Ok, I'll keep posting my advice on EHV static machines. Here are some thoughts based on what I'm reading here in the last few posts.
1. _ANY_ sharp points, thin wires, excessive roughness etc on voltage-charged parts will prevent buildup of EHV because such areas result in high field concentrations and will spray off corona -- often invisibly. Smoothness is critical. Even voltages as low as 600V can produce invisible corona discharge from sharp edges or points.
2. Moist air is not actually bad in itself; I've done electrostatic demonstrations in 100 percent RH with rainstorms outside. But moist air leads to invisible moisture adsorbed onto surfaces, particularly dusty surfaces, and this moisture will be conductive at EHV and will short out your machines, preventing charge buildup.
3. Once a conductive path forms, then allowing the machine to discharge along that conductive path, especially on plastic surfaces, causes an invisible carbon trail to form on the surface, which of course remains conductive enough to prevent charge buildup.
4. On VDG machines, the top spherical capacity has a sharp edge where the support column enters. To prevent this sharp edge from killing your charge buildup by corona spray, manufacturers of VDG machines use a "re-entrant" geometry, where this edge is forced up into the sphere itself, where the "faraday bucket" effect will cause the edge to be at a low relative potential and hence not be spraying corona. The only places a VDG machine should have sharp points are the "brushes" (which really aren't brushes because they really should not actually contact the belt.) Don't neglect the bottom enclosure part either; this can be connected to a good Earth ground like a cold water pipe (metal of course!), but if "floating" it too should have geometry and smoothness like the top.
So to get the best performance from your machines:
1. They must be _clean_ and _dry_. No dust must be present, and you can dry them effectively with an ordinary blow-dryer, carefully directing warm air over all surfaces inside and out. Careful not to melt parts! This will work even in high humidity but may need to be repeated when the parts have again adsorbed moisture from the air. I don't think I'd recommend putting the whole thing on a stove to dry it. Of course if you have a temperature-controlled drying oven big enough to contain the whole machine... that's a different story. But just use a blow-dryer; the warm airflow helps to evaporate moisture at the same time it warms the surfaces, and you can direct the airflow easily, like down the inside of the column, etc.
2. If you have invisible carbon trails caused by unintended sparking across surfaces, you may even need to replace the part, or abrade the surface followed by re-smoothing in order to get rid of the trail.
3. Thin wires, whether insulated or not, are not good for conducting your EHV charge to your load objects. Remember, at voltages of tens or hundreds of kV, the only effective insulation is distance. (Ordinary insulating materials are practically useless. Glass and smooth ceramic surfaces, like standoffs, etc. are the first things to accumulate and adsorb moisture from the air and will rapidly become conductive in humid environments.) Use bead-chain with large balls for your flexible "wires" to make connections from the machine to the load. Their resistance is negligible at EHV, and their effective diameter, being much larger than thin wire, reduces corona loss due to small curvature radius of the thin wire. Bead chain is also very flexible even in larger sizes.
4. If you use alcohol to clean parts, be especially careful not to get it on acrylic plastic parts. It will cause crazing, cracking, premature structural failure, and the cracks can even become low-resistance pathways for unintended discharge. Yes, on VDG machines the belt must be kept clean, as it will become conductive itself due to dirt and carbon deposits and will limit or kill charge buildup. Sparking along the belt is a common fault that will of course prevent charge buildup on the top capacity.
5. If your VDG top capacity does not have the re-entrant geometry at the hole, you can still help prevent or limit discharge by the use of "field shapers" around the column, as I illustrate in my VDG tutorials:
http://www.youtube.com/watch?v=r3pgtYQo7VUhttp://www.youtube.com/watch?v=1xcxk0WKo9oAs to the issue of boosting VDG machines by spraying charge onto the belt from a conventional HV supply, yes, that works. Also increasing the capacitance of the "belt" works, see the Pelletron for how this is done.
In several of my demonstrations linked earlier you can see me using ping-pong balls that have been spray-painted with a nickel-containing conductive paint (grey). This stuff is readily available in electronic supply houses as MG Chemicals "Super Shield" :
http://www.mgchemicals.com/products/emi-and-rfi-shielding/other-coating-systems/super-shield-nickel-841I would not expect this to work on styrofoam balls without prior surface sealing, though; it will probably dissolve the styrofoam. The surface of styrofoam balls available at, eg, craft stores is too rough to be usable anyway. And things like Mylar balloons, with very thin metallic coatings, are also not really suitable because every spark will vaporize a pinpoint of the coating where the spark originates/terminates.
There are several companies that supply re-entrant, spun aluminum spheres for VDG machines. You can also improvise: I was using Chinese steel bowls from a restaurant supply house in Toronto, and I even found a bunch of chrome-plated steel baby-moon hubcaps at Active Surplus that were excellent performers (although not nearly as good as spheres with re-entrant holes.) I used hole saws and the lathe to cut the column holes in the bowls and hubcaps, followed by sanding with emery cloth to make the cut edges rounded and smooth.
If your load is not allowing charge to accumulate and voltage to build, use a small spark-gap to keep the load disconnected until the machine has built up enough charge to jump the gap and thus connect the load. You can see me using this technique in some of my videos. (Dirod + neon bank, for example: there is a little spark gap built into one end of the neon bank.)
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