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MOT hacking

Discussion in 'Electronic Design' started by Rich Grise, Jun 6, 2006.

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  1. Rich Grise

    Rich Grise Guest

    please manage followups intelligently, thanks.]

    There have been threads about hacking Microwave Oven Transformers, and
    there are a lot of websites, and I'm sure you've all seen my MOT: (the penny was just
    to see if there were any eddy current effects. None was perceptible. :) )
    So, today, I scrounged a hank of #18 magnet wire, from some guy back in
    the shop at . All I did was do a google
    search on "transformer rewinding whittier ca" and this outfit came up.
    Well, it's just down the street, and it's a pretty respectaby-sized shop,
    in a nice part of town, and I called them on the phone. "Well, I'm looking
    for about 25 feet of #18." So, just on my good looks, this guy hands me a
    remnant (about 2/3 of one layer, on a spool about 4" diameter and about 5"
    long) of wire, and it's on the house! I asked, "Do you want me to mention
    you in my blog?" (figuring that "blog" would be easier for a motor
    rewinder guy to grasp than "USENET group") and he said, "Sure!"

    So, I'm going to wind a 15-turn pie on my MOT right up close and personal
    to the existing primary, but first I have to get those pesky magnetic
    shunts out of the way.

    I'm thinking prop the tranny up on a couple of 1-2-3 blocks on the steel
    welding bench, and just whack them out. How do you guys get your magnetic
    shunts out of your MOTs?

  2. I just tapped them out with a flat tip screw driver away from the primary
    Comes out easy.
    Do keep us posted on your progress, my eye's are perked up!
  3. Tim Williams

    Tim Williams Guest

    Whaddya think yo better than me?

    I use wood and a chisel or screwdriver and hammer... :)

    If you want to get the windings off I recommend (I probably did already)
    hacksawing or grinding the weld off the "I" section, then lightly bashing
    (can I use those two words together?) the primary off. Then you can glue up
    a bobbin from cardboard and slip your secondary on much easier. A clamp
    from the shop keeps the I firmly on the E until you feel like welding it
    back on or whatever.

  4. Rich Grise

    Rich Grise Guest

    Eek! I'm nowhere near that point yet. It seems that the wire I scrounged
    is somehow stiffer than I remember #18 hookup wire, but it's motor wire
    after all. The insulation has the look and feel of "Heavy Formvar" or
    "Polythermaleze" or something. I've threaded a few turns by just threading
    the wire through the winding window, and it looks like crap - that is,
    what I've got for a "winding" is kinda tangle-wound, but it turns out that
    because of the stiffness of the wire, and the toughness of the
    insulation, it's not all that bad threading it through the eyes, albeit
    time-consuming. The tangle-winding I have there now I'm sure would work
    electrically, but it looks like crap, and would probably buzz. So, I
    dedided, since I intend to show it off, it should look neat, so I've
    decided to 5-minute epoxy each winding into place as I go along; this
    should only take 2.5 hours to do 15 turns. ;-)


    Oh, and of course, don't forget the boredom medicine of your choice, ;-)

  5. Chuck Harris

    Chuck Harris Guest

    Golly, Rich, make your life easy, and grind the welds away, and take the
    center out of your tranny. Then you can wind the windings very neatly.
    You don't have to weld the center back, you can toss some of your 5 minute
    epoxy at it, and all will be well.

  6. Doesn't the extra air gap cause problems?
  7. Tim Williams

    Tim Williams Guest

    I doubt it, those things saturate on the peaks anyway. L will be lower but
    Imax will be higher.

  8. Chuck Harris

    Chuck Harris Guest

    No, because you are only removing a small percentage
    of the lamination iron. If you are truly concerned,
    you can weld it back up.

  9. Ok. I'd still avoid epoxy because it falls apart with
    heat. A resorcinol or urea glue would resist that.
  10. default

    default Guest

    Uh . . . the air gap is part of the way the/a magnetic shunt works -
    as the secondary draws more current than you want, the magnetic field
    is diverted into the shunt (bypasses the rest of the core and
    secondary). Bigger gap sets the current limit higher. Skewed gap
    gives a sloppy "knee" in the current limit.
  11. Chuck Harris

    Chuck Harris Guest

    We weren't talking about the shunts. They are popped out. We
    were talking about opening the core so you can remove the bobbin
    and wind it easily.

  12. default

    default Guest

    Oh. MOT I'm using only had a weld on the shunt.
  13. Rich Grise

    Rich Grise Guest

    This one, the shunts just popped out - tap, tap, tap, toink! But,
    since I'm having so much fun threading this wire through the EI eyes,
    epoxying each turn into place, I really don't feel like cutting up
    the core. I can see where it's welded, and I have access to all manner
    of grinders and a chop saw, and my office opens onto a weld shop, so
    it's theoretically doable, but I can thread wire sitting down. ;-)
    Plus, I don't want to introduce any new gaps - I'd rather change as
    little as possible per experiment. :)

  14. default

    default Guest

    I know you're having fun. What is the volts per turn? or turns per

    American Science and Surplus had some very inexpensive 1 KVA toroid
    isolation transformers awhile back They are 0.84 volts per turn -
    Brand new for ~$12 with flat rate shipping. But you can't beat MOT's
    for cost effective.
  15. Rich Grise

    Rich Grise Guest

    Yup. Can't get much cheaper than free! ;-)

  16. ian field

    ian field Guest

    Its a bit late for the u-wave oven you got the transformer from, but a handy
    tip for curing the problem of the lights keep blowing when people are a bit
    heavy handed shutting the door - watch out for CFLs with slim bodies and
    check if they fit the space available before hunting everywhere for the
    proper bulbs - CFLs seem to be a little more robust in this application!
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