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Giant Wimshurst Machine - could use some guidance

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Hazmatt_(The Underdog)

Sat Jun 07 2014, 06:01PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I think you're going to need to test your PVC capacitors. They're probably very very leaky.

If you can find a large 1 or 2L beaker and turn those into Leyden jars, you would probably be better off.

But test the PVC ones, or better yet, get some doorknob caps, they would be a slam dunk. You would probably need 4 of them rated at 50KVDC, because when this think works, it's going to throw a big spark.

Static dissipation is your biggest enemy on these things. Wood, plastic, paper, glues... you want to have all of your resistances very very high, higher then 10M Ohms.

AwesomeMatt

Sat Jun 07 2014, 08:00PM

Registered Member #4454 Joined: Sun Feb 26 2012, 12:47AM
Location: Western Canada
Posts: 74

wrote ...
Do you have an estimation of how fast the discs can rotate before they are an explosion risk?

Nope. Sounds like a problem for an engineer.

wrote ...
Do you have an estimation of how fast a thirteen year old boy with attention deficit disorder can turn a hand crank?

13 year old? Bah.

They regularly host "Adult's Night" where they open the bar, everyone gets drunk and makes stuff and plays with the exhibits. And by "plays" I mean "tries to set records on how fast/high/far/extreme/etc they can get the exhibits to perform."

Do I have any idea how fast a team of drunken college boys can spin it?

Well, I invested several years of research into the matter one upon a time, for science...

*This* version won't be an issue. It won't be touched by the public, and the pulleys will slip rather than accellerate.

wrote ...
How close to this sort of spark can a digital watch get and survive? Will the device need a faraday cage?

I remember back in high school, a Van Der Graff generator reset my watch date & time to zeros.

The enclosure is specifically not my problem, some higher ups have already decided it doesn't need a cage or enclosure, just a rail. They've not been swayed by my opinion so, they can deal with that.

...

wrote ...
But test the PVC ones, or better yet, get some doorknob caps, they would be a slam dunk.

Part of the purpose of me building this, is to experiment with capacitance values to discover the sweet spots between the following variables:

1. Energy of spark (bigger is better)
2. Loudness (as big as possible without causing hearing damage)
3. Timing (5-15 seconds between sparks for an average visitor)

Experimenting is taking the largest Leyden jars and gradually taking a razor blade the outer foil on the leyden jars and peeling it off.

For what they'll be spending on it, I think I will just suggest they purchase a pair of capacitors custom-made from a shop. But I need to find them a value or a range first.

Eleccentric

Sat Jun 07 2014, 09:17PM

Registered Member #33460 Joined: Tue Aug 27 2013, 06:23PM
Location: Seattle
Posts: 46

The voltage strength of PVC is highly variable. If this design doesn't work out, I see you have a number of 5 gallon buckets sitting around - which are made from a much more suitable material: polypropylene. Here's an image of a smaller machine creating a very nice spark for its size using 5 gallon buckets as Leyden jars:

You would be covering a much smaller area of the buckets, so perhaps two or more on each side would work better.

You are correct in your thinking that the voltage will be shared between the two Leyden jars equally. You don't need to worry about the foil ablating or vaporizing from the current under normal use. If you were to draw a spark directly off the foil, though, that would increase the current density at the point of the spark sufficiently to vaporize a tiny bit of the metal.

I decided to investigate the possibility of using carbon-fiber core spark plug wires as a source of brush material, and am disappointed. The one I cut open and played around with appears to be made from nonconductive fibers impregnated with carbon powder. This is totally unsuitable, as the powder would stick to the discs and eventually short out the sectors. I washed a section of the fibers and they eventually turned white. The resistance of the same bundle went from about about 5k to 5Meg after after washing

Ash Small

Sat Jun 07 2014, 10:27PM

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

Eleccentric wrote ...

I decided to investigate the possibility of using carbon-fiber core spark plug wires as a source of brush material, and am disappointed. The one I cut open and played around with appears to be made from nonconductive fibers impregnated with carbon powder. This is totally unsuitable, as the powder would stick to the discs and eventually short out the sectors. I washed a section of the fibers and they eventually turned white. The resistance of the same bundle went from about about 5k to 5Meg after after washing

Thanks for checking this, Eleccentric. I did experiment with home made resistors a decade or so ago, and tried this then, but I did notice it didn't 'feel' like carbon fibre, and some of the black came off on my fingers (It did make good resistors, though).

Maybe Patrick has some carbon fibre offcuts he could post to Matt?

I PM'd Patrick and he replied "i have a shit load... 1x3, 2x4, cloth, uni-directional, just a shit load."

PM him, and he'll post you some.

AwesomeMatt

Sun Jun 08 2014, 01:16AM

Registered Member #4454 Joined: Sun Feb 26 2012, 12:47AM
Location: Western Canada
Posts: 74

wrote ...
I see you have a number of 5 gallon buckets sitting around

They were on sale for $1. I bought 8 of them just in case they'd come in handy for this.

I don't have lids for them though. I could probably go buy some. Or whatever... I've spent probably $1200 in materials so far, $4/bucket is trivial.

They're only 13" tall though. So, no way for me to get 18" of separation (the mouth of the bucket is ~11", so, I could steal 5 inches to center, leaving 13" of gap down the outside.. I'd get just the bottom plate covered. Better than zero I suppose.

The PPE is very thin though. Very very thin.

What I've been thinking of doing for a week now, as my backup plan, is stacking one bucket inside another, to double up the insulation and add some airgap too... but I thought I might lose almost all my capacitance that way (plates too far apart).

Ordering thicker acrylic or PVC is also an option.

I really want to try the garbage can idea you or whoever came up with. Just to create some comically maniacal thunder. Break some windows maybe :p

On a related note, I bought a decibel meter last year. I imagined using it at a party to confirm to police that in fact, the music was *not* too loud. Instead, I will use it for science!

wrote ...
PM him, and he'll post you some.

Neat, except, I don't have time to wait for things to arrive. Everything is go go go. 1/2 the stuff I've been buying and trying for all kinds of parts on the Wimshurst is in the mindset of "I need to make *some* progress today" or "I need to do whatever might get it done today/tomorrow." This where I'm clearly trying a worse solution when I already know the better one.

Budget isn't an issue really. I paid $100 for some O-ring cord yesterday...

Ash Small

Sun Jun 08 2014, 01:27AM

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

If you can't get any carbon fibre locally, surely he can post it for next day delivery?

If he posts it Sunday, you'll get it Monday, or tuesday at the latest.

Surely you have 'next day delivery' in the States?

EDIT: Even if you have to use aluminium foil or something now, you'll have the carbon in a day or so.

AwesomeMatt

Sun Jun 08 2014, 03:00AM

Registered Member #4454 Joined: Sun Feb 26 2012, 12:47AM
Location: Western Canada
Posts: 74

wrote ...
Surely you have 'next day delivery' in the States?

Canada.

It's atrociously expensive. Like, over a hundred for a letter. Plus it has to clear customs and the like.

Plan is, get it working with what I can, ASAP, then just do an order and replace all the junk with better stuff.

...

Progress:

- Tried to find a 5/8" drillbit to drill out the new 6" pulleys (1/2" axle) so I could throw a 1/2"-5/8" bushing on them. 21/32" is the closest I could find. Nope. Abandon that.
- Figured that I might as well try 2x the drive torque by doing the crossed belts thing (both pulleys are drive pulleys, versus before, one was slipping and all torque came from a single large pulley).
- Took off old belt. New belts had to be built in-place, because there's no way I'm pulling those discs down by myself to get the belt around the upper axle. Thankfully the lower axle is easy to remove. I'm re-using the washer fluid hose rather than the Oring, because I'd rather practice and destroy the $15 hose than the $100 Oring cord.
- Tried adjusting the pulleys, they're stubborn. The keyway is damaged from set screws being oversized.
- Belt #1: Good! Very tight, barely able to stretch it onto the pulley.
- Belt #2: Impossibly tight, starting to fray. Might not have waited long enough for the superglue, or might just not be long enough.
- Belt #2: Throw away, build a new one.

What a difference! 1/2 the pulley ratio and 2x the drive friction (both pulleys are powered by the lower axle now).

It's a sensible ratio. 90% of my power isn't going into heating up a slip pulley without a bearing (not unsolvable, but crossed belts has no slip pulleys to even deal with). It's 1/2 the total belt length so less stretch and slop (no horizontal pulleys). The belts are far, far tighter because I pushed them harder. Just everything is superior.

And I can get it going fast enough that wobble becomes a problem. Probably 180 RPM? Terrifying fast. Something's ticking regularly and I don't know what it is. Don't care, it's not a Porsche.

I came up with a solution to the neutralizer bars: 2x4s and zipties. Frame along one side and lock it down.

Will try the carbon fiber neutralizer brushes.

Wish I had a capacitance meter to check the Leyden jars so I can do some calculations and research on the energy there (I guess that can be done now, even as open ended tubes?). Mine's packed away somewhere.

Patrick

Sun Jun 08 2014, 03:22AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i can send more than youll use, just need to know what length the scraps should have as a minimum. 1 inch may not do you much good for example.

in any case, i measured a single "strand" of 3,000 fibers. 6.5 inches long / 30 ohms = 4.6 ohms per inch. Its ultra soft fabric, like silk. i dont know how youll attach it though. i love silver epoxy for low current, you could mash it into the CF real good, so that might work.

AwesomeMatt

Sun Jun 08 2014, 03:30AM

Registered Member #4454 Joined: Sun Feb 26 2012, 12:47AM
Location: Western Canada
Posts: 74

Okay okay, I'll just post the video too...

...

Oh, hey Patrick. That's awesome, thanks a bunch! Let me figure out if I need it or not and I'll PM you if I do.

Eleccentric

Sun Jun 08 2014, 05:28AM

Registered Member #33460 Joined: Tue Aug 27 2013, 06:23PM
Location: Seattle
Posts: 46

I think you'd be better off using two complete 5 gallon bucket Leyden jars in series for each side, rather than doubling up buckets for single jars. I'd invert the bottom one, so it's butt-to-butt with the one on top, with the outer foils in direct contact.

The 18 inch spark will draw equally from each Leyden jar - either jar would only give half that spark distance to ground on its own. So, you may not need to put two in series after all.

The voltage strength of PP is much more consistent than PVC. Depending on what exact type of PVC you have, it may be far better or far worse than the PP.

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