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Re: Disc Construction



Hi Bob,

Sorry for the late reply. Comments below.

> Original Poster: bob golding <yubba-at-clara-dot-net>
> At 08:11 06/05/99 -0600, you wrote:
>  Why not go for a few thinner
> >sheets and
> >simply bolt them together (like I had originally planned)? What diameter and
> >motor
> >speed will you be running it at? 1/2" sounds like a little overkill here.
>
> hi Reinhard,
> maybe 1/2" is overkill. What I am planning is to fix the rotary on to the
> end of a rotary inverter. The speed is 3000 rpm and the wattage is
> 600watts. I haven't decided on the design yet, i am thinking of using a
> variation on Brett turners design. a lot depends on whether i can get a
> machinst to do it for me or if I have to do it myself. i only chose 1/2"
> based on other peoples designs. I haven't decided on the  final diameter
> yet. Any feedback on this will be most welcome. The idea of fixing it to
> the end on the rotary converter is that it will be a true synchronous gap
> regardless of the running speed. This means that I can control the power to
> the coil from the dc supply or with the variac on the output. Can anyone
> see any problems with this arrangement?

I think 1/2" is overkill unless you run a very large diameter (say 12+") disk
and large tungsten electrodes (weight) at your rated 3000 rpm. G-10 has
fibers inside making it VERY tough. If you have ever tried to snap a PCB
made of G-10 in half, you know exactly what I mean. You can almost fold
it in half before it breaks apart. The final diameter (of the disk and the
electrodes) depends largely (I think) on your input power. For 5-7kVA or
so, I think a 8-9" disk with 5mm tungsten electrodes (using 8 in this case) is
hefty enough.  Of course, the applied voltage is also a disk diameter factor,
esp. if your rotating electrodes are isolated from each other (in my setup
they are all on the same potential (above ground) Getting thoriated electrodes
might be difficult as they are being ruled out (because the are "radioactive",
just like the "socks" of the high candle power pressure kerosene fed Petromax
lamps {best camping light around}. Although I doubt you could receive a
"lethal"
dosage, it is a "finding" for law and money makers). However, the good news
is, that you can get special tungsten mixes (I have no idea to the exact
composition) that are just as rugged as the old thoriated ones. 5mm electrodes
are designed for 250A TIG welding (I know, as I build my own welders.... itīs
cheaper and more fun than buying a finished device). However, this is a
continious rating, so for coiling use you should be able to run them at higher
power levels w/o a problem. You might want to consider running them in
"side-pick-up" configuration. This increases the arc surface and reduces wear.

As to your rotary invertor idea: it sounds neat. I have no real conception
of how such a device works, but one thing that pops to mind is the time
in which the speed changes. During this speed "pickup" time, your gap
cannot be in sync. If this time is long enough for the cap to build up a
(i.e. small cap, big xformer) dangerous voltages, it might die. Can you give
me more details on this construction?


Hope it helped some.

Coiler greets from germany,
Reinhard