I knew that if you drove a three phase induction motor attached to the line at above line frequency it would change from a motor to a generator. So one day it popped into my mind that if you put capacitors across the terminals of a three phase motor and drove it above the L-C resonant frequency, it should be self exiting. But for a long time I couldn't find a motor suitable for testing. Then one day I was in a surplus place and saw a little three phase 400Hz squirrel cage blower that would fit in you hand. I could hook it up the the vacuum cleaner and test my idea. I made a crude measurment of L and a back of the envelope (literally) calculation said about .1mfd for the capacitors. I hooked everything up, the first run didn't work, but when I took the bag out of the cleaner the speed went past critical and the voltage soared up to 210, pretty good for a 208 volt motor. Changing caps brought the voltage up to 300. So the basic idea worked just fine, but on careful consideration I decided that it was of limited use. In most applications for alternators you want to closely regulate the speed and output voltage, which would be difficult to do in this case. Also the capacitors needed for a sizeable unit would be large and expensive. It might be useful in some wind driven battery charger applications where the rotor speed would tend to be regulated by operation of the alternator and the minimizing of moving parts would be important.