Alexanderson alternator

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An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson for the generation of high frequency alternating current up to 100 kHz, for the purpose of radio communication. It is on the list of IEEE Milestones as a key achievement in electrical engineering.

1 History
2 Theory of operation
3 Performance advantages
4 Disadvantages
5 See also
6 External links
7 References

[edit] History

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 U.S. Patent 447920 , "Method of Operating Arc-Lamps" (March 10, 1891), described an alternator that produces high-frequency current for that time period, around 10,000 cycles per second (later to be known as hertz). A forerunner to the Alexanderson alternator, it produced frequencies which were in the longwave broadcasting range (VLF band). Tesla continued research into higher frequency alternators and, by early 1896, he attained the means to produce CW (or "continuous wave") waves around 50,000 cycles per second for radio transmission. ^[1]

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 U.S. Patent 1008577 in 1911 for his device. The Alexanderson alternator followed Fessenden's rotary spark-gap transmitter as the second radio transmitter to be modulated to carry the sound of the human voice.

Until the invention of vacuum tube (thermionic valve) oscillators in the 1920's, the Alexanderson alternator was an important high-power radio transmitter, and allowed amplitude modulation radio transmission of the human voice. The last remaining workable Alexanderson alternator is at the VLF transmitter Grimeton in Sweden.

[edit] Theory of operation

The Alexanderson alternator operates by variable reluctance (similar to an electric guitar pickup), changing the magnetic flux linking two coils. The alternator has a circular laminated iron stator carrying two sets of coils, in a C-shape. One set of coils is energized with direct current and produces a magnetic field in the air gap of the stator. The second set of coils generates the radio-frequency voltage. The rotor is a laminated iron disk with holes or slots cut into its circumferance. The openings are filled with non-magnetic material so as to reduce air drag. The rotor has no windings or electrical connections.

As the rotor turns, either an iron portion of the disk is in the gap of the stator, allowing a high magnetic flux to cross the gap, or else a non-magnetic slot is in the stator gap, allowing less magnetic flux to pass. These changes in flux induce a voltage in a second set of coils on the stator.

The RF collector coils were all interconnected by an output transformer, whose secondary winding was connected to the antenna circuit. Modulation or telegraph keying of the radio frequency energy was done by a magnetic amplifier, which was also used for amplitude modulation and voice transmissions.

[edit] Performance advantages

A large Alexanderson alternator might produce 200 kW of output radio-frequency energy and would be water- or oil-cooled. One such machine had 600 pole pairs in the stator winding and the rotor was driven at 2170 RPM, for an output frequency near 22 kHz. To obtain higher frequencies, higher rotor speeds were required, up to 20,000 RPM.

Unlike the spark-gap transmitters and arc converters also used at the time, the Alexanderson alternator produced a continuous wave output of higher purity. With a spark transmitter, the electromagnetic energy is spread over very wide sidebands, effectively transmitting on several frequencies at once. With a continuous-wave transmitter such as the Alexanderson Alternator (or the Poulsen Arc type), the energy is concentrated onto a single frequency, greatly improving the transmission efficiency.

The frequency of the transmitted signal was directly related to the rotor speed, so an automatic speed regulator was always employed to maintain a stable transmit frequency; the speed regulator was designed to compensate for the effect of keying (and the subsequently varying load) upon the rotor speed.

[edit] Disadvantages

Because of the extremely high rotational speed compared to a conventional alternator, the Alexanderson Alternator required continuous maintenance by highly skilled personnel. Efficient lubrication and oil or water cooling was essential for reliability, difficult to achieve with the lubricants available at the time. In fact early editions of the British Navy's "Admiralty Handbook of Wireless Telegraphy" cover this in considerable detail, mostly as an explanation as to why why "The Navy" did not use that particular technology. The technology was however, widely used by the US Navy.

Other major problems were that changing the operating frequency was a lengthy and complicated process, and that, unlike a spark transmitter, the carrier signal could not be switched on and off at will. The latter problem greatly complicated "listening through" (that is, stopping the transmission to listen for any answer). There was also the risk that it would allow enemy vessels to detect the presence of the ship.