That's variable speed control. My variable speed well pump is digitally controlled by varying the frequency. It was this system that clued me in that there was a problem when I flipped the trans switches to start the well controller. It would throw up a a big FAULT alarm but didn't really explain what the issue was. Obviously the generator was suspect.
I didn't mean to start off on a tangent, but this experience got me thinking how I mostly take frequency for granted.
I agree, I don't want to steer the thread to far off into the woods. You do have something that will tattle on your generator, which is good! There are VFD controls available that can correct the output of a genset and do so automatically with or without adjusting the speed of the engine, but they are pricey!