How Alternators Work

In the early days, cars used generators rather than alternators to power the vehicle's electrical system and charge the battery. That's not the case anymore.

As automotive technology evolved, so did the need for more power. Generators produce direct current, which travels in one direction, as opposed to the alternating current used for the electricity in our houses, which periodically reverses directions.

As Nikola Tesla proved in 1887, alternating current becomes more attractive as it generates higher voltage more efficiently, something necessary in contemporary automobiles. But car batteries can't use AC power since they produce DC power. As a result, the alternator's power output is fed through diodes, which convert the AC power to DC power.

The rotor and the stator are the two components that generate power. As the engine rotates the alternator pulley, the rotor spins past three stationary stator windings — or wire coils — surrounding a fixed iron core that makes up the stator. This is referred to as a three-phase current. The coil windings are evenly spaced at intervals of 120 degrees around the iron shaft.

The alternating magnetic field from the rotor produces a subsequent alternating current in the stator. This AC current is fed through stator leads into a connecting set of diodes. Two diodes connect to each stator lead to regulate the current. The diodes are used to essentially block and direct the current. Since batteries need DC current, the diodes become a one-way valve that will only allow current to pass in the same direction.

Three-phase alternators have three sets of windings; they're more efficient than a single-phase alternator, which produces a single-phase AC current. When working properly, the three windings produce three currents that make up the three phases. Adding all three together produces the total AC output of the stator.

The two basic stator winding designs are delta wound and wye style. Delta wound are easily identifiable by their shape, as they're triangular. These windings allow for a high current flow at lower RPM. Wye windings resemble the flux capacitor seen in "Back to the Future." These windings are ideal for diesel engines, as they produce higher voltage than delta stators at even lower RPM.

After the AC/DC conversion, the resulting voltage is ready to use in the battery. Too much or too little voltage can damage the battery, as well as other electrical components. To ensure the correct amount, a voltage regulator determines when and how much voltage is needed in the battery.

One of two types of regulators are found in most alternators: The grounded regulator works by controlling the amount of negative or battery ground going into the winding in the rotor, while a grounded field type works the other way around — by controlling the amount of battery positive. Neither poses an advantage over the other.