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Constant Voltage Transformer Operation?

Discussion in 'Electronic Design' started by amdx, Aug 6, 2012.

  1. amdx

    amdx Guest

    I recently repaired a marine battery charger. It was set up to charge 3
    separate 12v batteries. The repair was simple the charger had a *shorted
    40uf 660v ac capacitor. The capacitor is connected across a winding on
    the transformer. This is the usual constant voltage transformer
    configuration. This is the extent of what I think I know.

    Can someone describe the operation of a constant voltage transformer
    that has 3 windings, input, output, and a third winding that is
    connected across a capacitor.
    I've seen these for decades but never have understood the operation.

    Mikek

    *bonus, why didn't the shorted capacitor (**measured 1.5 ohms) blow a
    fuse or overheat the winding on the transformer.

    **1.5 ohms plus 0.7 ohms lead resistance.
     
  2. Jamie

    Jamie Guest

    Apparently you're unsure of yourself, more or less!

    Jamie
     
  3. Tim Williams

    Tim Williams Guest

    The transformer has two sections: a core section around the primary winding,
    and a section around the secondary. These are coupled somewhat looser than
    a regular transformer, so (without the capacitor) the secondary voltage
    would be lower and "squishier" than a regular transformer (i.e., higher
    series inductance). Adding the correct value capacitor cancels the series
    inductance, forming a resonant tank. But the increased voltage difference
    across the barrier forces the transformer deeper into saturation, thus
    limiting voltage.

    The transformer needs to be designed so that, over the design range of input
    voltage (usually +/-10%) and load (0-100% current), the transformer must
    remain in saturation, without overheating. Finally, because the secondary
    voltage is generally lower (in terms of volts/turn), a lot more copper is
    needed. Iron operated in saturation also has high core losses. This makes
    these transformers particularly large and low in effeciency.

    One upside: the effective LC filter between primary and secondary isolates
    harmonics and transients; although the input and output are still a bit
    distorted, they make reasonable sine waves, so it doesn't matter much if
    your primary waveform is part square wave, or that your load has similar
    behavior (like a capacitor-input rectifier).

    Tim
     
  4. Jasen Betts

    Jasen Betts Guest

    Current in the regulatiing winding saturates the core and decouples the
    primary and secondary windings.
    the primary starts off poorly coupled and is rated for higher current
    than in an ordinary transformer to survive the saturation.
    0.7 is the meter leads?
     
  5. amdx

    amdx Guest

    Ya, added that to make it *clear that the short was a little long!
    I removed the 0.7 ohm meter lead resistance and still had a 1.5 ohm short.


    Mikek

    * apparently I didn't make it clear. :)
     
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