surface vs. inner flux path

Discussion in 'General Electronics Chat' started by samjesse, Feb 12, 2010.

  1. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    Hi


    How much current is needed in a primary before the secondary can “feel” any flux in a transformer core?
    Does core saturation needs to happen first before flux start using the air around it thus reaching the secondary?

    thx
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    A tiny amount of current will cause a tiny amount of flux and a tiny voltage across the secondary. I mean if there is current throug the primary the secondary will 'see' flux (maybe very small) regardless how small the current is.

    What do you mean in the second question?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    If core saturation is reached, increasing current in the primary will not result in significantly more current in the secondary.
     
  4. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    Is not the flux path inside the core when the primary current is at minimum and thus the flux will not radiate out to cut the secondary windings which are located outside the core?

    I am missing something..

    thx
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I think this is a fair question.

    But remember, Sam, the B-H curve is S shaped. So the slope is very low at the beginning (your initial question) and the end (saturation).

    But I am not sure what you mean by 'how much current'. How are you proposing this is limited? Transformers are designed to have a sensibly resistance and be limited by their reactance.
     
  6. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    What I am trying to understand is that at school we were told that current will be induced if flux cuts a wire.
    let this wire be the secondary which is wand around the core. and lets introduce flux in the core, be it by using a primary winding or a permanent magnet.
    once the flux passes through the core it will make a complete flux path. if that flux "stays" inside the core, then how will the secondary produce current?

    does not the secondary wire need to be cut by the flux in order for current to be "generated" in it. if the flux is inside the core and not outside it "where the secondary is" then how will the current be induced in the secondary?

    my question "assumption" about the point of saturation is my personal effort to justify that a saturation needs to happen inside the core and when that happens then there is no more material to carry the excess flux then and only then the flux leaves the core and uses the air surrounding the core "where the secondary is" and only then will the secondary gets cut by the flux producing current in it.
     
  7. mik3

    Senior Member

    Feb 4, 2008
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    You are right that in saturation of the core any increase in the flux is conducted by the sourrounding air.

    However, this is not how voltage is induced on the secondary. If you want to think that the flux cuts the wire and induces voltage it will like that: Because the density of the flux varies (AC current through primary) the magnetic lines are changing in intensity (if you imagine them). Therefore, as the magnetic flux density varies the magnetic lines are seen (by the coil) to move in and out from the sides of the coil (not long its axis). Thus, its like they cut the coil and induce voltage.
    Another, way to see it its with the amount of flux density which passes through the coil.

    e=-N*dΦ/dt Faradays Law

    As you can see, the induced emf (e) depends on the rate of change of the flux through the coil's cross sectional area. Because the primary's current is AC the flux (Φ) varies and thus voltage across the secondary is induced.
     
  8. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    Thanks mik3

    I understood that the secondary coil acts as if it were a "radar" which detects any flux intensity changes inside the core since it is wand around it as close as possible.

    Now lets imagine that instead of being wand around it, we make a cut in the core such that the flux path will have to "jump" from north to south to complete its path, and place the secondary coil in that gap. Will that "produce" as much current as in the first setup?
     
  9. mik3

    Senior Member

    Feb 4, 2008
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    If the current through the primary remains the same the flux through the core will reduce because of the low permeability of the air. Thus, the induced emf on the secondary will be reduced too.
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    This is changing the goalposts from the original questions which was about low current.

    Let's try this another way.

    The magnetic flux is produced by the (primary) coil, not by the core. The presence or absence of the core does not affect the total amount of flux produced, only its position in space.

    Magnetic flux, just like electric current, divides itself through all possible paths available. The greatest part passes through the path of least resistance. In magnetic terms this is termed reluctance. This path is of course the core. But there is always some air flux, particularly near the core, just as there is always some current in high value resistor in parallel with a low value one.
     
  11. mik3

    Senior Member

    Feb 4, 2008
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    I do not agree.

    Flux=N*i*μ*A/l

    This says that flux depends on μ and thus on the material of the core. It is the MMF and H that are not affected by the core.
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    That is only the flux density within a solenoidal winding.

    I said the total flux is the same. I think this a direct consequence of the law of conservation of energy.

    In other words the triple intergral over all space is the same.

    My point was that the total must be the same, it is just more spread out in an air or vacuum cored coil.
     
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