[Amps] Real time tests to see if an RF transformer is saturating?

[Chris Wilson]

Hello Manfred,

Again I thank you for an incredibly detailed reply! Firstly, the
"boxes". I used some die cast alloy boxes at picked up at a rally, I
bought his whole stock, he was an ironmonger and unaware of what some
places charge for them so i got what I considered a bargain, I'll get
some photos up later. I have digested all you said and think that for
future proofing I might take advantage of some space behind the
plasterboard walls of my upstairs (in a bungalow.... ;)) shack. I
think I might try air cored coils, left in free air (not boxed), for
the LPF. How far apart should I mount them so as to not interact? I
presume having the middle one at 90 degrees to the outers will help
too?

I posted previously about impossible to strip Litz wire I was loaned,
I have subsequently found some fairly similar Litz with an overall
diameter of about 2 mm, in a silk like covering. Very similar to the
loaned stuff, but this stuff tins fine in a solder pot :) I was
thinking of using it to either make an air cored LPF and to re-make
the transformer for the combiner, unless you think PVC covered 2.5 mm
stranded mains cable would be better for the LPF? One question that
comes to mind is what effect wire diameter has on impedance? I am
assuming negligible?


Is heat produced in an LPF indicative of wasted energy to the antenna,
or  is  it just the function of dissipating the unwanted harmonics and
will be produced by any filter designed to remove them?

Finally I have some bigger E cores in 3C90 material, they are:

http://uk.farnell.com/ferroxcube/etd49-25-16-3c90/ferrite-core-etd-3c90/dp/3056417

and the data sheet is at :

http://www.farnell.com/datasheets/1468409.pdf?_ga=2.63813673.769747149.1497613104-701848857.1475733716

Would  they  be  of use to try as an alternative output transformer T1 in
the  two  amps  (not as a comniner transformer, as a better substitute
for the toroids)? The amp info is at:

http://www.w1vd.com/137-500-KWTX.html and

http://www.w1vd.com/137-500-KWRev3.0.pdf

I am now looking at optimising everything as it appears that with your
help all is now very reliable :)

Thanks yet again Manfred.

On Tuesday, June 13, 2017,  you wrote:

> Chris,

>> Thanks  for  clarifying  that,  I  have  the  20 turn winding with the
>> grounded  end coming out on the side of the core with no terminal pins
>> and joining the 8 turn winding on the side with the pins.  I can't see
>> it will affect things though.

> I think you have it correct, but I'm not sure. As long as it works, it's
> fine, though! :-)

>> Whilst I have your magic ear maybe you
>> can  advise  on  the next heating issue please? With two amps combined
>> the  centre  toroidal  core  and winding get pretty hot if I use OPERA
>> with  its  32 minute transmission. The two end cores and windings stay
>> cool.  I  wonder  if you can suggest a better core or whatever please?

> I can try.

> In that lowpass filter, the center inductor has twice the inductance and
> works at twice the voltage but the same current as the two end
> inductors. Yet the designer in his infinite wisdom made all three on
> identical cores. Thus the center inductor has 1.41 times the turns as
> each of the end inductors.

> I don't remember your absolute power level anymore. Assuming 2kW, the
> voltage on the load is 316V, and that's also the voltage on each end
> inductor, while the center one has 632V on it. This is at the design
> frequency of 136kHz. The voltages get higher at higher frequencies,
> which might be important in case the inductors came out a little low in
> inductance and thus are working above the filter design frequency. But
> let's assume this is not the case.

> The end inductors have 52 turns each, so they work at 6.1 volts per
> turn. Instead the center inductor has 73 turns, and given twice the
> voltage runs at 8.7 V/turn.

> Considering the effective cross sectional area of the T-225A-2 core,
> this causes a flux density of 37mT for the end inductors, and a core
> loss of 9.7W in each. Instead the center inductor works with a flux
> density of 52mT, and a core loss of 20.8W. No wonder it gets hot, but I
> don't swallow that the end inductors stay cool! Surely not as hot as the
> center one, but at nearly 10W core loss they should get pretty warm too.

> So much for the cores. The wire is #14, is working at 6.3A, the end
> inductors have around 3.8 meters of it, while the center one has around
> 5.3m of it. The AC resistance at 136kHz is 0.08Ω for the end inductors,
> resulting in 3.1W of wire loss, while the center coil's AC resistance is
> 0.11Ω, resulting in 4.4W wire loss.

> Clearly core heating is most of the problem, rather than wire heating.

> Much larger toroids won't fit in that box, but it looks like a stack of
> two of those cores would fit. Using two cores, and 52 turns, you would
> get the correct inductance, all cores would work at identical flux
> density, and the wire length on that stack would be roughly 5.7m. As a
> result the center inductor would have a core loss of 19.4W and a wire
> loss of 4.6W, so the total loss would be almost identical as with a
> single core! The only thing causing some temperature reduction would be
> the greater surface area available. It doesn't look like a good solution.

> Let's try another material. Generally when a core material is too lossy,
> you have to go to a lower permeability one. This would be material 6.
> It's not available in T-225A size, but T225-6 cores exist. Stacking two
> of these is just a tad taller than a single T-225A. So let's try that.

> The slightly lower permability is almost compensated by the slightly
> taller stack, so that you would need 75 turns. The core loss comes out
> as 16.8W, and the wire loss as 4.6W. So there is a small reduction in
> heating, but not enough to be worthwhile.

> There aren't many iron powder materials available in such big toroids,
> so I don't see much more to do there. Your last option without changing
> the box might be using two of those toroids, individually wound with 52
> turns each, connected in series. That has the same core loss as stacking
> the two cores, has higher copper loss than the other solution, but gives
> you the full dissipation surface of both cores. And with some luck it
> might fit in that box.

> Let's try our luck with a compact ferrite core instead of a powdered
> iron toroid. For simplicity, I will try with the same core you used for
> your autotransformer, and I will assume that you have tested it enough
> to confirm that at 2kW for all the time you need, it doesn't overheat,
> but does get quite warm, which would indictae that the design is about
> optimal.

> Ferrite has a much higher permeability than powdered iron, and that
> calls for a different technique to build inductors. You absolutely need
> a split core (not a toroid), and you basically calculate turns number so
> as to get acceptable flux density, and then you introduce an air gap to
> reduce the huge resultig inductance to the value you need. This air gap
> also gives enough stability against temperature-induced variations in
> the permeability, and aging.

> We can base the turns calculation on your existing autotransformer. You
> have 28 turns there, for 316V. The center coil in your LP filter works
> at twice that voltage, so we need twice the turns. As simple as that.
> This will result in the same amount of core loss as the autotransformer
> now has.

> The Fair-Rite 6295420121 core has an AL value of 7.5µH for one turn. So,
> 56 turns will give you a whopping 23520µH! But you only want 115µH. That
> means that you need to reduce the effective permeability by a factor of
> around 205. Given that the material permability is 3000, that means a
> new effective permeability of roughly 14.6. Since 3000 is so high
> compared to 14.6, we can simplify the calculation by considering that
> the ferrite has no reluctance, and that all of the reluctance will be in
> the air gap.

> This means that if the air gap had the same cross section as the ferrite
> core, the gap would need to be the core's path length divided by 14.6.
> With the core having a path length of 73.8mm, this means an airgap
> length of 5mm. Since separating the core halves introduces a gap both in
> the center leg and in the outside return legs, this calls for separating
> the core halves by 2.5mm.

> But what will happen in this case is that the flux bulges out in the
> gaps, using a greater cross sectional area than the ferrite, and as a
> result the inductance will end up larger than desired. You need to
> further lenghten the gap to achieve the correct inductance. The only
> practical way to determine the required gap size is to connect the wound
> coil to an inductance meter of any kind, and adjust the gap size to get
> the correct inductance. You might end up with a core half separation
> much larger than 2.5mm, possibly reaching the point where the core
> halves barely enter the bobbin!

> If you wind that coil on that bobbin, with wire so thick that the bobbin
> is filled up completely, the resulting inductance in air, without a
> core, would already be almost half of the 115µH you need! This
> illustrates how little you will need to insert the core halves.

> This raises the idea of using an air-core, multilayer coil instead. The
> problem is that such a coil isn't self-shielding, and needs to be
> mounted at a fair distance from the metal surfaces of the box. And your
> box is too small for that. The core halves will deliver a fair amount of
> self-shielding, even if only slightly inserted.

> If my calculation is correct, you could use wire roughly 1.3 to 1.4mm
> thick (AWG #16), or rather a bundle of thinner wires totalling the same
> diameter, so you can wind it... I calculate roughly 0.12 ohm AC
> resistance for that wire, so at 6.3A you would get roughly 4.8W loss in
> the wire, plus the 2W core loss assumed for calculation last time. This
> represents a considerable improvement over the inductor you have now -
> but before calling this a success, you have to try it. There are too
> many details where I might have made mistakes.

> If you try it, you will need to find a way to fix the core halves in
> exactly the right position, holding the air gap constant. For testing
> it's probably OK to tape small pieces of cardboard to the cores, like
> struts, but a good definitive system might be using non-corrosive
> silicone caulk, forming 4 beads of it that join the core halves and also
> bind to the coil surface. This is stable over time, heat resistant, and
> the silicone's elasticity allows fine-tuning the inductance value by
> using any kind of clamp with a screw that presses down on the core.
> There you have my new invention: A compression variable inductor, a
> great counterpart to the well-known mica compression trimmer! :-)

> If you prefer not to try this ferrite kludge, at least you can improve
> the cooling of your powdered iron core. Those toroids are mounted in a
> way that blocks air flow through their holes, and also impedes free air
> flow on their sides! Get rid of that, and make some mounting that allows
> free air flow.

> And then go and buy a can of flat black spray paint, and liberally paint
> the coils and the box (inside and the outside) flat black. That will
> drastically improve heat transfer by radiation! I do that even with the
> innards of my computers, so that I don't need noise fans! Your very
> shiny boxes are formidable heat reflectors!

>> The  LPF  is detailed here, and they are in individual sealed die cast
>> alloy  boxes, 

> The photo shows sheet metal boxes, not die cast. But that's irrelevant
> in this case.

>> so  air  flow is none existent, but I suspect even if I
>> mill two openings in the centre box and put mesh over the apertures it
>> will  still be marginal. A fan sound like a kludge. Ideas very welcome
>> indeed!!

> I would take my drill press and drill a few hundred holes in those
> boxes. In addition to painting everything flat black, except the contact
> surfaces!

> And after that, stop worrying, because iron powder toroids and enameled
> wire can both take some heat.





-- 
Best regards,
 Chris                            mailto:chris at chriswilson.tv