Alternator: parts, how it works, how to check it

El Alternator of a car is one of the main elements of the electrical circuit. Its purpose is to transform mechanical energy into electrical energy to charge the battery and provide the necessary electrical supply to the car when it is running.

To find out how does an alternator work, first we must know what parts form it and what mission they have.

parts of an alternator

parts of an alternator

The basic parts of a standard alternator include a rotor, stator, bridge rectifier and brushes, in addition to the presence of the regulator. Below we detail each one:


It is the element that receives, through a auxiliary strap, the mechanical force generated by the car engine with the crank pulley.

It is attached to the alternator shaft and its purpose is to move the rotor inside it. In the new generation alternators, there is a small fan that helps them cool down and it is moved in turn by this pulley.

alternator pulley

rotor or inducer

It is made up of a electromagnet that produces a magnetic field as a result of the current it receives from the regulator through the slip rings located on the axis.

When this magnetic field is active, the stator coils (fixed part of the alternator) react by producing the necessary electrical current.

alternator regulator

As the current generated by the alternator varies according to the revolutions of the motor, it needs a voltage regulator. If not, it could go up to 18V or more, when what the battery requires is normally between 13,5V and 14,5V.

The voltage allowed by the regulator depends on how charged the battery is already and the temperature to which it is That is, several parameters are taken into account to take care of the battery properly.

alternator of a car


Is induced and fixed element of the alternator. The three-phase winding that allows the reaction and therefore the electric current is located in it. Its shape can be triangle or star.

diode bridge rectifier

This system is responsible for transforming the alternating current that is created in the alternator into direct current (which is what the battery needs and is used by the different car systems).

How the alternator works

Once we know the main elements that make up an alternator, we will move on to describe its operation in a very simple way.


Inserting the key into the ignition and turning it the alternator rotor coil receives current from the battery. Once we start the engine, the rotor coil begins to rotate and goes from receiving energy to generating it. In this simple way, it goes to be self-sufficient and provide the necessary energy to meet the demand for systems that facilitate driving and life on board the car, such as air conditioning, power steering or the audio system.

If Alternator works properly the light on the instrument panel will turn off. If after starting the car the light does not turn off, it could be an indication that the alternator does not provide current to auxiliary systems. If so, it will be necessary to check that the 12 volts from the battery reach the input of the regulator and that the diodes of the rectifier bridge are not in poor condition. In addition, also it could happen that the brushes of slip rings are in bad shape and therefore the necessary current would not reach the rotor.

How to check a car's alternator (video)

Checking an alternator is very simple. In the following video you can see how to do it in just 1 minute with a multimeter. If you do not have one, there are very cheap that fulfill this function and others more than enough.

Images 3 and 4 – Ben Chun, Robert Couse-Baker

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  1.   tanks said

    very good information

  2.   power_claudy said

    Hi, I have a citroen c 15 petrol company, the regulator is bad. I don't have a battery charge anymore.

  3.   Kleber-Viera said

    Simple and practical description, congratulations. Can you explain to me why there are regulators that have an active lamp type and others that are inactive?

  4.   romer vecchionacce said

    Good, I have an A127 Prestolite alternator. I would like to know what vehicle this type of alternator is for.

  5.   DANIEL said


  6.   brandor said

    what is its operation

  7.   Manuel Gutierrez said

    I measure the voltage and the alternator sends between 12.7 to 14 volts, the battery is new but the light on the battery and parking brake panel illuminates dimly. what is it that is bad

  8.   LILIANA said


  9.   fernando gomez said


  10.   fernando gomez said

    excellent information take it to THE CLINICA CONHCETUMARE

  11.   Ana Maria Martinez MARTINEZ said

    very explanatory. you have to think to retain the technical specifications congratulations

  12.   George said

    I have a hyundai elantra, I changed new alternator and battery.
    The problem I have is that the alternator sometimes reads and sometimes not.
    what can it be???

    Thank you

  13.   steven silva said

    very good information ubiesen complemented by a diagram. Thank you