What is a car alternator and how does it work?

What is it, how does it work and what defects can occur in a car alternator?

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What is a car alternator?

The alternator provides the electricity needed to operate a vehicle. When starting the car the battery it is the one that provides the necessary energy, after which the alternator comes into action. It charges the battery continuously, providing the necessary voltage for all electrical and electronic components of the car.

The alternator is actually a electric current generator. Its rotor is driven by the car engine through a drive belt.

By rotating the rotor, which is actually a magnet, inside the stator, a alternative induced voltage. The alternating voltage is transformed into direct voltage and stabilized by means of diodes and voltage regulator.

The constituent elements of the alternator

  1. Housing
  2. Stator
  3. Rotor
  4. Voltage regulator
  5. Bearing
  6. Manifold rings
  7. Diode rectifier bridge
  8. Rear fan
  9. Front fan
  10. Bearing

The alternator is a compact equipment, with small dimensions and relatively simple composition. Modern cars, which incorporate many electronic systems, require a fairly large amount of energy, so alternators must produce up to 2 kW / h.

Alternator housing

The alternator housing is usually made of aluminum and has the role of housing the other pieces inside. Through it, the alternator is mounted inside the engine compartment.

Another role of the housing is to protect the sensitive elements inside from the penetration of water and dust that could cause less or more damage.

Alternator stator

The stator is a metal body consisting of several thin sheets, over which are wound several turns of copper. This piece has the role of taking over the electric current produced by the movement of the magnets over the copper network. As the name suggests, the stator is an immovable part.

Alternator rotor

The rotor is moving part of the alternator, placed on a shaft inside the stator. It has the role of producing a magnetic field by spinning. On the rotor there is a rotor winding of copper wire and two apparent magnets, which generate a magnetic field.

The power supply is made with the help of some collector rings, the contact being ensured through a two graphite brushes.

Alternator voltage regulator

The voltage regulator of the alternator has the role of providing power with a constant voltage, regardless of engine speed. By means of this part the intensity of the magnetic field of the rotor is regulated, which leads to the production of an electric current with a regular voltage of about 14.2 V.

The voltage regulator is mounted on the alternator housing, which also has the role of cooling the part.

Car axle rectifier

The rectifier bridge contains 6 diodes integrated in an aluminum radiator. For each phase of the alternator, 2 diodes are used to transform the alternating current into electric current.

Also the rectifier bridge with more diodes has the role of blocking the flow of current from the battery to the alternator, if the alternator voltage drops below the battery voltage. The rectifier bridge is integrated in the alternator housing at the rear.

In addition to these main components, there is another bearing in the composition of the alternator which ensures the smooth and effortless rotation of the rotor, two fans in the front and rear for additional cooling of the assembly and a pulley on the outer end of the rotor, which connects the motor and alternator through the accessory belt.

Signals transmitted by a faulty alternator

An alternator that no longer works properly can lead to other defects, the repair of which can be much more expensive than checking and repairing the alternator.

In the following, we present to you the symptoms of an alternator that no longer works in the parameters.

Weak lights

If the alternator no longer works properly, you will notice that the car's lights (headlights, taillights, turn signals, on-board lights, etc.) will be dimmed from normal and the power windows will open heavily.

Rubber smells would make strange noises

If the alternator bearings have failed (seized, broken, worn, etc.), the rotor can no longer sustain the rotational movement imposed by the motor and then the drive belt begins to slip and friction occurs. By this friction, it also heats up gives off a "burning" smell.

Also, if you hear strange noises, the drive belt is worn or loose.




  1. There are two types of defects - mechanical and electrical.
    Bearing seizure is the most common for mechanical defects and for electrical defects starting from the loading relay, coal consumption, burning of the fusible fuse, burning of one or more diodes and less often stator frying.

  2. The rotor is electromagnetic. The stator has three-phase windings. The resulting voltage is rectified with a three-phase bridge.
    The rotor is energized by the voltage regulator which limits the charging voltage to 14.4V. When the voltage reaches the preset load value, the current in the rotor decreases, thus generating a lower voltage in the stator.
    The rotor coil is fed through two rings and two brushes (two coals).

  3. It is a three-phase synchronous generator. The rectifier is with three-phase rectifier bridge. Although it rotates in bearings, to produce 2kW, it consumes more than that, over 2,5 KW. A magnetic braking field must be defeated. That is, the alternator consumes 3.35 hp of engine power.

  4. The rotor is an electromagnet! By controlling the voltage applied to the rotor, the value of the voltage in the stator can be controlled, depending on the motor speed.

  5. Alternators produce a 3 phase AC voltage and then use a rectifier bridge to convert it to DC. A quick test to check an alternator is to set your digital volt meter to AC, if it reads more than .5 volts the rectifier is shorted. I have rebuilt alternators for years. If anyone has any questions feel free to ask.

  6. Battery charger, regulated by 12v, it is a dynamo or generator operated by other rotating machine or motor to produce alternating current.

  7. Common deffects: faulty voltage regulator, carbon brush, cut in rotor winding, burnt stator windings rare deffect on rectifier diodes.

  8. Vintage cars had DC alternators that use a built-in commutator to provide DC, rather than have an AC alternator using diodes to provide DC.

  9. The alternator is designed to maintain system voltage at a specified level and supply power when electrical demands increases as lights wipers a / c ect… By design the alternator is not designed to charge depleted batteries on a continuous basis. It will charge the battery but if the battery is bad, or an excessive parasitic drain causing the alternator to work harder than normal generating excessive heat is damaging the diodes or the stator sometimes both. Seldom does a rotor burn up but they can. Some alternators by design use cheap diodes rectifiers bridge assemblies, some of the truck or heavy equipment alternators use extreme duty diodes with much higher amperage ratings.

  10. Maybe we should find out more about the alternator.
    An apparatus, a three-phase generator with an excitation coil whose supply voltage is electronically controlled to obtain a variable voltage as needed. In addition to charging after starting, the alternator practically supplies any electrical consumer on the car, from the moment it is started. And, a feature to know, for newer cars, the alternator can provide power consumption over 100 A. Something we should know is that in fact the bulbs on the car also consume all the fuel. The culprit is the alternator that consumes mechanical energy to generate power for electricity consumers, this makes the motor consume more.

  11. The rotor is an electro-magnet! I don't want to be smart but I had to correct! only magnets work on magnets, that's why they are called that! 😉

  12. Tnx for sharing. I got an overview of this electrical piece of hardware on vehicles. Reading the comments on the article itself will give you further clarity.

  13. It is an alternator, it works by converting mechanical motion into electrical energy and the main defect would be if it quit alternating… .🤪

  14. Old ones don’t much to them to actually fail. Just bits that move wearing and corrosion.
    Modern ECM controlled can be a fair bit more complex. Engine bays have harsh environments for electronics.

  15. What defects can occur? Overheating due to excessive heat idling on traffic with the ac running full blast. Poor rebuilding techniques such as soldered slip ring terminals instead of spot welding. Too big pullys installed not allowing proper cooling and causing excessive field amperage. Some of these issues are well addressed by advanced regulation and software algorithms but this inevitably causes other issues such as battery reliability and electrical accessory peformance.

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