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How Do Automotive Alternators Work?

By Dale Marshall
Updated May 23, 2024
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Automotive alternators transform the mechanical, or kinetic, energy of an engine into electrical energy that’s used to charge the battery and operate the car’s electrical devices such as headlights, radio and air conditioning system. Before the development of automotive alternators, cars depended on generators to accomplish this, but they had some significant drawbacks. Developed in the early 1960s to overcome generators' shortcomings, automotive alternators were rapidly adopted by automobile manufacturers as a lighter, more reliable replacement.

Generators and alternators both operate on the same principle — when a conductor passes through a magnetic field, an electric current is produced in the conductor. A generator consists of a complex arrangement of wires wound around a framework that’s spun on a pulley by a belt connected to the engine. These "windings" are spun inside a magnetic field created by powerful magnets placed in a fixed shell around the winding framework, creating an electric current in the wires. The current is passed to the battery and the rest of the electrical system by means of a commutator, a system of metal brushes attached to the windings which rub against metal contacts as the windings spin.

An automotive generator’s output, however, is insufficient to charge the battery while the engine is idling. This is a serious drawback, especially in cold climates where cars need to be warmed up before they can be driven. Operators would sit in their cars and rev their engines to produce enough power to keep their batteries charged. This was a dilemma for most drivers — they had to decided if they should waste gas to rev the engine or let the engine idle and risk the battery draining before the engine was properly warmed up. It was this significant disadvantage that alternators were designed to overcome.

The greatest difference between generators and alternators is what gets spun. Where generators spin the windings, automotive alternators spin the magnet; the windings are contained in the shell within which the magnet, or rotor, spins. Even though the windings are fixed, the magnetic field moves around them, and the electric current is created the same as with the generator. The first advantage of this construction is that the connection to the car's electrical system is much easier, with no moving parts to wear out. From the perspective of manufacturers and operators, the most significant advantage is that automotive alternators produce a much stronger current and are able to charge a car’s battery as well as run the various electric components of modern vehicles, all while the engine is idling.

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