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Re: Tranformer efficency VS Frequency



Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 
> Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<jimlux-at-earthlink-dot-net>
> 
> You want to watch out for inductance and non unity power factor.  I assume
> you were operating into a resistive load?  Oddly, the manufacturer of a
> transformer tends to optimize its design for the intended frequency. As the
> frequency goes up, core losses due to things like eddy currents goes up:
> laminations that are thin for 50/60 Hz might not be thin for 400 Hz.
> Mechanical vibrations that are negligible at 50/60 Hz might be significant
> at higher frequencies (either due to loose laminations or magnetostriction).
> skin effect definitely gets worse as the frequency goes up (hence the
> popularity of Litz wire in very high frequency switching power supplies).

Even ignoring losses at the core, that certainly grow with frequency
(high-frequency transformers use fancy materials as ferrite for cores,
not laminated iron), there is the large inductance of low-frequency
transformer windings. The high inductance is necessary to reduce the
magnetizing current to acceptable levels (current that flows in a
winding when the other(s) are open). A transformer can be modeled as
an ideal transformer, that would work at any frequency, with one side
shunted by an inductor that controls the magnetizing current, and
one of the terminals in series with an inductor that is function of
the winding inductances and the coupling coefficient (use a fixed font
to see):

      k                      1:n
o----+ +----o    o-----L1----+ +----+----o  
     | |                     | |    |
    La Lb     =              ) (    L2
     | |                     | |    |
o----+ +----o    o-----------+ +----+----0

L2=Lb causes degradation at low frequency, shunting the windings with
low impedances (and impeding operation at DC, obviously). 
L1=La*(1-k^2) causes degradation at high frequencies, increasing
the input and output impedances. Remember that La is large, and k
is close to 1, but not 1.
n=sqrt(Lb/La)/k, doesn't cause degradations.

Add to this the winding capacitances, that add even more attenuation
at high frequency.

Antonio Carlos M. de Queiroz