The rotor of induction motors will always have SOME residual magnetic field.
This means that they can be used to generate power, also.
The technique is to feed back the induced current -- field shifting with the capacitors is most helpful -- until the rotor is actively re-polarized. The slip of the rotor versus the field poles will keep it magnetized.
WRT your automobile alternator: it operates exactly this way. It is, in fact, a 3-phase asynchronous alternator with a built-in package of 3 diodes. The field strength is modulated electronically -- other wise you might rev it up to 90 VDC!
Every issue that is involved in your project is replicated -- in a simpler way -- in every automotive alternator produced since the mid-1960s.
