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MOT dissapointment

Discussion in 'Electronic Design' started by ehsjr, Dec 14, 2008.

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  1. ehsjr

    ehsjr Guest

    In the thread "Need linear supply 3.3V/15A or 5V/25A"
    that Joerg started on 12/2 I posted the idea of rewinding
    a MOT for ~6V. I've never done it, but decided to try
    it. If anything good comes from it, Joerg gets the credit
    because his question planted the seed.

    Things went so well with the initial experiments that I
    decided to actually make a useful transformer for a
    13.8V ~25A (peak) supply.

    To do that, I need about 100' of #14 enamelled wire.
    (Gonna add 24-30 turns on the primary to knock the
    5.62 magnetizing current down to 1.4A or lower. 24
    turns knocked it down to 1.4. The secondary will be
    3 or 4 18 turn windings in parallel.)

    The dissapointment: I found another oven yesterday.
    Today I ground off the welds and pried off the primary.
    Now, get this: it looks like #14 copper, but weighs
    only 6 ounces! It has about 132 feet of wire (based
    on turns count x average perimeter of the winding)
    The damn stuff must be aluminum.

    So I'll have to keep on scrounging until I find one
    that has real copper. The rewinding will be a bitch,
    so I'm going to wait until I can get the real stuff.

    If anyone has any pointers to offer - about the winding,
    getting the wire, whatever, I'd love to hear them.
    By the way, the ~ 25A is not a requirement. I just
    want to get the maximum current I can from it, reaonably.
    There's no point in going through the effort for
    say 10 amps, and my main experimental interest was in
    finding out what it would take to get an MOT into
    "re-windable condition" . Now it is sort of a challenge
    to try to get the most out of it reasonably. I already
    know that Joerg could get his 15A at 3.3 or 25A at 5V
    from this xformer. It will easily take 50 feet of #14
    enamelled wire, with room to spare.

  2. James Arthur

    James Arthur Guest

    Richard the Dreaded Librarian ;-) has written about MOT
    rewinding here in sed:

    MOT hacking

    Boy, do I feel stupid!

    James Arthur
  3. You might consider using an old Powerstat or Variac (which are really
    toroidal transformers), and then add whatever number of turns you need
    through the donut hole. I have made transformers that put out 1000 amps
    continuous and over 10kA pulses by using one or two or four turns of bus
    bar as the secondary. You can also use copper pipe and run water through it
    for cooling. Even a burned up Powerstat can be repaired or even totally
    rewound, or you can get a complete kit from about 80 VA to 1.4 kVA from Toroids are notoriously quiet, small, and efficient. It's
    not too much hassle to wind a low voltage secondary. They are generally 0.2
    to 0.8 volts per turn.

  4. Joerg

    Joerg Guest

    A microwave oven? Considering that they often retail for $49.95 they
    will cut every corner they can. If non-copper windings save only one
    penny they'll do it.

    What I usually try to get away with is snip some windings out of a
    transformer to reduce the dissipation in the linear regulator, not wind
    a new one. Ok, I have wound some transformers but I do not enjoy that.
    Good transformers will have their laminations in with alternating
    orientation. So the last 4-5 must be tapped into the pack with a wodden
    mallet. Often the last one or two just don't want to go in and then it's
    always knocking wood that it'll be enough and won't hum.

    By the way 14AWG for 25 amps is, ahm, quite brazen ... watch for any
    "amperage stench" when drawing lots of current for more than a few
    minutes. When it ticks like a wood stove better turn it off ;-)
  5. ehsjr

    ehsjr Guest

  6. ehsjr

    ehsjr Guest

    These MOT transformers don't fall into the "good" category.
    The core is all E's in one direction with the I's across
    the open E's.
    Naw - I want to run the #14 windings in parallel to share
    the current. I'm hoping for 4 windings, 18 turns each, at
    a bit over 6 amps each - about 72 feet of #14 to fit on
    the core. Otherwise, I'll have to come to you to ask for
    sound effects of a melting transformer. :)

    I'm also looking to wind another ~ 25 turns in series with
    the primary, so I need about 100 feet of #14. As I struggle
    to get all that wire wound into the existing core openings,
    I will _not_ need any sound effects. I'll supply those myself.

  7. Guest

    ok this is tricky...If you are talking about delivering 25 Amps DC at
    the output, then the RMS current in the secondary may have to carry
    can be as high as 2x that. It depends on the type of input filter on
    the rectifier. If you use a big C, the xformer and rectifer will
    deliver the current in pulses, and to deliver 25 Amps average current,
    the instantaneous value of the pulses is a lot higher than 25 Amps.
    When you figure out the RMS value of the pulses (which determines how
    hot the wire will get) you will see its can be much higher than 25
    Amps. This is the detailed part of designing a high current

    The situation is actually better if you use a smaller filter C so the
    current pulses are longer in time and lower in magnitude, but then you
    have more ripple on the output. This is not a simple problem.


  8. ehsjr

    ehsjr Guest

    Yes I understand that. It won't be a problem *if* I can get four
    secondary windings on there, plus the extra 25 turn primary winding.
    The secondary resistance will be low - about 0.0113625 ohms if the
    wire tables are right. That's 4 windings at 18 feet of #14 copper
    all in parallel. Even at 45 amps, that's only ~ .54 volt drop,
    and the peak computes to 20.29, so ~ 17.29 after the rectifiers.
    So ripple needs to be less than 3.49 - 80,000 uF should do.

    But that's all on paper. I don't know how the damn thing will
    perform, and it's really an experiment to get the most I can
    from a scrounged MOT. All I've been able to do so far is
    reduce the no load, no secondary winding current draw from
    5.62 to 1.4 amps by temporarily winding 24 turns and putting
    them in series with the primary.

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