The Alternator
An automotive charging system is made up of three major components: the battery, the voltage regulator and an alternator. The alternator works with the battery to generate power for the electrical components of a vehicle, like the interior and exterior lights, and the instrument panel. An alternator gets its name from the term alternating current (AC).
Alternators are typically found near the front of the engine and are driven by the crankshaft, which converts the pistons’ up-and-down movement into circular movement. Some early model vehicles used a separate drive belt from the crankshaft pulley to the alternator pulley, but most cars today have a serpentine belt, or one belt that drives all components that rely on crankshaft power. Most alternators are mounted using brackets that bolt to a specific point on the engine. One of the brackets is usually a fixed point, while the other is adjustable to tighten the drive belt.
Alternators produce AC power through electromagnetism formed through the stator and rotor relationship that we’ll touch on later in the article. The electricity is channeled into the battery, providing voltage to run the various electrical systems. Before we learn more about the mechanics of the alternator and how it generates electricity, let’s look at the various parts of an alternator in the next section.
Alternator Components
For the most part, alternators are relatively small and lightweight. Roughly the size of a coconut, the alternators found in most passenger cars and light trucks are constructed using an aluminum outer housing, as the lightweight metal does not magnetize. This is important since aluminum dissipates the tremendous heat generated by producing the electrical power and since the rotor assembly produces a magnetic field.
If you closely inspect an alternator, you’ll find it has vents on both the front and back side. Again, this aids in heat dissipation. A drive pulley is attached to the rotor shaft on the front of the alternator. When the engine is running, the crankshaft turns the drive belt, which in turn spins the pulley on the rotor shaft. In essence, the alternator transfers the mechanical energy from the engine into electrical power for the car’s accessories.
On the back side of the alternator you’ll find several terminals (or connecting points in an electrical circuit). Let’s take a look at those:
S terminal – Senses battery voltage
IG terminal – Ignition switch that turns the voltage regulator on
L terminal – Closes the circuit to the warning lamp
B terminal – Main alternator output terminal (connected to the battery)
F terminal – Full-field bypass for regulator
Cooling is essential to an alternator’s efficiency. It’s easy to spot an older unit by the external fan blades found on the rotor shaft behind the pulley. Modern alternators have cooling fans inside the aluminum housing. These fans operate the same way, using mechanical power from the spinning rotor shaft.
As we start to disassemble the alternator, we find the diode rectifier (or rectifier bridge), the voltage regulator, slip rings and brushes. The regulator distributes the power the alternator creates, and it controls the output of power to the battery. The rectifier bridge converts the power, as we’ll learn in the next section, while the brushes and slip rings help conduct current to the rotor field winding, or wire field. Now let’s crack the coconut open.
Opening the alternator reveals a large cylinder with triangular finger poles around the circumference. This is the rotor. A basic alternator is made up of a series of alternating finger pole pieces placed around coil wires called field windings that wrap around an iron core on the rotor shaft. Since we know the pulley attaches to the shaft, we can now visualize how the rotor spins inside the stator. The rotor assembly fits inside the stator with enough room or tolerance between the two, so the rotor can spin at high speeds without striking the stator wall. On each end of the shaft sits a brush and a slip ring.
As we touched on briefly, alternators generate power through magnetism. The triangular finger poles fixed around the circumference of the rotor are staggered, so the north and south poles alternate as they surround the wire rotor field windings. This alternating pattern creates the magnetic field that in turn induces voltage into the stator. Think of the stator as the catcher’s glove as it harnesses all the power created by the spinning rotor.
All these components work together to give us the power we need to run our vehicles. Tesla captured this electrical energy and used it to light up cities, but we only need enough volts to power our stereo, lights, windows and locks. Let’s take a look at how the alternator produces that power in the next section.
Understanding Alternator Power Output
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 for the electricity in our houses, which periodically reverses directions. As Tesla proved in 1887, alternating current became 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 produce 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.
With so many components working to create the electricity vital for our vehicles, it’s safe to say the alternator is a crucial component under the hood. But like many parts on our cars, they fail. The next section will give you an idea of how to determine if you are about to be stranded and what you can do if you need to replace your alternator.