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E Field calculations for VDG top terminal

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Russ Edmonds

Sat May 30 2015, 05:47PM

Registered Member #46264 Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61

I have two 14 inch Ikea bowls to make a VDG top terminal with. Before I cut a hole in one, I would like to know the best shape for the hole edge in terms of minimizing the max E field and ease of making.
I wrote a program to solve Laplace's equation using a finite difference over relaxation algorithm. A test case was setup to calculate the field between two concentric spheres where the exact solution is known. The program matched the exact solution to within 0.05 percent. The attached plot shows the E field magnitude for a 10 cm radius sphere with a 10 cm hole held at a voltage of 150 KV. The center of the sphere is at a height of 50 cm from the ground plane. The current hole edge is 1 mm thick and is not curved in to reduce the E field. I'm working on adding this inward re-entrant geometry to the program.

Dr. Slack

Sat May 30 2015, 06:31PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Don't forget that the TC 'grades' the e-field, so there isn't the high field at the sharp edge you've shown. Do another simulation with this taken into account. You might not have to use the large radius that you see on VDG toploads

Russ Edmonds

Sat May 30 2015, 08:35PM

Registered Member #46264 Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61

This is for a VDG not a tesla coil. Of course some grading rings could be added to the VDG column.

Dr. Slack

Sun May 31 2015, 09:35AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Doh, VDG, not TC, failure to read properly. I wasn't posting while drunk, I was posting while the wife was saying 'stop playing with your geek friends and do something with me!'

Once grading rings have taken care of the field above ground, there is still the field between belt and topload as it passes through the hole to take account of. Here it's a case of large radius good, small radius bad.

You mention an 'ease to make' criterion. The main thing that affects it which you haven't mentioned, but might be significant, is how pretty you want it to look. Alli or copper sticky tape burnished down over almost any former can get you a large radius, but you'll need to invest in metal spinning if you want it to look nicer than that. However, unlike a TC where the topload has to be low loss due to big circulating currents, VDG topload e-field trimming can be very high resistance, due to DC, low currents and high voltages, which rather expands the range of materials you could use in their raw state. I don't know whether wood is conductive enough for the whole top terminal, fat sparks to a single point probably need metal, but it's probably good enough for electrically blunting the aperture, and turning that would be a lot easier than spinning stainless.

While you are in kitchen supply shops, how about a ring jelly mold, if you can find the smooth type, and if the hole's big enough, one that would cast something like this

, and if you can fix it nicely to the right size bowl - a lot of ifs.

Ash Small

Sun May 31 2015, 12:42PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

If the hole is 10cm diameter, then it should be possible to use, for example, four 90 degree copper tubing 'bends', of 2" (~5cm) radius (bend radius, not tube radius), with slots cut into the outer periphery, and silver soldered to the stainless bowls.

If the copper tubing is tinned with solder, the finished result could be made to look quite neat.

It will require the correct aggressive fluxes and silver solder to work, though.

Something like this?

Russ Edmonds

Sun May 31 2015, 01:05PM

Registered Member #46264 Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61

Dr. Slack, lol, that sounds like my wife. My current VDG terminal uses a aluminum ring jello mold for the bottom and a aluminum bowl for the top. I did some hammer forming on both of them to remove some ridges. It's only about 20 cm in diameter, which is why I want to use the 14 inch Ikea bowls. I thought about using wood for the bottom of the terminal but the question still remains: what is the best shape for the bottom of the terminal ? This question is made harder by the fields from the upper roller, column and belt like you mention.

Ash, the 10 cm hole on the 20 cm diameter sphere is only for E field modeling. I am going to scale the results of the program to the 14 inch ( 36 cm ) diameter Ikea bowls.

Ash Small

Sun May 31 2015, 01:59PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Russ Edmonds wrote ...

Ash, the 10 cm hole on the 20 cm diameter sphere is only for E field modeling. I am going to scale the results of the program to the 14 inch ( 36 cm ) diameter Ikea bowls.

Ok, I mis-read it too, but the principle still holds. It would probably require a large pipe bender, though.

omegalabs

Sun May 31 2015, 02:13PM

Registered Member #1521 Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128

I used a flange for this job, bought from here : Flange
But of course, because of the smaller radius, sparks did only appear at the flange, when no other object was around.
The best (and probably the biggest) topload can be made with 4 half spheres and a plate in this style: Link
I'm going to build a big VDG some day and will use a big topload like this.

Russ Edmonds

Sun May 31 2015, 03:13PM

Registered Member #46264 Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61

omegalabs wrote:
I'm going to build a big VDG some day and will use a big topload like this.

I can't see the picture of this.

omegalabs wrote:
The best (and probably the biggest) topload can be made with 4 half spheres and a plate in this style: Link

This design is great for high current but has some sharp edges limiting the voltage. Currently my E field program can not model this shape because it has no vertical axis of symmetry.

Finn Hammer

Sun May 31 2015, 10:06PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Russ,

If it can be of any interest, I had a batch of VDG toploads made at a local metal spinner. They are made from 2mm alluminium, and 340mm diametre, hole is 160mm. Reentrant curve starts downward from the seam with a 50mm radius, then inwards and up with a 40mm radius. top is a semi ellipsoid. Come in 2 parts, with a l-section centering ring. Easy to glue together, and moderate work to finish the seam, as there is a difference in diametre of 0.5mm or so.
I can post pictures if you are interested
.

Cheers, Finn Hammer

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