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LAN_403

Part Scavenger

Mon Jun 11 2007, 10:02PM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Sure! Here's a pic. They're about a foot long and you can get this pair in just about any Laser printer. I got this set from a LaserJet 5P I think. I've tried to make VDG brushes before, and yeah they're annoying.

They're not as hard as the ones you have, but I don't know if that makes much of a difference.

Ken M.

Tue Jun 12 2007, 12:14AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

The powder idea might not e such a good one, because if you've ever heard of grain silos exploding thats the reason why, the ground corn or wheat granules, the really fine ones are floating around and I believe a significant spark or other wise flame would ignite the power and since it'd be in an enclosed area the expanding gasses would rupture the outside as well as make a nifty flaming mushroom cloud.

GreySoul

Tue Jun 12 2007, 03:27AM

Registered Member #546 Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239

You could probably use a non-flammable powder of some sort. Organic powders could ignite, sure... but what about some silica flour (SiO2), mica powder, PVC, or sodium polyacrylate?

You could also use nitrogen or CO2 as your high pressure "air" source. After an initial purge you'd run very little risk or explosion.

Or maybe a closed loop system using Sulfur hexafluoride?

...hrm.... so now I have a coil gun, a VTTC, and a VDG to make this summer.

yay me.

My Art degree at work?

-Doug

Antonio

Tue Jun 12 2007, 11:21PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

A nice construction. I liked the idea of a control in the terminal to activate the machine, and also the top current doubler. I find a bit strange the 50 uA charging current. This is a lot of current for a VDG. My recently built machine is about the same size and works well in hair-rising experiments with just 5 uA. It can go to 10 uA, but the noise and vibration become inconvenient. I have made experiments with printer brushes as described. They work well, specially in doublers. There is some risk of belt contamination, depending on the kind of material used in the brushes.

Finn Hammer

Wed Jun 13 2007, 05:20AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Welcome to 4hv, Antonio

Antonio wrote ...

I find a bit strange the 50 uA charging current. This is a lot of current for a VDG.

Consider the driving pulley being 100mm diameter rotating at near 3000RPM, belt width 80mm.

Elcal says:
Van de Graaff generator calculations
Maximum voltage: 540.0 kV
Terminal voltage: 540.0 kV
Belt length: 151.4 cm
Charging current: 32.16 ÂµA
Terminal capacitance: 20.02 pF
Stored energy: 2.9187 J
Terminal charge: 10.8098 ÂµC
Electrical power: 17.3668 W
Pulley rotation speed 2891.83 rpm
Charging time 0.34 s
Maximum load 16790.7 MOhmss

Does Elcal assume charge doubler?

Antonio wrote ...

My recently built machine is about the same size and works well in hair-rising experiments with just 5 uA. It can go to 10 uA

Your terminal has a better shape, so your max voltage is likely higher than mine. I can only draw 125-- to 150mm sparks.

Antonio wrote ...

but the noise and vibration become inconvenient.

I am a toolmaker. When I produced the pulley`s I made sure that all surfaces are true within 1/100mm. (0,0004"). This insures vibration and noise less operation even with the top pulley at 6000RPM. I am considering a 200mm pulley at 3000RPM in the next machine.
With proper top terminal.

Cheers, Finn Hammer

DrZoidberg

Wed Jun 13 2007, 05:29PM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106

Damn, why don't I have a machine shop?

It doesn't look like it assumes a charge doubler.
And as far as I know with your belt width and speed, 32uA is already very good.

In my opinion your machine performs pretty much perfect except for the spark length but that's because of the sharp rim at your sphere.

Finn Hammer

Wed Jun 13 2007, 07:00PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

DrZoidberg wrote ...

In my opinion your machine performs pretty much perfect except for the spark length but that's because of the sharp rim at your sphere.

I have been wondering how best to cave the rim in.
I used the technique posted by Lyonel Baum some years ago, where I cut the hole in the bowl, then draw a metal cone down into the hole with a nut and bolt. This creates a 10mm radius.
Now I realise that the hole has to come "all the way in" because there is also leakage from the charged ascending belt to the terminal. Also, the distance from the belt to terminal has to be substantial to acheive full doubling.

I set up a 110mm sphere gap:

This is the longest distance the generator will arc reliably between this diameter balls, without creating too much corona at the rim of the hole,
According to Elcal this results in a breakdown voltage of 280kV
Under these circumstances, with 32uA charging current, the charge time ought to be 0,17S

I have here a short video that shows how the machine behaves under this load:

I counted 69 discharges in 10 seconds. (If you count, you will count 57 discharges, the rest is apparently lost during processing at youtube)
That`s 0,14 S charging time, corresponding to about 40uA charging current.

Considering that the measured charging current is short circuit, meaning no corona loss at the terminal top, indicates that the corona los at 280kV is around 10uA.

Cheers, Finn Hammer

Antonio

Thu Jun 14 2007, 01:42AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Finn Hammer wrote ...

Does Elcal assume charge doubler?

No.

Finn Hammer wrote ...

Your terminal has a better shape, so your max voltage is likely higher than mine. I can only draw 125-- to 150mm sparks.

This depends on the polarity of the terminal and on the radius of curvature of the "target" too. With a negative terminal it's easy to draw long sparks to a small ball. I can obtain 30 cm with that terminal and a 2.5 cm ball. With a positive terminal I get just corona at more than ~3 cm. With a large ball the distance increases. Between two identical terminals I can obtain no more than what you obtain. I didn't measure the frequency of the sparks. Will try. Something strange with a negative terminal and a small grounded target is that most of the sparks are faint, barely visible, but some are stronger and clearly visible.

Finn Hammer wrote ...

I am a toolmaker. When I produced the pulley`s I made sure that all surfaces are true within 1/100mm. (0,0004"). This insures vibration and noise less operation even with the top pulley at 6000RPM. I am considering a 200mm pulley at 3000RPM in the next machine.
With proper top terminal.

Ok. Good mechanics are necessary for high speed. My construction was really not specially precise. I mounted the roller above the box containing the motor, connected by pulleys and a belt with it, with the motor fixed to the bottom of the box. A perfect recipe for a lot of noise.

Finn Hammer

Thu Jun 14 2007, 09:08PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

During today, the performance of the generator kept dropping, at an alarming rate, untill I was down to 38uA charging.

Somethng had been rumbling in the back of my head since I read Antonios write up about his latest VDG, something about aluminum contamination of the belt.

Disassembeled, the belt did show faint traces of black ruboff from the alluminium pulleys.

I took the belt into the kitchen sink, and gave it a good scrubbing with a green abrasive sponge and dish washing detergent.
Rinsed well and dried it with a towel.

While the belt was drying, I covered the pulleys with a layer of electrical tape to keep the belt clean from alluminium traces.

Once assembled again, the generator now sports a whopping 59 uA.

During the weekend, a Freu toroid is going to loose it`s life and become part of a hopefully perfect bottom end of the top electrode. Interesting if I can manage to hit the 64 uA which will indicate that the doubler works 100%.
I will also cover the pulleys with shrink wrap.

That learning curve is getting flatter

Cheers, Finn Hammer

Steve Conner

Fri Jun 15 2007, 09:14AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Good luck shrinking that shrink wrap evenly to within 0.01mm!

Your generator looks awesome, BTW.

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