How Do Automotive Alternators Work

Alternators are electromechanical devices that convert mechanical energy into alternating-current (AC) electrical energy. This process is useful in an automotive context as it allows the vehicle to self-charge its battery while being driven.

In an automotive alternator, the mechanical energy is delivered by the vehicle’s crankshaft, which rotates. This rotational energy is passed via a drive belt and pulley to the alternator, and replicates it in an internal rotor shaft.

The turning of the alternator’s rotor shaft causes an attached iron core, surrounding field winding and set of staggered magnetic claw poles to rotate at high speed (up to thousands of times per minute). This entire assembly is referred to as the alternator’s rotor, with it slotting into another element called the stator.

The alternator’s stator is a laminated soft iron, roughly spherical component wrapped with, typically, three sets of copper phase windings. The stator, unlike the rotor, is fixed in place, attached to the inside of the alternator’s housing. As mentioned, the rotor sits within the stator while it spins, with the two offset slightly to avoid any direct contact.

As the rotor assembly rotates the staggered magnetic claw poles (with north and south poles alternating) generate a magnetic field. Because the field lines continuously change, however – due to the north-south polarity of the claw poles – the flux within the stator changes too, inducing an alternating current to flow through its phase windings.

Automotive AlternatorsAs the current in the stator’s phase windings is alternating, it needs to be converted into direct current (DC) for use in battery charging. This is achieved by feeding the alternating current in each phase winding through stator leads and into a set of diodes (two for each lead). Known as rectifiers, these diodes ensure that current flows in a single direction.

The total flow of direct current from each of the phase windings combined is controlled by a regulator unit. This prevents an excess of direct current from being fed into the vehicle’s battery – something that if left unchecked would cause it to overcharge and potentially explode.

Alternator anatomy

Casing – The outer housing of the alternator is made from aluminium. This material is used as it reduces weight, dissipates heat and does not magnetise.

Regulator – This controls the distribution of the electrical energy that the alternator produces, ensuring a safe power supply to the vehicle’s battery and electrical systems.

Diode assembly – The diodes convert the AC energy produced by the alternator into usable DC by only letting current move in one direction.

Stator – The stator is a stationary set of copper coils (phase windings) that the alternator’s rotor slots between. The stator acts as an armature, inducing voltage due to the influence of the rotor-generated magnetic field.

Rotor assembly – The rotor is made up of claw poles placed around a series of field windings and an iron core. The poles alternate in a staggered pattern to induce flux, and thus current, in the stator.

Pulley – The pulley holds the engine’s drive belt, which is connected to the vehicle’s crankshaft. This supplies the alternator’s rotor shaft with rotational energy.