Higher Voltage Pole Pig

Malcolm (and others):

A while back, Malcolm described a system whereby he planned to put a 
fixed gap across a rotary to prevent the gap voltage from becoming too 
great under worst-case misfire conditions. I'm thinking of doing the same 
thing but for a slightly different reason. I've got a 19.9/34500 10 KVA 
pole pig that I plan to use for both Tesla Coiling and other high voltage 
experiments. I also have a 60A 140 volt ganged variac and a 28A 240 volt 
single. This gives me the option of running 0-140 V in on the X1:X3 leads 
to get about 20,000 Volts, OR running off the 240/280 Volt variac to get 
up to 34500/40250 volts for non-Tesla use. My home-made LDPE plate-type 
caps have 128 mils dielectric made up of a total of 32 sheets of 0.004" 
thick LDPE sheet material, immersed in Shell Diala-X transformer oil, and 
are designed to run with off a 20,000 Volt RMS source.

After thinking about your proposed static-rotary gap configuration for 
awhile, I did a "thought" experiment. Suppose you had a well-quenched 
static gap (for example an air blast or vacuum gap) with a wider gap 
distance (say 0.60 inches) connected in parallel with a series rotary gap 
having a minimum total gap distance of 0.50 inches). Suppose further that 
you ran the rotary at a speed that gave you 600 - 800 potential 
breaks/sec. If you "just missed" hitting the rotary breakdown voltage 
at the current minimum electrode position and would have to wait for the 
next break, the static gap would then fire, preventing potential 
overvoltage stress on the tank cap. This approach would, in effect, clamp 
the maximum cap voltage to the peak voltage breakdown of the protective 
static gap (Vgapmax). Normally the rotary would govern the operational 
breakrate except for the misfire cases, which should be efficiently 
handled by the static gap. 

Now suppose that the pole pig secondary voltage is significantly 
increased (40 KV). The capacitor would charge to Vgap for a longer 
portion of each half-cycle, and assuming the pig/ballast could deliver 
enough charging current, the capacitor would re-charge at a quicker rate 
since it would be heading to a much higher peak value before being 
discharged at Vgap - we should get more "hits" per 60 Hz half-cycle, and 
correspondingly, more effective power transferred to the secondary.  
Slightly more stress would be placed on the primary cap since its duty 
cycle would increase, but in all cases it would be clamped to Vgapmax. I 
can't see a problem with this approach, but I haven't tried it out "for 
real", and am somewhat reluctant to do a smoke test without first 
bouncing the idea off the experts in the group.

Comments, flames, constructive criticism are appreciated... :^)

-- Bert --