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Re: [TCML] Telsa coils + capacitor bank = ?



Hi Josh,

I'll give it a try...  :^)

Josh Bailey wrote:
Hi Bert;

I recently made friends with Greg Leyh and attended the recent Norcal Teslathon - 'twas grand (I made my first tiny coil back in July so have had a... rapid introduction to the field).

It looked like quite a fun event! And Greg's lab makes me drool... :^)


While there we worked on using Greg's largest coil to strike a graphite rod, connected to ~100uF of capacitors charged to ~20kV.

I had some naive questions about what we observed, and Greg suggested you might have some interpretations? I'm a network engineer by trade so please forgive being a newbie - just want to understand what is happening.

Here are two series' of photos (frames from an MJPEG capture running at about 30 fps):

http://vandervecken.com/images/capbanktest1/
http://vandervecken.com/images/capbanktest2/

Q1. What conditions must exist for the capacitor bank to discharge? We noticed that sometimes the graphite was struck multiple times before it discharged. We also noticed that sometimes the bank was not fully discharged (still had 3-4kV on it).

In general, the brightness of a spark is a proportional to the amount of current passing through it. The base of air discharges from a Tesla Coil (TC) is the brightest and hottest part since the sum of all the branching discharge currents pass through the main channel. As you move away from the terminal, the discharges begin branching, with each branch carrying a portion of the total current. As the current flowing through more distant branches becomes progressively lower, the channel diameters become smaller, and the discharges become more purplish and dimmer. The tips of the discharges transition to a bluish glow (the streamer-corona region) where the gas temperature is barely above room temperature.

Unconstrained sparks and arcs in air have a negative resistance characteristic: the more current you force through them, the hotter their cores become, and the lower their electrical resistance. Because the more distant channels are cooler and smaller in diameter, these remote channels are considerably poorer electrical conductors.

When a Tesla Coil is operating near the limit of its spark length, you may see sparks sporadically/weakly connect the TC terminal to ground without the characteristic brightening that we usually associate with TC ground strikes. In this case, although the spark has indeed bridged the gap, the combination of high resistance from the discharge path, and the fact that we are close to being max'ed out on TC terminal voltage, prevents any major increases in channel current - even though we've actually bridged the gap. This will occur for regular coils and those using a high energy capacitor bank for secondary boost. It takes a significant increase in channel current to start the runaway process that culminates in the hot arc that's necessary to discharges the capacitor bank: the increased current heats the channel, reducing the channel resistance, which further increases channel current, further heating the channel, etc. etc.

While a barely connecting spark may not provide a sufficient current "bump" during the final jump to initiate the runaway process, a slightly "stronger" spark will. Once a bridging arc has been initiated, it will then persist, discharging the capacitor bank pretty much independently of further Tesla Coil operation.


Q2. What is the sequence of events at discharge?

Assuming that we've bridged the gap, initiated the runaway process, and formed a bridging arc, the energy in the capacitor bank begins flowing through the secondary, terminal, and arc, completing a series LC circuit that oscillates at a low audio frequency (that you can easily hear when the capacitor discharges). The combination of secondary winding resistance and arc resistance eventually dissipates most of the stored energy from the bank.

During each oscillation, the arc current passes through zero, temporarily extinguishing the arc. Usually it is quickly reignited by the high voltage from the capacitor bank. As the energy in the system decreases, so does the arc current, causing the arc resistance and overall arc voltage drop to rise. The voltage drop across the arc is also proportional to its length. As the bank energy decreases, a point is eventually reached where the arc can no longer be reignited, and the arc goes out for good.

Once the arc finally extinguishes, residual energy will still be left in the capacitor bank. This is typical behavior for an LC and spark gap system. Because of the relatively long arc involved in Greg's system, the stranded bank voltage is proportionally higher. You may also see significant variation in residual bank voltage if the Tesla coil continued to operate during the bank discharging process, since it would assist in reigniting the arc channel.


Q3. The arc changes colour several times (before and after the strike). What's going on there?

There may be several things occurring here. The arc itself changes color depending on how much current is flowing through it. Low current arcs tend to be purplish, transitioning to a whiter color and then a brilliant blue white at increasing current levels. In addition, the carbon rod that was used as a target may contain metallic salts in the core and a copper jacket - these may add other colors to the arc, particularly at the end of the arc nearest the carbon rod.


Q4. I was thinking about trying to get a better photographic record of what's happening - other than longer lenses/faster cameras etc - please might you have any suggestions on how to better capture what's going on?

Part of the problem is the huge optical dynamic range - you may need to use 2-3 cameras simultaneously, each with different neutral density filters in order to capture the entire discharge sequence. The main arc channel is primarily a black body radiator. However, you may be able to detect various spectral lines emitted by vaporized electrode materials and excited gases near the arc core using a slit and a prism or diffraction grating. Good luck!


Thanks,


Bert
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