How does an Alternator Work?

Let's look inside an alternator and examine all of its individual parts.
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In this article, we’re going to talk about a major component of many electrical power or charging systems, the Alternator. In particular, we’re going to look closely at a typical vehicle alternator.

We’ll look inside an alternator and examine all of its individual parts. Then we’ll discuss how these parts work together to do the work that the alternator is intended to do.

What is an alternator?

First of all, let’s discuss what an alternator is and its purpose. By definition, an alternator is a device that converts mechanical energy into AC electrical energy.

What is an alternator

In a vehicle, the engine turns a drive belt that rotates a pulley attached to the alternator. But wait a minute… Doesn’t a vehicle require DC voltage? Indeed, it does… but we’ll get to that later.

The alternator actually generates power for the vehicle. As the alternator rotates, it creates a DC voltage primarily to charge the vehicle battery. The battery provides the huge current required to start the vehicle engine.

Once the vehicle is running, the alternator assists by providing power to run the vehicle’s electrical systems.

Vehicle’s electrical system power.

Alternator vs. generator

What’s the difference between an alternator and a generator? Well, as we said earlier, an alternator is a device that converts mechanical energy into AC electrical energy. By definition, a generator is a mechanical device that converts mechanical energy to either AC or DC electrical energy.

DCS basic structure

So, by definition, it’s probably safe to say that an alternator is a generator.

Alternator major components

As you can imagine, there are differences in alternator construction and as such, there are a different number of components depending on the vendor.

Alternators have three major components and they are the Rotor, stator, and rectifier. There are more components and we’ll get to those as we progress through our discussion.

Alternator major components

1. Alternator rotor

Let’s start with the rotor. A drive belt pulley system spins the rotor on a shaft while the vehicle engine is running.

Alternator rotor

At the heart of the rotor is an electromagnet often referred to as field windings. So, what’s an electromagnet? An electromagnet consists of a length of conductive wire wrapped around a piece of magnetic metal.

Voltage is applied to the coiled wire creating a current in it. This creates a magnetic field around the coiled wire. Much like a permanent magnet, there is a North and a South pole.

Rotor electromagnet

A rotor also has a series of alternating North and South finger pole pieces placed around the field windings that wrap around the iron core on the rotor shaft.

Rotor poles

2. Alternator stator

The rotor fits inside the Stator. The stator is a stationary part of the alternator. The rotor spins inside the stator without physically touching it. On each end of the shaft sits a brush and a slip ring. We’ll talk about those later.

Alternator stator

The Stator consists of three separate coil windings with one end of each winding connected together.

Stator coil windings

The stator coil windings are evenly spaced at intervals of 120 degrees around the iron shaft.

Stator coil windings angles

Ok… so now we’ve got this rotor spinning inside a stator consisting of three coils of wire. How does that generate a voltage?

A wise scientist by the name of Michael Faraday discovered that a voltage can be induced in a coil of wire if you move that coil through a magnetic field.

Michael Faraday Law

If the coil of wire is stationary as in the stator, you get an induced voltage in the coil if you move the magnetic field past the coil. Interestingly, the faster the magnetic field changes, the more voltage is induced.

Stator magnetic field

The rotor spinning inside the stator will cause an induced voltage across the stator windings due to a rotating magnetic field.

Stator induced voltage

Brushes and slip rings

Hold on a second… where does the magnetic field come from? That’s where the brushes and slip rings enter into the picture.

Stator brushes and slip rings

Remember we said earlier that the rotor is an electromagnet? That’s true once we apply a voltage to the field winding. How do we do that? We apply the voltage to the field winding through the slip rings.

Alternator electromagnetic field

You might ask… Where does the field voltage come from? Hold on!… we’ll get to that shortly.

Alright… back to our electromagnet rotor spinning inside the stator. There will be a voltage induced in each stator winding. The induced voltages will be AC because of the electromagnetic pole changing during the rotor rotation.

We end up with three voltages each 120 degrees out of phase from each other because of the winding physical locations around the stator iron core.

Now we’ve got three AC voltages being produced by our spinning rotor. But, we need DC voltage to charge the battery and operate the vehicle’s electrical devices.

Electromagnet rotor spinning inside the stator

3. Alternator rectifier

OK… let’s look at how the vehicle alternator produces a DC voltage. How do we convert AC to DC? By using a Rectifier. What’s a rectifier? A rectifier consists of several diodes.

Alternator rectifier

Let’s look at how a diode works. In very simple terms, a diode only allows current to flow in one direction. A diode has two terminals, the Anode and the Cathode.

If the Anode is more positive than the cathode, current will flow through the diode. But if the Anode is more negative than the cathode, current will not flow through the diode.

How a diode works

Alright, let’s see what happens if we apply an AC voltage to a circuit with a diode in it. We end up with an output voltage that is not AC, but a bumpy DC voltage. It’s not a very pretty DC voltage, but we can fix that later.

AC to DC by diode

If we convert AC to DC, we have performed rectification. So, a diode is a type of rectifier.

The Alternator rectifier has more than one diode. More often than not, the alternator rectifier has six diodes. The six diodes are mounted in a heatsinking material to protect them from burning up.

Alternator rectifier diodes

Why so many diodes? Recall that we have three AC voltages being produced in the stator windings. Why not use all three of these voltages? In fact, the diodes are configured in such a way that we rectify and convert both half-cycles of every stator voltage AC voltage.

Alternator diodes.

Diode trio

Recall earlier in this article we mentioned slip rings and brushes that sit on the end of the rotor shaft? Let’s talk about what they do.

As we discussed earlier, the rotor field coil is an electromagnet. How does it become an electromagnet? A DC voltage is applied via the slip rings from two different sources.

The first source is from the battery when the engine is started. The second source is from the alternator itself once the rotor is spinning via a component called the diode trio.

Alright… let’s discuss the diode trio and another component called the voltage regulator.

Diode trios come in all different shapes and sizes but all have three diodes inside.

Diode trio

Exactly like the rectifier, the diode trio input terminals are connected to each stator voltage output. The output terminal of each diode is connected together. The diode trio converts part of the stator output voltages to a DC voltage.

Diode trio Connection

Voltage regulator

The diode trio output is fed to the voltage regulator and becomes the energizing voltage for the rotor electromagnet once the engine has been started and is running.

Voltage regulator

What does a voltage regulator look like? Just like the rectifier, the voltage regulator comes in all different shapes and sizes depending upon the vendor and alternator model.

Voltage regulators

So, what does the regulator do? If you recall, the faster the rotor spins, the more voltage is induced in the stator.

The voltage regulator is an electronic device that acts like an alternator voltage monitor as it keeps an eye on the battery voltage.

The purpose of the voltage regulator is to adjust the electromagnet energizing voltage so that the stator output voltages are held relatively constant regardless of the rotational speed of the rotor.

Purpose of the voltage regulator

Why do we want the stator voltage to be constant? The stator voltage is rectified and then used to charge the battery. The battery and other electrical devices could be damaged if the voltage is too high!

How do all parts work together?

Ok… It looks like we’ve described all of the alternator parts. So… let’s see how all of the parts of an alternator work together:

– The Ignition switch allows the battery to energize the rotor

– The Rotor spins faster as the engine revs up…

– The stator voltages rise

– The Rectifier output battery charge voltage rises

– The Voltage regulator senses the battery voltage rise

– The Voltage regulator reduces electromagnet energizing voltage, and

– The Stator voltages drop

Please keep in mind that our descriptions, drawings, and animations might not exactly match your alternator. As with any electrical device, there are different configurations.


Let’s summarize what we’ve learned:

– An alternator is an electromechanical device that generates a DC voltage and maintains a vehicle battery voltage

– Main alternator components are Rotor, stator, rectifier, diode trio, and voltage regulator

– The rotor spins inside the stator creating three separate AC voltages.

– The stator AC voltages are converted to a DC voltage by the rectifier and applied to the battery and vehicle electrical circuits

– The voltage regulator is an electronic device that keeps the alternator output voltage constant.

– The diode trio output becomes the energizing voltage for the rotor electromagnet once the engine has been started and is running.

If you have any questions about the alternator or about Vehicle’s electrical system or any electrical system in general, add them in the comments below and we will get back to you in less than 24 hours.

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