An alternator consists of two concentrically wound coils. The inside coil, known as the rotor coil, rotates, and the resultant magnetic field creates, or induces, an electrical current in the outside, or stator, coil.
The alternator draws current from the battery only long enough to start producing its own electricity and, thereafter, is self-sustaining. The purpose of an alternator is to charge the battery, so the alternating current (AC) produced is converted to direct current (DC) by a device known as a diode bridge rectifier.
Initially, no power is transferred from the alternator to the battery, because the alternator does not generate enough voltage to overcome the reverse bias of the diode bridge rectifier. However, as the speed of the alternator increases, so does the current flowing to the battery.
A typical automotive alternator rotates between two and three times faster than the engine. Thus, for a typical engine, which idles at around 700 revolutions per minute (rpm), the alternator rotates at anything up to 2,100 rpm.
