You asked, "...is roaster's controller able to overcome voltage variations?"
Yes, it should be. That's the reason for a Temperature Controller with a
temperature sensor instead of a variable power transformer (Variac). No
controller in FR, only a timer switch that you can reset to your heart's
Something is causing the power variations, however, and the Variac is not a
regulator. If the Gene Cafe has a temperature controller, I'd be surprised.
It would be immune to the roast size, and would direct you to load whatever
quantity of beans, and dial in (X) temperature, for (Y) time. You'd hit Cool
and log the time when you thought the roast was done. You could set that
time and ignore the roast if you have something more important to do while
roasting green coffee. There's always F* and *$.
Cheers -RayO, aka Opa!