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Re: Best cap size for a sync gap



Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Mark,

At 09:44 PM 4/29/2004, you wrote:
>On Thu, 29 Apr 2004 16:49:38 -0600, Tesla list <tesla-at-pupman-dot-com> wrote:
>
>>Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> I doubt that the nature of 
>>the load is significant, as the charging is done before the gap fires and 
>>the load is connected.
>>
>>I think the issue may be that the "recommended" value may have been 
>>derived from simulations which do not take into account the non-linear 
>>behavior of the NST when the volume is set to "11".

Exactly!!  "Normal" running LTR coils match up to the recommenced LTR cap 
sizes very well.  But if things start to saturate, all bets are off!!


>I would argue that simulations are fairly poor for any transformer 
>regardless of "volume setting...."  I have yet to see any SPICE package 
>correctly simulate a transformer with measured R and L components.  Case 
>in point if I short (nearly) the output of a "pole transformer" model 
>using the info in Terry's NST PFC Test spreadsheets, I get a primary 
>current of only a few Amps (23A in my model).  When I simulate an NST 
>whose outputs and inputs have been measured (except actual open load 
>voltage), I never get the same short circuit current to within 30%.

It "should" work, but there are some tricks.  The biggest problem with NSTs 
is finding the odd coupling factor.  You may be interested in this:

http://hot-streamer-dot-com/TeslaCoils/OtherPapers/TeroRanta/


>I recall that the "recommended" values were arrived at empirically.

They were computer modeled first and later confirmed.  LTR coils were first 
predicted by computer modeling and later confirmed to actually work for 
real.  Sort of the first "triumph of computer modeling" in Tesla coiling ;-))

>Terry would be able to explain in more detail, but simulations were used 
>to "what if," then MMCs were used to test and tune.  The best performance 
>seemed to happen at about 2.6 * Cres, which was assuming some resistive 
>losses in an RC filter.  Lately the trend has been to go for 3+ * Cres, 
>which I haven't seen any evidence to support other than SPICE.

The predicted value is pi (3.141...).  But losses lower that.  Best to use 
the calculators that take into account many real world limits.  Far static 
gap LTR, pi / 2 seems to work fairly well.


>I think results of the same test with the input set to 120V would be 
>telling as well.  ;)

Yes!!  That may return the NST to normal LTR operation.  Maybe we need to 
run NSTs off 240VAC variacs to reach a voltage to saturate the cores!!  I 
am not joking there...


>Thanks for the work, Gary!

Yep!!!

Cheers,

         Terry


>Mark Broker
>Chief Engineer, The Geek Group