Re: safety gaps at high power

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

Original poster: "Edward Wingate by way of Terry Fritz <twftesla-at-uswest-dot-net>" <ewing7-at-rochester.rr-dot-com>

Tesla list wrote:
> 
> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<Tesla729-at-cs-dot-com>
> 
> In a message dated 1/14/01 1:14:34 PM Pacific Standard Time, tesla-at-pupman-dot-com
> writes:
> 
> <<  "Edward Wingate by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> <ewing7-at-rochester.rr-dot-com>
>   >>
> 
> << My magnifier uses a 14400 volt pole transformer ballasted with an arc
> welder alone and will exhibit the same tendency to power arc the
> transformer safety gap when I slow rotary gap to approximately 7000 RPM
> or less from the normal 7500 to 8000 RPM operating range when the
> voltage input to the system is at the maximum 235 volts. >>
> 
> Ed,
> My classic Tesla system basically has the same power supply
> that you have (14400 volt, 10 kVA pig ballasted with an arc welder
> only) but my rotory RPMs are much lower. My twin rotory discs
> each have (6) 3/8" dia brass rotory electrodes and the the motor
> turns at a max of 3450 RPMs and I usually run around 3000 RPM.
> So, I guess I would definitely have that problem since yours starts
> to exhibit this property at < 7000 RPMs. I never thought about a
> lower motor speed causing this. I also use a 12 kV rated utility
> line lightning arrestor in parallel with the pig's output to the Tesla
> primary circuit and this really helps to clip the overvoltage spikes
> generated by the tank circuit. I think a slightly higher voltage rat-
> ed arrestor would work a little better though, as it starts clipping
> before I reach the full input voltage with my variac. I haven't installed
> a safety gap across the terminals of my capacitor. It is rated at
> 84 KVDC and the max. clearance between the opposite terminals
> is about 2". I don't think it could exceed an 84 kV potential with
> only this much clearance without the terminals flashing over to ea.
> other, so I think the installation of a safety gap across the cap. ter-
> minals would be moot with my setup.
> 
> David Rieben

Dave,

The minimum rotary gap speed at which safety gap power arcing will occur
is determined by other things like total gap spacing, capacitor rating
(MFD), and voltage/current input. Every system has it's own unique
signature.

I would say you are safe with the 2" spacing of your capacitor terminals
on your setup. With any other configuration like axial terminals or
greater spacing, I would definitely recommend safety gaps. I have used
safety gaps on both my capacitors and pole transformers for years with a
variety of coil systems and have had both transformer and capacitor
safety gaps fire at random times when experimenting and tuning and
suspect that my use of them has saved me many $$ worth of blown
components.

BTW, the LA's you are using are built by sandwiching a dozen or more
large MOV's inside the body of the insulator. An LA used on a 14400V
pole transformer will actually be rated at or close to 18KV.

Ed Wingate RATCB