Breakdown Voltage of Air at given Distance

WBahn

Joined Mar 31, 2012
29,978
Technically the coil is AC because its a transformer, but the "forward transformation" is insignificant compared to the flyback pulse that occurs when the LT circuit breaks and the field collapses.
So are you saying that the SPARK is AC?

For that matter, at what point in the cycle does the direction of the current in the coil reverse?

NOTE: This gets into the semantics of what is meant by AC and DC.
 

Johann

Joined Nov 27, 2006
190
Although we talk about the gap in air, it is worthwhile to note that when testing transformer oil insulation, water in the oil will have an effect of premature arching. This is one method to determine that either water or some other foreign matter has contaminated the oil and changed its dielectric strength.
From this I deduct that moist air would perform in a similar way, thus arching across will take place at a reduced voltage.
 

ian field

Joined Oct 27, 2012
6,536
So are you saying that the SPARK is AC?

For that matter, at what point in the cycle does the direction of the current in the coil reverse?

NOTE: This gets into the semantics of what is meant by AC and DC.
Like it or not - the ignition coil is a transformer with a primary and a secondary, but as I stated clearly enough for (almost) anyone; the forward transformed voltage is insignificant compared to the flyback pulse when the primary current is interrupted.

The secondary voltage *DOES* make both positive and negative excursions, but the positive excursions are orders of magnitude too small to jump any gap.
 

WBahn

Joined Mar 31, 2012
29,978
Like it or not - the ignition coil is a transformer with a primary and a secondary, but as I stated clearly enough for (almost) anyone; the forward transformed voltage is insignificant compared to the flyback pulse when the primary current is interrupted.

The secondary voltage *DOES* make both positive and negative excursions, but the positive excursions are orders of magnitude too small to jump any gap.
So, again, where in the cycle does the CURRENT in the coil reverse polarity?

And most ignition coils (at least traditionally, I don't know about modern systems) only have a single winding -- they are autotransformers. But no one is claiming that they aren't transformers.
 

WBahn

Joined Mar 31, 2012
29,978
Yes. And AC and DC are steady state.

A spark would be considered a transient. So, neither.
That's the definition that YOU are choosing to apply. But what is meant by AC and DC is very context sensitive. YOU are choosing to claim that they only apply in pure steady state and that anything else would be a transient. Most people that take such a narrow view also insist that even in steady state that AC ("alternating current", after all) requires that the direction of current flow must alternate between one way and the other.

The wider and more general view is that any waveform can be decomposed into the superposition of a DC component and an AC component where the DC component is simply the average value (over the time frame of interest) and the AC component is simply everything else. What you choose to call "a transient" can be fully represented as a DC value plus an AC signal and, if we want, we can represent the AC signal as the superposition of a set of pure sinusoidal signals. But sometimes we don't care so much about overall DC and AC components, we are concerned with separating the transient and steady state components. So there we do something similar -- we first determine the portion that is the steady state response and then everything else is the transient response. And both of those can be broken down into DC and AC components if we wish.
 

joeyd999

Joined Jun 6, 2011
5,234
That's the definition that YOU are choosing to apply. But what is meant by AC and DC is very context sensitive. YOU are choosing to claim that they only apply in pure steady state and that anything else would be a transient. Most people that take such a narrow view also insist that even in steady state that AC ("alternating current", after all) requires that the direction of current flow must alternate between one way and the other.

The wider and more general view is that any waveform can be decomposed into the superposition of a DC component and an AC component where the DC component is simply the average value (over the time frame of interest) and the AC component is simply everything else. What you choose to call "a transient" can be fully represented as a DC value plus an AC signal and, if we want, we can represent the AC signal as the superposition of a set of pure sinusoidal signals. But sometimes we don't care so much about overall DC and AC components, we are concerned with separating the transient and steady state components. So there we do something similar -- we first determine the portion that is the steady state response and then everything else is the transient response. And both of those can be broken down into DC and AC components if we wish.
It's been about 100 years since I've argued with a professor, and tonight I am too tired to have at it. I'll just say that your assumption of what I think is wrong, and your interpretation of AC/DC/steady-state is worthy of future discussion. Just not here, not now.
 

WBahn

Joined Mar 31, 2012
29,978
My assumption of what you think was that you think that AC and DC are steady state. I can't image where I got that idea. I guess whoever told me that didn't know what they were talking about. :D
 

cmartinez

Joined Jan 17, 2007
8,218
It's been about 100 years since I've argued with a professor, and tonight I am too tired to have at it. I'll just say that your assumption of what I think is wrong, and your interpretation of AC/DC/steady-state is worthy of future discussion. Just not here, not now.
If the two of you decide to have that long discussion in a bar... then drinks are on me if you let me watch Emoji Smiley-78.png
 

WBahn

Joined Mar 31, 2012
29,978
If the two of you decide to have that long discussion in a bar... then drinks are on me if you let me watch View attachment 91703
It's a discussion that has happened many many times before over the last couple of centuries. It tends to have all of the major defining characteristics of a religious war -- and, like any good religious war, it can definitely be fun to watch.
 
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