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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Insulator or dielectric?
They mean more or less the same thing, Things like mica, plastic, glass, oil, SiO2 could all be described as either. The difference comes when you use them in an application.
Where you are just interested in the bulk resistivity, and possibly the mechanical properties, perhaps for protercting wires, or mounting terminals, then you call it an insulator.
Where the dielectric constant matters, both real part (capacitance) and imaginary part (losses), you tend to be using it in a capacitor, or between lines in a fast processor chip, and call it a dielctric.
So a sheet of FR4 GRP board. If you mount mains terminals on it, you'd probably call it an insulator. If you ran a 50 ohm track across it, you'd call it a dielectric.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
AwesomeMatt wrote ...
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?
The full voltage appears between the two sides of the machine, identical in both disks. They are oppositely charged just in the top and bottom, at the space between the neutralizers, but there the voltage is low. The machine can spark across the center if the sectors are too close to the center, or across the sectors (and the neutralizers) if they are too close. The distance between the disks controls the rate of the charge buildup. At some distance the charge multiplication factor falls below one and the machine doesn't work. If the disks are too close sparking can occur between the disks at the top and bottom quadrants, wasting energy. There is an optimum spacing, between these limits. For a small machine it is as close as possible, but for a large machine can be 1 cm or more. Use ball bearings mounted in the bosses that hold the disks. The shaft (upper axle) can be conductive. With symmetrical voltages at the terminals it is at ground potential. The bosses are better if made of insulating material, but this is not critical.
Registered Member #4454
Joined: Sun Feb 26 2012, 12:47AM
Location: Western Canada
Posts: 74
Dr.Slack wrote ... Insulator or dielectric?
They mean more or less the same thing,
So to be clear, while they have different terms in different applications, they are the exact same property, right? There is no case of something insulating well but being a poor dielectric, or vise versa.
Antonio wrote ... There is an optimum spacing, between these limits. For a small machine it is as close as possible, but for a large machine can be 1 cm or more.
Not doubting, but I don't understand how this can be true. Maybe I missed something.
Suppose: - The disc is separated by, oh, 2cm, and, - The sectors are 1cm away from the edge of the disc, and, - The machine will generate 350,000v which normally shorts across the spark gap.
With only 4cm total airgap between sectors from one disc and around to the other side of the other disc, how does the machine not short around the disc edges as soon as the voltage potential is sufficient to do so (long before it bridges the spark gap)?
Registered Member #2018
Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74
Hello Matt
This is a very interesting project indeed I've built two wimshurst machines, one which is extremely simple with CD-Discs, the other one with acrylic glass discs with 30cm diameter.
In practical, the real challenge in building such a big wimshurst machine will be the parallel rotating of the two discs with a gap between them as less as possible. This will be by far the most difficult issue in the building process. Of course there are some other aspects as for example that every conductor must be rounded down at these voltage levels, because the very small power of such a machine shouldn't be wasted witn corona discharges. But in fact, the mechanical aspects are the most important ones.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
AwesomeMatt wrote ...
Antonio wrote ... There is an optimum spacing, between these limits. For a small machine it is as close as possible, but for a large machine can be 1 cm or more.
Not doubting, but I don't understand how this can be true. Maybe I missed something.
Suppose: - The disc is separated by, oh, 2cm, and, - The sectors are 1cm away from the edge of the disc, and, - The machine will generate 350,000v which normally shorts across the spark gap.
With only 4cm total airgap between sectors from one disc and around to the other side of the other disc, how does the machine not short around the disc edges as soon as the voltage potential is sufficient to do so (long before it bridges the spark gap)?
At the 9 hour, and the 15 hour position, where the charge collectors are situated, there is no potential difference between the sectors on the 2 disks, therefore no possibility to arc. The potential difference is between 9hour position and 15 hour position.
The machine works by establishing a charge difference between sectors on opposite plates at the neutralisor position, where there is a relatively big capacitance between these sectors, then move these sectors as far away from each other as possible, lowering the capacitance, and, in turn, increasing the potential.
Registered Member #4454
Joined: Sun Feb 26 2012, 12:47AM
Location: Western Canada
Posts: 74
Kilovolt wrote ... In practical, the real challenge in building such a big wimshurst machine will be the parallel rotating of the two discs with a gap between them as less as possible.
I think my solution to this will be to glue large (1" thick) acrylic circles onto the piece, inside of the sectors near the axle. I can't do that between the discs, but on the outside there should be no size constraints. This will help provide some rigidity along the length and help keep it square and 90 degrees to the axle.
I might even make several such inner disks, even 1" at 48" diameter is a pretty thin wheel.
For me personally, your comments strike a particular note as I often handle the electricals no problem, even the large components easily, but it's a matter of "Okay, now how does it mount/couple?" and that takes up most of my time.
Build a 170VDC power supply from junk? No problem. Few minutes to wire it. Mount it inside a case somehow? Rest of the day.
Finn Hammer wrote ... The machine works by establishing a charge difference between sectors on opposite plates at the neutralisor position, where there is a relatively big capacitance between these sectors, then move these sectors as far away from each other as possible, lowering the capacitance, and, in turn, increasing the potential.
Ahh, so if I've understood correctly, the maximum potential *is* between discs, but 180 degrees across the opposite disc. So, they will be actually close to 48" between max potentials (and decreasing the closer they get, self-solving the issue).
If so, I do understand it better now, yes. Thank you.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
About the distance between the disks, I made this machine, where the distance can be easily adjusted: Both driving pulleys are at the back side, with one turning the axle, fixed to the front disk, and the other the back disk. Four ball bearings are used, two to hold the axle and two in the boss fixed to the back disk. I tried also the sectors covered by an insulating layer and accessed through buttons. The idea worked very well.
Registered Member #4454
Joined: Sun Feb 26 2012, 12:47AM
Location: Western Canada
Posts: 74
Antonio wrote ... I tried also the sectors covered by an insulating layer and accessed through buttons. The idea worked very well.
Oh I see now. Interesting. I presumed the amount of current pulled off the wheels was related to the amount of brush contact area (bro-science heard elsewhere), but it seems evident from your experiments that the size of the sectors determines current per speed and the pickups only need to make small amount of contact to transfer the charge.
Sadly, aesthetically, I think having visible sectors is desirable. If I could cover them with a clear film (48" wide packing tape) I would do that. It would keep the sectors nice and purdy but, I guess it would look like old packing tape after a while.
I think I'm getting the hang of this.
On spacing between sectors...
Maximum voltage is at 180 degrees on a wheel, right?
So, the space between sectors must cumulatively add up to more than the desired spark gap, else it will short across the disc (another reason to laminate an insulator over them), right?
Is that basically the only reason you need a bigger wheel? Just to have sufficient space between sectors to actually get that arc, or are there other elements of physics in play?
I'm curious about the number of sectors. Why have, for example, 24 instead of 240? What impact does the number of sectors have on a machine?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
If you want to see what a machine looks like in the limit of an 'inifinite' number of sectors, have a look at the Bonnetti machine. It's basically a sector-less Wimshurst. The drawback is that whereas a wimshurst will self-start, a Bonnetti needs to be started (needs a few kV to get the cross conduct brushes emitting, whereas the metal contact on the wimshurst cross conductors will conduct from nearly nothing). Ceratinly from a looks point of view, a winshurst is a better proposition for a gallery.
There are discontinuous points along the wheel, such as the reverse side where neutralisation happens. I'm under the impression that the gap between sectors needs to be a minimum amount how ever many you have, so it's a fight between arcing, corona and having enough metal to move the charge (unless you are spraying as in the Bonnetti machine).
I've suspected for a while that because charge is being moved against a voltage gradient and then is collected on the inside of a conductor that Van de Graaf action is happening too. Antonio's point about the angle of the neutralisers would seem to support that theory.
I think a lot of theoretical factors are less important than rules of thumb and trial and error, since what is going on is not simple.
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Giant Wimshurst Machine - could use some guidance
