[Prev][Next][Index][Thread]
Re: TC Criticall Coupling
Hi John,
Always good to hear from you.
I am under the impression (perhaps an illusion :-)) that my research coil
is pretty "optimal". So I ran through the calculations and dug up the
measurements for it to get the following numbers:
Rp = 3 ohms (the real primary resistance)
Rs = 320 ohms (the real secondary resistance)
K = 0.1475 (primary to secondary coupling coefficient)
Lp = 80 uH (primary inductance)
Ls = 75.4 mH (secondary inductance)
Lm = 362 uH (pri to sec mutual inductance)
Fo = 111000 Hz (system resonant frequency)
Xp = 55.8 ohms (primary impedance at Fo)
Xs = 52586 ohms (secondary impedance at Fo)
Xm = 252 ohms (mutual impedance at Fo ???)
This is for a "large cap" (27nF) system with a sync gap run from a
15kV/60mA neon.
So my Rp and Rs differ by two orders of magnitude! and the impedances
differ by three orders! I don't see how I could ever get Rp to match Rs.
Perhaps I have completely "missed the boat" here???
I know many people don't like the old radio equations for Tesla coil use
because they fail to take into account the very high secondary losses in
Tesla coils. Also, the pulsed energy nature of disruptive Tesla coils is a
far cry from the steady state conditions that most of the old radio
equations rely on.
I ran you program (version 2.3) on the numbers you suggested with the
following results:
JHCTES Version 2.3 program 12/93
05/05/99 19:44:03
Tesla Coil Design System
Transformer Type = NEON
Wattage = 900
Primary type = SPIRAL
PRIMARY INPUTS ----------------------------------
Volts in RMS = 15000
Capacitance in uF = 0.0100
Radius in inches = 4.30
Width in inches = 10.10
SECONDARY INPUTS --------------------------------
Radius in inches = 2.60
Turns = 255
Turns per inch = 13.00
Bare wire dia (in inches) = 0.0200
Sec Terminal (pF) = 11.00
Rec Pri/Sec Clearance (in inches) = 3.25
SECONDARY OUTPUTS -------------------------------
Operating Freq - KHZ = 880.83
Sec coil lgt - ins = 19.62
Sec total cap - pf = 16.32
Sec coil self cap - pf = 5.32
Sec Q Factor = 119.99
Sec Log Dec = 0.0262
Cycles to 10% ampl = 87.95
Sec Volts - KV = 358.95
Rec wire insul - ins = 0.0028
Your tot wire dia - ins = 0.0769
Sec Wire - Ft = 347.15
Sec ind - mh = 2.00
Sec L/D <4 = 3.77
Sec reac - ohms = 11075
Effective Res - ohms = 92.3
Power Factor % = 0.83
10% Cycle time - us = 99.84
Sec volts/turn = 1408
Secondary peak amps = 32.41
PRIMARY OUTPUTS -------------------------------
Inductance - uh = 3.26
Reactance - ohms = 18.05
Q factor = 0.0618
Wire Size - sq ins = 0.0192
Mut ind - uh = 29.68
Watts /ft of spark = 646.37
Sec Spark Lgt in ins = 16.71
Overall % Efficiency = 30.94
Turns >1 = 5.07
Primary Peak amps = 830
Effective Res - ohms = 292.07
K Factor = 0.37
Mut react - ohms = 164.18
Sec Kv Per Inch Spark = 21.48
I then ran the system with a few guesses on my own pitiful program I wrote
late last year to get:
Terry's Tesla Coil Analysis Program
05-05-1999 19:35:56 <---- Note the Y2K compliance :-)))
Charging Circuit Input:
Transformer Output Voltage (RMS): 15000
Transformer Output Current (RMS): 0.06
Primary Capacitor Value (uF): 0.01 <<= 0.02
Primary Capacitor Firing Voltage (volts): 21210 <<= 21210
Break Rate (BPS): 120
Charging Circuit Output:
Primary Capacitor Firing Voltage (volts): 21210
Primary Energy per Bang (J): 2.249321
Primary Power (W): 269.9185
Primary Circuit Input:
Primary Inner Diameter (in): 8.6
Primary Turns: 2.9 <<= 2.899998
Primary Winding Pitch (in): 0.25
Primary Cap Value (uF): 0.01
Primary Circuit Output:
Primary Fundamental Frequency: 792013.9
Primary Inductance (uH): 4.038084
Primary Capacitance (uF): 0.01
Primary Current (RMS): 746.4549
Primary Reactance (Ohm): 20.09498
Primary Voltage (RMS): 15000
Secondary Circuit Input:
Secondary Diameter (in): 5.2
Secondary Length (in): 19.61
Secondary Turns: 255
Secondary Top Capacitance (pF): 11
Secondary Circuit Output:
Secondary fundamental Frequency: 793764.9
Secondary Inductance (mH): 2.002592
Secondary Self Capacitance (pF): 9.07542
Secondary Total Capacitance (pF): 20.07542
Secondary output Voltage (Peak): 473378.3
Secondary Reactance (Ohm): 9987.663
Secondary Current (RMS): 33.51931
The secondary effective resistance in your program was 92 ohms but the
primary effctive resistance was 292.07 ohms??? You didn't miss the 2 in
front of 292.07 did you ???
My program give a little less Fo since it adds some primary inductance to
account for streamer loading but the results are pretty much the same. I
was confused as to how your program could say that the primary resistance
was 292 ohms when the primary current was 830 amps. If you multiply those
out, you get 242360 Volts which is rather high for the primary?? Your
program said 830 peak amps, was that supposed to be RMS amps??
Thoughts?
Got a little carried away on this response :-))
Terry
At 07:25 PM 5/5/99 +0000, you wrote:
>
> 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
>>
>>
>>
>