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Effects of gaps in inductors and transformers

Discussion in 'Electronic Design' started by Paul E. Schoen, Feb 22, 2007.

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  1. In a thread in SEB there was a discussion on transformer failure modes that
    also mentioned gaps in the magnetic path. I never fully understood the
    function of gaps in the core, but I observed that they are generally
    present in iron core inductors, but not in most transformers.

    I found some information at, where it is
    explained that the gap size can maximize energy storage in an inductor by
    balancing the point of magnetic saturation (and core heating) with winding
    losses. It seems that a wider (or longer) core gap extends the point of
    magnetic saturation by allowing more current to flow through the windings,
    so the effect is to lower the inductance. A smaller gap will have higher
    inductance, but will saturate the core much more quickly, resulting in less
    energy storage.

    As an inductor is used more for energy storage, a gap (whether actually cut
    in the magnetic material or distributed as with powdered iron), allows more
    energy storage by allowing more current flow, and energy is proportional to
    the square of the current. For a transformer, as I understand it, the
    energy is transferred from the primary to secondary by mutual inductance,
    so the absense of a gap results in higher inductance and a higher volts per

    More information can be found at,
    which describes filter inductor design.

    I would like to get a better understanding of the characteristics of
    transformers and inductors to know how best to design high current 50/60 Hz
    transformers as well as switch mode boost converters using inductors.

    The transformers I have made use toroidal primary cores with 120/240 VAC
    windings, and secondaries consisting of several turns of bus bar or welding
    cable to produce up to 10s of thousands of amps. They will usually produce
    15 to 30 times their nominal output currents for short pulses.

    The switch mode boost converter I have designed uses a 10 uH inductor at
    100 kHz to boost 12 VDC to 25 or 45 VDC at about 800 mA. However, I
    recently found that a small pot core inductor rated at 6.7 amps seemed to
    work better than a larger toroidal inductor rated at 10.8 amps. I think
    this might be because the smaller inductor starts to saturate sooner,
    lowering its inductance but allowing more current to flow, resulting in
    higher energy storage. The larger inductor is probably allowing much less
    current and hence less energy, so it cannot produce the power for the
    higher voltage load. I can probably drop the frequency to 75 kHz or 60 KHz
    and maybe get the output I need.

    Thanks for any thoughts and discussion.

  2. MassiveProng

    MassiveProng Guest

    You observed incorrectly.
  3. Genome

    Genome Guest

    Ahem, since he is talking about low frequency power and has used the words
    'generally' and 'most' his observation is not entirely incorrect.

    In fact, since it is his observation and you do not know what he has
    observed then he is perfectly correct.

  4. Guest

    This is not quite the right way to describe what is going on.

    The maximum magnetic field you can build up in the magnetic path is
    independent of the gap - it is limited by the saturation flux for the
    core material. The number of ampere-turns of current through the
    winding required to generate that flux depends on the magnetic path
    length. The magnetic path length through the core itself is divided by
    the relative permeability of the core (about 1000 times air for
    ferrites, and 10,000 times air for iron) so even a small air-gap can
    dramatically increase the magnetic path length.

    A ten-fold increase in magnetic path length allows a ten fold increase
    in current through the winding before you ht stuaration, and reduces
    the inductance of the assembly by a factor of ten, thus allowing a
    factor of ten increase in the energy stored in the inductance (LI^2)
    before saturation sets in.
    A gap in a transformer core increases the leakage inductance, which is
    usually undesirable.

    Moreoever, a transformer isn't usually used as an energy storage
    device, so increasing the energy storage capacity is rarely a design
    At 100kHz you probably need to worry more about inter-winding
    capacitance. The detailed structure of the windings can get to be very
    important at this sort of frequency. The pot core may well have a two
    or four section former with the windings built up as two or four
    successive sections, while the toriod is more likely to have its
    windings built up as successive layers, one on top of another, which
    gives a higher winding capacitance and a lower self-resonant

    At even higher frequencies, you have to restrict yourself to single-
    layer windings to keep the interwinding capacitance within bounds, and
    eventually you have to go over to transmission line transformers.
  5. With the flyback converter being the counterexample for both. As more and
    more electronics are powered by switchers instead of linear supplies, and as
    most of those are flybacks, the day may come when transformers are indeed
    "usually" used for energy storage.

  6. Fred Bartoli

    Fred Bartoli Guest

    Robert Latest a écrit :
    Sloppy word usage.
    Flybacks don't use transformers. They use coupled inductors.
  7. I guess you're right.

  8. John  Larkin

    John Larkin Guest

    No, he's generally correct. Power and audio iron-lam transformers are
    almost never gapped; ferrite power transformers are usually not; iron
    core and ferrite inductors are usually gapped, either with a distinct
    gap or just an open magnetic structure; it prevents dc saturation and
    helps better define their inductance.

    Ferroresonant power transformers were gapped, but they're pretty rare
    these days.

  9. OK. That is very helpful in understanding the principles involved. The gaps
    I saw in some large C-core inductors I have are about 0.1", and the
    laminated steel has a length of about 10". So if the magnetic path length
    is increased to 1000, that is a 100 fold decrease in inductance, allowing
    100 times the ampere turns, and thus 100 times the energy. This is a 100 mH
    inductor rated about 10 amperes.
    Thank you for that information. I had posted on SEB that some small
    transformers may be made with a gap to make them impedance protected in
    case of an output overload or short. There are probably other ways to
    achieve this effect with looser coupling. As I had posted there, it comes
    at the price of low efficiency and poor regulation, but that's what is
    desired for self-protection.
    The toroid has only about 10 turns of approx #16 wire on a core about 0.75"
    x 0.37". I don't know the internal construction of the pot core, but it is
    only about 0.5" square and 0.3" high. It most likely has several layers of


  10. In class A audio amps the transformers are normally gapped.
    this to prevent saturation because of the DC component.
    Class A audio amps were _very_ popular in the tube ages, and later
    in small transistor radios.
    Even today people ...
  11. John Larkin

    John Larkin Guest

    Right. The only reason to add gaps is if there's DC present, or to
    better define the inductance.
    Is there no limit to audio insanity? Don't look like it.

  12. For 6500 $ a piece, I was thinking, 'how many does he sell'?
    I can hire a guy to assemble many of these a week.

    Did you notice he feeds the heaters with 250 kHz AC to prevent hum?
  13. D from BC

    D from BC Guest

    Saw a small tube amp for ipod at London Drugs Systems&CS_ProductID=2094647&ProductTab=1
    or use "vacuum" as keyword..
    I actually saw somebody stare at this thing for 5 minutes!!
    D from BC
  14. *S-Video* output?????????????????????????????
    On an audio amp?
  15. Rich Grise

    Rich Grise Guest

    I've worked for a battery charger manufacturer who wound his own ferros,
    and none of them was gapped. They do, however, have "magnetic shunts"
    that let some of the flux bypass the secondary; apparently this has to
    do with having a regulated output.

    I was testing one one time, with a BMF variac and AC volt and ammeters.
    I cranked up the variac, and the primary current went way high - it
    worried me, so I mentioned it to the client (who designs the things, and
    he beamed: "It's regulating!"

    The rest of ferro design, of course, is Black Magic. :)

  16. MassiveProng

    MassiveProng Guest

    Thank you.
  17. MassiveProng

    MassiveProng Guest

    The gaps are very small. For pot cores, it can be anywhere from
    half a mil to around 5 or even 10 mils. Without it, problems do
    In a switcher, it keeps the crossover from banging into each other.
    A big source of switcher noise, and LOST efficiency.
  18. John Larkin

    John Larkin Guest

    Small, impractical nit: Won't the flux continue to increase with
    current, at a declining effective permeability, approaching u=1 at
    full saturation of the iron? Admittedly, this is a pretty low slope,
    but I don't think it goes to zero.
    I know a guy who has a secret process for treating metglas, up to a
    permeability of about 1e6.

  19. MassiveProng

    MassiveProng Guest

    Prevention of saturation is the main reason, even if there is no DC
    component involved.

    One can also affect how a switcher pulse is handled.
  20. MassiveProng

    MassiveProng Guest

    Nope. Saturation can occur in transformers that do not have any DC
    component as well. It all comes down to Ampere Turns, and whether or
    not the switcher has any overlap on its pulses. (most do) Gapping
    opens that overlap up, and actually improves efficiency by reducing
    the losses caused by said overlap.
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