That's interesting about the "spike" capacity in the rotary inverter.
Well, the rotary inverter will drop the frequency as it slows down, but since it's a shunt motor at heart it will pull more current to "speed up" automatically. Plus the momentum of the rotor spinning helps to keep the power up to start the big motor.
A ferro-resonant UPS (like an APC Matrix) has a 100 pound transformer in the bottom that serves a similar function.
Glad to have had it. C