2009年1月22日星期四

Alexanderson alternator


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.

History
Prior developments
In 1891, Frederick Thomas Trouton gave a lecture which stated that, if an electrical alternator were run at a great enough cycle speed (later to be known as hertz), it would generate wireless energy . Nikola Tesla developed alternators with up to 50,000 hertz output. A forerunner to the Alexanderson alternator, his devices, by early 1896, produced continuous frequencies that were in the longwave radio frequency range of the VLF and LF bands.


Construction
In 1904, Reginald Fessenden contracted with General Electric for an alternator that generated a frequency of 100,000 hertz for continuous wave radio. The alternator was designed by Ernst Alexanderson. 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. In 1906 the first 50 kilowatt alternators were delivered. One was to Reginald Fessenden at Brant Rock, Massachusetts, another to John Hays Hammond, Jr. in Gloucester, Massachusetts and the another to the American Marconi Company in New Brunswick, New Jersey.

Alexanderson would receive a patent 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 1920s, 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.

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