Van der Graaff generator details. (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 17 Oct 2003 11:29:20 +0200
From: Finn Hammer <f-h@xxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Van der Graaff generator details.

Lyonel Baum wrote:

<J.G. Trump gave an excellent example of the correct design of a Van de
<Graaff high voltage terminal. Instead of a sphere, the terminal is a
<flat topped
<toroid.( 500 kV @ 200 microamp. is generated by this 1938 design ).
<www.lateralscience.co.uk/VDG/trump.html

Lyonel, thanks for that link, I think i will make a replica or
adaptation of this design. I already have the toroid half shells
available, and I can get the curved top spun on a combination of
existing tooling, so it will not get too expensive.


A question about the charging arrangement inside the top terminal:

The description is about, how to get a charge of opposite polarity to
adhere to the belt on it`s decent from the terminal.

I will describe with my own words, to test if I understand it right, and
then ask if it is right.

The top roller is isolated from the top terminal and brought to a
potential slightly higher than it, by this method:

The charge from the belt is taken off with a corona comb, slightly
before the belt touches the roller. The comb is connected directly to
the roller, and to the terminal trough a resistor and a corona gap.

It is the voltage drop across this combination that keeps the terminal
below the roller potential.

Another corona comb, connected directly to the terminal, is situated at
the top of the roller. Since the roller is at a higher potential than
the terminal, it is more negative than the terminal, and therefore it
will attract a positive charge from the terminal, trough the corona
comb. It will be intercepted by the belt and brought down to the base of
the machine.

This way, the machine will transport charge both up and down, so it will
be able to deliver a larger current into a load.

Did I get it right, or is this completely wrong?

I called a conveyer belt company, and ordered a 8 inch wide polyester
reinforced polyurethane belt which will fit between rollers that are 1.5
meters apart. This way, the R2D2 look of the original 1938 design will
be lost, but the high potential end of the machine gets elevated up in
the air where I want it. I hope this material choise will prove functional.

This machine should be able to reach around 450kV with the terminal
dimentions that I have to offer, and with a top capacity (or is that
capacitance) of around 32pF it will hold a charge of 3.24 Joule.
Painfull, but not deadly.

Cheers, Finn Hammer