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Coupling vs. Racing Arcs
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.
=========================================
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.
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.
This may also, explain why out of tune coils will arc but when they are
brought into tune the arcs disappear.
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