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Magnetic Cancellation

Discussion in 'Electronic Basics' started by Randy, Jul 15, 2003.

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  1. Randy

    Randy Guest

    Greetings:

    About a year ago, I built a small welder. During the assembly of the two
    transformers something got my attention. When I connected the two
    secondaries in series, I got less than half of the required output voltage.
    I switched the output wires and the voltage was there. I reasoned that the
    outputs were out of phase with each other and canceled a percentage of the
    magnetic field. What I failed to check was the effect it had on the output
    current.

    Now I'm wondering, can I use magnetic cancellation to my advantage in
    controlling current in high power circuits?

    To test this theory, I used a small 3volt output transformer and a powerful
    magnet to see what effect the magnets' magnetic field had on the
    transformer. I tried both poles.

    Result: zilch! The voltage didn't even flicker from the 3 volt reading.
    However, the closer I put the magnet to the coils, the greater the
    vibration. That transformer was dancing across the table like Michael
    Jackson on crack.

    I'm thinking, the only way to reproduce the cancellation is to run the
    output through a magnetic field that is the same frequency but, out of
    phase and variable.

    This second experiment is just a thought right now. What I'm seeking is
    opinions.

    An Inquiring Mind,

    Randy Gross
     
  2. Fred Abse

    Fred Abse Guest

    In a word, yes, but not the way you tried it.

    There's a thing called a saturable reactor, aka magnetic amplifier. It
    works like this:

    An iron cored coil (inductor) is connected in *series* with the load. Its
    inductance is chosen so as to present a large reactance (high impedance)
    at the frequency of operation, hence very little current flows.

    Also wound on the same iron core, are many turns of quite thin wire,
    which are fed with a small DC current. Because there are many turns,
    although the DC current (we'll call it the controlling current) is small,
    nevertheless there are a lot of amp-turns, hence a large magnetizing
    force. The DC magnetization takes the core into saturation, hence the
    reactance of the winding in series with the load (the controlled winding)
    is reduced, and more current flows into the load.

    In this way, a large alternating current can be controlled by a small
    direct current.

    Taking the principle further, a permanent magnet can be used instead of a
    control winding, making a reactor that is controlled by positioning the
    magnet. Such devices have been used in TV receivers as adjustment for
    horizontal scanning current.

    Unfortunately, trying to do the same thing with a transformer will just
    mean that the reactance of the primary will reduce as you bring up the
    controlling field. That'll eventually blow a fuse somewhere. Not the
    desired result :-(

    In the (good?) old days, there were mag amps controlling such things as
    big multi-kilowatt electric furnaces. These days, they use big triacs.
     
  3. Randy

    Randy Guest

    <>...
    :
    : There's a thing called a saturable reactor, aka magnetic amplifier. It
    : works like this:
    :
    : An iron cored coil (inductor) is connected in *series* with the load. Its
    : inductance is chosen so as to present a large reactance (high impedance)
    : at the frequency of operation, hence very little current flows.

    I found some data on the principles of the saturable reactor and it looks
    like it just might be what I'm looking for.

    My goal is this; to produce a welder with a control circuit that doesn't
    hack up the sine wave or, use circuitry that will choose a time to fail
    that could not be worse. I worked for the company that built the equipment
    to construct Bolder Dam. The stick welders they used then are still there,
    and work. Welders (those with the fancy digital readouts) in the shipyards
    I've worked in, were stacked up on death row like that scrap pile behind
    R&D, either dead in the water or, running full steam ahead - nothing in the
    middle.

    Thanks for the tip!

    Randy Gross
     
  4. My, oh my.. Oh my oh my.. :) That just totally made my day. The picture
    in my head of a dancing Michael Jackson on crack.. What a sight that would
    be!

    LOL..

    --
     
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