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Winding a conventional AC line frequency transformer

Discussion in 'Electronic Design' started by Eeyore, May 13, 2007.

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  1. Eeyore

    Eeyore Guest

    It seems to have been my destiny in life to have to learn more about
    transformers (and wind a fair few by hand) than I ever suspected I'd need to.

    I learnt quite a bit about switchmode type transformers - I suppose actually you
    could consider flyback types more like tapped inductors actually - from various
    application notes and also here. Epcos's Ferrite Designer application is
    additionally an excellent tool for pumping in the numbers for a wide range of
    cores in various materials.

    It so happens I'd like now to apply some of the techniques I've learnt to
    traditional line frequency transformers.

    I have several specific things in mind.

    A. Reduced flux operation. Notably with toroids this can avoid the typical
    'switch on surge' and it also occures to me that stray flux will be reduced both
    in proportion to the reduction in magnetisation force and additionally because
    operation at lower flux levels keeps core permeability values higher.

    B. Improved coupling by using bifilar windings with triple insulated wire
    eliminating the need for traditional insulation barriers.

    C. Higher than 'typical' VA ratings for a given core size by using more
    copper. Again this works well with toroids.

    D. I'm sure there was something else but now I've forgotten.

    The trouble is that I don't know of any single source of data on cores that's
    comparable to the Epcos Magnetic designer. And then again, you can wind almost
    any size core you like for toroids which will be my main area of interest.

    Suggestions ?

  2. Phil Allison

    Phil Allison Guest


    ** Inrush surge will only be eliminated by doubling the number of turns on
    primary and secondary.

    Equates to using a 480 volt tranny on 240 volts - results in only half
    the normal VA rating.

    ** Completely absurd for a 50/60 Hz supply transformer where high
    frequency coupling has no benefit.

    ** Fan cooling works to add VAs too.

    ** Exotic core materials ???

    Increased VA to size ratio.

    .......... Phil
  3. Eeyore

    Eeyore Guest

    A true 200% doubling is only necessary if the comparison transformer is already
    operating on the edge of saturation.

    Only if you stick to using the same amount of copper. I regularly 'overwind' to
    reduce losses and gain VA rating.

    It's also possible to use higher temperature insulation if you're pushing for VA
    of course.

    And the harmonics ?

    It certainly would do but I'd like something that's not noisy.

    Could be interesting.

    Always useful.

  4. Phil Allison

    Phil Allison Guest

    "Eeysore Fuckhead "

    ** How tediously pedantic.

    This utter fool does not want not want anyone's input.

    ** Which is the whole point of the comparison.

    ** At the expense of output voltage regulation.

    With a wire temp of say 150C, winding resistance and hence peak IR drop
    increases by 50% over the room temp value.

    ** Yawn ..........

    Another Graham Fuckwit Stevenson contextless & meaning free question.

    ** So YOU go invent it and patent it - fuckhead.

    This PITA fool REALLY does not want not want ANYONE's input.

    ** Do a Google search on it - fuckwit.

    ........ Phil
  5. There is also a switch-on surge caused by remanent magnetism in the core,
    which depends on the net DC current caused by odd number of positive and
    negative half-cycles. It can be reduced by reducing the applied voltage at
    turn-off, or using soft-start turn-on techniques. Another helpful method is
    a capacitor or RC snubber on the primary which provides a damped
    oscillating current path at turn-off. Toroids also exhibit this, at least
    the ones I use in our high current test sets.

    Bifilar windings are most useful for primaries used in parallel, to
    equalize the current. Secondaries are coupled very well because the flux is
    concentrated in the small hole in the donut.

    Toroids exhibit the best VA/weight and VA/volume because of the efficient
    tape wound steel core, typically made with high quality and very thin
    material. The core is very easy and inexpensive to make. Adding the
    windings is expensive, because it is labor intensive, requires special
    machines, and is difficult with heavy wire. Most of my secondaries are made
    of bus bar, so the difficult part is completing the turns outside the hole
    of the core.

    The size of the hole determines the size of the copper windings, and is
    also where the most heat will be concentrated. The outside windings are
    well separated and more easily cooled. I am experimenting with a fluid
    cooled design that runs thick wall copper tubing through the toroids.

    A good source of information is They have toroid kits from
    about 50 VA up to 1400 VA. The primaries are already wound on the core, and
    you add the secondary according to their design charts for selecting number
    of turns and wire size. Volts per turn varies from about 0.1 to 0.7. They
    also supply mylar tape for insulation, and steel washers and rubber
    bushings for center hole mounting.

    You can also use old Variacs or Powerstats as toroidal primaries. They tend
    to have larger than usual holes, so you can wind more copper on the
    secondary. Some of them are as large as 5 kVA. They have tapped primaries
    for 120/140 VAC, or 240/280 VAC. The higher taps can be used for lower
    primary current on 50 Hz.

    There are also some toroidal transformers that are wound with steel wire
    cores, which are supposedly even more efficient. Tape wound toroids have a
    rectangular cross section, but a true toroid has a round cross-section,
    which can be achieved with wire. I would think there would be more wasted
    space, however.

    Good luck. Let us know what you are trying to do, and what results you get.

  6. Robert Baer

    Robert Baer Guest

    I have info only on "E/I" and "C" core shapes (silicon steel
    laminations), nothing on toroids (sorry).
  7. Robert Baer

    Robert Baer Guest

    *Incorrect!* The flux is concentrated by the magnetic material - in
    this case the body of the toroid.
    Go back to a magnetics class...
  8. If you are picking nits, you are correct. Perhaps it is better stated that
    the location where a conductor will be most influenced by the magnetic flux
    in the core is concentrated in the hole. Or perhaps I should have said
    magnetic field, rather than flux. In any case, the point I was trying to
    make is that the conductors within this relatively small space have current
    induced in them only there, and it is a much smaller space than that of an
    equivalent size EI core.

    There is no need to be condescending.

  9. Phil Allison

    Phil Allison Guest

    "Paul E. Schoen"

    ** He ain't nit picking.

    ** Utter bollocks.

    ** Your proof of this absurd claim is where ?

    ** The usable copper window area is LARGER with GOSS strip toroidals than
    with similar E-I cores.

    My god - windings surround the ENTIRE core instead of just the middle
    stump or part of it !!

    ( Cricket allusion will mystify all Yanks ..... )

    ** Gotta bend down a long way to even SEE a Lilliputian like you.

    ........ Phil
  10. John Larkin

    John Larkin Guest

    Cricket has been explained to me many times. It still makes no sense.

  11. [snip]

    The imagery I have is that a straight piece of wire
    carrying a current has the magnetic field arranged
    as concentric rings around it. If a toroidal magnetic
    core is slipped over that wire then the 'rings' get
    naturally concentrated in the higher permeability core.

    I've done experiments with bar-turns and toroids.
    The interesting thing is that the current ratio is
    not particularly affected (<0.1% anyway) by the bar
    not being central, tilted, or even not round.
  12. That is a good description of a toroid being used as a current transformer,
    where a high current flows through a large conductor (or several turns of a
    smaller conductor) and the smaller secondary current is used for
    measurement. I recently tested a PCB mounted toroid with 1000:1 ratio, with
    a load of 10 ohms, and it was quite linear from about 1 ampere to several

    For the test sets we build, we use a Rogowski coil for current measurement.
    It is built using 8 air-core inductors positioned across the top and bottom
    of a 6" x 1/2" bus bar. The axis of each inductor is perpendicular to
    current flow. The output is a voltage which is proportional to dV/dT of
    current, so an RC integrator is used. The coils read the magnetic field,
    and can be affected by external fields and the presence of magnetic
    material nearby (like steel brackets and bolts), but once solidly mounted,
    the output is very linear and accurate over a range of about 50 amps to
    50,000 amps or more.

    The point I was trying to make is that only the very small portion of the
    conductor through the hole of the toroid is affected by its magnetic field.
    The conductors outside this small space serve merely to complete the
    circuit. There is no appreciable difference if the conductors are wrapped
    tightly around the toroid, or are routed far away from the outer surface.
    Of course, tighter wraps use less wire and there is less resistive loss.

  13. Fred Bartoli

    Fred Bartoli Guest

    John Larkin a écrit :
    What's that?
  14. Fred Bartoli

    Fred Bartoli Guest

    Eeyore a écrit :
    Bifilar winding isn't possible, unless you want a 1:1 xformer, or a
    total mess of series/parallel windings.
  15. Eeyore

    Eeyore Guest

    Why need it be a mess ?

  16. John Larkin

    John Larkin Guest

    Really, you are better off not knowing.

  17. I'm always suspicious of endeavors that require learning a new lexicon
    or require new clothes.

    Best regards,
    Spehro Pefhany
  18. Fred Bartoli

    Fred Bartoli Guest

    Eeyore a écrit :
    Suppose you want a 24V xformer (a 1:10 ratio). You'll need ten winding
    sections, each primary part in series, each secondary part in parallel.

    I qualify this as a mess, since this level of complication isn't needed
    at all for the purpose.
  19. Robert Baer

    Robert Baer Guest

    As long as the cross-sectional area of the wire used (not the shape
    as you correctly mentin) is the same, this is correct.
    Where one can see slight modifictions of this, is when a substantial
    percentage of the wire cross-section is rather close to the higher
    permeability core.
  20. Robert Baer

    Robert Baer Guest

    Bifilar windings for N:M ratios have beendone for at least 30 years;
    even using coax cable!
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