Re: coupling losses ? (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 21 Sep 2007 23:04:31 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: coupling losses ? (fwd)

Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Fri, 21 Sep 2007 21:24:01 +0100
> From: Chris Swinson <list@xxxxxxxxxxxxxxxxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Re: coupling losses ? (fwd)
> 
> HI Bert,
> 
> Thanks for your in dept analysis...
> 
> 
>> If you include system losses, a portion of system's energy is lost over
>> each cycle, leaving less available to make it to the secondary. Since a
> 
> Resistance, heat losses ?

Yes. The largest is spark gap losses (about 100-200 volts/gap pretty 
much independent of current). Other losses include wire resistance, tank 
cap dielectric losses, secondary base ground resistance, etc., etc...

> 
> 
>> Wrong interpretation. The energy in the primary transfers to the
>> secondary over a number of RF cycles. In most systems, the gases) in the
>> gap(s) are still quite hot and conductive. As a result, the gap
>> "reignites" at relatively low voltages - perhaps less than 10% of the
>> initial ("cold") breakdown voltage. Re-ignition causes a failure to
>> quench, which lets energy cycle back and forth between P->S, then S->P,
>> many times. For most Tesla coils, this back and forth energy cycling
>> process may continue many times before the gap finally stops conducting
>> (quenches).
> 
> 
> If we assume then that the spark gap is the main loss in the system, then if 
> we replace with for example solid sate, then use a low K factor, we will 
> need many cycles of course to transfer the energy, but what are the cycle by 
> cycle losses then ?

A SISG substitutes a chain of solid state switches (IGBT's) for the 
spark gap. This approach significantly reduces switching losses, but the 
rest of the system losses still remain. Since the overall losses are 
lower in an SISG coil, a greater portion of primary "bang" energy can 
make it to the secondary, and with proper adjustment, quenching can also 
be tightly controlled. As a result, an SISG coil is measurably more 
efficient (when measured as output spark length versus input power) than 
an otherwise identical coil that uses a spark gap switch.

> 
> In theory then at least, if we had zero losses, we can transfer energy in 
> any number of cycles over any amount of coupling ?

In theory, yes. However, if the intent is to make long sparks, there are 
other dynamics that come into play, such as the rate at which energy can 
be transferred from primary to secondary. If the topload voltage cannot 
be replenished quickly (once an initial spark leader is formed), further 
spark growth can be adversely effected. The cooler channel tips cool 
down before the topload voltage can recover enough to force additional 
spark growth. And, once streamers begin to load down the secondary, low 
coupling "starves" energy transfer into existing leaders to help keep 
them hot and conductive. The overall result is shorter sparks.

> 
> As for spark gap conduction, If we have a 10KV source, the RSG will break 
> down first at 10KV, now as the gaps move closer the voltage must drop ? In 
> effect the spark gap may not actually turn off until the volts have droped 
> to 8KV. Which would in effect mean we only put 8Kv into the primary ? 

NO. When the gap first fires, it starts the energy transfer process 
between primary and secondary resonant circuits. The gap continues to 
conduct until there's no more energy left in the primary circuit - and 
the energy transfer process continues until all of th system energy 
resides in the secondary LC circuit. At that instant the gap is no 
longer conducting, since the primary current is ZERO - remember COE 
applies. The energy that is initially available (called the "bang 
energy") is 0.5*Cp*(Vp^2) where Cp is the primary tank capacitance, and 
Vp is the voltage across the primary capacitor when the gap began to 
fire (i.e., 10 kV in your example).

Or did
> we pump 10KV as that was the inital fire voltage ? 

Yes. When the gap stops conducting only governs the point where we shut 
off further P->S or S->P energy transfers. See the Quenching" section on 
Richie's site.

I have trouble with this,
> as the spark gap will not quench until the voltage has drops to maybe 8KV, 
> in which case we only trapped 8KV in our primary system ?

Once the gap fires, it will NOT stop conducting until the primary 
circuit energy has fallen to zero (sometimes called the first current 
"notch"). Even though there are brief zero current crossings at the RF 
operating frequency while energy is being transfered to the secondary, 
the gap always reignites. When the gap finally stops conducting, the 
remaining "stranded" primary voltage will be 200 volts or less...

> 
> 
>> You may wish to study the extremely well written write-ups on Richie
>> Burnett's page on TC Operation and Quenching to get a better
>> understanding about spark gap coil operation, energy transfer, and the
>> roles of coupling and quenching:
>>
>> http://www.richieburnett.co.uk/operation.html#operation
>> http://www.richieburnett.co.uk/operatn2.html#quenching
>>
> 
> Thanks for that, I did read it a while ago, but It did not seen to answer a 
> lot of things. I got confused as my small 12V coil testing shows that a low 
> coupling of 10% translates to only 0.1V. On this basis there is a loss. So 
> it is confusing to say the least!

Although TC operation may be a bit complex, Richie's write-up is one of 
the most accurate descriptions on the web. It's worth re-reading it 
until you understand it. Otherwise, you'll be operating under a number 
of significant misconceptions.

> 
<snip>
> 
> Going by the archives, the general thinking Q factor does not matter. 
> However as Q is divided by resistance, a typical coil has around 30 DC ohms, 
> small dia wire will give higher AC resistance. So I would have thought using 
> a resistance of less than 1 ohm would be a far superior design ?

Good Tesla coil design is a balancing act. Since secondary Q is 
proportional to the ratio of its inductance divided by its AC 
resistance, if you can increase the inductance more quickly than you 
increase its AC resistance, Q will increase. Focusing only on low 
resistance does not guarantee that you'll have a coil with a high Q, 
since the wire must be larger, it will have fewer turns for a given coil 
height. And, while DC resistance scales with the number of turns, the 
inductance scales as the square of the number of turns.
>  
> 
> 
>> However, in spark gap coils (be they 2-coil classic or 3-coil
>> magnifiers), it's actually COE (less losses) that ultimately governs the
>>  output voltage. Since each bang has a fixed amount of energy, this is
>> the maximum that can be transferred to the secondary if there were no
>> system losses. Although having a high Q secondary is good design
>> practice and helps to reduce overall system losses, system losses are
>> actually dominated by the spark gap in the primary circuit and streamers
>> (once you achieve breakout).
>>
> 
> 
> This is where it gets confusing. I ran a lot of simulations and found that 
> while inductance is good for voltage gain, resistance goes up by almost a 
> equal amount which makes more inductance actually bad. Tesla even states 
> this in CSN book, Tesla then goes on to say that a large toroid is then 
> needed to overcome the larger inductance.

Be very careful when using simulation tools - you can easily get Garbage 
In - Garbage Out (GIGO). Although maximum voltage gain occurs when you 
have NO topload, longest sparks occur when you have a substantial topload.

> 
> 
>> The Q of a coil is what Tesla sometimes called the "magnification
>> factor". Tesla ultimately wanted to excite his largest systems from
>> continuous (CW) RF sources, but unfortunately the technology to do so
>> did not exist at the time. He knew that the higher the Q (of his
>> secondary or tertiary coil), the higher the output voltage would be when
>> excited from a constant amplitude CW source. A resonator with a Q of
>> 300, when base excited from a low impedance 1,000 volt source, could
>> develop 300 kV at the top (assumings no breakout). This is the "magic"
>> of resonant systems. Sometimes called Q-multiplication, this process is
>> how energy builds in the secondary of most vacuum tube and solid state
>> coils. Even in these systems, the eventual output voltage is governed by
>> a balance of energy: Energy into the power oscillator versus energy lost
>> by system and streamer losses. The "effective" Q of the secondary
>> plummets after breakout occurs, and so does the maximum output voltage.
>>
> 
> 
> http://www.future-technologies.co.uk/IMPULSE/graph/
> 
> My results show that a low resistance works better than a higher inductance 
> and once you get below 1 ohm you really start to double up on the output 
> voltage. While these are simulated tests, this does not include the actual 
> practicalities in construction.

The values you appear to be using in your secondary simulation (10 
turns, 100 (uH) seem to be quite far removed from values used by either 
Tesla or today's researchers. Did you take into account skin or 
proximity effects at the assumed (relatively high?) operating frequency 
in your simulations? Larger diameter wires show larger changes in the 
RAC/RDC ratio than small wires.

> 
> Cheers,
> Chris
> 
> 
Bert
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