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Modifying transformer

Discussion in 'Electronic Basics' started by Jon Slaughter, May 6, 2007.

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  1. I tore apart the battery transformer I mentioned here a while back. I'm
    going to remove a few turns on the primary to increase the voltage across
    the secondary(its 24.2VAC now and I'd like to get that to about 30VAC or
    so).

    I'm wondering if removing turns is a good idea? The core already saturates
    at about 110VAC and I would imagine that removing turns on the primary can
    only make things worse? Although since it already saturates and wastes
    about 1A or 120W I'd imagine that since I'm not loading the secondary to
    anything near what its suppose to be that it won't be to bad? (its suppose
    to be rated for 15A but I'll probably draw about 4A max on rare occasions).


    Another issue was that the top part of the transformer was welded to the
    bottom part. Now wouldn't this defeat the purpose of laminates? Surely by
    welding them they would be electrically connected and therefore reduce the
    reason to use laminates? When I cut the welds I was thinking that maybe it
    would be better not to weld it shut to remove any currents between the top
    and bottom halfs? The magnetic flux should still concentrated in the core
    because the gap would be very small?

    I guess I'm going to play around with it and see what happens though. Try
    without core first then piece it together. Just want to get a few things
    clear before I wrap everything up.

    Thanks,
    JOn
     
  2. lol, now I see why the welded it... anyone know?
     
  3. John Larkin

    John Larkin Guest

    If it's already close to saturation, removing primary turns could be
    dramatic. 30/24 = 1.25, and few transformers are designed 25% away
    from saturation.

    Saturation isn't an absolute event, but losses climb radically as the
    core is driven harder. Smoking varnish is a reasonable expectation.

    John
     
  4. jasen

    jasen Guest

    good! it's not.
    yup.

    it might be better to re-wind the secondary with more turns
    (maybe use thinner wire)
    if the weld is thin (not right across the core) it doesn't form a shorted turn
    for eddy currents.
     
  5. I'm not sure if its the laminates that are the main issue or the turns? I

    yeah, but what I'm saying is that if I would have added turns to the primary
    then I could have reduce the saturation? Of course I would have decreased
    the secondary voltage too but...
    Well, I will try to look into that and see. In any case its a good learning
    experience and things always make more sense when I get my hands dirty.


    Thanks,
    Jon
     
  6. default

    default Guest

    If the core is indeed saturating, you'll only make it worse by
    removing turns. It's anyone's guess how it will turn out - mine would
    be badly, but if it is only to be used occasionally for short periods
    it may work. The transformer is rated at 15 amps at 24 out raising to
    30 will lower the output rating as well. 100 watts of waste is a fair
    amount of heat and cost if it is to run for protracted periods.

    Putting a small 4 amp 6 volt transformer phase wired to boost the
    secondary is safer and probably easier.
    Transformer manufacturers do take some short cuts and welds are one of
    them - you see it a lot in el-cheapo 12V battery charger transformers.
    microwave ovens, and other bulk produced consumer stuff. Not as bad
    as it may seem at first because the weld usually has very shallow
    penetration and is on a part of the core where the field is weaker -
    but it will waste some power and I wouldn't try to re weld it. It is
    also hard to do unless you have a TIG setup and are very good and
    fast.
    You can add turns to the secondary if there's room.
     
  7. As a learning experience what you are doing is fine, and maybe for a "rough
    and dirty" power supply it will be OK. I looked at your previous post in
    Mid-March (although the nym was BobJones, so maybe not you), the
    transformer was a 12 VCT at 15 A, and another was 25 VCT at 2 A. The
    purpose there was for a variable supply 5 V to 12 V.

    Anyway, much depends on what you intend to use the transformer for. A
    variable DC supply will be more efficient if you run the primary off of a
    Variac or Powerstat. At low outputs you will be below saturation and it
    will be very happy. You can also boost the 120 input to 140 and push the
    output at the expense of saturation.

    You could also use a phase modulated triac controller designed for
    inductive loads.

    If you just want to get higher DC voltage out, you can connect the output
    in various ways. A full wave bridge with 12 VCT will give you about 15-18
    VDC, while a full wave CT will give you half the voltage and twice the
    current. You can make a doubler circuit using two capacitors and two diodes
    to get 30-35 VDC. The regulation will be poor but such a circuit is also
    somewhat current-limiting, which may be good. You will need about 2000 uF
    per ampere.

    Have fun with your experiments.

    Paul
     
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