Re: coupling losses ? (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Sat, 22 Sep 2007 10:08:25 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: coupling losses ? (fwd)

Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Sat, 22 Sep 2007 11:19:39 +0100
> From: Chris Swinson <list@xxxxxxxxxxxxxxxxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Re: coupling losses ? (fwd)
> 
> 
> HI Bert,
> 
>> 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.
> 
> the SISG seem to output a spark length at 100watt input compaired to my 500W 
> tesla coil, it was looking to me like the main losses were in the spark gap. 
> a couple 100 volts isn't going to matter that much if thats the drop over a 
> spark gap.

Remember that the peak tank current can be many hundreds of amperes, and 
many systems use a series of two or more spark gaps. Since EACH gap has 
its associated voltage drop, a system that uses 5-10 air cooled static 
gaps will have a combined voltage drop of the order of 1000-2000 volts. 
Since this occurs while passing several hundred amps of tank current, 
the peak power dissipated within the gaps can be 300 - 600 kW. Spark 
gaps make robust HV switches, but they ARE lossy when conducting.

Since the voltage drop across a series of IGBT's in an SISG is a 
comparatively small fraction of this, significantly more energy makes to 
the secondary. When designed properly for the coupling coefficient of 
the coil its working with, the SISG can also quench perfectly at 
whichever current notch is desired. It can also operate at a minimum of 
2X line frequency (like a synchronous gap). All of these combine to make 
an SISG coil extremely efficient at producing long sparks.

> 
> 
> 
>> 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...
> 
> Right, so in effect once the gap fires at 10KV, you can pretty much say its 
> a wire link until current drops to zero and the "wire link" cannot be 
> anylonger maintained.

Yes - but actually more like a pair of 100-200 volt Zener diodes 
connected back-to-back in series. Once the gap fires, they'll continue 
to conduct current as long as the applied voltage is greater than the 
gap sustain (similar to Zener) voltage.

> 
>> 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.
> 
> 
> skin effect , or resistance, was a lumped sum of resistance, All I did was 
> increase each factor one at a time to see what would happen. I suppose I 
> should relate it to AC resistance which might not be so linear ?

Skin effect and proximity effects can combine to cause the AC resistance 
to be 3-6 times (or more) than the DC resistance for a single layer 
coil. Neither are linear functions of frequency or turns, and 
unfortunately, there is no simple closed form equation that can be 
applied to quickly calculate proximity effect. A fairly accessible 
practical treatment for estimating proximity effect in single layer 
coils can be found in Terman, "Radio Engineers Handbook, 1943, 
McGraw-Hill. It takes a bit of patience, combined with using a number of 
tables, but I've done it for a couple of coils.

> 
> cheers,
> 
> chris
> 
> 
> 
> 

Bert
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