LTR cap explanation? / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

LTR cap explanation?

Move Thread

LAN_403

NickCesar

Mon Dec 15 2008, 06:25AM

Registered Member #1799 Joined: Thu Nov 06 2008, 02:20AM
Location:
Posts: 23

I can't seem to find any good explanation as to why the LTR cap size causes more power throughput. I did the static.sch Microsim from richieburnett.co.uk and it doesn't show increased power throughput at LTR size. It seems like most of the links I find that would have good explanations are all dead links.

NickCesar

Mon Dec 15 2008, 02:16PM

Registered Member #1799 Joined: Thu Nov 06 2008, 02:20AM
Location:
Posts: 23

This is interesting. I'm modeling a 15kV 60mA NST (663H 7400R) I have the input voltage set to 15k*sqrt(2) =21213 and frequency set to 60Hz. If I set the breakdown voltage of my gap to 15kv, instead of 21213, with a 16.6nF cap it has a perfect 120bps.

Power Throughput and Bang Size Comparisons:
BPS x (1/2)CV^2
120 x (1/2)16E-9*15000^2 = 216W = 1.8J LTR
60 x (1/2)16E-9*21213^2 = 224W = 3.735J
240 x (1/2)10.6E-9*15000^2 = 286.2W = 1.2J Resonant
150 x (1/2)10.6E-9*21213^2 = 357.74W = 2.38J
120 x (1/2)13.4E-9*21213^2 = 361.79W = 3J

Am I doing something wrong?! It seems like my true LTR is 13.4nF ? Which is not conducive with the (pi/2)*(resonant) LTR size!!

Can someone please help me? Is my microsim setup incorrectly?

Steve Conner

Mon Dec 15 2008, 05:20PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

First of all, no double posting inside of 48 hours please :P

Second, LTR isn't the value that gives maximum power throughput, as far as I know. The special property of LTR, and the reason why Terry Fritz promoted it, is that it doesn't resonate the NST terminal voltage above its unloaded value.

A tank cap resonant at your line frequency gives virtually infinite power throughput, which means in practice that you just keep opening up your spark gap and getting more power until you destroy your NST from overvoltage.

NickCesar

Mon Dec 15 2008, 07:03PM

Registered Member #1799 Joined: Thu Nov 06 2008, 02:20AM
Location:
Posts: 23

Sorry about the double post!

In a static gap, LTR is supposed to be the capacitance value for 120bps operation.

I'm well aware of what the effects of using a resonant size capacitor are. The capacitive reactance is just "canceling" out the inductive reactance and current is only limited by the winding resistance and if energy is not removed from the system a resonant voltage rise can very quickly occur. It wouldn't be very special if all it did was prevent resonant voltage rise and through all the old documents and pupman list threads I've been sorting through, LTR is supposed to provide a high level of power throughput that is atleast comparable to or greater than the power throughput of using a resonant size capacitor. I think it has something to do with the bps being 2x line frequency, but I'm not sure, which brings me back to my original question.

Hazmatt_(The Underdog)

Tue Dec 16 2008, 01:05AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Here ya go man:

LTR optimizes with power throughput at about 1.4 to 1.5 the resonant cap size. The hard part for most people is finding their extracted equavilent impedance of the source, and it is quite difficult if you don't have measurements of open/short circuit conditions.

I still need to post more on all of this, but my issue is working 55hrs a week, my projects, and energy to do it. I'm trying to find the time.

Hope this helps.

Matt

NickCesar

Tue Dec 16 2008, 02:58AM

Registered Member #1799 Joined: Thu Nov 06 2008, 02:20AM
Location:
Posts: 23

I had read your post earlier and was like oh cool! and then I realized I wouldn't be able to find my equivalent impedance.

The simulation is showing greatest power throughput at the 13.4nF cap size even though it isn't "LTR" or resonant. It is definitely showing it operating at 120bps though, so I may just go with the 13.4nF cap.

Hazmatt_(The Underdog)

Tue Dec 16 2008, 05:11AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

wellp... if you can find your resonant impedance of the transformer, 1.4x is about all you can do. Anything more would require some analysis or a decade capacitor of sorts.

Getting close is probably good enough anyway. I was looking for some emperical answers to the "why" and that's what I found, which holds true.
Hope the info helps a bit.

GeordieBoy

Tue Dec 16 2008, 04:00PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

First of all, static gaps _usually_ fire quite chaotically, so this makes assessing the relative merits of different tank cap sizes quite difficult. In my work I found that capacitor size was not all that critical with a static gap and anything around the resonant value worked okay, provided you don't open up the spark gap too far.

In the calm and controlled world of the synchronous rotary spark gap, things are much more well behaved. For 100bps (120bps in 60Hz countries) I found that the optimum capacitor size for NSTs was around 1.4 times the matches size for resonance at the power line frequency. Even this isn's so clear cut, as there is one value that gives you maximum power, and another that gives peak voltage, and yet another that gives optimum power factor. The latter meaning "biggest sparks for minimum line current draw."

For 200bps systems (240bps in the states) I found that the equivalent capacitor value was around 70% of the value required for resonance at the line frequency. The lower capacitor value in this case reflecting the reduced time for it to recharge between presentations in the rotary.

Really it is not the relationship between the capacitor size and the values that resonates at the line frequency that is important here. It is really the actually resonant charging frequency that is important here in determining how the resonant charging takes place. What I am saying here is that for any given BPS speed in the RSG there is a corresponding optimum resonant charging frequency which gives good power throughput and good power factor. If you change the motor speed and alter the BPS, then the optimum resonant charging frequency changes.

Likewise it is the characteristic impedance of the resonant charging circuit that defines the power throughput. So if you bolt together two identical NSTs, you should double up on the tank capacitance to keep the resonant frequency the same, and half the characteristic impedance. All things being well-behaved that will process twice the power with the same peak-voltage stress on the caps and NST, and with the same power factor.

There's lots of stuff about this on my web page, although I haven't done any work in this area for about 8 years now.

-Richie,

NickCesar

Wed Dec 17 2008, 04:57AM

Registered Member #1799 Joined: Thu Nov 06 2008, 02:20AM
Location:
Posts: 23

I've read through your website very thoroughly and feel like I have a pretty good understanding of what is being said. I just find it very interesting that I'm getting the most power throughput with a capacitor value that isn't "LTR" in the standard sense nor is it the matched resonant size.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob