Chris,
it's very easy to calculate the flux density. Easier than measuring it.
So, let's do the maths:
You have 2kW applied to a 7 turn winding. Assuming the impedance at that
point is 50 ohm, 2kW is 316V.
You have three FT-240-77 cores. That core has an effective cross
sectional area of 1.57cm². Three of them are then 4.71cm², or 0.000471m².
This is all we need to apply equation 4 from my web page
http://ludens.cl/Electron/Magnet.html
316V / 7 turns / 4.44 / 0.000471m² / 136000Hz = 0.159 tesla
That's the peak flux density in your cores, meaning that the total flux
swing is between 0.159 and -0.159 tesla. This is comfortably below
saturation, which begins roughly at 0.3T, but it's in a range where core
loss is high.
Let's consult manufacturer's data for core loss. Extrapolating the data
given in a graph in the material's datasheet, it looks like the
volumetric loss is around 600mW per cubic centimeter of ferrite. Each of
your cores has a volume of 22.6cm³, so you have a total of 67.8cm³, and
that means a power loss of slightly over 40 watts in those cores! That's
clearly FAR too much.
So, saturation is no problem, but power loss in the cores is excessive.
You need to either add more turns, or design a different transformer, so
you get lower core loss.
You might start looking at the material. Type 77 is really not so good
at 136kHz. There are newer materials that have lower loss at that
frequency, at the roughly the same permeability and other data. The
question is what you can buy...
If you want to keep the same cores, you need to increase the turns
number. At 136kHz the wavelength is enormous, so you won't run into
trouble from excessive wire length.
Let's take 5W as an acceptable maximum core dissipation. That would make
it warm, but probably not too hot. So you need 8 times lower core loss,
which is roughly 75mW/cm³. Consulting the loss graph given by the
manufacturer, the allowable peak flux density is roughly 50mT. You have
to triple the current number of turns to achieve this: 15 turns primary
and 21 turns secondary should work.
A lower-loss ferrite would allow you to get away with fewer turns for
the same core size, but never as few as you have now!
And a pot core, RM core or even EC core would probably be better than
stacked toroids - if you can find one large enough!
And at that frequency a bundle of thin wires has lower loss than a
single solid wire of the same diameter, and is far easier to wind.
Manfred
Hi. Two off 1kW quasi Class D push pull FET amps on 136khz feeding a
Wilkinson combiner via the amps own output transformers designed for a
50 ohm load. Combiner built from plans for one to combine two 500 Watt
amps. Combiner's output transformer is three stacked FT-240-77 ferrite
toroids, 5 turns 12 AWG enameled (2mm OD) wire primary. Secondary is 7
turns of the same wire. The combiner feeds a big LPF bank. At full
power the combiner's toroids get hot quite fast with WSPR 2 signal
applied. Needs a small fan to stay within the realms of sensibility,
even then it creeps over 70C if left too long. How can I tell in real
time, with measuring instruments, if it's saturating, and if so what
cores might be more suitable please? Hopefully the schematic of the
combiner is linked. Many thanks! Chris 2E0ILY in the UK.
http://www.w1vd.com/137-500-500WCombiner.pdf
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