Hi All,I’ve made a few mods to the ‘40ft coil’, to see how far the working tower materials can be reliably stressed, and to resolve some of the remaining mechanical issues.
Originally the secondary tower had nine 44in sections, for a total secondary length of about 33ft. The additional base height + toroid spacing + toroid height increased the overall height to about 40ft. The total secondary length was driven by what maximum voltage gradient I thought the tower materials could reliably withstand. Maybe I was a bit conservative though, since I've yet to see any surface flashovers at full voltage [~3MV.]
Given that the focus of this design study is to minimize costs, I tried removing one of the 44in secondary sections, and driving the remaining 8 to the full 3MV. No flashovers! So, I removed yet another section and drove the remaining 7 to 3MV… Still no flashovers! Maybe the toroid size helps more than I thought towards keeping the E-field uniform.
To get more real world stress data, I left the secondary at 7 sections for the entire trip last July to Wardenclyffe and Little Rock AR. The wilds east of the Mississippi proved to be a suitably harsh environment… with 95F days, and humidity over 90%!
Despite the adverse conditions, I didn’t see secondary flashovers at the Wardenclyffe coil demo, nor at the Little Rock demo, nor at the demos back here in Alameda. So now I’m compelled to get more runtime with the secondary at 7 sections, making it now the ‘32ft’ coil.
Although there were no secondary flashovers, the primary drive circuits suffered damage at both demos from wayward ground strikes -- both times during the finales while cranking the power to maximum.
We couldn’t determine the exact chain of events for the failure at Wardenclyffe. However in Little Rock, Terry Blake managed to capture [in the same image!] the arc attachment point and the exact moment a fault indicator light lit up on the primary drive box… all in wonderful hi-res slow motion!
The arc briefly attached to the corner of one power bank, which I had thought was tucked sufficiently close to the tower base. The secondary surge current then traveled down the DC mains and in to the contactor box, where it thoroughly fried the solenoid of the contactor associated with the bank struck in the video.
I’ve now added strike guards to all of the equipment on the ground.Ground strikes can deliver fairly high peak currents. I’ve measured surges of over 500A to grounded test points. For reference, the 280pF top capacitance stores over 1.2kJ at 3MV.
Another major problem we had at both demo sites involved the mechanical tower retraction. When the tower retracts during takedown, three friction drives uniformly guide the tower core back into the coil base. However the extreme humidity in both locations completely lubricated the friction drives, leaving us with the only choice of vertically ‘crash landing’ the tower. Fortunately, at both locations, there were enough brave volunteers to catch the entire tower and toroid as it vertically collapsed to the base!
Since returning from the trip to the harsh eastern climates I’ve been working on a positive drive retraction scheme that doesn’t rely on friction. I just finished it over the holidays and it seems to work okay, so I’m planning more coil tests at the full 3MV to make sure that the positive drive elements I added to the tower can take the full voltage gradients at 3MV.
If you’re in the area, stop by! I’ve put up the testing dates and details on http://www.lod.org/events.html
Cheers, GregPS: Here's a report of the Wardenclyffe trip if you haven't seen it, detailing msmts of Tesla's original grounding system and showing some of the ground strikes: https://youtu.be/QdZIuB9K6zE
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