How Does an Automotive Alternator Work?

Updated July 19, 2017

Your car's alternator supplies the electricity that keeps the battery charged and ensures that the ignition system has enough juice to repeatedly produce voltages in the thousands. An automotive alternator works by the basic principal that a magnetic field spinning inside a coil will produce an alternating current in the coil. But a car's alternator makes use of many parts to perform its function.

Alternator Rotor

As implied by its name, the rotor rotates, driven by a belt connected to the crankshaft pulley. The rotor is made up of field coil windings to which current is supplied via slip rings mounted on the rotor shaft (this has the effect of making the rotor into an electromagnet). To produce alternating poles on the rotor, two claw-shaped pole pieces surround the windings. This ensures that the rotation of the rotor will produce an alternating current in the windings of the stator.

Alternator Stator

The stator (think stationary) has three groups of windings (coils) wrapped around a soft, laminated iron core. When the rotor spins inside the stator, an alternating electric current is induced in the windings of the stator, from which the current is fed to the battery after passing through a rectifier and a voltage regulator. There are two types of stator---the Y-type and the delta-type. A Y-type stator's wire ends from the windings run to a neutral connection, while a delta-type's stator wires are connected end to end. Delta-types are typically used for high-output alternators.

Alternator Brushes

Riding on the slip rings are the alternator brushes. The brushes are the parts of the alternator that deliver current from the battery into the rotor windings. Since the amount of current drawn by the windings is small, the brushes in an alternator are much smaller than the brushes found inside comparably sized electric motors, which use a parallel principle for their operation, namely that current flowing through a coil can cause magnets inside the coil to spin.

Alternator Rectifier (Diode Assembly)

The rectifier's purpose is to convert the alternating current, which is the output of the alternator, into direct current, which is the type of current the car's electrical system requires to function. A common arrangement for a rectifier is to use six diodes (a diode is an electrical component that only allows current to flow in one direction) to bring about full-wave rectification, which makes possible direct current originating from both positive and negative currents.

Voltage Regulator

This part may be located either in or out of the body of the alternator. Essentially, it controls the amount of current going to the rotor windings so as to control the field strength acting on the stator. By doing so, the voltage charging the battery can be kept at a preset level. Voltage regulators may be electronic, or they may be electromechanical.

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About the Author

A reporter since 2005, David Eiranova wrote for "The Lunenburg Ledger," from 2007 to 2009 and has served as a correspondent for "The Lowell Sun." He holds a Bachelor of Arts in economics. Since 2007 he has been the director of publicity for the Acton Community Chorus.