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Re: LF: Re: Transformer problem; advice sought

To: [email protected], <[email protected]>
Subject: Re: LF: Re: Transformer problem; advice sought
From: [email protected] (Warren K2ORS/WD2XGJ)
Date: Sat, 03 Jun 2006 15:50:31 +0000
Cc: "James Moritz" <[email protected]>
Delivered-to: [email protected]
Reply-to: [email protected]
Sender: [email protected]
Dear Jim,
   Thank you for your detailed and thoughtful analysis of my transformer 
problem.  
  There is a very high potential between the primary and secondary and hence my 
use of Teflon wire on the primary, I estimate about 6kV on the secondary.

  I also received an analysis from Neils OZ8NJ who suggested the use of lower 
permeability material such as Mix 61 ferrite. Neils also suggested winding a 
secondary out of copper foil or braid first followed by the primary on top of 
the secondary.

  Very interesting on connecting  the cold side of the primary to one side of 
the secondary, but I seem to remember that Bill Ashlock thought that it was 
important to float the loop from ground although the reason escapes me at the 
moment. Perhaps I could still float the loop from ground by adding a second 1:1 
transformer to connect the feedline.

  I recently gave a talk about building large fuel cell systems to a group of 
scientists/engineers who had specialized in building small single cell fuel 
cells. I was criticized for simply building big systems - I won the group over 
by pointing out that there are things that you learn from big systems that you 
would never suspect from working with small units, that things often don't 
scale up in a simple fashion. The parallel to LF is obvious, you can build 
small QRP transmitters and associated matching units and antennas all day long 
and never suspect the issues that will come up when you go to high power!


--
73 Warren K2ORS/WD2XGJ
FN42hi
http://www.w4dex.com/wd2xgj.htm

 -------------- Original message ----------------------
From: "James Moritz" <[email protected]>
> Dear Warren, LF Group,
> 
> Taking the inductive reactance first - this is due to the leakage reactance
> of the transformer windings. In an ideal transformer, all the magnetic flux
> produced by a current in the primary winding would also pass through the
> secondary and vice-versa - but in practice, some of the flux always
> "escapes", resulting in a real transformer effectively having some series
> inductive reactance. This leakage inductance depends on the winding
> geometry, core permeability etc. and can't readily be calculated, but
> usually measures to be a small fraction of the winding inductance, and very
> roughly proportional to the winding inductance. Leakage  inductance is one
> of the things that limits the upper bandwidth of a transformer, so normally
> you try to minimise it.
> 
> In your case, assuming the cores are 4 x Fair-Rite 5978015901 or something
> similar, the 28t primary inductance works out to about 11mH (XL = 9.4k). The
> leakage inductance, assuming 25ohms inductive reactance per transformer, is
> about 29uH, which is quite low for that amount of primary inductance.
> 
> The first thing to observe is that saturation of the cores just isn't going
> to happen. Assuming about 1kW TX power, so about 224V primary voltage, the
> flux density in the transformer core comes out to only around 12mT, while
> saturation flux density will be over 300mT even at high operating
> temperature. The cores are certainly under-stressed from the magnetic point
> of view. Estimating from the Fair-Rite graphs for 78 mix ferrite gives a
> total core loss of less than 1W. So you definitely have over-kill in the
> amount of ferrite being used.
> 
> The second thing to observe is that the winding inductance is too high. The
> reactance of the winding won't make much difference to the overall
> impedance, provided it is more than about 10 x the transformed load
> resistance, in this case XL >=500R, or 580uH. But the trouble with having a
> much higher winding inductance like 11mH is that the leakage inductance will
> also be relatively high. This is why your transformer introduces significant
> inductive reactance.
> 
> The leakage inductance of the transformer may contribute significantly to
> the heating of the cores. The reactive power circulating in the assumed 25R
> of leakage reactance will be about half the total TX power being delivered
> to the 50R load resistance when the system is tuned to resonance, i.e.
> 500VA. The Q of the leakage inductance possibly isn't very high, at a guess
> maybe 10 or 20, leading to power dissipation of about 50W or 25W
> respectively somewhere in the transformer with 1kW TX pwr. This would
> certainly warm it up. Using bigger cores does not really help in itself,
> because bigger cores will also mean more leakage inductance
> 
> This situation ought to be improved by reducing the primary and secondary
> turns. An 11 turn primary and 2 turn secondary will give about the same
> ratio, with a primary winding inductance of 1.7mH, hopefully with leakage
> reactance correspondingly reduced, and a flux density of around 30mT, still
> very low. This will increase the total core loss, but only to around a
> couple of watts. As far as leakage inductance goes, a toroid is not the
> optimum transformer geometry for a transformer with only a few turns on the
> winding, especially when the wire is very thick and does not lie flat on the
> core. It is best to make the windings as compact and close to each other as
> possible, i.e. wind secondary directly on top of primary, don't spread the
> windings round the whole circumference of the core. For example, look at the
> type of transformers used in HF linears, where the low impedance winding is
> a compact tubing loop, with the high impedance winding threaded through it,
> and the ferrite cores wrapped closely around it. I think leakage reactance
> in your transformer could be reduced a bit more by arranging the cores as 2
> pairs side-by-side in "binocular" form, with the windings wrapped around the
> middle of the assembly where the cores are touching together. Another
> possibility is to have multiple secondaries of thinner wire distributed over
> the primary winding and connected in parallel, but this can be quite fiddly
> to produce.
> 
> Another possible cause of heating in the transformer is if there is a large
> RF voltage between primary and secondary windings. Ferrite is a poor
> dielectric, so will get hot if subjected to an intense RF field. This means
> that one side of the secondary should be connected to the "cold" side of the
> primary, effectively that the transformer should be inserted at a
> near-ground potential point on the loop.
> 
> Cheers, Jim Moritz
> 73 de M0BMU
> 
> 
> 
> ----- Original Message -----
> From: Warren K2ORS/WD2XGJ <[email protected]>
> To: rsgb lf reflector <[email protected]>
> Cc: LWCA LF Reflector <[email protected]>
> Sent: Friday, June 02, 2006 3:55 AM
> Subject: LF: Transformer problem; advice sought
> 
> 
> > Hello the list,
> >
> >    I have been experiencing heating problems with my step-down toroidal
> transformer that feeds my transmit loop. Fearing core saturaton issues I
> have been going to ever larger cores and more and more turns in an attempt
> to reduce the flux density.  My latest core is 4 stacked 4" diameter Mix 78
> toroids weighing a total of 3 pounds (1.34 Kg) with 28 turns of #10 on the
> 50 ohm winding and 5 turns of 16 mm2 wire on the loop side.
> >
> >  Now for the puzzle: As suggested by J.B., I hooked two transformers back
> to back ; the present transformer (described above) and the previous model
> (4 stacked FT-290-77 toroids with 17 turns on the 50 ohm side and 3 turns on
> the loop side.  I connected the two transfomers with the the loop windings
> from one going into the loop winding of the next. One transformer 50 ohm
> winding was hooked to a 50 ohm Bird Dummy Load, the other transformer's 50
> ohm winding went to an AEA VIA impedance bridge.  Much to my surprise I had
> a high swr and reactance !!!
> > The swr was on the order of 2.7:1 and the reactance was 45-55 ohm and
> increasing with frequency.
> >
> >  Any thoughts on why I should show a reactive component when one
> transformer went directly to a 50 ohm load?
> >
> >
> >
> > --
> > 73 Warren K2ORS/WD2XGJ
> > FN42hi
> > http://www.w4dex.com/wd2xgj.htm
> >
> >
> 
> 




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