Re: [TCML] Spark gap comparisons

[FutureT@xxxxxxx]

In a message dated 2/23/2008 3:33:39 P.M. US Eastern Standard Time,  
bartb@xxxxxxxxxxxxxxxx writes:

>Hi John,

>It's not surprising to me they performed about  the same. The sync rotary 
>and the trigger gap both are controlled  timed events of the mains 
>frequency. So in that respect are similar.  The rotary however does have 
>the adjustment of where to fire along the  waveform and I suspect the 
>trigger was not adjustable (to fire after  peak, maybe before peak). 
 
Hi Bart,
 
I'm not surprised that the results were similar, but I was  surprised
the results were the same because there is a view that a very  wide
gap such as the triggered gap used, would cause noticeably larger
losses.  Apparently the losses were similar in both cases, or  are
not really that important in this coil.  Both the rotary and the  
triggered gaps had an adequate range of adjustable timing.  



>Triggered gaps are also prone to the triac not turning off  periodically 
>without adding some extra circuitry to ensure turn off.  There are 
>probably some slight differences between the two gaps but  the cap 
>voltage was probably very similar.
 
The firing seemed very steady, at least as steady as the rotary.
I agree the cap voltage was probably similar.  

>I think  gap comparisons are very difficult to make especially trying to  
>compare different types of gaps, especially comparing a static gap to  
>either a rotary or a triggered gap. They are different beasts and the  
>static gap firing voltage is very dependent on thermal regulation,  
>surface, size, etc. where a rotary is not (even a trigger gap is not  as 
>it's firing is independent of the electrode geometry or  temperature).
 
My point was that despite any differences, the results (spark  lengths)
were the same for both gap systems.  This seems to suggest  that
the exact design of a gap is not too critical provided that it's a good  
design and suitable for the coil in question.  
To me it is significant that two such widely differing gap
systems gave the same spark lengths.  Regarding the static  gaps
that I tried, none were able to equal the spark lengths and overall
performance of the rotary or the triggered gap, in the TT-42 coil.   
Certainly for a coil that needed a higher break rate to perform,
neither this rotary nor this triggered gap would have performed  well.  

>I'm not sure I understand what you mean about rotary gaps not  capable of 
>firing at peak due to voltage rise upon electrodes  approaching. Can you 
>give more details to your hypothesis?
 
It has to do with the way the electrodes approach each other as the
voltage is rising across the gaps as the cap charges.  Depending  on
the electrode speed, the gaps may ionize and cause a premature
firing.  Certainly rotaries fire before the gaps align because of  the
excessive voltage, but this would be something beyond that effect.
Rotating the phase to try to delay the firing, may then prevent the
rotary from firing at all.  This would depend very much on the
way the voltage rises.  I've seen some NST systems where the
voltage rises, then levels off before firing, but in other systems  it
fires right as it's rising, yet a delay in phase will stop the  firing.
It seems to depend on both the particular NST and the cap value.
 It's a little hard to explain the overall 
concept in words.  It was something I had considered in an  attempt
to explain the results of equal spark lengths despite the very 
wide gap (high losses?) of the triggered gap.  Actually Mark  
Reszotarski had done an analysis of spark gap losses due to
gap width, and found that wide gaps caused very large losses.
It was around that time that I happened to do the comparisons,
and my thinking was along those lines.  I do not know of any
other comparison tests between a rotary and a triggered 
gap that were done, so I thought it might be of interest to those
who are thinking about various gap systems and their losses
and their advantages or disadvantages. 
 
John
 
 
Take care,
Bart

FutureT@xxxxxxx wrote:
> In one  experiment, I compared the performance of a 2 gap 120 bps sync   
rotary
> to a single 120 bps triggered static gap.  In both cases I  got a  42" max
> spark length using my TT-42 TC.  Using the  rotary, the gap light and  sound
> emitted was quite low.   Using the triggered gap, the gap light and  sound
> emitted was  intense.  The gap spacing when the rotary fired was  small.
>  The gap spacing of the triggered gap was large at 5/8".   Unfortunately  I 
> don't
> remember if I measured the firing  voltage in both cases, but input  power
> draw was the same.   The triggered gap
> may have been capable of firing at a higher voltage  for some reason.   I 
have
> speculated in the past that  rotaries may not be capable of truly firing  at
> a peak voltage  because of the way the voltage rises as the electrodes 
>  approach.  I'm not sure if that's a valid idea.  If both  systems  fired 
at the
> same voltage, then it's interesting that  they both gave the same  spark
> length despite the very wide  triggered gap spacing.  One might say  that
> the 2 gaps of  the rotary introduced extra losses.  Yet in other   
experiments
> I compared the performance of 2 gap and 4 gap rotaries,  and didn't  see
> a difference in spark length output.   Perhaps some slight  quenching
> differences occurred such that the  negative and positive aspects
> canceled.  Various multi-pipe gaps  were tried on this coil and the  sparks
> were considerably shorter  and more feeble looking.  I reported all  these
> results in  the past.  I'm re-posting them for the benefit of new list  
>  members.
>  
> John
>    








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