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Another Inductance winding Number question

  1. Mar 17, 2015 #1
    Hi,

    So if you wind a bifilar core in parallel and: connect it up in series, then it will have twice the N of if you connected them in parallel, i.e. The inductance is proportional to (2*N)^2 rather than just N^2, as connecting them up in parallel just increases the cross sectional area of the wire.

    But what is the value of N if you wound two separate windings around a core, of different number of turns to eachother, THEN parallel?

    So for instance I wind 30 turns on a core, then another 60 turns and parallel these sets of windings together, what's my value of N?
    I'll take a guess and say it's the average of both, N = 45 turns?

    Please refer to the link if necessry:

    http://en.wikipedia.org/wiki/Bifilar_coil


    P.S
    I'm reading my book's transformer section and it says
    "the windings should be bifilar wound in order to maximise the couplings and minimise the leakage inductances if there are no safety conciderations. In order to do this the wires should be twisted together. This is often carried out with multiple secondary windings to improve the cross-regulation."

    Is this saying that you wind the primary and secondary at the same time? Otherwise what would be the point in
    1. parallel-wound, series connected?
    It says it would 'magnify the potential between coils' but what would be the point of that?

    Thanks
     
  2. jcsd
  3. Mar 18, 2015 #2

    NascentOxygen

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    For a given transformer arrangement there can be only one volts/turn factor. So I think you're entertaining the same idea as: if I connect a 12v battery in parallel to a 6v battery what will their terminal voltage?

    In that moment before the explosion it may well be somewhere around midway ....
     
  4. Mar 18, 2015 #3
    I have a hard time accepting the battery analogy, depending on the size of the batteries I imagine rather than being the midway point it'd just be the larger potential of the two. I can imagine the potential of the larger overwhelming the smaller and a lot of current flowing back through it, probably distroying it, if it's not a rechargable battery. But in this case there's no opposing MMF or EMF.
    And I don't expect that thinking only of a scale where an explosion is possible, is the best way of addressing the theoretical situation, nor is the battery analogy too helpful.

    total MMF is some N * I, logically I is a function of the amount of paths and the resistance of the paths, so it would seem to follow that so would N be.
     
  5. Mar 18, 2015 #4

    Baluncore

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    If you analyse a transformer with two windings in parallel then you will see that if they have differing turn counts, part of one winding will be cancelled while the remaining turns will appear to be shorted. A significant current will flow in the “shorted” section.
    If n = 30, m = 60, n turns cancel, so m-n = 30 appear to be a shorted winding.

    A bifilar winding is used to get very close coupling between two coils. When bifilar wires are connected in series it produces a more accurately balanced centre tapped winding.

    That is not good. The voltage difference between the wires requires insulation. When a single wire is neatly wound the insulation needs only withstand the voltage between two adjacent turns. When bifilar windings are connected in series, half the total winding voltage appears between the bifilar twisted pair of wires.
     
  6. Mar 18, 2015 #5
    Ah, cool, so the bigger of the two is the turns number and the difference is shorted (with a lot of current in that section). Great answer thanks

    Right, so it was saying wind the primary and secondary together.



    Yeah, that's what I thought, I don't know why it's mentioned as an option if no one would use it, or atleast it should be stated that it has no practical common use.

    Thanks!
     
  7. Mar 18, 2015 #6

    NascentOxygen

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    You're implying that only some turns in one of the windings will carry a heavy current.
     
  8. Mar 18, 2015 #7

    NascentOxygen

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    It's stated because it's something you have to watch out for if you wish to take advantage of bifilar winding's attractions. These are often seen in transformers for matching RF equipment to antennas.
     
  9. Mar 18, 2015 #8
    Interesting point, if it were only some, how would you know, or how would it decide, which ones they'd be?
     
  10. Mar 18, 2015 #9

    jim hardy

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    You'll run across it in current transformers. The purpose is to give a non-integer turns ratio. It's mentioned in some 1920's patents but i havent yet found the one explaining it.

    You just said two windings on a core, not a transformer.
    For DC it's easy, current will divide between the windings in proportion to their mhos(1/ohms) and the fluxes will add.
    For AC it's not so easy. Current sharing will be a function of both winding conductivity and counter-emf.... old GE instruction leaflets for CT's are careful to differentiate whether transformers that have uniformly distributed windings. So i think mutual coupling and leakage flux come to the game. There are "tricks of the trade" here that only experienced people(well, more experienced than me ) ever encounter.


    We had a 2012 thread where a PF'er unwrapped a 50::5 current transformer only to find two secondaries connected in parallel , one of them ten turns as you'd expect and another of nine turns,.
    https://www.physicsforums.com/threads/how-many-turns-on-a-50-5-ct.586722/
    I tried at length to figure it out but never quite arrived.....

    Since there DO exist current transformers designed to provide voltage output, albeit low, for monitoring with a voltmeter across a low ressitance,
    i figure they design for a certain load impedance. That defines how much voltage the transformer needs to produce, which defines the flux level in the core at operating point. That gives the volts per turn which by controlling the winding resistances would allow a designer to achieve near perfect indication .
    If that 50::5 transformer needs a half amp-turn to make desired voltage, you'd shoot for a current effective turns ratio of 49.5 to 5, turns ratio of 9.9.

    In that thread the OP never characterized his core, sadly. Had we a measure of his volts per amp-turn we could have backward calculated the effective turns of his paralleled secondaries.

    Paralleling unequal windings in a power transformer where flux is set by applied voltage will result in high circulating current and probably smoke if there's no external resistance to limit current
    On just an inductor i dont know how to calculate it for AC current. It has to start with summing MMF's. But i wouldn't apply significant fixed voltage.

    Sorry, i wanted to help but probably just muddied things.

    here's a good magnetics page. He doesn't give away trade secrets but has good intro for CT's and Magamps.
    http://www.butlerwinding.com/resource-library/
    but hopefully this will trigger your "little gray cells"....
     
    Last edited: Mar 18, 2015
  11. Mar 18, 2015 #10

    Baluncore

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    The voltage is due to the count of turns that do not cancel. The current then flows in a loop through both windings, limited by the total resistance of the two windings.
    A better numerical example would be if n = 30, m = 34, n turns will cancel, so m-n = 4. The 4 turn difference produces a voltage difference that appears across the loop resistance of both windings in series, 30+34 = 64 turns. That loop current will heat the wire and is usually sufficient to saturate the core.
     
  12. Mar 19, 2015 #11

    The Electrician

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    What voltage are you referring to?

    In the first half of post #1, the OP first considers two windings of N turns each on a core, in parallel, giving some inductance proportional to N2. Then he notes that if the two windings are connected in series, the inductance is now proportional to (2N)2

    He then asks this question:

    "So for instance I wind 30 turns on a core, then another 60 turns and parallel these sets of windings together, what's my value of N?
    I'll take a guess and say it's the average of both, N = 45 turns?"

    He is supposing that the inductance of this configuration is also proportional to some N, but N is not 30 and not 60. He wants to know what the value of this new N is.

    It has been pointed out to him that paralleling two windings of different numbers of turns will effectively lead to a short, but no one has tried to calculate the value of N for him.

    Since he is considering an inductor, not a transformer, any voltage applied to the two paralleled windings of 30 turns and 60 turns will have to be applied externally, and not by induction from a third winding as could happen if it were a transformer, and there were a third winding, which there is not.

    So what does it mean to say "The voltage is due to the count of turns that do not cancel." if that voltage is not coming from a third winding?

    There will be a large current if the windings are excited by an external source, but I don't think the voltage driving the large current through the windings will arise in the manner you have described.
     
  13. Mar 19, 2015 #12

    Baluncore

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    Two windings on one core makes a transformer. If windings with different turn counts are connected in parallel and then excited, an internal loop current is driven by the volt/turn difference.

    The calculation of n becomes meaningless since there is a virtual short circuit across the inductor. Excitation of the inductor results in terminal currents and an internal circulating current that flows in a loop through both windings.

    30 turns in parallel with 30 turns on the same core will appear to be 30 turns. 30 turns in parallel with 30 turns on different cores will be equivalent to 21.213 turns.
    I agree it is a different situation but consider two magnetically independent inductors. 30 turns gives L=1, 60 turns gives L=4. Those inductors in parallel give L = 1/( 1 + ¼ ) which is equivalent to a turns count of n = 26.832

    That is certainly not the case. To analyse the magnetic coupling a third winding could be hypothesised that causes the same magnetisation as the two paralleled windings would if they were excited together. The circulating internal current loop could then be analysed without consideration of the loop terminal conditions.

    The OP wrongly interpreted the example to be an inductor, when in fact it must be analysed as a heavily loaded or short circuited transformer. That does not mean that I have to make the same mistake.
     
  14. Mar 19, 2015 #13

    jim hardy

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    Hmmmmm
    Paradigm shift here

    OP did not postulate a power transformer where volts per turn is fixed.
    Soooo,,,,,
    We must not apply not some fixed voltage to his postulated assembly because that'd let the smoke out of its windings, for the reasons you say.

    So instead we must apply some fixed current and let it divide between the coils as Mother Nature sees fit...
    If the applied current is limited, as in current transformers, no smoke will come out. Instead the flux will settle out to an amount equal to the sum of the mmf's divided by reluctance of the core. That flux will provide some number of volts per turn, and the currents will flow per the laws of Ohm and Kirchoff

    i think an approach might be to assume some AC induced volts per turn Φ
    and some ohms per turn R
    so voltage across each coil would be Nturns * (IcoilR +jΦ)
    then equate those two and see how current divides.

    Coils are 30 turns and 60 turns, he said?

    30 *( I30R +jΦ) = 60 *(I60R +jΦ)
    divide both sides by 30
    I30R +jΦ = 2I60R + 2jΦ

    subtract from both sides jΦ
    I30R = 2I60R +jΦ

    divide both sides by R
    I30 = 2I60 + jΦ/R

    divide both sides by I60

    I30/ I60 = 2+jΦ/( I60R)

    So at zero induced volts per turn(flux) current divides in proportion to their conductance ratio, 2, just like DC
    seems logical

    as soon as we allow flux we affect that current ratio.
    This has the form A/B = 2+jΦ/BR where A = I30 and B = I60
    Could i say that we apply total current A + B = 1 in some arbitrary measure ?
    Then A/(1-A) = 2 + jΦ/(1-A)R

    multiply both sides by (1-A)
    A= 2 -2A +jΦ/R
    3A= 2+jΦ/R

    sanity check,
    when Φ=0 then A=2/3 and B = 1/3, in accordance with the windings' conductance ratio of 2

    so
    i hypothesize that for OP's 2::1 ratio of turns and one unit of current applied
    A = 0.666 + jΦ/3R , the current in the 30 turn winding
    and B = 1 - A = 0.333 - jΦ/3R , current in 60 turn winding
    where Φ is a property of the core, volts-per-turn/amp-turn
    and R is a property of the wire, ohms per turn.

    Now that's interesting, it says real current divides in proportion to conductance of windings
    and reactive currents are equal and opposite. That sounds logical enough.

    Typing after midnight invariably gets me in big trouble.
    I might really regret hitting "Post Reply"

    but here goes anyway.

    Criticism is welcome, hopefully it'll be constructive.

    old jim
     
  15. Mar 19, 2015 #14

    Baluncore

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    Well done. Equal and opposite induced current suggests a circulating current of jΦ/3R.
     
  16. Mar 19, 2015 #15

    jim hardy

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  17. Mar 19, 2015 #16

    NascentOxygen

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    That is an interesting way to look at it, but this is still going to turn out to be an extraordinarily lossy inductor, is it?
     
  18. Mar 19, 2015 #17
    Thanks everyone, people have offered different perspectives or refinements to replies to the origianl question, all greatly appreciated, it's like real-time peer reviewing. I've read everyone's post several times.

    Even though I'm inferring a lot here, to see if I've got this concept down: 'a parallel turns imballance will cause a current to circulate between the sets of coils of a value [V / (difference in turn number) ] Amps? that flows through the resistance of m+n'
    If this happens at while the core is somehow not saturated, with respect to inductance what the 'N' value is 'm' turns?

    EDIT: (on seeing there'd been more posts after I started writing this) EITHER THAT OR (N = 'some MMF I don't know how to calculate' / 'the total current flowing through both coils: circulating and applied') I can't imaigine it being the sum of the MMFs because you can't calculate that except maybe if you did MMF1 = N1*(current divider for applied current to that coil + the circulating current through that coil) and the same for N2

    However the fact that there is a shorted turn component means that the inductance equation (and N value) is redundant because due to this circulating current, saturation has probably caused the inductance to drop right away?
     
    Last edited: Mar 19, 2015
  19. Mar 19, 2015 #18

    jim hardy

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    well i never got so far as the answer to your question, what is effective impedance.
    More algebra.... maybe Z of each coil = V/I , then Zparallel = Z60Z30/( Z60 + Z30 )

    With limited primary current we should be well away from saturation.
    Remember, that fellow unwrapped a real CT and found parallel secondaries.....

    so things like this do really exist

    i'd like to repeat that exercise for 9 and 10 turn secondaries then plug in some values for reluctance of a small toroid and resistance of a few feet of copper wire for the windings.

    But i make lots of mistakes and that's frustrating so i usually procrastinate .
     
  20. Mar 19, 2015 #19
    I think something's wrong with my browser, I'm seeing posts that I missed before and I might have been misreading a discord that doesn't exist.

    All very clever having to use a constant current condition rather than voltage, but I still have some questions.
    I'm a bit lost: "i think an approach might be to assume some AC induced volts per turn Φ"
    Is this AC induced 'Φ' volts per turn caused by the circulting current? And what was the 'j' in 'jΦ', like imaginary (-1)1/2? Also I don't see how the volts per turn is flux, wouldn't it be the derivative of flux: Φ' ?
    Seems weird that R and jΦ is the same for both coils, I'd have thought one coil would have 2R, but I'll take it on faith, so what was the current jΦ/3R, as a function of turns? was it for Φ = V developed across both windings / total number of turns?

    and what did this all mean for the total MMF?

    Thanks, this is really interesting.
     
  21. Mar 19, 2015 #20

    Baluncore

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    Modelling with spice shows that the behaviour of the circuit is highly dependent on the coefficient of coupling between the paralleled windings. If the coefficient is 1.000 then, when driven by the current source, there is in-phase voltage due to winding resistance but no reactive voltage across the inductors. Reducing the coupling to 0.999 shows a distinctive phase shift due to inductive effect. Coupling below 0.95 shows a dominantly reactive circuit as expected.
     

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