The alternator is universally used in automotive applications. It converts mechanical energy into electrical energy, by electro-magnetic induction.
In a simple version, a bar magnet rotates in an iron yoke which concentrates the magnetic field. A coil of wire is wound around the stem of the yoke. As the magnet turns, voltage is induced in the coil, producing a current flow. When the North pole is up, and South is down, voltage is induced in the coil, producing current flow in one direction.
As the magnet rotates, and the position of the poles reverses, the polarity of the voltage reverses too, and as a result, so does the direction of current flow.
Current that changes direction in this way is called alternating current, or AC. The change in direction occurs once for every complete revolution of the magnet.
Theory of operation
Alternators generate electricity by the same principle as DC generators. When magnetic field lines cut across a conductor, a current is induced in the conductor. In general, an alternator has a stationary part (stator) and a rotating part (rotor). The stator contains windings of conductors and the rotor contains a moving magnetic field. The field cuts across the conductors, generating an electrical current, as the mechanical input causes the rotor to turn.
The rotor magnetic field may be produced by induction (in a "brushless" generator), by permanent magnets, or by a rotor winding energized with direct current through slip rings and brushes. Automotive alternators invariably use brushes and slip rings, which allows control of the alternator generated voltage by varying the current in the rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor but are restricted in size owing to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator. Brushless AC generators are usually larger machines than those used in automotive applications.
Source: CDX Global & Wikipedia - en.wikipedia.org
