Saturable reactors vs. SCRs?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>

Hi all,

It occurred to me that there maybe isn't much difference between a 
saturable reactor, and a small fixed ballast inductance plus a couple 
of SCRs. This sounds silly but I believe it is true. To explain:

A saturable reactor isn't a linearly variable inductance like the 
slide chokes and gapped variacs some people use to ballast their 
Tesla coils. It's really more like an inductor that switches abruptly 
between a very high value (what you would measure on the transformer 
winding with the core in place) and a very low value (which is more 
or less the inductance of the transformer winding alone with the core 
removed: saturation pretty much makes the core "disappear" from a 
magnetic point of view.)

By varying the DC control current, you vary the amount of 
volt-seconds the core can stand before it switches from high to low 
inductance, and the effect is not unlike a phase angle controlled 
lamp dimmer. This is where the harmonics that some other posters 
mentioned come from: the fact that the inductance changes quite 
suddenly at a certain point.

But anyway, the question is: If a SR is similar to a phase angle lamp 
dimmer in series with a minimum of ballast inductance, why not just 
replace it with those items? The ballast inductance should end up a 
lot lighter than an equivalent SR, since it could be coreless and 
still give the same ballasting effect the SR would have when saturated.

Of course, one big advantage of the SR is that it is tough, with no 
electronics beyond a diode bridge. That makes it very hard to destroy 
from the usual RF spikes that Tesla coils always seem to feed back 
into the line one way or another. SCRs are about as tough as diodes, 
but the control circuit for them could probably get EMP'd quite easily.

I have made two different control circuits to do phase angle firing 
of large back-to-back SCRs. I used one of them with a 90A dual SCR 
brick and have run about 30A at 240V through it, and used it on my 
OLTC2 making arcs over 6ft. The drivers are designed for 50Hz, but if 
anyone is interested in adapting them for 60Hz I can help :-)

Steve Conner
http://www.scopeboy.com/