[Prev][Next][Index][Thread]

Bumping above 40%

 * Original msg to: Rwstephens-at-ptbo.igs-dot-net

Quoting "Robert W. Stephens" <rwstephens-at-ptbo.igs-dot-net>:

> Richard,

> I read your report of Wed, 13 Mar 1996 03:50:00 GMT on the 
> Tesla Group net with interest.  Congratulations on successfully
> re-wiring your pole pig and on your other Tesla 
> accomplishments!  I've seen some of your photos on the Web.  It
> looks like you have been doing this for some time.

I try!

> You described 'bumping' in the power cabinet after turning the
> variac beyond 40%.  I have experienced a similar situation and
> I thought you might like to hear what I found.

Very much interested. I have not had time yet to gather extensive
data on this. More on this later.

> In my case severe bumping was accompanied by primary RF winding 
> flashover between turns.  The primary was a flat spiral, about
> 10 turns of 0.75 inch aluminum CATV distribution hardline coax. 
> Spacing was approximately 3/8 inch gap between turns.

My flat spiral primary is 15 turns wound with a single 100 foot
length of 1/2 inch OD soft copper refrigerator tubing. The
spacing is identical to yours at 3/8 inch between the surfaces of
the conductors. However I could not associate the bumping
directly with flashovers. The setup was brand new, and the
primary was somewhat dusty, so there was some occasional
flashing; but I felt bumping and saw the ammeter jump at times
when there was no flashing at all... It was entirely possible
that the gap was ringed (shorted) with arc however. I did not
have my mirror/UV-filter in place to observe the quenching.

> I was not using an RFI line filter between my 16KV pole pig and
> the 230 volt variac at the time.  

I use two sets of high quality filters in the power cabinet; one
set before the variacs, and the second set after the variacs.

> When the bumping and TC primary interwinding arcing began, the
> area of winding on the variac that was currently selected by 
> the control rotor would smoke and burn. Dialing up a new power 
> setting would then immediately burn another winding area on the
> variac. 

I have not gotten into the cabinet yet to examine the windings.
Normally I can smell scorching when things are having problems,
as the vertical vents from the power cabinet are practically
right under my nose. However, given that the surging on the
ammeter was limited to no more than 25 amps (as the needle
read, instantaneous surges could have been much higher) on a pair
of variacs nominally rated at 56 amps when wired in parallel, I
don't think I have damaged anything yet. I did peer in through
the side access panel while bumping was occuring and I did not
see any visible arcing at the variac brushes.

> As I came to understand the situation I learned that there were
> two undesireable conditions occuring here at the same time.  
> I have dealt with them separately and successfully.

> The bumping and TC primary arcing was caused by the pole pig 
> and system RF capacitor jumping into 60Hz resonant charging.  

Ah humm!

> The 60Hz reactance in my variac (above a certain setting) 
> combined with the reactance of the power control reactor choke
> and primary winding of the pole pig on one side, and the 
> secondary reactance and RF capacitor on the other side 
> conspired by chance to resonate at 60Hz.

> When I dialled the variac up to only about 45 volts the pole 
> xfmer would suddenly bump and then emit an angry growling 
> noise.  

My 25 kVA pig pretty much just sat there. There was not enough
voltage or current on the primary yet to get it more than mildly
interested. The damned core is heavy.

> Measurement with a high voltage probe showed that the secondary 
> voltage coming out of my mild mannered 16KV pole pig was 
> suddenly 24KVRMS!  The transformer was in hard saturation. 

> Further increase in the variac voltage setting from 45 volts up
> to the full 220 volts made negligible change in the 24KV 
> transformer output.  Note that this is EXACTLY how a Sola 
> constant voltage transformer is made, and note also that they 
> make the same growling noise!

I can see where my bumping could have been due to the transformer
attempting to saturate. I was ballasted down with inductive
reactance to the point that complete saturation of a 25 kVA core 
was unlikely. Still it was acting as if it certainly wanted it. 

...

> The resonant situation was easily stopped by setting the 
> inductance of the power control reactor out of the range that 
> caused this resonant condition.  I changed it from 16 
> millihenries to 8 millihenries.  No more bumping or harmful 
> 24KV surges on my capacitors. Changing the value of the RF 
> capacitor would also do the trick. 

This I can certainly verify. The bumping ceases when I increase
or decrease the inductive ballasting. It only occurs over a
narrow range on the arc welder inductive ballast. My guess is
that to obtain smooth operation in the 20 amp range on this
system I will need to use non-inductive ballast to pass some of
the current.

Thanks a lot for your input.

Richard Quick


... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12