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Flat strip primary inductance

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Finn Hammer

Fri Dec 19 2008, 06:13PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,

I know this has been up before, but here goes:
A flat strip primary has lower inductance than one made out of round conductors. This is not new, but here is an extreme example, I think:
I kept hanging tank capacitors on to the primary assembly, without reaching the desired res. frequency. At last I had to investigate further.

The primary is an internal flat strip primary, for a BRISG coil, and it consists of 2 turns 180mm dia with 5mm spacing.

All simulators, INCA, javatc, wintesla predict 1.5ÂµH and since the primary cap is 7.5ÂµF, the anticipated resonanse was 47kHz.

However, the assembly resonated at 63kHz which corresponds to 0.8ÂµH

It baffels me that such an extreme difference could exist, so I made up a sanity check:

2Turns of thick speaker wire, and a 7.5ÂµF cap resonates at 43kHz.

Has anybody experienced this before?

Cheers, Finn Hammer

Proud Mary

Fri Dec 19 2008, 06:39PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Can you measure L and C directly, Finn, rather than deriving them from f?

Mads Barnkob

Fri Dec 19 2008, 07:07PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

A strip copper coil might have higher selfcapacitance, but that would lower the res.freq.... other than that, no clue :)

Finn Hammer

Fri Dec 19 2008, 07:42PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Harry wrote ...

Can you measure L and C directly, Finn, rather than deriving them from f?

Yes, C is 7.55ÂµF vs. 7.45ÂµF measured with Meterman 37XR.
Inductance so small, I don`t really trust the meter, that`s why I ressonate.

Do you se any problem with that approach?

MadsKaizer wrote ...

A strip copper coil might have higher selfcapacitance, but that would lower the res.freq.... other than that, no clue :)

Antonio mentioned that the strip could be viewed as several conductors in parallel, thus lowering the inductance.
I was expecting _some_ of this, only not so much.

Cheers, Finn Hammer

Mathias

Fri Dec 19 2008, 08:03PM

Registered Member #1381 Joined: Fri Mar 07 2008, 05:24PM
Location: Hungary
Posts: 74

You could try it with much thinner copper strip

Well i dont not know much about triggered-brick-sisg driven coils but why not more then 2 turns for the primer?
Are maximum peak currents nessesary?

Finn Hammer

Fri Dec 19 2008, 09:30PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Mathias wrote ...

You could try it with much thinner copper strip

Well i dont not know much about triggered-brick-sisg driven coils but why not more then 2 turns for the primer?
Are maximum peak currents nessesary?

Since there is only one IGBT to handle the voltage, which cannot exeed 1200V then, the tank cap should be as big as possible, in order to get a decent bang energy.
All within reason, of gourse.
That`s the reason for having so few turns.
The thicknes of the coil material: The max current is beyond 3kA so to keep temperature low during prolonged runs.....

Cheers, Finn Hammer

Finn Hammer

Sat Dec 20 2008, 01:53PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,
This is a legal double post, because I have new experimental data to share.
I just made another round conductor primary, similar to the one used in the sanity check, and placed it 45mm apart from the first one, axially aligned, of course.

This brings the combo into resonance at 55kHz, which is comparable to the resonant frequency of the strip primary.

I am using the term sanity check in the form that I am not spending time to make the test primaries _exactly_ to measure, only by eye measurement, and then I am satisfied if resonant measurements fall within 1 0r 2 kHz of the specimen being compared.

It would appear that a strip wound primary can be viewed as 2 individual primary`s in parallel, spaced apart by the same distance as the width of the strip.

That`s quite simple, then.

It is probably possible to arrive at this conclusion by calculation, and I`d appreciate if someone will step forward with that solution. I`m not the analytical type, more like the monkey wrench tootin`jack of all trades (and a master of only a few

)

Only snag as I see it is how to place the point on the coil, to base a calculation of coupling upon.
It seems possible to me, that coupling to a secondary coil might force current to prefer flowing in the lowest "virtual" conductor. But I`m may very well be wrong.

All this mounts up to me having to wind 2 new secondary`s for the v-twin, but that`s no big deal.

Cheers, Finn hammer

Steve Conner

Sat Dec 20 2008, 08:38PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Why bother winding new secondaries? Just pile more tank capacitance on to get it into tune, and enjoy the extra bang energy, which these OLTC type coils are usually dismally short of.

Tom540

Mon Dec 22 2008, 03:34AM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

It also appears the your primary is like a flat spiral rather than a solenoid. What I mean is your winding the flat copper so that each additional turn goes only out horizontally and not over the previous turn. You should try winding a 90 degree primary with the flat copper which will look a bit odd but might have more inductance.

Steve Ward

Wed Dec 24 2008, 05:44AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Only snag as I see it is how to place the point on the coil, to base a calculation of coupling upon.
It seems possible to me, that coupling to a secondary coil might force current to prefer flowing in the lowest "virtual" conductor. But I`m may very well be wrong.

"current bunching" is real. Id expect high current concentration on the top edge of the primary ribbon.

FEMM is a wonderful tool for just this kind of work! It'll even tell you inductance, and AC resistance at some frequency you define. Throw your secondary coil in the model too while your at it

.

On the same note, FEMM took me a couple days to learn how to use effectively. So if you are seriously interested in this problem i suggest you get the free download:

I found it to be wonderfully insightful when looking at primary "hot spots". See how much of that big wide ribbon of copper is really being put to use! Or maybe you cant handle the truth!!

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