magnetic intensity is inversely proportional to distance from point to wire. So if distance is increased then intensity would reduce as per Biot Savart's law.:oldconfused:
     
  21. Sep 12, 2016 #20

    jim hardy

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    Intensity at any single point, sure. Flux isn't intensity at any point, it's the integral of flux density over an area. When you add area by separating the wires you have a lot more points to tally up with your integral.

    Draw two end views with one having separation twice the other. Do the circles near the wires get farther apart ? Or do you get more of them in the middle ? Why would the circles nearest each wire change any? Have you tried superposition yet ? What's the intensity contribution from each wire midway between them, and what is it 1/10th of the way between them?
     
    Last edited: Sep 14, 2016
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