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Aluminium bobbin for choke. Is this a common practice?

Discussion in 'Electrical Engineering' started by John Crighton, Feb 21, 2005.

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  1. Hello All,
    How common are Aluminium bobbins for 50/60 Hz
    chokes and inductors? This is the first one I have seen.

    I was at a ham radio field day recently and picked
    up a heavy duty coil which I thought might make
    a good choke for small home made rectifier unit
    for a small welder. I will add the iron core when I
    find some laminations.

    The winding is aluminium strip 0.7 inches by 0.1inches.
    There are two strips of aluminium making up the 0.1 inch
    thickness. The strips are covered with some sort of
    fabric tape hand wound over the two strips.
    The whole winding is taped and painted so I can't see
    how many turns.

    The hole in the bobbin measures just over 4 inches by 1 inch.
    The bobbin is 3.3 inches wide.

    I didn't notice, since the whole coil assembly is covered
    in thick grey paint, but the bobbin is made of aluminium.
    I just assumed the bobbin would have been made of
    some sort of insulating material such as fibre glass or
    paxolin. An aluminium bobbin was a surprise to me.
    Is this a common practice to make heavy duty robust
    bobbins for low voltage applications out of Aluminium?

    Regards,
    John Crighton
    Sydney
     
  2. It doesn't sound like a choke to me.
    Al is a such a pain to machine or cast, ($$) plastic is widely used.
    need a pix.
     
  3. daestrom

    daestrom Guest

    I agree. The only way an Al bobbin would work is if it has a slot cut down
    the length of the hole and out the end faces to break the 'shorted turns'.
    The OP didn't mention such a slot though. As is often the case, "a picture
    is worth a thousand words".

    daestrom
     
  4. --

    -- Guest

    The coil appears to be a rather neat bit of creative engineering - better
    and lower cost.

    as to aluminum as the conductor and as the core

    conductor -

    1) The conductor in an inductor can be any material, since it is the
    orientation of the current, not the wire material, that does the work. An
    aluminum winding will work.

    2) And if you wish to get the oxide deep enough, I think it aluminum oxide
    is a fairly decent insulator. Aluminum welding machines use a high frequency
    low current high voltage rider on the AC/DC power to punch through the
    aluminum oxide for the low voltage power flow. It would have to be fairly
    deep oxide, I think, architectural grade or better anodizing. The tape
    would keep it from scratching both in handling and thermally.

    3) Theory says the extruded rectangular shape you can have from aluminum
    would let you get more field density than the round that copper is limited
    in drawn wire, and the coil would be more efficient.

    core -

    4) As I recall, aluminum is what is called paramagnetic (weakly magnetic).
    Where iron has a lot of its electrons oriented so as to spin in one
    direction over the other which then makes it magnetic, I think aluminum,
    nickel, cobalt, and a couple others have a few more spinning on one
    direction than the other, and they are called paramagnetic. (I would double
    check it in the book, but my kid borrowed it, and he has the book with the
    info in it over at the U)

    5) plastic in a coil ain't necessarily air-equal plastic as much as it can
    be various levels of ferrite imbedded in the plastic matrix- some probably
    akin to the levels of magnetism of paramagnetic aluminum -
    remember that there is more available to fill a core shunt than the yes
    of iron and the no of air - including iron-aluminum "alloys".

    putting a magnetic material in the core of a coil interferes with the
    magnetic field in that it :
    -changes the coil's inductance according to the current carried and actually
    drops inductance to zero at core saturation, It probably takes a LOT of
    field to saturate a paramagnetic core
    -decreases it to zero at certain frequencies so as to allow passage of those
    frequencies otherwise blocked by the coil,
    -severely drops it in the proximity of the coil (so tesla coils, sensors,
    etc. use non-magnetic/air cores) ,
    adds distortion and harmonics,
    reduces Q,
    lowers efficiency,
    reduces power handling,
    gives you the same inductance with fewer turns,
    lowers evil self-resonance,
    lower interference from stray field pickup,
    and
    it increases density near the coil proper.

    Aluminum in a core will interfere less than iron, iron either as ferrite
    (oxidized) or as element. And andozied deep enough and not scratched, I am
    pretty sure it is non-conducting. And it can be extruded into shapes whose
    edges leave no gaps.

    a fairly neat bit of engineering.
     
  5. AL losses are higher than copper, and it is the material that conducts the
    current.
    Deep oxide is brittle and causes fractures on AL wire. If you bend it
    slightly, it breaks through the surface.
    Ditto Copper. However extruding AL is more difficult that Cu, Cu is more
    malleable.
    Weakly anti magnetic. It causes L to decrease as it is put further into the
    core. (at high F)
    At low Frequency it is "transparent" to magnetic fields.
    nickel is magnetic.


    need a pix of the gizmo.
     

  6. Hello All,
    the best I can do for a picture is to refer you to this site
    http://website.lineone.net/~dave.cushman/transformers.html

    Half way down the web page there is a couple of pictures
    under Coil Formers or Bobbins.

    My coil former looks like the one on the left hand side.
    On my former, the side cheeks are made of thin aluminium sheet.
    The dark orange centre piece is again made of thin sheet
    aluminium (not an extrusion) with folded up 3/4 inch tabs to
    hold the cheeks in place. To stop the cheeks falling off the
    edge of the centre dark orange section.

    Just thinking about this coil. The fact that there was no
    iron core or scratch marks on the paint, it probably was
    never used or intended to be used for an iron core choke.
    The bobbin would be a shorted turn like you guys said.
    Not a good idea.
    I think someone has made this coil up and then
    realised, ooops! metal bobbin, scrap it!

    When I find some laminations to fit the 4" by 1" slot
    in the bobbin, I will experiment with the aluminium former
    and without it just for fun.
    All the hard work of winding the heavy conductor has
    been done, so tape alone will hold it together. I can
    still use the coil that way with a regular iron core.

    Thanks for all your replies, and discussion.
    Regards,
    John Crighton
    Sydney
     

  7. The strange part is the AL wiring on it. Seems like 1960's vintage.
    They used AL wiring in houses back then for a while
    It could be made to get hot and stay that way, and plastics will flow over
    time.
    But it should work fine with 50/60 Hz. (if the coils are not shorted).
    Some big equipment uses transformers and chokes (weigh 25 to 300 lbs. each)
    thick painted gloss black, for vibration tables, AM radio stations, some
    medical equipment, etc.
     
  8. --

    -- Guest

    but in the small amount used in a coil, that difference would be
    negligible.
    true about the oxide being somewhat brittle - I can't speak to fractures
    being caused by oxides on round wire and would imagine that would be
    possible, given the outer fiber deflection from the round shape turned in
    bending, but i find that unlikely on rectangular pieces, since the outer
    fiber stresses have more area to relax into in a rectangle than on a round.
    We never had any problem with fractures when anodizing and (generous
    radii) bending any of our rectangular architectural or exterior structural
    aluminum - mostly 6xxx and 5xxx series ( our sister companies were anodizing
    and extruding facilities).
    Extruding aluminum is fairly easy and a common practice, in my experience.
    I have had, and seen, many complex shapes with sharp corners extruded.
    I was under the impression from our sister company that copper would not
    hold a sharp corner coming out of the dies like aluminum does.
    Book is back from the U -
    According to Halliday, it is paramagnetic.
    also -

    http://www.rare-earth-magnets.com/magnet_university/magnetic_levitation.htm

    lists it as a paramagnetic material
    Being repelled by a magnet is referred to as diamagnetic in the physics
    text. It is present in all substances. See also the link above.
    I stand corrected - iron, cobalt, nickel, gadollinium, and dysprosium are
    all ferromagnetic elements.
     
  9. Roy Q.T.

    Roy Q.T. Guest

    you know Aluminum:

    how long have you had them ?

    An Aluminum bobbin may not do much under Normal Current conditions as
    you there suspect., but, betcha if you could keep boosting the
    ƒrequency to higher levels regardless Vin & control of Aout the
    circuit you'd get some notable results ....

    aren't those used in signal buzzers?
     
  10. Roy Q.T.

    Roy Q.T. Guest

    Try Your Phyics: aluminum has an explicit conductivity.

    you're to lost in electrical eng if you dare state it (Al) has no
    conductivity, perhaps you're taking the Clear Gel Subcoating of your
    sample for a given in your field.,

    [a boost in ƒ and you'll get the coil to sock or arc as customary]

    pierce the sample and you'll differ such utterances....

    Current: it will seem to stall under superfluos superresistive
    conditions but, believe it., it's there; perhaps in {another dimension
    or} frequency but it'll always be there if Voltage is applied.

    sheesh how can he have said it doesn't conduct ....?


    PARENTS: KEEP YOUR KIDS AWAY FROM THESE GROUPS !!!

    IF YOU'RE [not] A KID:)
    take care...this is a place where serious harm may result from misguided
    handling of products & information.
     
  11. --

    -- Guest

    I am not sure to whose response you are posting

    Try Your Phyics: aluminum has an explicit conductivity.



    you're to lost in electrical eng if you dare state it (Al) has no
    conductivity, perhaps you're taking the Clear Gel Subcoating of your
    sample for a given in your field.,

    [a boost in f and you'll get the coil to sock or arc as customary]

    pierce the sample and you'll differ such utterances....

    Current: it will seem to stall under superfluos superresistive
    conditions but, believe it., it's there; perhaps in {another dimension
    or} frequency but it'll always be there if Voltage is applied.

    sheesh how can he have said it doesn't conduct ....?


    PARENTS: KEEP YOUR KIDS AWAY FROM THESE GROUPS !!!

    IF YOU'RE [not] A KID:)
    take care...this is a place where serious harm may result from misguided
    handling of products & information.
     
  12. --

    -- Guest

    Try Your Phyics: aluminum has an explicit conductivity.

    electrical or magnetic?

    1) it does not conduct magnetic fields, although it is relatively permeable
    to magnetic fields

    2). Of course aluminum is an electrical condcutor . Aluminum oxide,
    however -

    you're to lost in electrical eng if you dare state it (Al) has no
    conductivity, perhaps you're taking the Clear Gel Subcoating of your
    sample for a given in your field.,

    to what are you referring?

    [a boost in f and you'll get the coil to sock or arc as customary]

    pierce the sample and you'll differ such utterances....

    Current: it will seem to stall under superfluos superresistive
    conditions but, believe it., it's there; perhaps in {another dimension
    or} frequency but it'll always be there if Voltage is applied.

    sheesh how can he have said it doesn't conduct ....?


    PARENTS: KEEP YOUR KIDS AWAY FROM THESE GROUPS !!!

    IF YOU'RE [not] A KID:)
    take care...this is a place where serious harm may result from misguided
    handling of products & information.

    that's for damn sure
     
  13. Roy Q.T.

    Roy Q.T. Guest

    I Exaggerated about the Parental Guidance,

    NOTE: Aluminum is not as impervious to Magnetism as suggested, (is
    Always a Conductor) in fact it has a low reluctance to magnetism, but,
    at High [ƒ] DC Voltages & Currents it has an Electro/Physical
    ("Magnetic Flux") Effect of it's own ....I believe it's measured in
    Gauss as well.
    perhaps a bit off the charts.

    Ever heard of the term: Ion Wind
     
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