Re: van de graaff tube (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Tue, 30 Nov 2004 18:11:20 -0600
From: Dr. Resonance <resonance@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: van de graaff tube (fwd)


Ed:

We were only running at 2,100 rpm so, at 3450 rpm, your current would be
higher.  These values listed were from the direct terminal to ground
measured short circuit current.

At 3450 the bearing noises become rather high for my liking.

Dr. Resonance

>
> "Assuming you are spraying the lower pulley at 5-7 kV DC, the 2.5 inch
> wide
> belt will deliver charge to the upper terminal at an approximate rate of
> 12-15 uA DC.  If you are using a self-excited design, the max charge
> rate
> will be approx 3-4 uA DC."
>
> I realize you're "in the business" and should know better than I, but
> the current you quote for a self-excited machine sounds low to me.  I'll
> have to look up my old notes but I have a self-excited machine using a
> belt just under 2" diameter which gave something like 15 microamps.
> Far, far from an optimum design.  The column is grey PVC, 4" OD and cost
> a small fortune when I built the thing back around 1963 or a year or so
> later.  The pulleys are about 7/8" OD and turned at a pretty high speed
> through a leather belt/pulley system running off a 3450 RPM motor.
> Original upper terminal was a "more or less" 15" sphere made from
> aluminum mixing bowls, later replaced with an oblate I got from Frank
> Lee.  I asked him about his experience with separately-excited machines
> and he told me the current wasn't necessarily greater than self-excited
> but that things were much more controllable.
>
> I have a smaller machine I bought from Science First which uses a belt
> about 1-1/4" wide which puts out about 6 microamps.
>
> Antonio has published some notes on VDG output current limits but I
> haven't bothered to compare notes with my old measurements.
>
> Ed
>
>
>
>