I believe there is a point tdmack is making and others are missing. Tdmak mentioned it once but has not been pushing it:
(If I understand this correctly) An overdriven induction motor is a legal way to backfeed the grid in many jurisdictions. Motors just DO this under some load conditions.
For example: An electric elevator with counterweights, a controlled DC motor, and an induction motor/DC genset. (These are the ones that make the sound used by the original Star Trek show for the photon torpedo launch sound effect: When they've been unused for long enough that the genset has shut down and then you call them, the first thing they do is start the genset, which makes that sound as it starts up.) When the elevator is decelerating the energy from its momentum is recovered and fed back to the grid through the DC motor-as-generator, DC generator-as-motor, AC induction motor-as-generator. You can hear the gen set speed up as this happens. Nothing special was needed to make this happen: It's a side-effect of the speed control in the DC system ramping down the elevator's speed. (I wonder if smartmeters are smart enough to accept short intervals of backfeed like this without triggering the "somebody's trying to cheat" alarms, or charging, rather than crediting, for the power given back if it exceeded the rest of the loads? B-) )
Doing this is legal (in some places) because it has little risk of islanding. In the case of elevators and other speed-controlled moving masses, there's a limit to the amount of energy stored, and thus the time it could potentially backfeed an island is limited. Even if there is a continuous prime-mover attached, unless the power-factor/excitation capacitors are big enough and they and the load are within a rather narrow range and STAY there, the motor will quickly lose excitation and spin up or down to a speed where it won't regain it. (Also: On the Breezy: Once the power is lost even momentarily the brake comes on and stops the mill.)
Because of this, where backfeed of overdriven induction motors is legal, it's something close to "just another motor" for regulatory purposes. Because it's a backfeed you have to get it approved, of course. But because it's just an induction motor you can get that if the motor, its controls (the main contactor stuff), and the wiring are all UL approved and to code. If you don't have UL approval for the REST of the system it may be an issue for your insurance (or some separate code about what kind of mills you can install in mill-hostile jurisdictions). But it isn't for getting the utility and inspectors to sign off on the backfeed.
For inverters doing backfeed safely and legally is a major engineering job, with extra components and algorithms and a full inverter system check required. So getting the inverter UL approved was an issue that cost the vendors a bunch of labor, bux, and testing time (including at least one device burned up to prove it won't set its surrounding on fire, explode, or poison the personnel.) And that cost gets passed on to you, in the form of several extra grand for the grid tie option. But for overdriven induction motors it's something that's been happening, and well understood, since about the time Tesla and Westinghouse deployed them. So it's already implied in the UL unit testing and you can often get approval for the application.
(Or at least that's how I understand it, having not actually DONE this or heavily researched the legality issue.)
