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Registered Member #46264
Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61
Finn, your design sounds close to a half torus on the bottom is that right? The half torus bottom is a shape I want to evaluate. Do you have any observations about were the corona appears on the terminal in the dark?
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Russ,
Yes, a half torus at the bottom.
There is hardly any visible corona, except on small imperfections on the surface, like scratches and such. But even the smallest speck of dust renders the sphere lossy, with audible and visible corona leakage as a result. Heep the terminal clean and free from specks, and it delivers the advertized stand off voltage. I once calculated it with Antonios software, probably INCA, you can get it here: but it is probably around 250kV
Registered Member #1521
Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128
Russ Edmonds wrote ...
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.
These edges face directly at a same potential, so there is no charge loss. High current can be only archieved with the belt and charging system, not with the topload. You need to model only a sphere, as the biggest field intensity will be at the sides, where the half spheres are (if the underside is well shaped with flanges, like in the wiki picture).
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
If you want to check your calculations, here is a simulation with my Inca program of a VDG terminal with the bottom formed as a half torus. This is probably not the best shape. It's better to extend the top terminal as a sphere for a greater angle, but the improvement is small.
Registered Member #46264
Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61
Very nice Antonio, could you tell me the dimensions of your terminal ? I can't pick them out from this graphic. Also are you modeling the ground plane? Thanks
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
The diameter is of 10 cm. The ground is at infinity. A sphere with 10 cm of diameter far from other bodies should have a breakdown voltage of 150 kV, assuming a maximum electric field of 30 kV/cm. That shape is reaching 113 kV. The values scale with the diameter.
Registered Member #46264
Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61
Here is my field plot for a half torus bottom on the terminal. My terminal geometry is different from Antonio's. 20 cm diameter and 10 cm hole. I assume 150 KV on the terminal and the ground (0 V) is 50 cm from the center of the terminal. In order to make a consistent comparison between the different terminal shapes I'm going to assume a 20 cm diameter hemisphere on top and a 10 cm hole on the bottom.
Registered Member #46264
Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61
I improved how the ground plane was modeled by setting the outer boundary conditions of my solution domain with an image solution to a charged 20 cm diameter sphere 50 cm above and infinite conducting plane. Before I was just setting the outer boundary condition to 0 V. Here is the best shape for the terminal I have found so far using the constraint of a 20 cm diameter hemisphere on top and a 10 cm opening on the bottom. Terminal voltage is 150 KV.
a bottom radius of 45 mm going into a 5 mm radius 180 degree bend at the opening edge.
Compare this to just a 5 mm 180 degree bend at the opening edge
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E Field calculations for VDG top terminal
