What is an automotive alternator?
The alternator is a component that is responsible for converting the rotation of an axis to electricity. It is considered as one of the most important elements within the car since it is in charge of generating and supplying the electrical current that makes the circuits that need it work, for example: windscreen wipers, lights, fuel supply, audio devices, windows, engine control system, etc.


The alternator is also used to keep the battery charged, this is used to start the car through the starting system and also to be able to use electrical and electronic devices while it is off.

battery charge

Alternator parts
Despite having a somewhat complex operation to understand, the alternator has only a few main components for its operation.

Although a rotor is practically a rotating shaft that incorporates a coil, two metal plates and two sliding discs. All of these materials are coupled together and used to create an electromagnet. The two disks are the positive and negative terminals of the coil and when we connect a direct current to them the plates are magnetized one at the south pole and the other at the north.

Alternator rotor

This ring-shaped part is fixed and houses three coils that can be connected in two different ways in star or delta.

Three-phase stator

Rectifier diode bridge

This is an essential part of the alternator since it rectifies the alternating current generated by the rotor and stator and converts it into direct current through an arrangement that can be made up of 6 or 9 diodes.

Diode bridge

Voltage regulator
Since the voltage generated by the alternator depends on the revolutions generated, it has considerable variation and that is why a device is needed that is capable of maintaining the voltage within the preset ranges.

Although there are different types of regulators, some older than others, the operating principle is the same. When the maximum voltage is exceeded this device disconnects the coil from the rotor to stop generating voltage and when it is below the minimum, reconnects it.

Alternator regulator

It practically serves to house and keep all components protected from external agents.

Alternator housing

Being in constant operation and due to electromagnetic induction, all the components heat up and that is why a fan is placed on the rotor shaft so that it rotates at the same speed and cools the components with air.


Distribution band
This band is placed next to the crankshaft pulley in order to transmit the rotation to the alternator, therefore these two always rotate at the same time and at the same speed.

Alternator band

How does an alternator work?
The operating principle of this device is based on electromagnetic induction through an electromagnet that is generated by electricity from the battery.


When we start the car’s engine, the rotor automatically begins to rotate through the distribution band and the coil is also polarized so that the electromagnet with its respective north and south poles is generated.

Alternator timing belt

If the rotor is rotating and magnetically polarized, an alternating current is generated in each of the stator coils. This current is usually triphasic because an array of three coils is placed and each coil generates a signal independent of the others.

Alternator signal

As a next step, there is an arrangement of rectifying diodes that convert the alternating current into positive but variable voltage, in order to generate a signal without variations, a regulator is placed that maintains it within the acceptable ranges, to be able to supply it to all electrical circuits and be able to charge the car battery.

Alternator diode bridge

Alternator Types

We can find different types that can be differentiated either by their characteristics and / or the type of vehicle to be used.

Compact first and second generation
They are alternators that are composed of 12 poles and were designed mainly for vehicles with high energy demand such as tourism, the difference between the first and second generation is that they were improved in terms of power and size.

Within this type we can also find the liquid coolant, this uses the engine coolant and usually has a higher power and lower noise due to its better cooling system.

Compact alternator

This type is very similar to the compact ones but it has a characteristic that makes it different and this is that it allows a higher turning speed. 18,000 rpm for compacts vs. 20,000 rpm for monoblocks.

Monoblock alternator

Pole interspersed with slip rings
This is practically the best known and the magnetic poles of the rotor are claw-shaped and, as the name implies, are interspersed.

Alternator rotor

Single pole with slip rings
These are used when high power and high current consumption greater than 100A and a voltage of 24v are required.

Note because we do not have an image of a single pole alternator, we put this image of a similar rotor.

With guide rotor without slip rings
Within this type, the main idea is to use the fewest parts and have the least wear possible, due to this concept the only parts that wear out are the bearings. This alternator is used in transports that have too long journeys and also for vehicles with high efforts.

Mechanics fans

Automotive electrical system

Book: Automotive Electricity and Electronics (Alternators) Bosch