Re: Building an RSG

Hi Alan, all

I have built two rotary gaps, one of which you saw running at the
Teslathon,  and am having a new bigger one made now.  My power level
is also around 2kW.

> I've got a sync AC motor out of an old LP turntable.
> I'ld have liked something a bit more powerful but I
> think that it will do.

These motors are usually "shaded-pole" induction motors,  which are
NOT SYNCHRONOUS and provide very little torque.  Does the motor look
like a ring of iron laminations with a winding at one end and the rotor
at the other end ?  You can tell it is shaded pole if it has thick
copper wires forming shorted-turns near the rotor end of the stack.  I
would get a 1/4hp or bigger induction motor and modify for synchronous
operation,  by grinding flats.  It is not hard to do and operates well.

> I was going to make  disks from two 1/4" thick paxolin
> boards.

This sounds OK to me.  I used 10mm Kite brand Tufnol from RS 194-1148,
it is quite strong, heat resistant and can still be cut with a
fine-toothed jigsaw.  This material is also useful as a baseplate for
mounting all the bits on, because of its high voltage standoff.

> 1) how much difference will it make to the coil? At present
> I can get 4 foot arcs at 2kW with a static gap.

That is a real big question.  If you just substitute a rotary for your
static gap the performance may drop because the system is not optimised
for the new break rate.  I think a static gap system will "find" its
own break rate in order to process the power available,  whereas a
synchronous system has to be designed to operate with good efficiency
at the chosen RPM.  What are the V and I ratings of your supply,  also
do you have a particular cap size in mind ?  Are you using neons or a
ballasted power transformer ?

I spent a LOT of money having my latest rotary disk machined,  and I
still do not get arcs any longer than with my forced air cooled static
gap, although operation is much smoother and easier on the components.
Your sparks with a static gap are bigger than mine are with the rotary!
Just look at Alex Crow's coil for an example of how well a static gap
can perform.  The single biggest benefit of the rotary in my opinion is
the smoothness of operation, which is great for taking measurements.
(Also it does not clog like a static gap.)

There are a lot of decisions such as disk size, rotational speed,
number of electrodes, electrode sizes, spacings, break rate. etc.
which all need careful thought. I think that 200BPS is far more
tolerant of incorrect phase setting than 100BPS, in terms of not
overvolting components,  and allows capacitor size to be smaller for
a given power, however the sparks may be slightly shorter. That is my
personal preference, no doubt many prefer 100BPS (or 120BPS).

I would tend to go for a 3000RPM motor if possible, and a large
electrode spacing,  as I have had problems with trailing arcs in the
past at high powers. The combination to aviod is low RPM, a small
disc, and lots of closely spaced electrodes.  This can cause the gap
to re-ignite after each firing,  and it will draw out an arc which
wastes power and ruins charging.

> 2) Is the project doomed because I only have a press drill
> and basic tools?

I have heard of people using only a drill press,  although I bit the
bullet and got my disk made on a lathe by a compitent machinist.  I
would say, do not underestimate the accuracy and tolerances required.
I tried to make my own disk for my first rotary and have learnt, now I
would always get an expert to make it.  The accumulation of mechanical
tolerances from the tips of the fixed electrodes through the mountings,
motor bearings, disk, electrode holders, to the tips of the rotary
electrodes is very critical to get a consistent small air gap.

> 3) Can an RSG work with a horizontally spinning disc?


Rotary gaps are something which I have spent some time looking into in
terms of efficiency of charging and power factor. I would be happy to
provide more information by private Email if you wish.  If you have a
"finished" design I can run it on the simulator and predict voltages
and power throughput before you throw the switch.

						- Richie,

						- In sunny Newcastle.