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About those custom wound line freq transformers - part 1

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

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  1. Eeyore

    Eeyore Guest

    Thanks for all your comments. They covered a wealth of areas so I realise I
    should have followed my own usual advice and concentrated on one aspect at a
    time.

    Firstly, for the most part I'm mostly interested in toroidal construction. So
    please ignore any aspects relating to EI and other constructions.

    Secondly, I'm especially interested in designs with LOWER FLUX operation, since
    stray field is a particular concern to me especially with 'restoring' and
    improving to more current specs certain older specialist items of 'boutique'
    audio gear for example. What may have been an acceptable noise floor a couple of
    decades ago looks very second-rate by modern standards you see.

    There is also the advantage of elimination of 'switch-on surge' but that's a
    relativley minor consideration for equipment of < ~ 100VA. but I am also
    interested in this for other applications).

    So, I like the idea of operation at reduced flux. The stray field will reduce in
    proportion to the reduced magnetising force but I presume will also reduce since
    the magnetic material retains it's permeabilty best at lower fluxes.

    Additionally, the distortion in Imag will be lower with a lower magnetising
    force resulting in less of the very annoying harmonic content in the stray flux.

    So, there are some reasons for wanting to work with a lower flux level.

    I'd be additionally interested in suggestions for other methods of reducing
    stray flux.

    The symmetry ? of the windings appears that it may have some influence and I
    noticed that transformers from a certain manufacturer which had a thick 0.4mm
    polyproplylene barrier between pri and sec appeared to have higher leakage flux.
    I'm hoping that I may be able to reduce this stray flux by improving coupling
    with the use of triple insulated winding wire which doesn't require a solid
    barrier like that.


    Graham
     
  2. Eeyore wrote:
    (snip)
    (snip)

    Make sure the windings (but especially the low voltage, high
    current ones) fold back on themselves to their starting
    point, instead of going around the hole one or more times.
    Such circles produce a magnetic field out the ends of the
    toroid.
     
  3. John Larkin

    John Larkin Guest

    All those things are good but they will all cost dearly in watts/pound
    (in both senses of "pound").

    Toroids leak so little flux it's probably not worth adding a lot of
    turns to reduce flux density. An evenly wound primary is probably
    worth the effort. If you use a bridge+capacitor off the secondary, the
    current surges in that wiring will usually radiate more nasty harmonic
    flux than the transformer. Transformers sometimes get blamed for this.

    John
     
  4. Eeyore

    Eeyore Guest

    Of little concern in this context.

    They leak enough to matter. I'd like to get my hands on some O-cores actually. Have
    you seen those ? R-cores are good too but try getting a custom one !

    That's the sort of thing I was getting at. How can one optimise the winding ?

    Indeed. In my recent 'experiments' I was adding external series resistance to stretch
    the conduction period.

    Graham
     
  5. Eeyore

    Eeyore Guest

    So, ideally you want each winding to be arranged equally through 360 degrees ? Or
    presumably, multiples therof.

    Graham
     
  6. John Larkin

    John Larkin Guest

    As John P points out, the secondary can be a single-turn winding in
    the plane of the toroid, pushing its flux through the hole. In that
    case, that rather large loop, fed with nasty rectifier surges, could
    be a mean source of radiated harmonics. More math to deal with.

    I've pretty much gone over to switchers. I can buy a 150 watt
    universal-input PFC corrected switcher for about what I'd pay for a
    150-watt toroid.

    John
     
  7. Robert Baer

    Robert Baer Guest

    Without regard to the shape of the core, remember how magnetic losses
    happen: movement of magnetic domains, if i remember correctly (been 30
    years) is a lossy mechanism.
    Rotation of magnetization in the domains may be a secondary aspect.
    The first appears to be non-linear, and the second appears to be
    linear; it is the non-linear aspects that will cause distortion.
    Therefore, it appears that the larger the field within reason, the
    smaller percentage the non-linear aspects will be = less distortion (due
    to magnetic effects).

    Concerning symmetry, especially for toroids, simplified theory seems
    to teach that one can have a winding "piled" high in a small arc of the
    toroid and achieve the same results as if uniformly wound on the toroid.
    As long as the permeability of the toroid material is at least 1000
    times that of air, then it would seem the error in that simplified
    theory would be less than 0.1%.
     
  8. Robert Baer

    Robert Baer Guest

    Good point, especially if the number of turns is fairly small (eg:
    N=100, wrapped uniformly full 360 degrees would give the equivelant of a
    one turn circle the diameter of the toroid for a 1% vertical field.
    Wrapping 50 turns for 180 degrees CW and the other 50 turns back the
    same 180 degrees CCW would give reasonable cancellation between the two
    halves.
    If one wants to be rather picky, wind the 180 degrees *bifilar* and
    connect the two windings appropiately.
     
  9. I was not addressing the fraction of the toroid covered by
    each winding. I was addressing the external field caused by
    windings that enclose the center hole. This can be
    prevented either by back winding a second layer over the
    first, till you get back to the starting point, or by just
    returning back over the winding with the end wire, around
    the outside circumference, so that no winding encircles the
    center hole, circumferentially. This prevents production of
    any field out the ends of the hole.
     
  10. Better to space those turns so that the first half of the
    winding covers 360 degrees and the second half of the
    winding retraces back around the core to the start. But it
    works almost as well to wind a 360 degree layer and wrap the
    wire around the outside circumference back to the start to
    either end, or start another layer.
     
  11. Eeyore

    Eeyore Guest

    I'm not following your terminology here. What do you mean by " windings that enclose
    the center hole " ?

    Nope. I didn't get that at all. It sounds like it might be important though.


    Graham
     
  12. Okay, forget the windings that encircle the cross section of
    the core. Those produce flux around the core, and that is
    what you want. But if the current path from start of
    winding to end of winding makes a circle all the way around
    the circumference of the hole, (winding covers entire
    toroid, and end exits near the start) then magnetic flux
    will also be produces through the hole (because 1 turn
    encircles the hole as a solenoidal winding) and that flux
    will spray out each flat side of the toroid.

    Two otherwise identical toroidal transformers can have a
    measurable difference in external flux if the winder took
    the trouble to retrace each full layer, back around the
    circumference to the starting point (or to wrap a second
    layer back to the start), to avoid encircling the hole.
     
  13. Fred Bartoli

    Fred Bartoli Guest

    John Larkin a écrit :
    If correctly wound (ie 360° for each winding) the surge induced mag
    fields cancel out. The single turn JP spoke about just sees the
    magnetizing current.
     
  14. Eeyore

    Eeyore Guest

    How can this be ? The surge is the result of temporary core saturation caused by the
    combination of remanent magnetism at turn-on and the undefined start point of the
    magnetisation cycle.

    Graham
     
  15. Fred Bartoli

    Fred Bartoli Guest

    Eeyore a écrit :
    Quoting JL :
    I wasn't speaking of the turn on surge, which is almost a no problem,
    but rather of the peak rectified current involved by the capacitor
    charging. In that case the secondary ampere-turn product compensate for
    the primary ampere-turn product, except for the magnetizing current.
     
  16. Eeyore

    Eeyore Guest

    Oh I see.

    Yes, a large loop will indeed radiate more of course. That's an interesting aspect I
    hadn't previously considered as such within the context of transformer design. I see what
    you're driving at now.

    I had perhaps imagined that the permeability of the core would preferentially retain most
    of that flux within the core adequately.

    Graham
     
  17. Haude Daniel

    Haude Daniel Guest

    Possibly, but John didn't mean the winding proper, but the additional
    one-turn winding formed by the fact that each winding forms a single
    360 degree loop around the perimeter of the core. Proper winding
    methods which are probably not employed for normal off-the-rack
    mains toroids can eliminate the stray effects of those though.

    --Daniel
     
  18. Fred Bartoli

    Fred Bartoli Guest

    Haude Daniel a écrit :
    I was too. Just picture the primary and secondary equivalent additional
    single turn, incoming currents...

    Oops, currents are in 1:n ratio, so you need nx360°/360° windings to
    balance this. Surely never done, and probably impracticle.
     
  19. John Larkin

    John Larkin Guest

    I almost did a post declaring that the primary NI canceled the
    secondary NI until I realized that most likely N=1 in both windings
    but "I" is sure different.

    John
     
  20. And if the winder is not aware of the effect, the two I*T
    factors may add instead of subtract. The direction the
    current takes around the toroid with each winding is
    completely optional.
     
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