An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature. Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In principle, any AC electrical generator can be called an alternator, but usually the term refers to small rotating machines driven by automotive and other internal combustion engines. An alternator that uses a permanent magnet for its magnetic field is called a magneto. Alternators in power stations driven by steam turbines are called turbo-alternators. Large 50 or 60 Hz three phase alternators in power plants generate most of the world’s electric power, which is distributed by electric power grids.
Alternators may be classified by method of excitation, number of phases, the type of rotation, cooling method, and their application.
There are two main ways to produce the magnetic field used in the alternators, by using permanent magnets which create their own persistent magnetic field or by using field coils. The alternators that use permanent magnets are specifically called magnetos.
In other alternators, wound field coils form an electromagnet to produce the rotating magnetic field.
A device that uses permanent magnets to produce alternating current is called a permanent magnet alternator (PMA). A permanent magnet generator (PMG) may produce either alternating current, or direct current if it has a commutator.
Direct connected DC generator
This method of excitation consists of a smaller direct-current (DC) generator fixed on the same shaft with the alternator. The DC generator generates a small amount of electricity just enough to excite the field coils of the connected alternator to generate electricity. A variation of this system is a type of alternator which uses direct current from the battery for initial excitation upon start-up, after which the alternator becomes self-excited.
Transformation and rectification
This method depends on residual magnetism retained in the iron core to generate weak magnetic field which would allow a weak voltage to be generated. This voltage is used to excite the field coils for the alternator to generate stronger voltage as part of its build up process. After the initial AC voltage buildup, the field is supplied with rectified voltage from the alternator.
A brushless alternator is composed of two alternators built end-to-end on one shaft. Smaller brushless alternators may look like one unit but the two parts are readily identifiable on the large versions. The larger of the two sections is the main alternator and the smaller one is the exciter. The exciter has stationary field coils and a rotating armature (power coils). The main alternator uses the opposite configuration with a rotating field and stationary armature. A bridge rectifier, called the rotating rectifier assembly, is mounted on the rotor. Neither brushes nor slip rings are used, which reduces the number of wearing parts. The main alternator has a rotating field as described above and a stationary armature (power generation windings).
Varying the amount of current through the stationary exciter field coils varies the 3-phase output from the exciter. This output is rectified by a rotating rectifier assembly, mounted on the rotor, and the resultant DC supplies the rotating field of the main alternator and hence alternator output. The result of all this is that a small DC exciter current indirectly controls the output of the main alternator.
Small-scale examples are ubiquitous in engine-driven motive-power applications. For example, early Honda four-cylinder motorcycles (CB750F, CB350F, CB500F, CB550F) used a brushless Hitachi 200W alternator. This had a fixed “rotor” winding on the outer cover; the outer end of the iron core was a disc that closed the outer rotor pole. The rotor comprised two intermeshed six-pole “claws” welded to and spaced apart by a non-magnetic ring. This was bolted directly to the end of the five-bearing crank via the hub of one pole. The other pole had an open end to receive the stator winding. The outer cover also held the three-phase stator windings. The magnetic circuit had two auxiliary air gaps between the rotor and its stationary core. The regulator was a conventional automotive type with vibrating points. As it had no slip rings, it was very compact and rugged, but due to the auxiliary air gaps, it had poor efficiency.
By number of phases
Another way to classify alternators is by the number of phases of their output voltage. The output can be single phase, or polyphase. Three-phase alternators are the most common, but polyphase alternators can be two phase, six phase, or more.
By rotating part
The revolving part of alternators can be the armature or the magnetic field. The revolving armature type has the armature wound on the rotor, where the winding moves through a stationary magnetic field. The revolving armature type is not often used. The revolving field type has magnetic field on the rotor to rotate through a stationary armature winding. The advantage is that then the rotor circuit carries much less power than the armature circuit, making the slip ring connections smaller and less costly; only two contacts are needed for the direct-current rotor, whereas often a rotor winding has three phases and multiple sections which would each require a slip-ring connection. The stationary armature can be wound for any convenient medium voltage level, up to tens of thousands of volts; manufacture of slip ring connections for more than a few thousand volts is costly and inconvenient.
Many alternators are cooled by ambient air, forced through the enclosure by an attached fan on the same shaft that drives the alternator. In vehicles such as transit buses, a heavy demand on the electrical system may require a large alternator to be oil-cooled.  In marine applications water-cooling is also used. Expensive automobiles may use water-cooled alternators to meet high electrical system demands; the additional cost of purchase and maintenance is not a sales liability in this market segment.
Most power generation stations use synchronous machines as their generators. Connection of these generators to the utility grid requires synchronization conditions to be met.
Diesel electric locomotive alternators
The traction alternator usually incorporates integral silicon diode rectifiers to provide the traction motors with up to 1200 volts DC (DC traction, which is used directly) or the common inverter bus (AC traction, which is first inverted from dc to three-phase ac).
The first diesel electric locomotives, and many of those still in service, use DC generators as, before silicon power electronics, it was easier to control the speed of DC traction motors. Most of these had two generators: one to generate the excitation current for a larger main generator.
Optionally, the generator also supplies head end power (HEP) or power for electric train heating. The HEP option requires a constant engine speed, typically 900 RPM for a 480 V 60 Hz HEP application, even when the locomotive is not moving.
Marine alternators used in yachts are similar to automotive alternators, with appropriate adaptations to the salt-water environment. Marine alternators are designed to be explosion proof so that brush sparking will not ignite explosive gas mixtures in an engine room environment. They may be 12 or 24 volt depending on the type of system installed. Larger marine diesels may have two or more alternators to cope with the heavy electrical demand of a modern yacht. On single alternator circuits, the power may be split between the engine starting battery and the domestic or house battery (or batteries) by use of a split-charge diode (battery isolator) or a voltage-sensitive relay.
High frequency alternators of the variable-reluctance type were applied commercially to radio transmission in the low-frequency radio bands. These were used for transmission of Morse code and, experimentally, for transmission of voice and music. In the Alexanderson alternator, both the field winding and armature winding are stationary, and current is induced in the armature by virtue of the changing magnetic reluctance of the rotor (which has no windings or current carrying parts). Such machines were made to produce radio frequency current for radio transmissions, although the efficiency was low.