Help with 3D printed anemic Van de Graaff generator

[Shokker]

Hi makers, first post here!

I'm trying to work on a project for a science exhibition at school. I want to make a van de graaff generator using 3D printed parts for the base. I'm just trying to get a working version, but I'm having some troubles.

Build:
There are a bunch of different ways to make it, but I'm basing it loosely on this

He uses an elastic band and PVC pipes, which is what I'm doing. I made rollers and cut the band into shape. I attached a small motor to one roller, and have ground and charging wires. I'm using an empty soda can for the collector of charges, and I was told the ground could be something simple like an aluminum foil or metal bar.

Problem:
In all the videos, it only takes a few cranks before there is enough charge to get strips of paper to move away from each other, and large sparks can fly out if you hold your finger close to the metal ball. In my device, I'm lucky to get even a small spark, and not consistently.

Clarifications:

-There is good connection between the wires and their endpoints (ground or the soda can)
-We live in a desert, so humidity isn't an issue
-When the ground and charging wire were hooked to a capacitor with a voltmeter, the voltmeter showed that there was a slight charge being built up, but it took ~10 seconds of running to motor to get to 3 volts. It also bled out fairly quickly.

I've attached an image (final will be 100% 3D printed but this is a placeholder). Please let me know what could be wrong.

Thanks
-Shokker

 

[JimB]

Welcome to ETO.

It is difficult to be specific with just a small picture to look at, but consider the following:

1 Some of your materials may not be very good insulators.
2 What are the rollers made from in your model?
3 do you have a "comb" of charge collectors at each end of the belt or just a single wire?
4 When you use the voltmeter and capacitor, the voltmeter has resistance which will leak away the charge.
The average digital voltmeter will have a resistance of 10meg ohm. This is OK for most purposes, but for a Van de Graff generator it is a very low resistance.
5 You seem to be keen on 3D printing parts of your final product. Do check the suitability of the 3D print material for use at high voltage, it may be "leaky".

Finally, a real Van de Graff generator demonstration, at the Electrical Museum in Mulhouse, France.

JimB

 

[Shokker]

Welcome to ETO.

It is difficult to be specific with just a small picture to look at, but consider the following:

1 Some of your materials may not be very good insulators.
2 What are the rollers made from in your model?
3 do you have a "comb" of charge collectors at each end of the belt or just a single wire?
4 When you use the voltmeter and capacitor, the voltmeter has resistance which will leak away the charge.
The average digital voltmeter will have a resistance of 10meg ohm. This is OK for most purposes, but for a Van de Graff generator it is a very low resistance.
5 You seem to be keen on 3D printing parts of your final product. Do check the suitability of the 3D print material for use at high voltage, it may be "leaky".

Finally, a real Van de Graff generator demonstration, at the Electrical Museum in Mulhouse, France.
JimB

Thanks for the feedback.

To answer them:
1/2. I'm using PLA from my dad's collection. Google says it is a strong insulator at room temp. Other materials are wood (for base), pvc rollers, elastic from a theraband, and styrofoam (which is a placeholder).
3. I didn't have a dedicated comb but one iteration of the device had wire bundles that I spread out to make a comb like structure. I changed it to regular wires so I could do testing with capacitors.
4. The capacitor experiment was there to make sure that something was working. While a voltage is being built up, it's pretty low and slow.
5. The materials are thermoplastics, so they should be super high resistance. Google says PLA is an insulator up to 70C.

I could take more pictures if you wanted, just let me know what you would like to see.

Thanks.

 

[gophert]

What kind of capacitors are you using. They may be (probably are) leaking.

 

[rjenkinsgb]

What do you have for the upper terminal?

The top collector dome or sphere is absolutely critical in a Van de Graaff machine.
It should not have any sharp edges anywhere and it should ideally be curved or rolled inward at the underside opening for the best possible results.
They will work to some extent with a less ideal shape, but the belt must do more work to compensate for charge leakage.

If it's working right, the belt transports electrons to the interior of the top sphere.
Due to an oddity of the way charges move the interior of a conductive sphere never has a charge, so electrons from the belt are never repelled, they just migrate to the outside which carries the high voltage charge...

In theory, the charge can build forever, or until something gives!

The machine arrangement must be vertical for it to work properly, with the upper sphere supported from the inside, so nothing allows charge to leak from the outside - and the upper roller & collector comb must both be inside the upper terminal.

And, if there are any points or sharp edges facing out anywhere on the sphere, they will act like the needles in an ioniser and leak the charge to the surrounding air.

See these drawings - they show examples of the ideal shapes, with the opening edge facing inwards; a lot of simplified diagrams do not show that detail, and many cheap kits do not bother as the proper shape is more expensive.

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSgbxWl_0nDMgRsXq-KsFZ_fXqqmbKF4D9sOzmrvgwO8_wI1bYd&s

https://cds.cern.ch/record/2203629/files/Bilder_vdG_1.png

 

[shortbus=]

Is your top roller and bottom roller the same material? They need to be made of different materials. Van De Graff's work by something called the "triboelectric effect" https://en.wikipedia.org/wiki/Triboelectric_effect The most effective generators have the rollers for the band made from materials that are as far apart in the triboelectric series as possible.

Here is a chart of the most common materials in the triboelectric series -

The TriboElectric Effect Series

A table of the triboelectric effect series that can be used to select materials that will minimize static charging.

www.alphalabinc.com

If your using a standard capacitor I don't think that will work. The static electricity created by a Van De Graff, is too high a voltage for most caps. You need to use a "Leyden Jar" https://en.wikipedia.org/wiki/Leyden_jar

Another thing if your using a PVC pipe for the column make sure to wash it good with hot water and soap then get it dry. There is some kind of chemical film used in the making of the pipe that is conductive that needs to be washed off.

 

[Shokker]

What do you have for the upper terminal?

The top collector dome or sphere is absolutely critical in a Van de Graaff machine.
It should not have any sharp edges anywhere and it should ideally be curved or rolled inward at the underside opening for the best possible results.
They will work to some extent with a less ideal shape, but the belt must do more work to compensate for charge leakage.

I'm using a soda can that I sanded down. The surface is conductive, and there are no major sharp edges that I can see. I used it because it's simple and other projects recommend it. I would like to use something better, but is there a good option for $20?

The machine arrangement must be vertical for it to work properly, with the upper sphere supported from the inside, so nothing allows charge to leak from the outside - and the upper roller & collector comb must both be inside the upper terminal.

Could it absolutely not work if it was horizontal? I thought the main issue was that the belt carries the charge.

Is your top roller and bottom roller the same material? They need to be made of different materials.

I based the current version off of the video I linked in my OP, which seems to use PVC for both rollers. I tried to have one roller 3D printed, but that wasn't working as well (probably since I'm not too good with CAD yet )

Another thing if your using a PVC pipe for the column make sure to wash it good with hot water and soap then get it dry. There is some kind of chemical film used in the making of the pipe that is conductive that needs to be washed off.

I washed it, but didn't scrub it too carefully. Maybe that's an issue?

Anyway, thanks for the feedback folks. Some of the issues seem to be the top roller isn't inside the collector, the collector is a coke can (but it seems to work for others), and I need to switch up at least one roller.

Another design uses a glass fuse and a rubber band. Do you guys think that might be more fruitful?

 

[Beau Schwabe]

Due to an oddity of the way charges move the interior of a conductive sphere never has a charge, so electrons from the belt are never repelled, they just migrate to the outside which carries the high voltage charge...

This "oddity" is based on a simple principle that may or may not be obvious... Since LIKE charges repel one another the least path of resistance is to go to the outer most region of the sphere where the like charges can be furthest apart. Since all of the charge is on the outside of the sphere, the center remains relatively neutral therefor accepting any small charge that accumulates on the belt. Google "Faraday's ice pale experiment"... This is essentially the working fundamental concept of a van de graaff generator.

Another design uses a glass fuse and a rubber band. Do you guys think that might be more fruitful?

The further away your materials are for the "bottom roller" and the belt on the triboelectric series the greater the charge you can develop. The top roller can be metal.

I have used two nylon bed rollers for the top and bottom roller with decent success... The bottom roller was coated with Teflon tape (Not plumbers tape, actual Teflon adhesive tape). The top roller was aluminum adhesive tape. For the belt I used a roll of transparency film (Acetate) I got at Hobby Lobby and had to cut it down to 2 inches wide. Since the transparency film is rigid and does not flex like a rubber belt, the top roller was essentially a spring loaded idler pulley. I chose the transparency film as a belt material for something that would be more durable than a rubber belt. The van de graaff generator was powered with a sewing machine motor on a speed controller.

For combs, you need one at the top connected to the sphere and one at the bottom connected to ground... it is important that the comb does not touch the belt or roller. (an 8th to 16th of an inch gap is sufficient )
Pin Headers work great for a comb ... https://www.mouser.com/ProductDetail/Harwin/M20-9990246?qs=Jph8NoUxIfWjw4WmyRvzag==&gclid=EAIaIQobChMIisaPjb-36AIVJoNaBR3xVgzpEAQYAiABEgK6Q_D_BwE

Note for the Acetate rolls, make sure you don't get the Acetate Alternative it's cheaper, but not the right stuff. Below is a link to an example of the right material.

Restricted Access 2023

Also another tip ... to keep the belt centered on the roller. If the roller is slightly higher in the center, the belt will automatically center itself with only a small amount of belt tension.

 

[rjenkinsgb]

Could it absolutely not work if it was horizontal?

If the can is supported from the inside and has plenty of clearance from other items, it could work.

The picture you posted shows a "naked" horizontal belt, rather than how the complete machine is assembled.
A soda can should work to a moderate extent.


For readily-available materials, can you try glass (eg. the fuse tube) for the top and keeping PVC for the bottom?
If you want a larger glass tube, look at guitar bottleneck slides on ebay - they are cheap and a good size for a roller, if you add something inside for bearing supports.
eg.

3Pcs Clear Glass Bottleneck Slide Set for Acoustic Electric Guitar Accessories | eBay

High quality glass guitar slide bar. -The glass slide can provide silky bluesy slide sounds. -1 Set guitar finger protection slip slides glass slider. Color: Transparent. Material: Glass. ãClothing shoes & accessoriesã.

www.ebay.com

 

[Shokker]

Thanks for the advice! I'll do some redesigns and see how they work out. I'll keep you posted...

 

[Diver300]

Could it absolutely not work if it was horizontal? I thought the main issue was that the belt carries the charge.

Physical orientation doesn't matter. The physics master at school made a horizontal Van der Graaf generator with two spheres, a positive and a negative.

 

[Nigel Goodwin]

Going back to the hair picture in post #2, we did similar in physics at school - the teacher stood a (long haired) pupil on a wooden stool (must have been a girl?, as there were very strict hair requirements at my Grammar school) and ran a wire to the Van De Graff generator.

Just as in the picture her hair obviously ballooned out nicely, and the teacher (a really nice guy called Mr. Gregory) was walking about talking and gesticulating, waving his arms about.

Unfortunately he got a little 'too close', and as he waved his arm up a huge spark jumped from the girls nose across to his finger, knocking him to the floor and sending the girl flying off the chair - pretty dangerous for her, but she wasn't injured. No idea now what the girls name was, but I remember the incident as if it was yesterday - needless to say, we all found it extremely amusing

 

[Beau Schwabe]

As a school experiment for my daughter when she was in the 5th grade, I shocked the entire 5th grade ( about 150 kids ), in unison.

The experiment involved a homemade capacitor made from a plastic dinner plate and adhesive foil on both the bottom and the top. The plate was charged to about 25kV and the class formed a human chain... The instructions were to use only ONE arm from hand to elbow to form the chain as to prevent the voltage going across the body.

During the presentation I had a Van De graaff Generator, Marx Generator, electrostatic corona motor, Franklin bells, Jacobs ladder, etc.
The Franklin bells used another plastic dinner plate capacitor as described above and ran throughout the entire presentation ( about 45 min ).
Needless to say it was a huge hit and everybody loved it.

Here are a couple of videos of the high speed corona motor ... the second video shown the Marx generator, but it is not active... The Marx and the corona motor used the same power supply is all.

High Speed Electrostatic Corona Motor #1

High Speed Electrostatic Corona Motor #2

13-Stage Marx Generator (7.25 inch sparks)

Mini Jacob Ladder

Cockcroft--Walton generator 21kV

 

[shortbus=]

I like the Wimshurst machine myself, built two of them years ago. Have some plans for a Dirod generator but haven't made it yet.