Hi Bert,
I always enjoy getting your input on this subject matter, as I always seem to acquire more info that I was previously unaware of. I was completely unaware that there are TWO LTR methods of running a coil system. From your description, the inductive kick LTR sounds pretty persnickety, to say the least! Sounds like standard LTR is the way to go and I assume that is what most coilers are referring to when they speak of LTR tuning. I learn something new every day ;-)
Seasons greetings,
David
Sent from my iPhone
> On Nov 22, 2018, at 12:27 AM, Bert Hickman via Tesla <tesla@xxxxxxxxxx> wrote:
>
> Hi Dan,
>
> If the safety gaps are set properly, they should seldom fire in a stable LTR system. If your safety gaps are firing frequently, partial cancellation of the NST current limiting function and main resonance are likely occurring.
>
> There are two types of LTR approaches: standard LTR and inductive kick LTR. A good discussion of standard versus inductive kick LTR systems (versus classical mains resonant systems) can be found in the archives:
> https://www.pupman.com/listarchives/2000/August/msg01085.html
>
> A regular LTR system just barely charges the tank cap to the maximum rated output voltage of the NST twice on each mains cycle. Either a static or rotary gap works well with this setup. The tank cap is ideally sized to 1.57 times the mains resonant value, so for your 15/120 system, this would be about 33.3 nF.
>
> Because of the heavier capacitive load on the NST, the size of the PFC capacitor bank can be significantly reduced or PFC can be eliminated entirely. A standard LTR setup with a much smaller PFC bank is the configuration I would recommend. The PFC bank may, in fact, be causing part of the problems you're seeing.
>
> If you want to use an inductive kick LTR setup, the tank cap size can be estimated using the following formula:
>
> C = 0.83*I/(BPS x V)
>
> Where:
> C = LTR Tank cap (Farads)
> I = NST bank short-circuit output current (Irms, in amperes)
> V = NST output voltage (Vrms)
> BPS = Break rate (2X mains Hz or 120 BPS for your system)
>
> For your 15/120 NST bank the ideal LTR size works out to be about 55 nF or about 2.6 times the resonant cap size.
>
> However, there are a couple of significant disadvantages using an inductive kick LTR system. It requires a properly phased SRSG where the gap fires 3-4 milliseconds after zero crossings. More importantly, the SRSG can be difficult to "start". The tank cap will only charge up to about 30% of the NST open-circuit peak voltage if the gap doesn't initially fire. However, if the gap does start firing, then the tank cap voltage will significantly increase to near the level of a standard LTR setup. This occurs as energy (stored in the magnetic field in the NST's secondary inductance) provides an inductive "kick" that boosts tank capacitor voltage. But this can only occur IF the gap starts firing at the right time.
>
> If your rotary gap is configured to have a total of four gaps in series (two sets of gaps located 180 degrees apart on the disk), the total SRSG breakdown voltage may be too great to "start" your LTR system. This sounds like the problem you encountered when trying to drive your Maxwell 60 nF cap. To alleviate this, you may want to use only ONE set of stationary electrodes (i.e., only 2 total gaps). This will allow you to maintain adequate mechanical clearance while also allowing the gaps to initially fire at the reduced starting voltage in your LTR system.
>
> I don't recommend an inductive kick LTR approach since its benefits may not justify the added setup complexity and gap firing difficulties.
>
>
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