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Registered Member #4454
Joined: Sun Feb 26 2012, 12:47AM
Location: Western Canada
Posts: 74
Eleccentric wrote ... Reducing capacitance isn't going to be too difficult with Leyden jars, depending on how you construct them.
Yeah, it's just a function of surface area so, increase or decrease that and I'm set. I'm building a prototype to tinker with, so, lots of tinker time.
Was actually considering using aluminum duct foil (not duct tape, the better stuff). Which reminds me, I was also thinking of using that on the wheels for the metal sectors. Any reason not to? Alum creates that continuous black AlumOx ruboff. Any reason aluminum is usually used? Any reason not to use a more aesthetically pleasing copper or brass or phosphor bronze, or a more durable stainless steel?
And speaking of materials.. not set on wheel material either. Plexi, acrylic (fragile), polycarb, glass (heavy and maybe dangerously fragile at the speeds it'll be moving, shrapnel hazard). Any suggestions?
wrote ... An 18" discharge isn't necessarily deafening, but the more capacitance behind that spark, the louder it will be. The goal should be to use the least amount needed to achieve your desired spark.
Heh. Until recently I was thinking the opposite. Spend as much of the budget here as possible, since it will create the hottest and brightest and most intense sparks. When weighing time/effort of cranking the wheel hard enough, I was tentatively pessimistic about that. When weighing safety I backpedaled, and now weighing noise I'm in full retreat to in line with your comments.
wrote ... This means absolutely no sharp points or bends in your HV conductors (except your collection combs), and though this is less critical for the neutralizers, you don't want sharp points on anything anywhere near your lovely gently curving mirror-smooth conductors.
I've been trying to figure out how much of skin effect occurs for this kind of thing. If it's 1/5 to 1/15 hz, there's no RF in the conventional sense, but, the sudden discharge and, the whole deal with it being static has me uncertain.
For conductors I was considering copper or brass pipe for aesthetics, but also simply welding cable (can buy it in thick segments and is easy to bend). I was also considering making a twisted cable out of many strands of enameled wire (basically emulating Litz wire). Is there any point in that? Any ballpark size for my conductors? I was figuring half inch just to be safe.
wrote ... Ground in this sense is going to be halfway in potential between the two sides. If you want to attach it to earth ground, do so to the outer foil of both Leyden jars. This will equalize the voltage on each side of the machine, reducing corona and ion wind discharges to surrounding objects.
They asked me about a safety ground and, I was mostly sure it didn't need one really, because it's not mains powered, but I thought there might be some other reason to do it.
Do I have to worry about visitors touching a single conductor, or just getting caught between both conductors? With static charges, it seems that, relative to the visitor, either side is still at a massive charge difference and will discharge to or from either end.
wrote ... Consider using chain drives or enclosed gear drives, as most belt materials will break down into a fine dust that will stick to everything,
Well, another hobby is making things from junk bicycle parts, so, I'm all aboard with that. It's actually what the original plan was, for the wrong reasons, then moved into belts, and now probably back full circle. Oh well, right things for the right reasons.
Antonio wrote ... The polarity of the machine is really random, unless some form of precharge is used. Terminals with two balls allow adjustment of the terminals so the positive terminal inclinated in the direction of the negative works as an asymmetrical gap.
Ahh, so stacking the small balls on top of the large let it discharge whichever way allows spark quickest? Or, are you saying they have to be adjusted every single time the wheel is cranked?
Would it be possible to cheat and just hook a low voltage power supply up to the caps (or a DC generator also driven by the machine, or... something) to ensure one terminal to always be positive and the other negative? I don't care if the contraption is built on a cavern of lies, if it helps the overall result.
Thanks again for the feedback, I'm learning tons. You guys are great.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
This here paper may help you out. It has both Wimshurst theory and plenty of practice and practical that goes as far as engineering drawings, optimizing disk speed and size, choice of materials for conductors and insulators, single and multiple disk machines, comparison with belt-type electrostatic machines - in a word, the whole nine yards:
Cosman, B.J., Disk Type Electrostatic Generators MIT MSc Thesis, May 1937
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Antonio, that's the machine I had in mind. Note the big balls added to the conductors behind the machine.
I feel I don't know enough about Wimshurst machines to understand why the cross conductors on that one are not at 45 degrees, like they are on almost every other machine I've seen.
As the machine produces a balanced bipolar output, 'ground' is neither terminal. If you look up formulae for capacitance a 'sphere' has ground at infinity, the integral does converge and you do get a finite capacitance. The formulae for two concentric spheres has some 1/R terms on the bottom, and what happens is once the enclosing sphere gets much bigger than the internal sphere, it is the diameter of the inner that dominates the value.
The formula you need is the concentric spheres one, with the inner sphere being whatever you add to the conductors, and the outer 'ground' sphere being a crude average of the radius of the enclosing room, and a theoretical 'zero voltage' virtual ground plane between the positive and negative spheres. It would be impractical to compute an analytical value for the outer radius, but it's bounded above by half the room diameter, and below by half the distance between the added spheres, and doesn't affect the value much, so you don't need to be too careful. You might worry that the room isn't really a ground conductor, but it has wires and pipes in the walls, and building materials of normal moisture content are perfect conductors compared to megavolts and microamps of the Wimshurst.
There are two problems to this project a) getting it working b) still having it working in a year's time, or ten year's time
I'm sure you'll manage the first, a bit of advice, enthusiasm, care, no trouble.
Still having it working in a year, when dust has settled, people have put greasy finger-prints on it, both of which attract surface moisture, and all the charge is leaking across the insulators before it can jump across the gap, will be your biggest issue.
You might want to consider exactly how the machine is physically built, so that it's either a) very easy to keep clean or b) very easy to dismantle and reassemble for a thorough clean from time to time.
Will there be anybody in the Science Centre to 'love' it, give it care and cleaning, when you're not around?
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
Proud Mary wrote ...
This here paper may help you out. It has both Wimshurst theory and plenty of practice and practical that goes as far as engineering drawings, optimizing disk speed and size, choice of materials for conductors and insulators, single and multiple disk machines, comparison with belt-type electrostatic machines - in a word, the whole nine yards:
Cosman, B.J., Disk Type Electrostatic Generators MIT MSc Thesis, May 1937
This work is very interresting, but it deals with a version of a Van de Graaf generator using a disk instead of a belt. The charging system is electronic. Some of the theory and construction details are valid for a Wimshurst machine too, of course.
AwesomeMatt wrote ...
Ahh, so stacking the small balls on top of the large let it discharge whichever way allows spark quickest? Or, are you saying they have to be adjusted every single time the wheel is cranked?
Would it be possible to cheat and just hook a low voltage power supply up to the caps (or a DC generator also driven by the machine, or... something) to ensure one terminal to always be positive and the other negative? I don't care if the contraption is built on a cavern of lies, if it helps the overall result.
The terminals have to be adjusted depending on the polarity. They have to be adjustable anyway, because the machine will not reach the same maximum spark length every time, depending on humidity, pressure, temperature, cleanliness, etc. Precharge can be done by spraying charge from a HV power supply through a point opposite to one of the neutralizers. It's more practical to adjust the terminals.
Dr. Slack wrote ...
I feel I don't know enough about Wimshurst machines to understand why the cross conductors on that one are not at 45 degrees, like they are on almost every other machine I've seen.
As the machine produces a balanced bipolar output, 'ground' is neither terminal. If you look up formulae for capacitance a 'sphere' has ground at infinity, the integral does converge and you do get a finite capacitance.
The angle of the neutralizers controls the maximum voltage, high angle increasing it, up to a point where the machine ceases to work because the charge increasing mechanism stops. In a large machine the neutralizers can be almost vertical. The capacitance of an ideal sphere far from other bodies is ~55 pF per meter of diameter. It's directly proportional to the diameter (an not to the surface area, as it may look).
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
Proud Mary wrote ...
Interesting to see that a patent for an improved Wimshurt machine has been granted in this century, in 2002, in fact:
I have seen this patent before. Apparently a Wimshurst machine with several pairs of neutralizer bars instead of just two. Would rise the output voltage faster, but would produce less voltage due to the decreased insulation and at most the same current of a conventional machine, as it's not possible to increase the maximum charge density in the disks. The proposed structure seems very strange, with a partition between the disks and the sectors mounted on the sides of the disks that face each other.
AwesomeMatt wrote ...
Was actually considering using aluminum duct foil (not duct tape, the better stuff). Which reminds me, I was also thinking of using that on the wheels for the metal sectors. Any reason not to? Alum creates that continuous black AlumOx ruboff. Any reason aluminum is usually used? Any reason not to use a more aesthetically pleasing copper or brass or phosphor bronze, or a more durable stainless steel?
And speaking of materials.. not set on wheel material either. Plexi, acrylic (fragile), polycarb, glass (heavy and maybe dangerously fragile at the speeds it'll be moving, shrapnel hazard). Any suggestions?
Aluminum foil or adhesive tape for the sectors is easy to find and easily replaceable. Aluminum oxide does not appear to be a problem. Copper, bronze and brass are good too, but tarnish easily. Stainless steel would be ideal, but is more difficult to find in thin foil. May be more difficult to glue precisely too. For disks, I have used only acrylic. Policarbonate probably works too, or any other rigid plastic. Glass is heavy, difficult to cut and drill, breaks easily, and may be too hygroscopic, and so conductive (use Pyrex glass).
Registered Member #33460
Joined: Tue Aug 27 2013, 06:23PM
Location: Seattle
Posts: 46
AwesomeMatt wrote ... Was actually considering using aluminum duct foil (not duct tape, the better stuff). Which reminds me, I was also thinking of using that on the wheels for the metal sectors. Any reason not to? Alum creates that continuous black AlumOx ruboff. Any reason aluminum is usually used? Any reason not to use a more aesthetically pleasing copper or brass or phosphor bronze, or a more durable stainless steel?
Aluminum is cheap and easy to cut. So long as your neutralizer brushes are soft enough the oxide rub off shouldn't be a problem. I use carbon fiber brushes from printers, normally used to remove charge from the paper - they are exceedingly soft and springy, and plenty conductive at these voltages. Other than being more prone to tarnish and corrosion, copper or brass will work fine as well. Stainless will be more difficult to work, but it perhaps the most durable choice of easily available sector materials.
More important than the metal the sectors are made from is the corona loss from the edges of the sectors. They have a sharp edge all around. I have seen designs that laminate the sectors between two thin discs, and then only make contact on the edge of the disc or through a small raised bump that protrudes through the outer laminate surface.
AwesomeMatt wrote ... And speaking of materials.. not set on wheel material either. Plexi, acrylic (fragile), polycarb, glass (heavy and maybe dangerously fragile at the speeds it'll be moving, shrapnel hazard). Any suggestions?
I have used vinyl, bakelite, acrylic, and HDPE. All could be made to work, but the best results by far were with HDPE. According to R.A. Ford's book "Homemade Lightning" the most ideal material would be G-10 fiberglass laminate, which is very similar to material used for making printed circuit boards. HDPE is somewhat hygroscopic, but G-10 is minimally so. Of course, these suggestions won't work if you want to stick to transparent discs.
AwesomeMatt wrote ... I've been trying to figure out how much of skin effect occurs for this kind of thing. If it's 1/5 to 1/15 hz, there's no RF in the conventional sense, but, the sudden discharge and, the whole deal with it being static has me uncertain.
For conductors I was considering copper or brass pipe for aesthetics, but also simply welding cable (can buy it in thick segments and is easy to bend). I was also considering making a twisted cable out of many strands of enameled wire (basically emulating Litz wire). Is there any point in that? Any ballpark size for my conductors? I was figuring half inch just to be safe.
The skin effect does not apply in this case. Litz would be a waste of time and money and likely be a source of leakage. The rapid discharge will create some RF, but not until the spark has already appeared. Large diameter metal pipe will work very well, with gently curved bends and any joints buried inside spheres, ideally with involute edges - like the way the underside of a Van de Graaff generator's terminal curves inward around the insulating pipe/pedestal/belt support.
If you want to use cable, go for high-voltage rated silicone insulated wire, not welding cable. It is easily available at voltage ratings of 100kV, and I imagine that some searching could find it at even higher ratings. You could also sleeve it in appropriately sized silicone tubing to add more voltage strength.
On my best-performing machine I use 1" diameter copper pipe. The larger, the better. But, the smoothness of the joints is more critical. A tiny fleck of metal shaving on the terminal of an otherwise perfect Van de Graaff can completely spoil the output, as it bleeds off the charge.
AwesomeMatt wrote ... Do I have to worry about visitors touching a single conductor, or just getting caught between both conductors? With static charges, it seems that, relative to the visitor, either side is still at a massive charge difference and will discharge to or from either end.
YES. While the output of this machine will almost certainly not be lethal to a healthy person (in my nonprofessional opinion), it would definitely be very painful experience for anyone, and could be disastrous for someone prone to seizures, with a pacemaker or implanted defibrillator, or otherwise with some sort of neurocardiogenic vulnerability.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I was interested to see in the Disk Type Electrostatic Generators paper I posted above, the authors' use of conductors made of wood sprayed with zinc paint.
I don't recall seeing zinc spray cans on sale, but there is certainly conductive nickel metal spray available for instant screening and shielding.
As we are only talking μA with these electrostatic machines, this antique method could still be very useful to the home constructor like myself who has a small wood lathe, but not one for working with metal.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
Zinc paint exists too Wood is a so bad insulator that in some applications can be safely used as conductor without treatment. Wood can be dried in an oven and boiled in insulating resin, or just paraffin, becoming a reasonable insulator. Alessandro Volta made an electrostatic machine entirely in wood (but didn't leave good documentation of it).
Registered Member #4454
Joined: Sun Feb 26 2012, 12:47AM
Location: Western Canada
Posts: 74
Proud Mary wrote ... This here paper may help you out.
I gave it a read. Hard to tell sometimes what they're talking about in particular, but it was low level enough that I made some sense of it. I liked how it was written a bit as a trial-and-error thing, so you got to see why things were the way they were.
Dr. Slack wrote ... Will there be anybody in the Science Centre to 'love' it, give it care and cleaning, when you're not around?
Well I won't be "around" ever really, but, yes, there is a staff, presumably maintenance staff too. I'll be sure to have a chat with them about regular maintenance and how that affects the design now.
Antonio wrote ... They have to be adjustable anyway, because the machine will not reach the same maximum spark length every time, depending on humidity, pressure, temperature, cleanliness, etc.
Initially what they were looking for was a perfect, calculated design they could send off to a fabricating lab. One that would engineered perfectly with the proper numbers from the proper formulas and then just built.
I talked them out of that immediately. That's just not how these things work. Almost everything needs to be adjustable, tweakable, etc. It's not like this is ballistics or chemistry where you just know and calculate everything ahead of time. The machines are finicky and the science is fudged. Any numbers used would be approximations and a lot of the constants aren't constant.
I also pointed out that interacting with the machine and seeing how changes influence it would be interesting to visitors, but I don't know if I was as convincing in that.
I suggested if they are worried about it, to just set the spark gap to some unambitiously close worst-case amount such that it always fires. I.E. Design for 24", set it to 18". Or, make it variable.
wrote ... The angle of the neutralizers controls the maximum voltage, high angle increasing it, up to a point where the machine ceases to work because the charge increasing mechanism stops. In a large machine the neutralizers can be almost vertical.
Fascinating. Another one of those things I not only didn't know the answer to, I didn't even know it was a question to be asked.
wrote ... Aluminum foil or adhesive tape for the sectors is easy to find and easily replaceable. Aluminum oxide does not appear to be a problem. Copper, bronze and brass are good too, but tarnish easily. Stainless steel would be ideal, but is more difficult to find in thin foil. May be more difficult to glue precisely too.
Well I'm sure if I shop around I can find whatever I need. Even if it's just stainless foil that I have to superglue myself.
wrote ... For disks, I have used only acrylic. Policarbonate probably works too, or any other rigid plastic. Glass is heavy, difficult to cut and drill, breaks easily, and may be too hygroscopic, and so conductive (use Pyrex glass).
Glass scares me. High edge speed, sudden catastrophic failure, etc. No thanks. Luckily they agreed.
I have a dumb question about the term "Dielectric". Is it synonymous with "electrical insulator"?
I see them used somewhat interchangeably, but not entirely. In some contexts it seems as if you want to avoid using a material with a high dielectric value but a low insulative value. In other places it appears as if when you have one, you have the other.
Eleccentric wrote ... I use carbon fiber brushes from printers, normally used to remove charge from the paper - they are exceedingly soft and springy, and plenty conductive at these voltages.
I had a copier tech and fellow HV enthusiast suggest the same thing to me. I'm very much leaning that way now, as I have a bottomless supply of copying machines.
wrote ... More important than the metal the sectors are made from is the corona loss from the edges of the sectors. They have a sharp edge all around. I have seen designs that laminate the sectors between two thin discs, and then only make contact on the edge of the disc or through a small raised bump that protrudes through the outer laminate surface.
Hrm, you lost me.
By edge you mean, if the sectors were paper, it would be the edge that gives you the papercut, yes?
That is because, being foil, and thin, they are a sharp (narrow anyway) edge?
If so, I'm good until the lamination part, then I can't picture it.
wrote ... According to R.A. Ford's book "Homemade Lightning" the most ideal material would be G-10 fiberglass laminate, which is very similar to material used for making printed circuit boards. HDPE is somewhat hygroscopic, but G-10 is minimally so. Of course, these suggestions won't work if you want to stick to transparent discs.
Ahh, I'm familiar with G-10, it's used by knifemakers when building prototypes.
I'm fairly sure, but not positive, that transparent discs are highly desired, if not an absolute need. It functions largely as a display piece.
wrote ... The skin effect does not apply in this case.
I didn't think so, but, I know there's contexts that I'm missing, so sometimes I get suspicious and ask about things I don't think matter.
wrote ... On my best-performing machine I use 1" diameter copper pipe. The larger, the better.
Do you (or would it be beneficial to) fill the pipe with any additional conductor (wire), or is the resistance of the pipe low enough to not be a concern?
I can easily source some large copper pipe, probably as large as required. 1-2" seems reasonable.
What is your method for bending the pipe smoothly? I was going to pack with sand and wrap around an appropriate mandrel. I've tried the water-->ice fill method and for short lengths it's basically no better than empty. I could borrow some conduit benders but they have such strict radii.
...
One additional question that's come to mind...
The spacing between wheels. Across the sides of the wheels is the full voltage potential. It doesn't appear that machines are built with the wheels a sparkgap's distance between them, but, how then do they not short? Do I need a larger insulator on the wheel?
Related, I also read that the farther apart the discs, (and the thicker their material?), the lower their power generating capacity. So, seems that you want to put them as close together as possible without becoming a hazard.
Oh, and one more... attaching the wheels to an axle. Are there issues with bearings and the shaft being conductive, or is the middle of the wheel, by geometry, sufficiently far away from the HV to not mess with it?
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Giant Wimshurst Machine - could use some guidance
