Re: TC for VDG injection

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 

>         They do.  This is why 'working' (accelerator, etc,
>         van de Graaffs use them. 

The main reason is to have good control on the charging current. A self-
excited system would charge up until the saturation limit, but surely
is more sensitive to ambient humidity conditions, dirt, etc.

> There is however a
>         clarification:
> 
> > The maximum current is limited by the maximum charge
> > density that the belt can sustain without producing ionization
> > (26 uC/m^2).
> 
>         concur.
> 
> > A friction system in dry ambient quickly reaches this
> > limit too.
>         'quickly' is a matter of debate.  While the max current
> 
>         is belt limited (as noted), the _average_ current
>         is higher, approaching this limit more rapidly,
>         with a charge injector.
> 
>         The added current is more notable in 'pretty lights'
>         operation (sparks) that 'working' (Current to
>         load, eg accelerator tube) applications.
> 
>         Also an injector makes things more 'predictable',
>         as its less subject to humidity anomalies.

Ok. Better control, quick startup, more reliable operation, are
certainly advantages of an active charging system. I only managed
to make my VDG operate reliably with friction excitation when I
installed a Teflon roller. With other materials the performance
was always poor, but it always operated well with active charging.
With the Teflon roller, there is no difference, except for a bit
longer startup while the roller accumulates charge.
Note that an active charging system was the fundamental difference 
between the original Van de Graaff machine and previous electrostatic
machines.

Antonio Carlos M. de Queiroz