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Mains transformer goodness

Discussion in 'Electronic Repair' started by N_Cook, Mar 20, 2009.

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  1. N_Cook

    N_Cook Guest

    Something to do while watching some less engaging TV. Unwound another large
    toroidal mains transformer. Suspecting doubled up winding of the primary and
    then for UK use joining opposite ends and so relying on 2 thicknesses of
    lacquer to resist high voltage. Of course somewhere near the middle at some
    point it fails catastrophically. Yes, burnt spot weld buried in the middle
    of the primary. Anyone know what this duff winding technique is called ?
    (reduces the number of shuttle passes by 2 must be the reason). Is there a
    way of testing an unknown , but good , transformer for this winding pattern
    How come this wiring procedure is not outlawed ?
    More generally, someone in production told me that a "goodness" test for a
    mains transformer is an open secondaries, no load, monitoring of the primary
    current is useful, any truth/rationale in that. ?
  2. Guest

    Not that I know of.
    Most of the transformers wound in this way don't burn out?
    It tells you the inductance of the transformer. It doesn't tell you
    how much heat is being dissipated in the windings and the core - for
    that you have load the seconaries and watch how fast the output
    voltage falls off with incresing load.

    Since the permeability of the core declines with increasing
    temperature, the inductance falls away as the transformer heats up, so
    that magnetising current - and the power dissipation - goes up as the
    transoformer gets warmer.
  3. Phil Allison

    Phil Allison Guest

    ** " Standard practice " ??

    ** Only by testing to destruction.
    ** It works, is not a recognised safety hazard and saves cost.

    BTW: I have a 300VA toroidal with EXACTLY the same kind of failure here
    at the moment, waiting for a replacement to be made. But it is lasted 8 or
    9 years before shorting.

    ** Yes.

    If you checked a run of transformers and found one with a significantly
    higher I mag than the others - then the core assembly or the primary
    winding has been bodged. For example - an E core tranny where the
    laminations are not fully interleaved or tightly enough packed to eliminate
    air gaps between the Es and Is will show high I mag on test.

    Toroidals normally have very low I mag figures, 10 to even 100 times lower
    than similar sized e-cores. However, if the primary voltage exceeds the
    maker's design value, expect to see I mag go through the roof. It will also
    go through the roof if you try to use a toroidal made for 60 Hz mains power
    on 50Hz.

    ....... Phil
  4. N_Cook

    N_Cook Guest

    I'm amazed they last as long as they do , 400 turns or so of paired primary
    wires layed together touching and thermal cycling / chaffing possibility
    along the lacquer . Certainly not produced in a clean-room so any amount of
    microscopic grit between the wires, what is the thickness of 2 layers of
    lacquer to resist pk-pk voltage of 350V , let alone mains surges?
  5. PlainBill

    PlainBill Guest

    Way back in my dark past I worked in a plant where we wound our own
    transformers. Indeed, the doubling of the primaries is standard
    practice. Note that if your primary voltage is 240VAC the voltage
    between the adjacent wires will be approximately 180V maximum.

    Of course, these were using 'I and E' laminates. We tested the wound
    bobbin for the correct number of turns and leakage between the wires
    of the paired windings. After the laminations were added the entire
    transformer was vacuum impregnated with varnish. Not much chance for
    movement between the windings after that!

  6. Eeyore

    Eeyore Guest

    You are describing all transformers.

    The lacquer will resist plenty more than that.

  7. N_Cook

    N_Cook Guest

    Did you ever metal saw across any of them to actually check on the
    impregnation. ? Not transformers, but I've seen such a check on a similar
    process with much the same viscosity of fluid and most of the interior was
    not impregnated, vacuum gave out to viscosity down fine pathways.
  8. Eeyore

    Eeyore Guest

    You're worrying about stuff that's not important.

  9. N_Cook

    N_Cook Guest

    In the ideal yes, but this Tx and another similar there is no interlayer
    insulation between each of the 3 or 4 primary "layers".
    Compact winding only on the inside diameter, not the outside , where
    could easily slip a "layer". So worst case voltage could be 240/0.707 , peak
    to peak, without any mains spikes.
  10. Ecnerwal

    Ecnerwal Guest

    Then you'd be somewhat less than brilliant. No mention of transformer
    oil was involved, and nobody is likely to uncan an oil filled
    transformer in their living room. A dry, varnished/lacquered transformer
    has no oil, so no PCBs.
  11. N_Cook

    N_Cook Guest

    I did think elf 'n' safety a week ago, opening a pack of silicone rubber
    sleeving. All this fine glistening deposit everwhere , will I get silicosis
  12. Phil Allison

    Phil Allison Guest


    ** Totally off with the fairies bollocks.

    ** Totally off with the fairies bollocks.

    ....... Phil
  13. Phil Allison

    Phil Allison Guest


    ** Nevertheless, vacuum impregnation works very well with transformers.

    The basic idea is to remove all the air spaces between windings and fill
    them with insulation material - so the unit will never suffer from "corona"

    Corona discharge is the biggest killer over time of transformers that handle
    AC mains or higher voltages - particularly valve output transformers and
    EHT transformers.

    Until fairly recent times, vacuum impregnation was standard practice for all
    mains voltage transformers - toroidal types were wound with porous cloth
    tape insulation to allow this step to be done.

    Now makers use polyester tape insulation along with tougher, flexible
    enamels and impregnation is impossible.

    IMO - a mains toroidal with dual primaries ought to have them wound
    separately and with at least one layer of poly tape in-between.

    ...... Phil
  14. Jasen Betts

    Jasen Betts Guest

    peak to peak is half of that between series-connected windings.
  15. Adrian C

    Adrian C Guest

    Though if the deposit had been from something dowsed down from a fire at
    over 300degC, it's likely to be bloody painful amounts of Hydrofluoric
    acid. Be ready to lose fingers :-(
  16. Fred Abse

    Fred Abse Guest

    Probably French chalk, AKA talc. Put there as a lubricant.
  17. Eeyore

    Eeyore Guest

    Yup, totally normal.

  18. Eeyore

    Eeyore Guest

    Well not ALL, but lots.

    5 - 10 kV according to this site.

  19. whit3rd

    whit3rd Guest

    There's three issues with PCB oils. First, they're fireproof (a
    safety plus).
    Second, the oil can overheat and decompose, into dioxins and other
    toxins/carcinogens. This, is a safety minus. I've heard of a
    animal feed episode that got LOTS of political attention. The third
    is kinda scary: like DDT, the compound persists and spreads in the
    natural environment, so it can cause problems in future decades or
    centuries if it isn't collected and neutralized. No one wants to
    take the risk of causing the next species extinction and/or lawsuit
  20. N_Cook

    N_Cook Guest

    Anyone care to speculate on the timeline of failure?
    There was no local hotspot and no lacquer damage more than a mm from the
    "spot weld", and a bit of very localised smoke staining travelling a cm or
    so along the affected wires in each direction. Nothing to suggest that the
    initial bridge was between the 2 bifilar wires of the primary, so not
    running a 120 volt primary in effect on 240V ac for any time. For 240V use
    the 2 primaries seriesed to give about 2.8 ohm originally , after failure
    then about 0.4 ohm.
    The 2 primaries broke into 5 lengths
    9.6,14.6,20.1,20.1 and 25.3m long , measured to about 0.2m accuracy.
    So originally probably 2 x 45m. Don't know for sure as did not think to
    check but the weld was probably 20.1m from one end, but I would suggest that
    bridge occured after an arc to another layer (higher pd) and then localised
    heating to bridge across to the bifilar fellow wire.

    In summary , no evidence that bifilar wiring itself was the reason for
    failure but more due to the lack of any interlayer insulation. Because of
    the uneven wire spacing between inside and outside faces of the toroid it is
    too easy for the layers to be jumbled.
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