What is an alternator and how does it work?

Many people think the battery powers all the electrical equipment on board a car, from the windshield wipers, headlights or stereo system. They may be surprised to learn that it is not in fact the battery but the alternator that supplies most of the electricity to the electrical equipment on board a car. The battery is mainly used for the starter motor to start the engine and to run electrical equipment when the car is switched off.

The alternator is an essential component of the electrical system of a car, and it is good to know how it operates within the car.

What is an alternator?

An alternator is a device whose purpose is to generate electricity and supply it to the various electrical equipment in the car, and also to recharge the battery. All cars with an internal combustion engine have an alternator with the exception of some hybrid cars which have large battery packs that store electrical energy. The alternator is mounted at the front of an engine and is run by a belt that draws power from the rotating crankshaft of the engine.

The alternator is a component that converts mechanical energy into alternating current (AC) which is a kind of electrical energy.

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How does an alternator work?

The engine’s crankshaft has a belt that drives the alternator. The alternator generates electrical current for the vehicle. While the alternator rotates, a direct current (DC) voltage is created which recharges the battery. The battery does the job of starting the engine. However, when the engine is running, it is the alternator that powers all the electrical devices and components in the vehicle.

What is the difference between an alternator and a generator?

What are the major differences between an alternator and a generator? An alternator is an electrical component that converts mechanical energy into alternating current (AC). On the other hand, a generator is an electrical component that converts mechanical energy into either direct current (DC) or alternating current (AC).

So technically speaking, an alternator can be called a generator.

Major components of an alternator

Depending on the type of alternator, its components may vary. But there are some parts that are found in all types of alternators. This is the rotor, stator and rectifier. There are more components related to an alternator which we will get into subsequently.

1. Rotor

While the vehicle engine is running, there is belt and pulley attached to the crankshaft. This drives the rotor of the alternator. The drive belt pulley system rotates a shaft attached to the rotor on the alternator.

In the middle of the rotor is an electromagnet which is often referred to as the field winding. It is called the field windings because of a wire coil that is tightly wound around the electromagnet. The electromagnet consists of field windings wrapped around a magnetic material.

An electric voltage is applied across the electric coil wires which produces a current in it. A resultant magnetic field is created around the coiled wires. Just like a permanent magnet, a North pole and South pole is created.

The rotor also has alternating North and South pole pieces placed along the electrical field windings wrapped around the iron core on the rotor.

2. Stator

The rotor is placed inside the stator. The stator is so called because it is stationary within the alternator. The rotor rotates inside the stator of the alternator without actually touching it. On either end of the stator is a brush with slip rings attached to it.

The stator has three different wire coil windings with one end of each of the coil windings being attached to one another.

The stator coil windings are placed at intervals of 120 degrees around the iron core. The rotor spins within the stator with the three coils of wire and produces an electric current. But how does it provide this potential difference to drive a current?

Physics explains that a voltage can be induced in electrical coil windings if the coil is rotated through a magnetic field.

The coil is stationary within the stator, and an electric voltage is induced in the coil if a magnetic field is rotated past the coil. The faster the magnetic field rotates past the coil, the higher the voltage that is produced.

The rotor spinning within the stator causes an induced voltage across the electrical coil windings of the stator due to the rotating magnetic field of the rotor.

3. Slip rings and brushes

The brushes and slip rings are required for the magnetic field. The rotor is an electromagnet due to an applied voltage on its field windings. The voltage is applied to the field wires via the slip rings. But where does the field voltage come from?

We have to look back at the rotor, which has now become an electromagnet spinning inside the stator. A voltage will be induced in each of the three wire coils of the stator. The induced voltage will be an alternating current (AC) because of the rotor switching its poles every time it makes one full rotation.

What we end up with is three separate voltages each 120 degrees out of phase with each other. This is due to the placement of the wire coil windings within the stator iron core.

The result is three alternating current (AC) voltages produced by the rotating rotor. However, a direct current (DC) is required to recharge the battery and power the electrical components in the vehicle. So, how is this direct current produced?

4. Rectifier

A rectifier is required for the alternator to produce a direct current (DC). The question is how is an alternating current converted into direct current. The conversion of AC to DC is managed by the rectifier.

What is a rectifier? A rectifier consists of a number of diodes.

Let’s have a look at how a diode works. A diode allows current to flow in only one direction. The diode is made of two terminals. A cathode and an anode.

The anode is the positive terminal and the cathode is the negative terminal. If the anode has a positive charge, a current is allowed to flow through the diode. But if the anode receives a negative charge, it will not allow current to flow through the diode.

What will happen when we apply an alternating current (AC) voltage to a circuit with a diode in it. We receive a direct current (DC) as the output voltage. The DC is not exactly stable, but we can rectify the bumpy signal further.

Conversion of AC to DC is known as rectification. A diode produces rectification in a circuit. So, it is safe to assume that the diode acts as a rectifier.

The alternator often has more than one diode in its circuit. Usually, the alternator will have six diodes in its assembly. The diodes are placed in a heat sinking material component to keep them from burning up due to the intense heat that is produced in them.

The reason we have so many diodes in the alternator is because we have three voltages separated by 120 degrees produced in the stator. So, in order to make use of all three voltages we will dedicate two diodes for each phase of the current. The diodes are designed in such a way that it will rectify and convert both half cycles of all three stator AC voltages.

5. The three diodes

Brushes and slip rings are present at the ends of the rotor shaft. The rotor electrical field coil windings are an electromagnet due to the DC voltage applied through the slip rings at either ends of the rotor shaft from two different sources. The battery provides the first source to the rotor when the engine is started. Once the rotor starts spinning, the alternator produces a voltage which is supplied as the second source on the other end of the rotor shaft. The component that is responsible for the second source is known as the diode trio.

There is another component here along with the diode trio known as the voltage regulator. Diode trios are essentially made up of three diodes. Just like the rectifier, the diode trio input terminals are connected to the stator output terminals that produce the voltage output. The output terminals of the stator are all connected together. The diode trio converts the alternating current (AC) from the stator output into direct current (DC) voltage.

6. Voltage regulator

The output from the diode trio is fed to the voltage regulator and becomes the second source for the rotor electromagnet once the engine is started and running. The voltage regulator looks just like the rectifier. It comes in different types depending on the vendor.

As mentioned earlier, the faster the rotor spins, results in a higher voltage produced by the stator. The voltage regulator in an alternator acts as a monitoring device for the voltage as it follows the battery voltage.

The main purpose of the voltage regulator is to adjust the energising voltage of the stator so that the output voltage remains somewhat stable irrespective of the rotational speed of the rotor.

We need the stator voltage to be constant because the stator voltage is rectified before being sent to the battery. If the output voltage from the alternator is too high due to high rotational speeds, then the battery and other electrical devices could be damaged.

7. Cooling fan

Alternators produce a lot of heat and need to be cooled down efficiently in order to output the right voltage. Alternators are built with vents and fins in their aluminium casing in order to dissipate heat produced by them. They are also fitted with cooling fans to get rid of this excess heat. New alternators have internal cooling fans, while older models of alternators use external cooling fans for the dissipation of heat.

How do the components of an alternator work together?

After having described all the parts of the alternator in detail, let’s see how all of them come together.

  1. The ignition switch is turned on and the rotor get energised.
  2. The rotor of the alternator begins to spin faster as the engine rpms rise.
  3. The voltage across the stator begins to rise.
  4. The output at the rectifier and the voltage across the battery starts to rise.
  5. he voltage regulator senses the rise of voltage across the battery.
  6. The voltage regulator stabilises the electromagnet energising voltage.
  7. The stator voltage drops and stabilises.

How does the alternator charge the battery?

The voltage coming from the alternator is converted to direct current (DC) voltage before it arrives at the battery. Car batteries operate on direct current (DC) while the voltage from the alternator is alternating current (AC). The rectifier is used for this conversion.

The direct current (DC) voltage is used to recharge the battery. Like all vehicle components, the alternator undergoes wear and tear. If the alternator is not outputting the correct voltage, the battery may not fully recharge and this may lead to starting trouble the next time you try to crank up the engine.

What is the difference between a dynamo and an alternator?

Alternators and dynamos are both devices that convert mechanical energy into electrical energy. Dynamos have the same major components as compared to alternators with a few differences. However, the main difference between them is the type of current they output. Dynamos produce direct current (DC) whereas alternators produce alternating current (AC) which is rectified into DC before it charges the battery and powers the electrical components in a vehicle.

Additionally, the dynamo is not as reliable as an alternator and a dynamo-based system is less efficient than an alternator-based system.


The car battery is an essential component if you want to start the engine, but once the car is up and running, it is the alternator that does all the electrical magic to keep the car running. The alternator is responsible for powering all the electrical components when the car is running or idling with the engine on. Components like dashboard instruments, windshield wipers, stereo, power windows, power steering, headlights, etc are all powered by the alternator in the engine. The alternator supplies these components with direct current (DC) and the alternator is also responsible for recharging the battery of the car.

Like all components in a vehicle, the alternator is subject to wear and tear. A worn-out alternator will not be able to recharge the battery and will have trouble powering all the electrical equipment on board. So, it is a good idea to keep the alternator well maintained by having it inspected by a certified mechanic periodically as the car goes in for service.

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