[DonRotolo]

First, what flameout said about safety isn't to be ignored.

Modern alternators require 'bootstrap' current to excite, so a battery of some sort is required, perhaps as small as 5 Ah might be enough. I'd think that even a small alternator would have the power output to run two wheelchair motors.

An alternator doesn't have a permanent magnet, instead it uses coils of wire to create a magnetic field. These electromagnets are spun on the 'rotor', past the coils on the 'stator'. The stator wiring is where the power output comes from.

Since the power generated must be 3-phase (you'll learn why in Electrical Engineering class), it needs to be rectified (converted to pulsating DC), using diodes.

The battery not only provides the energy for that first bit of magnetic field on the rotor, it acts like a giant capacitor to smooth out much of the pulsating DC from the alternator.