How does this specific Van de Graaff generator work?

[greypilgrim]

Hi.

I have access to following Leybold Van de Graaff generator:

The instructions and data sheet can be found here, but they don't give much insight.

I have found different working principles of VdG generators. Some need the rising and falling sides of the belt to be closely together from top to bottom to act as a capacitor, others don't rely on this at all. I haven't found other descriptions of VdG generators like this one where the sides are brought together only at one point (6).

I thought it might be an improvement to the following kind of VdG generators:

They might have brought together both sides of the belt where they placed the lower electrode such that more positive charge gets attracted to the outside of the belt (since now there's more negative charge on the inside of the belt close to the electrode).

However, I found that the dome actually gets charged negatively. Also, all three rollers seem to be made of the same kind of plastic.

 

[davenn]

However, I found that the dome actually gets charged negatively

how did you determine this ?

Also, all three rollers seem to be made of the same kind of plastic.

3 rollers ? I have only seen ones with 2 rollersDave

 

[greypilgrim]

how did you determine this ?

I first charged an electroscope positively on the plus pole of a high voltage source (I later repeated this using the positive charge of a wool rubbed glass rod and got the same result). Then I charged a small isolated sphere on the dome of the generator and approached the top part of the electroscope, which made the spreaded leaves in the electroscope approach each other. If my understanding of electrostatic induction is correct, this means that the dome and the electroscope must be of opposite charge.

I repeated this with different parts of the generator, mainly the in- and outside of the belt and it seems to me that all charges are opposite compared to the second image I posted. So I guess they just used materials such that the belt is closer to the positive end of the triboelectric series than the rollers.

3 rollers ? I have only seen ones with 2 rollers

There are different working principles, some even use 4 rollers:

But yeah, 3 rollers seem unusual. I still think the outside roller on my generator was added to attract more electrons from the lower electrode, because they now feel more positive charge from inside the belt (not only from the part going upwards but also from the part going downwards).