Primary Resonance


This is directed primarily to Scott Myers (as suggested by Malcolm Watts)
but it's open for discussion. 

I believe this topic has been discussed before (prior to my joining) but it
is really giving me a headache. I am running neon sign trannies (4, 15kV
30mA) through a 15A variac. I am using an air quenched spark gap and have
taken care to build all components carefully and properly. 

When I begin to turn up the variac (at about 20%) it begins to fire,
indicating 15kV across the secondary. Obviuosly I do not want to open my
gaps as this would be disastrous. I know that with matching capacitance the
system should want to resonante at 60Hz, however there is something else
that I am observing. With capacitance almost matching there is a distinct
beat discharge. I investigated this with a HV probe and the scope and found
a superimposed frequnecy on the 60Hz cycle. This frequency increased (as did
the beat - as expected) when the variac was turned up. I immediately assumed
that the changing variac inductance was somehow influencing this. By
calculation based on these observations I estimate the output voltage would
be 130kV at full power (if it could have gone this high)!

I have just fried one of my caps and was wondering how people have tackled
this problem in the past? Malcolm has had the same frustrations and I guess
now it's my turn. I never used a variac in the past and never had this
problem. Any ideas would be most welcome.



[ NOTE: Scott is currently on vacation. -- Chip ]