I saw you posted that earlier, did you have a patent reference?
No, we didn't patent VW for furnace power supplies back then. We didn't think VW was patentable 'cause it was too obvious. However, there was another aspect how we did what we did that we thought was patentable because wasn't obvious. In fact, pretty much everyone that saw how we were doing what we were doing said: "That can't possibly work!" Well, it did work, and there was a trick to it that we probably could have patented, but we didn't pursue patenting it due to limited resources.
Was your furnace element control actual power control, or was it power regulation?
It was a power supply. You could command it from 0% to 100% of rated power using an industry-standard 4-20mA input and it would produce that power level at the heating elements inside the furnace, taking the resistance of the elements into account in real-time. Conceptually, it wasn't much different than what's described in Evolv's patent. I still have the original schematics and source code, and I think there are still a few of those power supplies in service almost 20 years later.
FWIW, VW wasn't really part of the original design criteria; our primary objective was to build a power-factor corrected power supply to replace the variable-reactance transformers or phase-controlled SCRs that were in common use at the time. Both have horrible power factors when they're not full on, and utility companies were starting to penalize industrial customers who presented large reactive loads onto their grids. But another part of the design criteria was that you had to be able to drop a crow-bar (literally) across the output of the supply without damaging it, and in fact, without blowing fuses or tripping breakers. To do that, I had to monitor current in real time. At that point, I thought: Hey, it wouldn't it be easier for the customer to dial in his temperature-control PID loop if we control actual power instead of voltage or current (because the resistance of the elements changed dramatically with their temperature), so that's what we did.
. To each their own, I suppose, though.
