"If the generator is the kind where the speed of the motor determines the frequency of the wave, then it must be a sine wave, right?"
Agreed. Good point. (I think you mean "shaft speed" there instead of "speed of the motor", though.)
I've gotta think about that some more.
I believe even pretty high-powered generator sets nowadays are inverter outputs, with the active solid-state switches being high-powered Silicon-Controlled Rectifiers (SCRs). What frequency the generator actually produces doesn't matter with an inverter. I would bet that the actual generators nowadays are really alternators like a car's, where the frequency (due to rotational speed) doesn't matter much because it is then rectified and fed into a 110V (or whatever) 60hz (or whatever) inverter. Yours is a good point, and I don't know offhand how much frequency in an inverter would be affected by load. The SCR switching could be controlled with a crystal oscillator, but then you wouldn't have the frequency problem that 280plus is having in the first place.
I know they use SCRs in some pretty high-powered ultrasonic and induction heating systems, like in the many-kilowatt range.
I believe you can generate a pretty close sine wave with a square-wave input (or better, a triangle-wave input), but you need some pretty good pulse-modulation techniques to do it. Maybe they've gotten that far in consumer-type gensets.
On the other hand, you could get a fairly good basic sine wave of a specific frequency out of an alternator regardless of the rotational speed by modulating the rotor itself with an input sine wave of that specific frequency. The alternator then acts as an amplifier, rather than a generator. (Input power in this case is mechanical, rather than electrical, as in a conventional tube or transistor amplifier --there would be no "B+" or "Vcc.")
I actually put some of the above in my post, but deleted it.
See, I figured by now a "real" techie knowledgeable in actual and practical genset technology would enter the stage and have "real" answers, but noooo.... So I can theorize all I want, I guess.
Terry, 230RN