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Re: Coupling vs. Racing Arcs
Hi Terry,
> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
> Hi All,
>
> I would like to submit the following as a possible mechanism for racing
> arcs being worse with higher coupling. I will use an analogy of a spring
> mass system to describe this.
>
> Imagine a spring mounted upside down that has a significant mass. This
> spring and its mass will have a natural oscillation frequency due to the
> spring rate and the mass. This is analogous to a secondary coil and its
> self capacitance. Now we add another mass to the bottom to represent the
> top terminal. This additional mass will lower the frequency of the system
> just like our top terminals do.
Interesting analogy. I use a steel ruler in a bench vise with a mass
attached at the top.
> =========================================
> I ground
> ---------
> | |
> | |
> | |
> | |
> | | spring with
> | | mass
> | |
> | |
> | |
> | |
> | |
> ---------
> I
> --------------
> | | top mass
> --------------
>
> In order to drive this system, we induce a movement at the base (the top
> in this case since it is upside down) to push the system into oscillation.
> The further the top mass moves, the high the voltage in our analogy. The
> primary system basically acts to induce a small movement at the base that
> is rung up into a much larger movement at the top terminal.
>
> Now, if there is too much movement at the base, the windings of the
spring
> will touch and we can consider that as being like an arc condition. In
> other words, we pushed too hard. The "coupling" controls mostly how hard
> to push. Loose coupling gives a gentle push that takes a long time to get
> the secondary moving. Tight coupling give a much harder push that gets
> thing going much faster. However, couple too tight and you get an arc.
Well, if the transfer is largely lossless, k shouldn't make any
difference to the final amplitude if premature breakout doesn't
occur. This suggests to me that wiith an improved efficiency due to
greater k in the real case, secondary amplitude is increased beyond
what the windings can stand. That this can occur in a free standing
resonator with sufficcient primary energy also suggests this is the
case.
> It has been noted that adding a larger top terminal will reduce arcing.
> This would have the effect of lowing the frequency of the system and
> allowing the spring more time to transmit it's energy to the system before
> the windings would crash. Thus, a larger terminal will reduce arcing.
A larger terminal also = a lower voltage across the resonator for a
given Ep. Also may be better shading the windings.
> This may also, explain why out of tune coils will arc but when they are
> brought into tune the arcs disappear.
I've made several resonators flash over their entire length.
> While it is true that further on in the firing cycle a given place in the
> spring will be moving a lot, Later in the cycle the whole system is moving
> as well and thus there are no small tight spots in the spring. Once the
> oscillation is setup and everything is moving smoothly, you can have very
> high displacement without windings hitting. It is just at the initial
> start where too much movement will simply push the base windings together.
>
> Well that's it. That was the easiest way I could think of to explain the
> idea. Of course, all this has a direct correlation to a real coil system
> that is electrical instead of mechanical.
>
> One possible test for this, is to see where in the firing cycle the
racing
> arcs occur. If they occur very quickly, just after the gap fires, it would
> tend to support this. In other words, the initial jolt will be the most
> likely time for the windings to be over voltaged as opposed to further
> along in the cycle. Unfortunately, my big oversized coil cannot do racing
> arcs so I can't check... :-(
>
> Thoughts, comments, other :-)
>
> Terry
I think it's more likely they will occur when most of the energy has
arrived at the secondary. Anyone taking bets? I'd like to know.
Cheers,
Malcolm