Fundamentally, an alternator produces electricity by spinning a magnet inside a wire coil. The rotating magnet is actually an electromagnet – it requires a small amount of electricity to generate its magnetism. As this inner rotating wire coil – the rotor – spins, its magnetic field sets up an electric current in the stationary coil on the outside of the alternator – the stator.
The voltage regulator drives the rotor coil with a field current. By varying the field current, the voltage regulator changes the strength of the magnetic field and therefore also the amount of electricity that is output. Since the rotor is rotating and the voltage regulator isn’t, the field current actually is supplied to the rotor through a pair of brushes – graphite contacts that ride against the spinning rotor. The rotor is supported by and spins inside two bearings.
A stator comprises three separate and interwoven coils of wire, and the electricity it produces actually is alternating current (AC, and thus the name “alternator”). Since batteries operate on direct current (DC), the AC output of the stator is first rectified (converted to DC) by three pairs of diodes before being applied to the battery. Diodes are electricity’s equivalent of check valves, allowing the flow in one direction but not in the opposite. A condenser (or capacitor) reduces radio interference generated by all these whirling wires.