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Transformer primary

Discussion in 'Electronic Basics' started by Jamie, Jun 13, 2006.

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  1. Jamie

    Jamie Guest

    Another transformer question:

    What prevents current overload on a primary side of a transformer
    when there is no current flow in the secondary windings? As I understand
    it, the current flow in the secondary opposes a flow of current in the
    primary. If there is no load in the secondary, isn't there basically an
    open circuit in the primary, or does the reactance of the primary prevent
    current flow until a secondary field exists in the secondary windings?

    The Moon is Waning Gibbous (98% of Full)
  2. Pooh Bear

    Pooh Bear Guest

    Yes. See magnetising inductance.

  3. The term there is counter electromotive force. the magnetic field generated
    from the voltage applied when reversed, counters the flow of current on the
    ac swing. Balances out till some of the flux field is used by the secondary
    load etc.

    How well the transformer core is made determines how much magnetic flux is
    lost to ( eddy currents ) generated in the core material. Which ends up
    being heat. If you are going to try and build your own, get a good core.
    Thinner the slices on the core, will have fewer eddy currents, as in the
    core of a microwave transformer has fairly thick slices and gets hotter from
    my exp. with them.
  4. Jasen Betts

    Jasen Betts Guest

    Nope, (when dealing with AC) current flow int the primary opposes current
    flow in the primary, allowing a current in the secondary counteracts
    some of that and allows more in the primary.

  5. When the secondary has no load, the primary winding acts like a very
    large value inductor. It draws a small (relative to full load)
    inductive (current delayed almost 90 degrees relative to the applied
    voltage) current.
    That is roughly backward. load current through the secondary
    increases the current through the primary.
    Not an open circuit, but a high inductive impedance.
    Here are the basic relationships.

    1. The flux in the core is proportional to the total, instantaneous
    amperes wrapped around it (amperes times turns)

    2. The voltage produced across each turn is proportional to the rate
    of change of the flux through the core.

    A load current through the secondary does not oppose the primary
    current, it opposes the core flux generated by the primary (cancels
    some of the flux created by the primary current). The reduced rate of
    change of flux generates slightly less voltage across the primary
    windings, which holds back the primary current less, so the primary
    current rises.

    But even with no secondary current, there must be a little primary
    current, because something must generate the core flux, so there can
    be a rate of change of that flux to generate the voltage that bucks
    most of the primary voltage, to keep the primary from looking like the
    resistance of a piece of wire across the applied voltage.
  6. Jamie

    Jamie Guest

    you just about answered your own question.
    you don't have a complete open circuit
    but what ever the inductance is in the primary
    placing a load on the secondary side will cause
    load effects on the primary side..
    one neat thing i did years ago was to use a
    primary side of a xformer as part of a low freq
    oscillator and cause load effects on the secondary
    side to change the inductive value of the primary
    thus change the frequency of the oscillator.
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