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Core selection

Discussion in 'Electronic Design' started by Tim Williams, May 10, 2009.

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  1. Tim Williams

    Tim Williams Guest

    Question the first: where to find transformer (or inductor) cores?
    Mouser and Digikey don't sell them plain. Who's the magnetic
    equivalent of Digikey?

    Other question: how to select cores? Specifically, I'm looking for
    something in the 10kHz (square wave, PWM), 10kVA range, with high
    permeability to minimize the number of turns. Saturation should be
    fairly high, given the low operating frequency. Ratio 4:1, with about
    200Vrms (fundamental component) primary and 50Vrms secondary. The
    secondary will be copper tubing, 1/4 or 3/8" dia., so a one-turn
    secondary is advantageous.

    Given the permeability and saturation, I'm thinking something like
    Metglas. I know I need something in the 2-3" range, and I can
    calculate things like A_L and A*t(sat) from the properties, but it
    would be a whole lot easier if I had both parameters laid out in a
    table of standard shapes. I can't really make any estimates on what
    size I need if I don't have a standard formula for the geometry, so I
    don't even know what inductance and magnetizing current and saturation
    I have to look for.

    And why do they never specify amp-turns saturation? It's always in
    B. I can measure amp turns, I can't measure Teslas. They give A_L by
    the core, but not saturation, what the hell?

  3. James Arthur

    James Arthur Guest (ham and hobbyist-friendly)

    James Arthur
  4. I get most of my cores from Eastern Components, they're an
    industrial supplier.
    I think they will sell small quantities - you can see.

    A place that does sell small quantities, but with more limited
    selection, is Amidon.
  5. Tim Williams

    Tim Williams Guest

    At a glance, they don't seem to offer anything suitable. Given the
    poor layout, I could be missing it by a mile...
    Just as vague a layout, too. Geez, is it traditional for magnetics
    companies to have terse, cryptic layouts? And,

    This website doesn't even appear to have a catalog. So, I'm better
    off browsing the manufacturers' websites themselves (which appears to
    be all the information on Eastern's website), looking for a material
    that *I think* is right, and then what? I can't even tell what they
    stock. I could call them, but geez, this is 2009, everything is
    supposed to be online!

    Right now I'm eyeing the F-290-W at CWS, which tells me that it's a
    2.90" o.d. toroid (and there's some other stuff in its row),
    and...**** all else.

    If it's an average u = 2000 up to B = 0.5T, then H = 0.5 / (u_o*2000)
    = .5/2000*pi*4e-7 = 199 A/m. The average path length is ((2.9 +
    1.53) / 2) * pi = 7.0 inches, or 0.177 m. So evidently, saturation
    might be around 199 * 0.177 = 35.2 At. Sounds like a typical ferrite
    toroid. If that's the case, then for one turn, I need I_m(max) <
    35Apk, or under 25Arms. At 50Vrms, that's about 2 ohms X_L, which at
    10kHz is about 32uH. Claimed A_L is 16,280mH/T^2, which is so full of
    shit that it's obvious they meant nH/T^2, so it's actually 16.3uH/
    T^2. So I'd need 2 turns to get the minimum inductance. But then At
    would double, so I need double the inductance, so I actually need
    2*sqrt(2) turns to be on the edge of saturation. Call it 4 for some
    margin. Or I could stack a pair of these cores with one turn down the

    So, is that right?

    God, it's no wonder people are afraid of inductors, they sure try hard
    making them impossible to use. Seems to me there could be big
    business simply setting up a website that actually *tells* you
    things! Parametric search, even (including A*t saturation)!

  6. Guest

    Hello Tim,

    You said 50Vrms at 10 kHz, 1 turn. This means you need peak flux = 1.2
    mT. So using a ferrite up to B=200mT, you need a cross section of
    about 1.2m/0.2 = 0.006 m^2. That is a square with 3" sides. I never
    saw ferrite cores with that large cross section.

    Probably you have to increase the number of secondary turns and/or use
    a low loss material that has higher Bsat (so you can design at higher
    peak flux density). This will result in less cross section for the
    magnetic path.

    Given the power, a complete electrical / thermal design is required
    when the duty cycle of this transformer will be high.

    Best regards,

  7. Hammy

    Hammy Guest

  8. Thanks, Hammy!

    Best regards,
    Spehro Pefhany
  9. Tim Williams

    Tim Williams Guest

    Ah yes, they have some, and I recall Allied has similar products. But
    that's EMI stuff, I need bigger. The biggest toroid I see there is
    about an inch across- I'd need a lot of those.

    Surprisingly, the Fair-Rite website seems to be fairly new. The
    scripting runs really slow though. At least it contains information,
    for once!

  10. legg

    legg Guest

    Or putting it more generally, for fully reversing AC flux,

    Ae = V / (2.f.n.Bpk)

    Ae = core cross-section in square meters
    V = average volts applied
    n = number of turns
    Bpk = permitted peak flux determined for core loss limited
    or saturation-limited design.

  11. James Arthur

    James Arthur Guest

    They offer a range of E-cores, toroids, in a variety of materials.

    I assumed you'd want a mess of ferrite E-cores in mat'l #77--you'll
    have to stack 'em to get to 10kVA.

    James Arthur
  12. legg

    legg Guest

    Or putting it more generally, for fully reversing AC flux,

    Ae = V / (2.f.n.Bpk)

    Ae = core cross-section in square meters
    V = average volts applied
    n = number of turns
    Bpk = permitted peak flux determined for core loss limited
    or saturation-limited design.

  13. Rich Grise

    Rich Grise Guest

    Good Luck!
  14. Tim Williams

    Tim Williams Guest

    #77 is starting to look like the material of choice. Or something
    similar, like 75 or 78.

    The largest E-core Amidon offers is rated for "about 200W", which
    suggests I'd need roughly 50 of them for the 10kW level I'm interested

    On an indirectly linked page, I discovered the data:

    This says the largest core has a winding window of 2 * 0.593 x 0.375
    inch (using an E-E arrangement). A stack of 50 would be 50 * 0.605 =
    30" thick, which is certainly possible, but would stick out one side
    of my chassis. On the plus side, I would certainly be able to push
    all the voltage through one turn. A single piece of 3/8" tubing would
    fit without too much trouble, though leakage inductance to the primary
    wouldn't be great (though it doesn't need to be). Evidently, A_L
    would be 5.3 * 50 = 265uH/T^2, which would be fairly "ideal". But it
    seems like an awful lot of overkill, not to mention way too expensive
    ($312 for 50 E-cores? no thanks).

    Where does cross sectional area fit into this, anyway? Isn't that
    absorbed into A_L? So, as long as I am given A_L, I can calculate
    inductance and saturation at will? And saturation only involves path
    length, right? -- by amperes per meter, they mean *A/m*, not A.m/m^2
    (like how resistivity is actually ohm.m^2/m)?

    Ok, so, this is Usenet, right? If I've made an error, surely there
    would have been fifty people in the first hour telling me what an
    idiot I am -- since this has not happened, I can only assume my
    calculations are correct??? Then why do I calculate that a moderately
    sized toroid (like the FT-290-W) will suffice, whereas others have
    suggested that I need something approximately as thick as my ankle?

  15. Guest

    Hello Tim,

    Regarding the Usenet. Be happy that there are also polite people that
    don't start to roar immediately. In addition it can also be that
    others are not competent enough to tell you whether you are right or
    not, or don't have the time to do the complete math. Magnetics is
    mysterious in the eyes of many people.

    I belong to the people that think that for a 1Turn secondary you need
    a large cross section to avoid core saturation. Designing transformers
    is balancing between core losses, copper losses, availability of
    materials, money, etc.

    Best regards,

    please remove first three letters of alphabet in case of PM.
  16. Fred Bartoli

    Fred Bartoli Guest

    Rich Grise a écrit :
    At least you didn't suggested a PIC!
  17. Tim Williams

    Tim Williams Guest

    Damn, I like that. You'll, or no wait, Jan :) will have to send me
    some PICs. Then I can put them across this transformer's secondary
    and see what happens. >:-D

  18. Nico Coesel

    Nico Coesel Guest

    Perhaps nobody has experience in this field? I tried the same thing
    you are doing for low power stuff. As someone already typed before:
    there are a huge amount of units which need to be converted and each
    vendor seems to have their own specification method. Its a world of

    That leads to the question: why do you want to wind your own
    transformer? It might be much easier to have a transformer wound which
    meets your specs than failing a couple of times. At the power levels
    you are talking about, a failure is likely to cause collateral damage.

    If you are serious about winding your own transformer I think you
    might need to buy some cores first put some windings on them and
    verify your calculations. Beware that the method of winding also
    influences the behaviour of the transformer.
  19. Tim Williams

    Tim Williams Guest

    That leads to the question: why do you want to wind your own
    I'm not too concerned about that (which, you're right to observe,
    seems kind of odd given the ten kilowattedness of my ambition). I've
    done this plenty of times before- junk ferrites are plentiful, but
    small. The biggest cores I have are probably worth 2kVA together (and
    that at 100kHz). Testing saturation and inductivity is easy, making
    transformers is easy (especially when they have only ten turns). I
    really just need the core to do it.

    Getting a transformer wound feels like a huge waste of resources,
    seeing as I just need the core (whichever size it has to be). I don't
    need someone else to design it (assuming I get the correct core), and
    I don't need someone else to wind it -- do winders even do 1/4 or 3/8"
    copper tubing? -- I can do all that myself, no need to quadruple the
    price of this project on services. Consider, BTW, that I am of the
    age group where "money is short and time is free". ;-)
    That should also not be a problem, about five turns of copper strap
    around the toroid with one pipe down the center would be pretty fine I
    think. If I need several turns secondary, I can either wrestle the
    pipe through, or cut it into segments and fix it back together with
    fittings (compression or flare, I'm thinking). The extra connections
    won't do any favors for conductivity, but oh well.

    Whereas, I suppose if I bunched all the primary turns to one side of
    the toroid -- which is tempting, stiff as that copper strap is -- that
    side would be more prone to saturation, which might produce
    unfavorable results (sooner saturation, higher LL, etc.?).

  20. James Arthur

    James Arthur Guest

    No it's not safe to conclude your calcs are right--I didn't
    check them.

    Here's Terry Given's take on a similar app, complete with
    worked out examples and wisdom of the ages:

    James Arthur
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