Re: SRSG break rate

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

On 21 Jun 01, at 18:28, Tesla list wrote:

> Original poster: "Bill Vanyo by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<vanyo-at-echoes-dot-net>
> 
> After reading Steve's original question, I'm still confused by the
> answer.  I never gave it much thought before, but his question (or
> rather the lack of an answer that I can grasp) has me puzzled.  Suppose
> one break is at a positive peak.  Now, after that break, there is
> positive voltage for a quarter cycle, then negative for a quarter cycle,
> then the next break at the negative peak.  Won't the positive and
> negative 1/4 cycles cancel each other out, as far as charge delivered to
> the cap, before the next break?  It seems if you were to use 120bps (for
> 60hz), or one break per half cycle, you would want the breaks to be at
> the 0 voltage points.  This way, between breaks, you have either all
> positive or all negative voltage, getting as much charge to the cap as
> possible.

The cap has to be discharged while it still has a high terminal 
voltage. That rules out firing at zero voltage. What really happens 
is this: suppose your gap fires when the cap voltage hits a peak (say 
90 degrees after the zero voltage crossing point). Momentarily while 
energy transfers are going on in the TC itself, the ballast 
inductance of your transformer is being fed current from the 
transformer in a closed loop. It is storing some energy during this 
period. When the gap opens again (as the transformer voltage is 
climbing back down, the ballast energy together with the transformer 
begin recharging the cap. The recharge can be quite significant and 
allow another gap fire in an async rotary system if the breakrate is 
fast enough. Suppose however that the gap does not (more-or-less) 
realign until the next transformer peak (180 degrees hence). The 
energy stored in the paritally recharged cap is cycled through the 
ballast and combined with more energy coming from the transformer to 
recharge the cap with the opposite polarity. Note that this mechanism 
can boost charge voltage beyond the opencircuit voltage of the 
transformer if the gap timing allows it to. So the energy is not 
wasted, instead it is circulating between reactive components in a 
tuned circuit (primary cap and ballast inductance).
 
> Of course, I'm new at this, and either:
> a) I don't know what the heck I'm talking about, or
> b) Everybody already knew this.
> 
> I'm interested, because I'm thinking of going to rotary (synchronous,
> with NST's), and I want to get it right.
> 
> BTW, is the voltage/current at the secondary of my NST's in phase with
> the mains?  If not, is it always out of phase by the same degree? 

Phasing depends on cap size and leakage inductance (ballast) is the 
simple answer.

 I'm
> just trying to get a handle on how to adjust the phase of a synchronous
> rotary gap, without fancy equipment I don't have (like oscilloscopes). 
> I don't want to fry my NST's.  If I use the fluorescent-light-as-strobe
> technique, the light will flicker in phase to the mains - hopefully the
> same as the phase of the NST's high voltage side.

Be aware that adjusting the phase of a sync gap to peak coil output 
is doing exactly the same thing as opening up a static gap and could 
have the same consequences. This is very easily demonstrated. Place a 
safety gap across your sync rotary and set it to fire at the open 
circuit voltage of the transformer with no capacitor attached. Now 
run the coil, adjust your sync gap phase and the static gap will let 
you know when you are trying to push the transformer margins.

NSTs are not designed to withstand lightning strikes whereas pigs 
have a designed-in tolerance to this sort of abuse. Pig deaths are 
rare in the electrical world.

Regards,
malcolm