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Efficiency of a transformer with increasing frequency

Discussion in 'Electronic Basics' started by CNM, Apr 23, 2005.

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  1. CNM

    CNM Guest

    Why does the effciency of my low voltage transformer very much
    decrease (to eg 10%) when increasing the frequency to around 40000hz.
    Why does this increase in frequency affect the efficiency value so
    much?
     
  2. Lord Garth

    Lord Garth Guest

    The impedance of the transformer is Xl=2*PI*F*L if F go up to 40KHz, Xl also
    goes way up. Your power transformer was designed to work efficiently at
    line
    frequency, not 40KHz.
     
  3. John Larkin

    John Larkin Guest

    Core loss and skin loss. Cheap, thick steel laminations in the core
    work fine at 60 Hz but have lots of eddy-current loss at hf. The wire
    itself has skin and proximity effects, too. I'm guessing the core loss
    is the biggie.

    Besides, if it works at 60 Hz, it has far too many turns to be optimum
    at 40K. A ferrite torroid or pot core would be much better up there.

    John
     
  4. The most likely reason is that eddy current losses
    in the core go up and magnetizing current goes up.
    In effect, the shunt inductance of the transformer
    goes down as frequency goes up when eddy
    currents become signficant. Those currents are
    (nearly) in phase with the applied voltage and
    represent a loss.

    You probably have a transformer designed for
    line frequency with the lamination thickness set
    accordingly. At 40 KHz, that thickness is way
    too large.
     
  5. Don Kelly

    Don Kelly Guest

    --
    Don Kelly

    remove the urine to answer
    --------
    Actually the "magnetising" or inductive component of the current decreases
    because of lower flux density for a given voltage, at higher frequencies
    (The inductance won't decrease much if at all) .but the total exciting
    current increases due to hysteresis and eddy current loss increases. This
    increased loss current will coupled with skin effects will also lead to
    higher I^R loss.
    ---------------
    -----------
    Right on- design a 40KHz transformer for 40KHz- don't use a 60Hz transformer
    and expect good operation. It is more than just lamination thickness, it is
    excessive iron.volume as well as capacitive effects. It is a wonder if a
    60Hz transformer is not completely useless at 40KHz.>
     
  6. I agree with your correction on terminology. I was
    incorrectly referring to the shunt current term as
    "magnetising current". As for inductance not being
    reduced, I have to disagree. If you consider that
    eddy current prevents flux from penetrating into
    the interior of the laminations, you should be able to
    see that the effective area of the core is reduced.

    This effect shows up as a non-polynomial relation
    between impedance and frequency, such that the
    impedance rises approximately as sqrt(frequency).
    If you take the imaginary part of the impedance in
    any part of that curve, you will find that inductance
    (defined as that component divided by radians/S)
    is indeed decreasing with frequency.
    Agreed.
     
  7. Don Kelly

    Don Kelly Guest

     
  8. Bob Eldred

    Bob Eldred Guest

    Core loss is a part of it and core loss goes up with frequency, but flux
    density goes down with increasing frequency which helps to ameliorate core
    loss. Whatever the core losses are at a given frequency, they would be much
    worse if the flux density were maintained at its low frequency value.

    Another important loss is driving of the winding capacitance by the
    resistance of the windings and by the impedance of the leakage inductance.
    This can be a very important loss factor because the AC resistance of the
    copper (not impedance) goes up frequency because of skin effect and may
    become excessive at 40KHz. The capacitance was most likely ignored in a line
    frequency transformer design but can be significant in a 40KHz transformer.
    Furthermore higher voltage windings often have higher capacitances and
    smaller wire with higher resistance exacerbating the loss effects.

    Leakage inductance is the inductance that does not link the primary to the
    secondary and therefore no transformer coupling occurs across it. It's just
    a series impedance in the way of transferring power across the transformer.
    It drives the winding capacitances as mentioned but also is in series with
    the load reducing the transfer. Like all inductances, it's impedance
    increases with frequency.
    Bob
     
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