Alternator

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.

History

Alternating current generating systems were known in simple forms from the discovery of the magnetic induction of electric current. The early machines were developed by pioneers such as Michael Faraday and Hippolyte Pixii. Faraday developed the "rotating rectangle", whose operation was heteropolar. Thompson, Sylvanus P., Dynamo-Electric Machinery. pp. 7 The first public demonstration of a more robust "alternator system" took place in 1886.Blalock, Thomas J., "Alternating Current Electrification, 1886". IEEE History Center, IEEE Milestone. (ed. first practical demonstration of a dc generator - ac transformer system.) Large two-phase alternating current generators were built by a British electrician, J.E.H. Gordon, in 1882. Lord Kelvin and Sebastian Ferranti also developed early alternators, producing frequencies between 100 and 300 hertz. In 1891, Nikola Tesla patented a practical "high-frequency" alternator (which operated around 15,000 hertz). Tesla, Nikola, "US447921 Alternating Electric Current Generator". USPTO. After 1891, polyphase alternators were introduced to supply currents of multiple differing phases. Thompson, Sylvanus P., Dynamo-Electric Machinery. pp. 17 Later alternators were designed for varying alternating-current frequencies between sixteen and several hundred hertz, for use with arc lighting, incandescent lighting and electric motors.Thompson, Sylvanus P., Dynamo-Electric Machinery. pp. 16

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.

A rotating magnetic field is a magnetic field which periodically changes direction. Up until the identification of this concept, generators, such as the homopolar generator and Gramme dynamo, operated by continually passing a conductor through a stationary magnetic field. The key principles of the rotating magnetic field to the operation of alternating-current motor allowed the magnetic field to rotate. In 1882, Nikola Tesla identified the concept of the rotating magnetic field. Tesla had eariler suggested that the commutators from the generator could be removed and the machine could operate on a rotary field of force. Professor Poeschel, his teacher, stated that would be akin to building a perpetual motion machine. "Tesla's Early Years". PBS. In 1885, Galileo Ferraris independently researched the concept. In 1888, Tesla gained for his work. Also in 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.

3phase-rmf-60fv2-airopt.gif of the magnetic field vectors of the stator coils produces a single rotating vector of resulting rotating magnetic field.]]

A symmetric rotating magnetic field can be produced with two coils, driven by alternating currents with a relative delay of one-quarter of one period of the current. Three coils will have to be driven by a symmetric 3-phase AC sine current system, thus each phase will be shifted 120 degrees in phase (one-third of the period) from the preceding phase. For the purpose of this example, magnetic field is taken to be the linear function of coil's current. The result of adding the fields (eg., sum of three 120-degrees phased waves) of the three coils is a single magnetic field, of constant magitude, which rotates in space at the same frequency as the applied currents, or in a synchronous manner. The position of the N pole of the rotor is adjusted to S pole of the stator's magnetic field, and vice versa.

A permanent magnet in such a field will also rotate so as to maintain its alignment with the external field. A rotating magnetic field can be constructed using two phase coils with 90 degrees phase difference in their AC currents. However, in practice such a system would be supplied through a three-wire arrangement with unequal currents. This inequality would cause serious problems in standardization of the conductor size and in order to overcome it, three-phase systems are used where the three currents are equal in magnitude and have 120 degrees phase difference. Three similar coils having mutual geometrical angles of 120 degrees will create the rotating magnetic field in this case. The ability of the polyphase system to create a rotating field for electric motors is the main reasons why three phase systems predominate in electric power supply systems.

Because magnets degrade with time, synchronous motors and induction motors use short-circuited rotors (instead of a magnet) following rotating magnetic field of multicoiled stator. (Short circuited turns of rotor develop eddy currents in rotating field of stator which (currents) in turn move the rotor by Lorentz force).

Note that the rotating magnetic field can actually be produced by two coils, with phases shifted about 90 degrees, but such field would not be symmetric due to difference between magnetic susceptibility of ferromagnetic materials of pole and air. In case two phases of sine current are only available, four poles are commonly used

Automotive alternators

Battery charging system

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.

Modern 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.

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

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.

Gas energy recovery systems

A gas driven alternator (or turbo alternator) was made public in 2005 by Foresight Vehicle. The company announced the turbo alternator and referred to it as the "Turbo-generator Integrated Gas Energy Recovery System" (TIGERS). The unit uses exhaust from internal combustion engine to generate electricity. The firm claimed it can reduce fuel consumption by up to 10%. The design can generate up to 6 kW of power at 80000 rpm.

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 ^*. Nikola Tesla's , "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). Tesla continued research into higher frequency devices and, by early 1896, he attained the means to produce undamped (or "continuous") waves around 50,000 cycles per second for radio transmission. Leland Anderson, "Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power", Sun Publishing Company, LC 92-60482, ISBN 0-9632652-0-2

In 1904, Reginald Fessenden contracted with General Electric for an alternator that generated a frequency of 100,000 Hz for 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 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

General references

Thompson, Sylvanus P., Dynamo-Electric Machinery, A Manual for Students of Electrotechnics, Part 1, Collier and Sons, New York, 1902
White, Thomas H.,"Alternator-Transmitter Development (1891-1920)". EarlyRadioHistory.us.