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Re: TC Criticall Coupling (was Overcoupling?)
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.
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
>
>
>