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Bumping above 40%

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.

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.

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.

I was not using an RFI line filter between my 16KV pole pig and the 
230 volt variac at the time.  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.  Dialling up a new power 
setting would then immediately burn another winding area on the variac.

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.  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.  
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!

Naturally, this excessive overcharge was hard on my RF capacitor.  
Luckily it survived to run another day.  With the transformer running 
in saturation at 24KV output, along comes the rotary break cyclicly dropping a 
dead short across the transformer.  If you think of magnetic flux in 
a transformer as sort of a spring between primary and secondary 
windings, and a saturated core as a spring at the extreme end of its 
travel, you can imagine how the rotary break closures would be 
reflected through the pole transformer back to the variac as 
'bumping'.  The current surges are enormous!  That was a big problem 
merely looking at the 60 Hz component.  Now consider all of the RF 
which also rides on the primary circuit from the pole transformer 
back to the variac and I think one has an explanation for the smoking 
winding where the wiper is selected!

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.

To stop RF and associated garbage from getting back to my variac, and 
the neighbourhood power grid, I put a 50 Amp, 250 volt RFI line 
filter in each line between the pole pig and the variac.  These are those big 
rectangular, oil fillled filters used for power conditioning as the 
mains enter RF shielded  test rooms.  They attenuate 100db from 14KHz 
to 10GHz.  They cost about $500.00 each but I've seen them at 
hamfests like Dayton for $50.00.  Try to score a pair of these if you 
can!  They are rated for 140% overload condition for about 15 
minutes.  I suspect a 200% overload for  normal Tesla coil duty 
wouldn't hurt them.

There is a lot of hyperbole out there in reference to Tesla's 
Colorado Springs work.  I wonder if the real cause of the power generator 
meltdown wasn't all that sci-fi that we've all heard, but rather the mechanism
which  I desribed above?  
 
Happy coiling!
R.W.S.