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Re: Bumping above 40%

Quoting jim.fosse-at-bdt-dot-com (Jim Fosse):

> I've just experienced bumping on my system: 12 amp 140v variac
> 15KV -at- 60ma neon, 11.4nF primary cap, 15.5 turns 30 degree 
> primary, 8" by 20" secondary, 28" by 4" toroid. When I go above
> 100% on my variac I experience bumping. This is just before I 
> overcouple my secondary.

Good point. I too noticed that my coil was overcoupled when
my bumping problems arose. One of my solutions to bringing the
bumping under control was to loosen the coupling some, this alone
seemed to help reduce the "bumping" down to simple "surging"
which was read on the ammeter but not felt on the variac wheel.

Maybe part of the problem with flashovers, bumping, etc., in the
other report here was overcoupling as well.

> Notice the big difference between out system power levels, yet
> it still occurs. 

I was probably operating at about 4 kVA, but it would seem the
root cause could be found in any system... Apparently even in
coils supplied with internally limited transformers...

> I would concur that this is related to HV transformer/ primary
> cap resonance at the mains frequency (60Hz in our case). Think
> about the recommendations for selecting a primary cap: Zc = 
> Vout/Iout. By setting the impedance of the primary cap to the 
> impedance of the HV transformer a resonant circuit is created.
> The "bumping" may just be a beat frequency between the mains 
> frequency and the HV/cap resonant frequency.

With a bit of iron and copper in between to help match the tune.
I will say this, resonance in the 60 cycle lines would pass right
through my top-of-the-line filters. They are designed to pass 60
cycles with very low impedance. The large surges were obviously
pulling more current through the inductive control reactance (arc
welder core) then the core shunting in the welder should have
allowed. The thought that the energy supplied to the bumping may
have been coming from a partial collapse of the welder field flux
came to mind as well... 

Very interesting discussion. 

Richard Quick


... If all else fails... Throw another megavolt across it!
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