Re: VDG (fwd)

["High Voltage list" <hvlist@xxxxxxxxxx>]

Original poster: <sroys@xxxxxxxxxxxxxxxxx>



---------- Forwarded message ----------
Date: Sat, 26 Feb 2005 22:25:30 -0300
From: Antonio Carlos M. de Queiroz <acmdq@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Cc: hvlist <hvlist@xxxxxxxxxxxxxxxxx>
Subject: Re: VDG (fwd)

High Voltage list wrote:

> From: robert heidlebaugh <rheidlebaugh@xxxxxxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Re: VDG
> 
> Gordon; The top roler and comb is used to charge the charge collecting
> sphere  The use of a metal roler at this point is reasonable as everything
> is at a common charge point. This is not the case at the bottem, Part is at
> ground potential , The comb is at 4-5 Kv so a metal roler at this point
> would be subject to short circuit problems of differing potentials. Metal on
> top, fine. Plastic on bottem, fine. Metal on bottem, NOT GOOD.

Allow me to disagree...
In a friction-excited machine, where the bottom comb is grounded and
there is no charging supply, it's perfectly possible to interchange
the two rollers to revert the terminal polarity. My double VDG works
exactly in this way:
http://www.coe.ufrj.br/~acmq/myvdg.html
If there is a power supply connected to the lower comb, the bottom
roller must be at least partially conductive, and grounded. There is
no problem in using a metal roller there (of course the charging
supply must be current-limited to avoid disastrous sparks).
If the bottom roller is connected to a charging supply and the
bottom comb grounded, again the bottom roller must be partially
conductive, or fully conductive.
There is no essential difference between what can be mounted at the
bottom and top ends of the belt. At both sides the belt sees a
grounded terminal. True ground below, and the interior of the terminal
above.

Antonio Carlos M. de Queiroz