Some of you may remeber that I posted to the list an enquiry about the suitability of GE protective power caps (2X 0.25 uFd @ 13.8 kVAC in series) for Tesla coil operation, probably several months ago now. Bert Hickman thought that they would be quite suitable for Tesla coil duty, so that was good enough for me to give 'em a try. ;^) Well, between being back to working 40 hours a week and what seems like a life time of miserable, rainy weekends, I finally got a good day to try out those caps in my coil. Since I was replacing a 0.1 uFd Maxwell pulse cap (measured C about 0.114 uFd) with a measured C of 0.135 uFd, the coil obviously needed retuning for optimal operation. Since I am forced to run it outdoors in my driveway and the primary tap takes several minutes to loosen and reattach each time I change it, I was really only able to get a preliminary tuning down to the nearest whole turn, but still good enough for a test run.
I have also added 0.33 ohms worth of power resistors on one of the 'hot' 240 volt input legs, in combination with my inductive ballast on the other 'hot' input to my pole pig since the last time I fired it (probably at least a year by now!) I was trying to further 'smooth' the operation of the coil and further suppress any nasty kickbacks that may find their way back to my control panel or even my home's wiring.
Well, she did run fairly well, although it seemed like the added resistance throttled it back a bit. I noticed that even with the variac wheel turned up to around 80, the coil was still only drawing around 60 to 65 amps and it seemd that this was about the 'limit' - (it was drawing around 85 to 90 amps before without the added resistive ballasting and with the original 0.1 uFd Maxwell cap with the variac wheel at this same setting). Of course, even with only 0.33 ohms of added resistance, per the I2R law of joule heating, at 65 amps, thats still about 1400 watts of wasted energy that does NOT make it to those beautiful streamers!
Also, there seemed to be more 'wah-wah' beating of the output with my typical 300 to 350 bps roary gap setting - (noticed this more from observing my panel ammeter flactuate than from the actual tone of the sparks). Never-the-less, the output (and current draw) was definitely smoother and more steady with my original setup. I tried varying the speed of the rotary gap drive to see if I could get out of the beat fluctuations and find a 'sweet spot', but that didn't really seem to make much difference. So it seems that the added resistance gave me the opposite affect than the 'smoothing' that I was looking for.
Anywho, I will probably have an audience next time I run it and the first thing that I will likely do is try bypassing those power resistors. I have never really liked the idea of resistive ballasting wasting power in heat anyway, but I have read that a small resistive component in the ballasting does tend to smooth out and knock the tops off of some of the nasty kickback transients. I have still occasionally observed an occasional spark inside my control panel where you DON'T want to see sparks! That's the only reason that I was trying the resistive ballasting approach.
Those GE protective capacitors DO seem VERY robust, though and never even broke a sweat - can you say 27,600 volts AC rating with never more than 17 kVAC input??!! (plus they have internal bleeder resistors, making them safer than the typical pulse cap) so at least for now, I'm leaning more toward staying with them and working out the few preliminary kinks that I am having with their operation than changing back to my original 0.1 uFd, 75 kV Maxwell pulse cap.
Any comments or suggestions from any of the resident geniuses and/or other experienced pole piggy coilers?
David
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