Troubleshooting a homemade Van de Graaff machine

[gregory112]

TL;DR Summary

I am building a homemade VdG machine, and it does not generate any charges.

I'm trying to build a Van de Graaff generator, but it does not generate any charges.
The problem is, I don't know which part is accidentally shorted, or grounded. I use both PVC pipes for the top and bottom rollers. The belt is neoprene rubber, 1mm thick. The rollers are bolted on aluminum shafts, one aluminum shaft is connected to the motor (is this okay? maybe it's shorting it?).

My room hygrometer says it's 67% humid.

The shafts are attached on bearings on standing acrylic boards. I made combs from copper wires. I want to test whether this is a humidity problem, maybe grounding problem, or something else? I can rub the PVC and it picks up some pieces of paper just fine.

The dome is cylindrical, with flat sides, made from steel plates and pipes that are welded cut open. In the picture here I haven't attached the dome. Can someone help by pointing me to the right direction? I don't have a high voltage ammeter, can I just use a normal multimeter?

Before I attached the dome, I tried to make a leyden jar and connect one terminal (the ball on top of the jar) to upper comb, and the ground terminal (the body) to the bottom comb. The jar does not seem to be charged however.

 

[anorlunda]

I did a search on "Debugging Van de graaf" and quickly found this.

http://amasci.com/emotor/vdgbug.html

 

[gregory112]

I did a search on "Debugging Van de graaf" and quickly found this.

http://amasci.com/emotor/vdgbug.html

I have read that. I have tried blowing a hairdryer and using a dehumidifier. It is also said that

For non-powered VDG machines, the surfaces of the rollers MUST be made from two different materials.

Well I tried to use stainless steel on the bottom and PVC on top, did not work. The video here shows that he is using both PVC pipes, and a pantyhose. It works, and he could charge a leyden jar even without a dome, which I think is the expected behavior. As far as I understand it is also required that you have two different materials far apart in triboelectric series. My generator is not providing any observable charge/spark that I cannot test which part is leaking, with something like my finger.

 

[hutchphd]

First: put the Leyden Jar AWAY. I speak from long ago adolescent experience with a machine far less capable than the one you have built. The first time I discharged the Leyden Jar that I had built it was clear to even pre-adolescent me that this was potentially LETHAL. Now that I am capable of the analysis it was certainly lethal. Put it away. Since you are not likely to listen to me, at least make a very small one (smaller than your fist).
The points made by @anorlunda are all very useful. You do understand that the upper comb needs to be internal to the ball to facilitate charge transfer?

 

[gregory112]

First: put the Leyden Jar AWAY.

Thank you for your advice, no worries, I am listening to you, mine is smaller than my fist, my multimeter registers like 0.1 nF of capacitance. But because I don't think it's charging I want to find another way to check if my generator generates charge.

You do understand that the upper comb needs to be internal to the ball to facilitate charge transfer?

Yes, my comb is connected with an alligator clip, and is wired and taped to the inside body of the dome. The wire is exactly like those that you see on my picture.

 

[hutchphd]

As I recall my generator had a latex belt (maybe 3/4 inch wide) polystyrene spindles and the lower(?) one was wrapped with yarn (maybe wool maybe acrylic). The sphere was probably 4 inch and the combs were quite rudimentary. I could pull a 3 inch arc to my finger on a good day (so maybe 100kV?) . Made by Remco Toys as I remember it. Have fun and stay safe.

 

[Baluncore]

What are the vertical side plates made from?
Can a surface protective paper cladding be removed?

 

[gregory112]

What are the vertical side plates made from?
Can a surface protective paper cladding be removed?

Acrylic, yes, they can be removed. Are they causing problems?

 

[Tom.G]

Acrylic, yes, they can be removed. Are they causing problems?

Yes, the paper is likely a large part of the problem. It absorbs some of that 67% humidity and becomes quite conductive.

I made one as a teenager out of varnished, kiln dried, wood that refused to work. After cutting the vertical pieces and inserting a few inches of Acrylic it worked fine. The pulleys were made of wood and the belt was a wide, white, ribbon from the fabric store (polyester? Rayon? really don't recall for sure). Anything Black probably contains Carbon and is at least slightly conductive, which means it won't work at all for a belt.

The combs were ordinary 14 gauge house wiring with plastic insulation. Ordinary straight pin from my mom's sewing box were stuck thru the insulation, contacting the 14 gauge wire. They were made the full width of the belt.

Having the motor electrically connected to the lower roller may be a problem. Try a flexible, insulating, coupling there. I used a few inches of rubber hose held on with small hose clamps.

To detect if it is generating a voltage, make a Gold Leaf Electroscope.
https://en.wikipedia.org/wiki/Electroscope
Since Gold leaf is scarce and expensive, I sacrificed the foil wrapper from a stick of chewing gum. It was actually a very thin sheet of Aluminum foil with a paper backing (at least it was several decades ago, I haven't looked lately.) With patience and practice, the thin foil can be peeled off the paper and used in the electroscope.

Another detector that is a little easier to make is a piece of popcorn suspended on a thread.

Have Fun! ... and let us know how it works out.

Cheers,
Tom

 

[Baluncore]

Acrylic, yes, they can be removed. Are they causing problems?

Yes.
But they might not be the only thing causing problems.

 

[gregory112]

I'm so proud of this community.

I did peel off the protective paper from the acrylic. Now I have a spark, but it's very tiny, not even visible, but I can hear it. I heard it when I hold a metal hammer with rubber handle very close to it, like very very close, not touching but almost. My dome is shaped like this.

 

[gregory112]

Anything Black probably contains Carbon and is at least slightly conductive, which means it won't work at all for a belt.

I did read somewhere that black rubbers may contain carbon, but my neoprene belt is (supposedly) uncolored. I don't know the "natural" color of a neoprene rubber, but mine is black. It's kinda greasy though, and the grease cannot be washed away.

 

[Baluncore]

It's kinda greasy though, and the grease cannot be washed away.

What have you done to de-grease the surfaces ?
Have you followed the procedure from the debugging VdG link in post #2 ?

 

[gregory112]

What have you done to de-grease the surfaces ?

I have read that too, I actually washed the belt with soap first. It's still greasy, and it's not skin oil. I have a roll of neoprene rubber and it's very greasy too, it makes me think that unprocessed neoprene rubber is always like this. I saw also in that post that belts that can be rubbed with other materials and then attract some materials are conductors.

To test your rubber for conductivity, simply rub a piece of it on your arm hair, then hold it near that hair and verify that the rubber surface attracts the hair and makes it rise.

That is why I replaced my belt with this rubber. Before, I used a cordura nylon fabric, it does attract some pieces of paper after I rubbed it to PVC, I guess this is what the quote is trying to say.

 

[Baluncore]

Maybe it is a water resistant silicon grease and you must use something like rubbing alcohol.

 

[gleem]

The cylindrical design that you have chosen for the "dome" is poor. The welded edges will be large sources of coronal discharge limiting the ultimate terminal voltage attainable.

 

[Tom.G]

As @gleem noted, the 'dome' shape you have chosen will leak off much of any charge that is generated.

You need a sphere that encloses the top of the generator, down to and including the brush at the top of the generator. You don't mention dimensions of the generator, but maybe you can find a metal globe of the Earth or Moon to use for the sphere.

Although not ideal due to the flat bottom, you may have to purchase some metal mixing bowls to create your own. Stainless steel bowls, which are the most common, are difficult to join and to shape so avoid them is possible.

Grind off the rims of the bowls, cut a hole in one to fit over the generator, then solder or weld the two together. The outer surface seam must be smooth to avoid loss of voltage due to corona discharge from rough spots.

A less ideal last resort is to put a hemisphere on each end of your current configuration so there are no edges at the cylinder ends.

Cheers,
Tom

 

[gregory112]

Maybe it is a water resistant silicon grease and you must use something like rubbing alcohol.

I just bought Isopropyl Alcohol and rubbed the belt a few times but it does still seem a bit greasy though.

The cylindrical design that you have chosen for the "dome" is poor. The welded edges will be large sources of coronal discharge limiting the ultimate terminal voltage attainable.

I tried to wrap all edges with electrical tapes. Unfortunately casting an aluminum dome is quite expensive. So I want to make sure that the belt and pulleys are all fine first.

You don't mention dimensions of the generator, but maybe you can find a metal globe of the Earth or Moon to use for the sphere.

It's a bit large, W x H x L = 30cm x 15cm x 40cm. I did grind the edge of the steel, and now wrap them with electric tape.

Stainless steel bowls, which are the most common, are difficult to join and to shape so avoid them is possible.

I thought the most common was aluminum? I guess casting an aluminum dome is the only way to generate decent arcs.

My pulleys are both PVC. Why does this configuration work? I thought you need to have both pulleys made from different materials far in the triboelectric series. I tried stainless steel, but because I think it's neutral in triboelectric series I chose to replace it, aside from the fact that my generator did not make any charges too. Would wrapping one PVC pipe pulley with electrical tape help generating more charge? If the dome already make an arc, but small, does that mean changing the dome is the only way?

 

[Baluncore]

Avoid using electrical tape for electrostatic insulation.
A spun metal spherical dome certainly looks good, but it does not have to be made that way. The dome can be any sufficiently large plastic ball, with a hole cut out for the top roller and brushes. Wrap the outside of the ball in kitchen foil, and connect a foil tail to the upper brushes.

 

[gregory112]

Avoid using electrical tape for electrostatic insulation.

Why?

A spun metal spherical dome certainly looks good, but it does not have to be made that way.

Alright, I'll try to find another dome and report as soon as possible

 

[Baluncore]

Avoid using electrical tape for electrostatic insulation.

Why?

At high voltages, charge flows through the tape, or across the surface of the tape. Also the adhesive has properties that change over time. There is no advantage gained as the charge will seek out the angled corner, passing through the tape. The corner will appear as sharp as it was before the tape was applied.