Re: TC Criticall Coupling (was Overcoupling?)

To: tesla-at-pupman-dot-com
Subject: Re: TC Criticall Coupling (was Overcoupling?)
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Fri, 07 May 1999 01:28:58 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme
Hi all,
        IMHO, the notion of critical coupling should forever be 
thrown away in the context of Tesla coils. It is obvious if one 
examines the waveforms on the scope that a TC can do *far* better 
than losing equal amounts of energy in the primary and secondary 
circuit. I have personally measured a transfer efficiency approaching 
90% in a small coil. You can see the primary ring down to zero while 
the secondary rings up and if ideal quench is effected, the energy 
is contained in the secondary circuit or lost in impedances attached 
to it, hopefully an output discharge. 

> Original Poster: "John H. Couture" <COUTUREJH-at-worldnet.att-dot-net> 
> 
> 
>   To All -
> 
>   Has anyone designed a Tesla coil that was critically coupled? Terman says
> that is the point of maximum secondary current. This should give a Tesla
> coil with the best possible output. Critical coupling occurs when the
> primary circuit resistance is equal to the resistance of the secondary
> circuit.  Also when:
> 
>   Rp = Rs  at critical coupling
> 
>   Kc = Lm/(sqrt(LpLs))
> 
>   Kc = Xm/(sqrt(XpXs))
> 
>   There are many possible TC parameter combinations that will give  Rp=Rs
> but numerous related calculations are required. To my knowledge no coiler
> has ever designed, built, and tested a critically coupled Tesla coil to
> verify that the  Rp=Rs criteria gives an optimum TC.

If one is talking about the static unloaded resistances present 
as being Rs and Rp, it doesn't.  Surely the goal is get the energy 
into the secondary as fast as possible and ideally in its entirety 
before the gap wastes it all.

Malcolm

>   For coilers with the JHCTES program you may want to try the following
> inputs that give Rp=Rs = 92 ohms, .21 coupling factor, 31% overall
> efficiency, plus other output parameters for the design of the coil. It
> should be noted that this feature of the program has never been verified.
> 
>   watts = 900    voltage = 15000
>   pri cap = .01 uf
>   pri rad = 4.3
>   pri turns/inch = 4.00
>   sec rad = 2.60
>   sec turns = 255
>   sec turns/inch = 13.00
>   bare wire dia = .0201
>   sec term pf = 11.00
> 
>   John Couture
> 
> -------------------------------------
> 
> At 10:36 PM 5/3/99 -0600, you wrote:
> >Original Poster: Gary Lau  03-May-1999 1507 <lau-at-hdecad.ENET.dec-dot-com> 
> >
> >>Original Poster: Terry Fritz <twf-at-verinet-dot-com>
> >
> >>>I was tuning up my coil again a couple of days back (I now have my other
> >>>NST back from repair and have stretched by alu duct toroid to 36" dia
- 2kW
> >>>in) and at full power (2kVA) it now tunes at around 8.3 turns (WinTesla
> >>>predicted 7.6). However when I hit this tune I think that I identified
> >>>symptoms of 'double-humping' - I saw a ring of heavy inter-turn corona and
> >>>mild sparking about 3/4 of the way up the secondary, and the occasional
> >>>spark all the way down the secondary coil.
> >>>
> >>>I raised the secondary up about 1/2" and this was reduced, although not
> >>>absent. The sec base is now about 1/4" above the level of the primary, and
> >>>the gap around the sec is about 1.5". My toroid's centre disc is
temporary,
> >>>not covered completely with foil, so maybe this is causing probs too.
> >>>
> >>>Anyone else have any experience with coupling adjustments? If you have any
> >>>advice I'd be happy - I don't want to reduce the coupling too much...
> >>>
> >>>PS Spark length so far is 60", although since I am in a confined space, if
> >>>I try for beyond this, the coil seems to favour the roof beams and
walls as
> >>>targets. I hope I can achieve more - do you think I can get that 6 feet??
> >>>
> >>>Alex Crow
> >
> >>Hi Alex,
> >>
> >>   When in doubt, I would reduce the coupling.  Too little coupling and
your
> >>sparks are short. Too much and your coil may burnup!!
> >>
> >>  The corona area you mention is probably a natural high field stress area
> >>that linear wound Tesla coils have about 2/3 up the secondary.  Raising or
> >>lowering the top terminal may help this if it looks like it is going to
> >>arc.  I would fill in the center disk covering as this may help the high
> >>field stress things you are seeing.  I think your coupling is really too
> >>high and the output sparks may easily grow longer if you reduce it.
> >>Overcoupling is really bad and can cause many problems.  It is always a bit
> >>odd to move the secondary away from the primary to get longer sparks but
> >>this is often the case...
> >>
> >>  Terry
> >
> >If the main consequence to overcoupling is damage to one's secondary from
> >the sparks that race along the length of the secondary, could this be
> >helped by adding several disks of an insulator, say LDPE, along the
> >length of the secondary, disk ID tightly hot-melt glued to the secondary,
> >to effectively increase the top-bottom surface length of the secondary?
> >I seem to recall seeing this done on someone's wep page, no idea who that
> >was.  Same idea as on the ribbed HV insulators we see on pigs, NST
> >bushings, etc.  In clear acrylic, it had that kind of futuristic, Jetsons
> >look to it too!
> >
> >Gary Lau
> >Waltham, MA USA
> >
> >
> >
> 
> 
>