Re: LTR question

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Steve,


>At 08:50 PM 10/26/2001 -0700, you wrote: 
>I have been reading a lot of posts lately about LTR (lower than resonant) 
>capacitors. This term does not make sense to me. Any capacitor in 
>combination with an inductor will form a tank circuit which will resonate at 
>a frequency determined by the values of the L and C. Is LTR refering to some 
>sort of stagger tuning where the primary tank circuit is resonant at a 
>slightly different frequency than the secondary tank circuit? If so, to what 
>end? In either case, a common suggestion that is offered to achieve this 
>"LTR" is to use a slightly smaller or larger capacitor than one would 
>normally use. The same result could be achieved by merely changing the tap 
>position on the primary inductor rather than going to the trouble of 
>changing the capacitor. Another reason often cited for using this "LTR" 
>value of capacitance is that it lowers the peak voltage that the capacitor 
>is exposed to. This does not make sense to me either. The resonant frequency 
>of the tank circuit should have little effect on the voltage passing through 
>the tank circuit. If stagger tuning is being referred to, then there may be 
>some rationale here. Any insights?
>

Larger Than Resonant cap sizes refer to the 60Hz resonance between the
NST's secondary winding and the primary cap.  This circuit is also a
resonant circuit just like between the primary coil and the primary cap but
it runs at 60Hz (50 Hz) instead of the hundreds of kHz of the primary
circuit.  The primary cap and NST can resonant up to like 80kV if the
system is not drained by the main gap firing.

Traditionally, the primary cap size was selected to be exactly resonant
with the NST's inductance.  A 15kV/30mA NST has an output impedance of:

15000 / 0.030 = 500000 ohms

At 60 Hz, a capacitor with this same impedance is:

500000 = 1 / (2 x pi x F x C)  Where C = 5.305nF

By selecting the capacitor at this exact resonant size, you can get a very
high power transfer through it.  However, there are two big problems.  It
is far too easy to keep widening the gaps or turning up the variac in such
a system which gives greater and greater arcs but it ends when the NST or
primary cap blows up.  Second, if the main gap fails and does not provide a
place for the resonant energy to go, the voltage will skyrocket to around
80kV.  That will destroy something instantly!  Long time coilers remember
well the many many complaints of NSTs blowing up...

LTR coil use a cap size about 1.5 X higher value than the resonant size.
This allows the NST to charge to full working voltage but the capacitance
is so high the voltage will not go much higher.  This removes all the
dangers and problems with resonant rise effects that have destroyed so many
NSTs.  Since the cap size is larger, it stores more energy which makes up
for loosing most the resonant effects.  You have the same arcs but without
many of the dangers to the NST and cap.  LTR coils tend to have better arcs
but they may be due to more careful tuning and cap sizing.  LTR coils like
this are typically used in static gap systems and the BPS rate is around 200.

Computer modeling of standard LTR coils predicted another effect.  If one
uses a carefully timed sync gap, you can pull out the energy stored as
current in the NSTs secondary winding (E = 1/2 x L x I^2) and use it to
help charge the primary cap too.  This is often referred to as the
"inductive kick" effect.  When it was actually tried, the computers were
right and "sync-gap inductive-kick LTR coils" were born.  These are the
most powerful and safe coils that can be built with an NST.  They can
charge very large primary caps (13.8nF for our 15/30 NST) with a very high
safety margin.  Oddly, if the main gap fails in these systems, the voltage
drops about 30% since the gap timing is essential to get the maximum
voltage out of the NST.  Inductive kick coils need rotary or the new
triggered gaps to provide the right timing to the main gap.  Although, a
few folks have managed it with fancy static gaps.

There is a chart of the values at:

http://hot-streamer-dot-com/temp/MMCcapSales.gif

Many thanks to Mark Broker of the Geek group for coming up with these numbers!!

There are also a bunch of technical numbers at:

http://hot-streamer-dot-com/TeslaCoils/Misc/NSTStudy/NSTStudy.htm
http://hot-streamer-dot-com/TeslaCoils/Misc/NSTStudy/NSTtest.xls

Cheers,

	Terry