Re: Recent s.s.t.c work

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>

Steve (& all others interested)-

An update on my on-going t.c. project (the one with the internal 
primary wound in a bucket):  I've had to rework, somewhat, my circuit 
board, to accomodate TO-220-size transistors for the coupling 
transformer drivers and also for the plus-going drivers for the 4 
IGBTs.  And, the better to keep the latter off between  sparks, I've 
replaced the op-isolators that gate them with darlington 4N30s.  Also 
added some bleeder resistors here & there to help obviate unpleasant surprises.

After ensuring that the IGBT drive signals are present & properly 
phased with no mains-voltage applied, I then briefly applied full 
mains voltage with the primary temporarily disconnected, to see if I 
have any common-mode IGBT current.  I measured none, which implies 
that my drivers are working as expected.

So I'm now at the stage of maximum trepidation: starting to pour the 
coal to, it so to speak, with the primary connected (but presently 
without a secondary).  And I find a worry already:

I have 3 means of measuring IGBT current: 1) the 1:100 (cored) 
feedback current-transformer in one leg of the primary circuit,  its 
output connected thru a 1 ohm resistor seriesed with a 1:1 
transformer primary whose secondary is shunted by a pair of diodes 
back-to-back; 2) a homemade 4 m-ohm resistor in series with the "-" 
mains-electrolytics; and 3) a commercial 100 mV/50 A current shunt in 
the common bus connecting the "+" and "-" mains-electrolytics.

With mains voltage set by variac at about 40% of max and the circuit 
pulsing, I measure, differentially with the scope, approximately a) 
10 V peak across that 1 ohm resistor, implying ~1000 A peak in the 
primary; b) 25 V pk across the homemade 4 m-ohm resistor, implying 
~6250 A peak capacitor current; and c) 25 V peak across the 100 mV/50 
A shunt, implying ~12500 A peak capacitor current.

All of the above cannot be true at the same time.  Any of the above, 
if true, is cause for concern since the mains input is only at ~40% 
of max.  In an H-bridge, the turned-on IGBTs, the primary circuit and 
the "+" and "-" mains-capacitors are all effectively in series, so I 
should measure the same current at the 3 places, more-or-less.  But 
they're significantly, not to say wildly, different.  The IGBTs I am 
using are Semikron SKM 400GB 123D's.

So...I haven't gotten to pursuing this further yet but it's going to 
take some looking-into before I start trying to make sparks.  In the 
interim, any comments will be appreciated.

Ken Herrick