Current is provided to the rotor by means of the slip rings and brushes.
When the engine is running, most alternators supply the field current directly by means of three extra diodes connected to the bridge rectifier circuit. Extra diodes are known as field diodes or exciter diodes and the alternator is said to be self-exciting. However, this self-excitation can only occur when the alternator is producing an output.
When the ignition switch is closed, current can flow from the battery positive terminal through the ignition switch and charge indicator lamp to the L terminal of the alternator. The circuit is completed through the slip rings and rotor field winding, and through the voltage regulator to ground on the vehicle frame.
The small amount of current flowing in the circuit illuminates the indicator lamp and provides the initial excitation of the field winding. This magnetizes the rotor pole shoes and produces a weak magnetic field.
When the rotor is driven by the engine crankshaft, the rotating magnetic field induces a voltage in the phase windings which is applied to the B positive alternator output terminal. However, this voltage is also impressed on the exciter diodes and current can now flow directly to the field circuit, restricted only by the resistance of the field winding. This strengthens the magnetic field and the output voltage rises quickly.
The voltage regulator now takes over to control the field circuit current and maintain a pre-set regulated output voltage at the B-positive terminal voltage of approximately 14 volts. As the voltage on each side of the charge indicator lamp is now equal, there can be no current flow through the lamp and the lamp is extinguished.
The aternator is now said to be charging and since the output voltage at the B-positive alternator terminal is greater than that of the battery, the current flows to the battery to begin the re-charging process.
The alternator output voltage can be controlled or regulated by varying the rotor current.