Re: New NST-sync tests, (Resonant charging effects)
On Tue, 22 Jun 1999, Tesla List wrote:
> Original Poster: FutureT-at-aol-dot-com
> I figured it was time for me to return to an NST based sync gap
> system since my speculations about NST charging were not fitting
> real world data. Now I'm seeing an NST LTR system mystery.
> I decided to set up the system to be similar to Terry's, except I used
> a 12kV, 30ma NST, and a .0147uF cap. This cap is about 2.2 times
> the reso-cap size, which is the same ratio that Terry uses, I think.
> operation was inefficient. It needed a radical shift in the
> sync gap phase. My PT system ran best at around 60 degrees
> ASVP (After the Supply Voltage Peak). Terry's works best at 75
> degrees ASVP. This new setup worked best at 135 degrees ASVP.
> .. a little later, the voltage rise is less. I did not expect this
> 135 degree ASVP phasing requirement.
This makes sense, because your cap size is now larger than that required
for resonance at 60Hz. As a result the phase is radically different. The
cap size was much less than resonant size with your ballasted PT. As
Malcolm pointed out the optimum phase can be calculated knowing the
resonant charging frequency. It is however easier to reference the firing
angle to the supply voltage zero crossing, so I will use ZC (zero
If Tank cap < Resonant size then best firing is just before supply ZC.
If Tank cap = Resonant size then best firing is at or near supply ZC.
If Tank cap > Resonant size then best firing is just after supply ZC.
If Tank cap < Resonant size then little resonant voltage rise but big IK.
If Tank cap = Resonant size then big resonant voltage rise and moderate IK
If Tank cap > Resonant size then little resonant voltage rise & little IK
The further your cap value is from resonant size, the further the optimum
firing angle moves from the supply voltage zero crossing. The graph of
optimum firing angle against cap size is actually an inverse tangent !
You can easily tell if a given ballast and capacitor is resonating above
or below 60Hz by looking at the firing angle.
I hope this makes the whole resonant charging thing a bit clearer.
- In sunny Newcastle.