RE: The OLTC II lives!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com> 

 >You say that coax already has low inductance due to the forward current
 >path being inside the return path.
 >I assume the inductance is so low, because this prevents the wires from
 >forming a loop, which could be viewed as a narrow coil with more
 >inductance, is this correct?

Yes. The inductance of a circuit is proportional to the loop area, and coax
has a very small loop area. I'm sure someone will point out that coax is a
transmission line with capacitance too. I do know this, but in my
application I only care about the inductance.

 >You then parallell a number of these conductors to tie the already low
 >inductances in paralel and make them sum up to an even smaller value, is
 >this correct?

Yes. But the main reason I used several coax's in parallel was to get less
resistance and stop them overheating.

 >Are the 4 primary coils isolated from each other to lower their total
 >inductance by parallelling?
 >If yes to above, do the separate turns share current equally, despite the
 >different coupling that I assume they will have.

Sort of. They are divided into two groups, one for each IGBT. The idea
behind this is that the resistance and leakage inductance of the two
circuits will help the IGBTs to share current.

http://www.scopeboy-dot-com/tesla/ol2primary.jpg
http://www.scopeboy-dot-com/tesla/ol2primary2.jpg

The primary design was a compromise. Tight coupling between the primaries
makes current sharing less effective, and also increases total inductance.
But with loose coupling, each primary will probably have a different
coupling to the secondary, so again the current sharing might be poor.

As you can see, I went for a tight coupling between the two primary
circuits, by interleaving them. I measured the collector current of each
IGBT with a Rogowski coil amd they are equal to within 5 percent. I later
found out that my IGBTs were a matched pair and I could just have connected
them directly in parallel and used a single copper strip for the primary.
|(  But the 4-tube primary looks nice and I had fun with my brazing torch
making it :)

Steve C.