An alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy.


Most alternators use a rotating magnetic field. Different geometries - such as a linear alternator for use with stirling engines - are also occasionally used. In principle, any AC generator can be called an alternator, but usually the word refers to small rotating machines driven by automotive and other internal combustion engines.

Theory of operation[]

Alternators generate electricity by the same principle as DC generators, namely, when the magnetic field around a conductor changes, a current is induced in the conductor. In a typical modern alternator, a rotating magnet called the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. 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 (usually in very small machines), 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.

Automotive alternators[]

Alternators are used in automobiles to charge the battery and to power all the car's electric systems when its engine is running. Alternators have the great advantage over direct-current generators of not using a commutator, which makes them simpler, lighter, and more rugged than a DC generator. The stronger construction of alternators allows them to turn at higher speed, allowing an automotive alternator to turn at twice engine speed, improving output when the engine is idling. The availability of low-cost solid-state diodes from about 1960 allowed auto manufacturers to substitute alternators for DC generators. Automotive alternators use a set of rectifiers (diode bridge) to convert AC to DC. To provide direct current with low ripple, automotive alternators have a three-phase winding.

Later automotive alternators have a voltage regulator built into them. Typical car alternators generate the field using a DC current through slip rings. The field current is much smaller than the output current taken from the fixed stator windings, and so heavy duty slip rings are not required. For example, in an alternator rated to produce 70 amperes of DC, the field current will be less than 2 amperes. The voltage regulator operates by modulating the small field current in order to produce a constant voltage at the stator output. In many older designs of car, the field windings are initially supplied via the ignition switch and charge warning light, which is why the light glows when the ignition is on but the engine is not running. Once the engine runs and the alternator is generating, a diode feeds the field current from the alternator main output, thus equalizing the voltage across the warning light which goes out. The wire supplying the field current is often referred to as the "exciter" wire.

This system is simple and avoids the need for a heavy duty switch in the main alternator output circuit, which can carry very high currents—up to 100 amperes (though typical cars have 40–60 ampere alternators). One drawback of this arrangement is that if the warning light fails or the "exciter" wire is disconnected, no priming current reaches the alternator field windings and so the alternator will not generate any power. However, some alternators will self-excite when the engine is revved to a certain speed. The driver may check for a faulty exciter-circuit by ensuring that the warning light is glowing with the engine stopped. Modern systems have more complex electronic monitoring and should alert the driver when such problems occur.

Modern automobiles have invariably AC generator instead of a DC generator. It has inbuilt rectifiers group to convert AC to DC current. This unit becomes more reliable because it avoids the commutator and carbon brushes of DC generators.

Very large automotive alternators used on heavy equipment or emergency vehicles may produce 150 amperes. Very old automobiles with minimal lighting and electronic devices may have only a 30 ampere alternator. Hybrid automobiles replace the separate alternator and starter motor with a combined motor/generator that performs both functions, cranking the internal combustion engine when starting, providing additional mechanical power for accelerating, and charging a large storage battery when the vehicle is running at constant speed. These rotating machines have considerably more powerful electronic devices for their control than the simple automotive alternator described above.

Radio alternators[]

In 1891, Frederick Thomas Trouton gave a lecture which stated that, if an electrical alternator were run at a great enough speed, it would generate wireless energy [1]. Nikola Tesla's Template:US patent, "Method of Operating Arc-Lamps" (March 10, 1891), describes an alternator that produces high-frequency current for that time period, around 10,000 cycles per second (later to be known as hertz). His patentable innovation was to suppress the disagreeable sound of power-frequency harmonics produced by arc lamps operating on frequencies within the range of human hearing. The frequency produced was in the longwave broadcasting range (VLF band).

In 1904, Reginald Fessenden contracted with General Electric for an alternator that generated a frequency of 100,000 Hz for "continuous" radio. E. F. W. Alexanderson designed the Alexanderson alternator, which produced such alternating currents at General Electric. The Alexanderson alternator was extensively used for long wave radio communications by shore stations, but was too large and heavy to be installed on most ships. Alexanderson would later receive Template:US patent in 1911 for his device. The Alexanderson alternator was the first form of radio transmitter to be modulated to carry the sound of the human voice. Like Tesla's high-frequency alternator, the Alexanderson alternator used the principle of periodically varying the magnetic permeability of the field circuit. Such alternators have no moving windings and so are not limited in speed by the presence of slip-rings and brushes.

External articles and further reading[]

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