Re: Van de Graaff generator safety question (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 15 Jan 2004 22:02:55 -0500
From: David Sharpe <sccr4us@xxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Van de Graaff generator safety question (fwd)

HV List

I ran into a very interesting "Van de Graff" type problem while
working on package conveying systems about 12 years ago.
After installing a carton conveying system, personnel kept
complaining of getting "intense" shocks off of conveyor
body, even though I was responsible for the power / control
design, and everything was correct (and double checked)
for compliance to applicable codes (IEEE, NEC, local electrical
code).  I even had AC utility voltage leakage measurements
(ground to frame) performed, voltages of << 1VAC/DC
were observed.

People started complaining of upstream machine jams, etc.
I went to site to investigate.  Pan of steel conveyor had a
polethylene wear surface (!!!) and a synthetic cotton belt
with metallic "alligator" style splice in it.  Belt was perhaps
6" wide, moving at 100 FPM.  Static charges were building
up on alligator clip + surrounding belt to level that you
could hear loud SNAPPP over the nearby running machinery
and BRIGHT ARCS measured better then 8" long were
reaching out to contact around the PE wear surface to the
converyor frame.  The charges were causing the cartons
to literally "stick" to the side guide metallic rails and physically
jam the upstream machines (could not manually move 20
cartons end to end, the attractive force was that great!!!!).

I strongly recommended shutting the conveyor down and
removal of the PE wear surface ASAP, due to personnel
safety risk, potential damage to product, and possible
conveyor bearing damage.  After these mods were
performed, manufacturing operations returned to normal.
Our Mechanical Engineering brethren had successfully built
an extremely efficient Van de Graff generator, and didn't
even know it.  Names of personnel, company, location and
date of occurence shall remain anonymous, to avoid
embarassment to the guilty parties...   :^D

This is a true story, I was responsible for the original EE
design of this conveying system, and the resolution
to this debacle.

Regards
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA

High Voltage list wrote:

> Original poster: Steven Roys <sroys@xxxxxxxx>
>
> ---------- Forwarded message ----------
> Date: Thu, 15 Jan 2004 09:55:19 -0800
> From: Jim Lux <jimlux@xxxxxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>, hvlist@xxxxxxxxxx
> Subject: Re: Van de Graaff generator safety question (fwd)
>
> You can run a chain much faster than a belt, for one thing, because the
> mecahnical dynamics are better.  (Chains aren't as "stretchy")  ALso, if
> you want to run parallel chains, it's much easier to do than using a big
> wide belt.
>
> There are some other schemes similar to pelletrons that appear in the
> literature, such as a belt carrying metal plates.
>
> In my recollection, Pelletrons charge by induction, as opposed to using
> brushes or charge spray bars.  Perhaps this is a more "efficient" way of
> getting charge on the belt, although, as Antonio points out, the
> fundamental limit is the surface charge density on what ever is carrying
> the charge.
>
> Chains might also be more consistent, making field control as you move up
> the column easier, so the field gradients are smaller.
>
> At 09:58 AM 1/15/2004 -0700, High Voltage list wrote:
> >Original poster: Steven Roys <sroys@xxxxxxxx>
> >
> >
> >
> >---------- Forwarded message ----------
> >Date: Wed, 14 Jan 2004 14:57:06 -0200
> >From: Antonio Carlos M. de Queiroz <acmq@xxxxxxxxxxxxxxxx>
> >To: High Voltage list <hvlist@xxxxxxxxxx>
> >Subject: Re: Van de Graaff generator safety question (fwd)
> >
> >High Voltage list wrote:
> > >
> > > From: Dr. Resonance <resonance@xxxxxxxxxx>
> > >
> > > Yes, we built one about 12 years ago.  Current increase, as compared to a
> > > belt, is approx 1,200%.
> >
> >This doesn't seem possible. The belt in a regular VDG operates already
> >close to the limit of charge density that would cause ionization of
> >the air. The conductive sections of a pelletron chain would at most
> >operate at this same density. The current is always directly
> >proportional to the area per second of charge transport surface that
> >reaches the terminal. For a given belt speed, it's very difficult to
> >get something better than what a belt can give.
> >
> >Antonio Carlos M. de Queiroz