An Alexanderson alternator is a rotating machine, developed by Ernst Alexanderson beginning in 1904, for the generation of high-frequency alternating current for use as a radio transmitter. It was one of the first devices capable of generating the continuous radio waves needed for transmission of amplitude modulated (AM) signals by radio. It was used from about 1910 in a few "superpower" longwave radiotelegraphy stations to transmit transoceanic message traffic by Morse code to similar stations all over the world.
Although superseded in the early 1920s by the development of vacuum-tube transmitters, the Alexanderson alternator continued to be used until World War II. It is on the list of IEEE Milestones as a key achievement in electrical engineering.
History
Prior developments
After radio waves were discovered in 1887, the first generation of radio transmitters, the spark gap transmitters, produced strings of damped waves, pulses of radio waves which died out to zero quickly. By the 1890s it was realized that damped waves had disadvantages: their energy was spread over a broad frequency band so transmitters on different frequencies interfered with each other, and they could not be modulated with an audio signal to transmit sound. Efforts were made to invent transmitters that would produce continuous wavesa sinusoidal alternating current at a single frequency.
In an 1891 lecture, Frederick Thomas Trouton pointed out that if an electrical alternator were run at a great enough cycle speed (that is, if it turned fast enough and was built with a large enough number of magnetic poles on its armature), it would generate continuous waves at radio frequency."Radiation of Electric Energy" by Frederick Trouton, The Electrician (London), January 22, 1892, page 302. Starting with Elihu Thomson in 1889, a series of researchers built high-frequency alternators: Nikola Tesla, Nikola Tesla "Alternating Electric Current Generator" (March 10, 1891) (1891, 15 kHz), Salomons and Pyke (1891, 9 kHz), Parsons and Ewing (1892, 14 kHz), Siemens (5 kHz), B. G. Lamme (1902, 10 kHz), but none reached the frequencies required for radio transmission, above 20 kHz.
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 kW alternators were delivered. One went to Reginald Fessenden at Brant Rock, Massachusetts, another to John Hays Hammond, Jr. in Gloucester, Massachusetts and 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 human voice. Until the invention of vacuum-tube (valve) oscillators in 1913 such as the Armstrong oscillator, the Alexanderson alternator was an important high-power radio transmitter, and allowed amplitude modulation radio transmission of the human voice. The last remaining operable Alexanderson alternator is at the VLF transmitter Grimeton in Sweden and was in regular service until 1996. It continues to be operated for a few minutes on Alexanderson Day, which is either the last Sunday in June or first Sunday in July every year.
World War I and the formation of RCA
The outbreak of World War I forced European nations to temporarily abandon development of international radio communications networks, while the United States increased efforts to develop transoceanic radio. By the end of the war the Alexanderson alternator was operating to reliably provide transoceanic radio service. British Marconi offered General Electric $5,000 in business in exchange for exclusive rights to use the alternator, but just as the deal was about to go through, the American president Woodrow Wilson requested that GE decline the offer, which would have given the British (who were the leader in submarine communications cables) dominance over worldwide radio communications. GE complied with the request and joined with American Telephone and Telegraph (AT&T), the United Fruit Company, the Western Electric Company, and the Westinghouse Electric and Manufacturing Company to form the Radio Corporation of America (RCA), giving American companies control of American radio for the first time.
Stations Thorn L. Mayes identified the production of ten pairs of 200 kW Alexanderson alternators, totaling twenty transmitters, in the period up to 1924:"200 kW Alexanderson Alternator Transmitters" (table), Wireless Communication in the United States by Thorn L. Mayes, The New England Wireless and Steam Museum, Inc., 1989, page 182. Includes the note "Call letters and wave lengths in meters from RCA listing Long Wave Stations, Dec. 5, 1928". The Frequency column has been added, using 300,000 meters per second as the speed of light for the calculations.Thorn L. Mayes. "The Alexanderson 200-kW High-Frequency Alternator Transmitters". "Ports O' Call" Vol 4. 1975. Appendix D. No.LocationCallsignWavelength (m)Frequency (kHz)InstalledIdledScrappedRemarks1New Brunswick, New Jersey, USWII13,76121.86/191819481953Replaced a 50 kW alternator installed in February 19172WRT13,27422.62/1920194819533 Marion, Massachusetts, USWQR13,42322.34/192019321961Replaced a Marconi timed spark transmitter4WSO11,62825.87/192219321969To Haiku, Hawaii in 19425Bolinas, California, USKET13,10022.910/192019301946Replaced a Marconi timed spark transmitter6KET15,60019.2192119301969To Haiku, Hawaii in 19427 Radio Central, Rocky Point, New York, USWQK16,48418.211/1921194819518WSS15,95718.819211948To Marion, Massachusetts 1949. Later Smithsonian Institution.9Kahuku, Hawaii, USKGI16,12018.619201930193810KIE16,66718.019211930193811Tuckerton, New Jersey, USWCI16,30418.43/192119481955Replaced a Goldschmidt alternator12WGG13,57522.119221948195513Caernarvon, Wales, UKMUU14,11121.24/1921193914GLC9,59231.31921193915Varberg, SwedenSAQ17,44217.2192419461960Initially 18.600 m, parallel connection16SAQ17,44217.219241946OperationalPreserved at Grimeton, Sweden17Warsaw, PolandAXO21,12714.212/1923Seized by German army 9/1939, who destroyed the stations in 194518AXL18,29316.4192319Pernambuco, Recife, Brazilnever1927Delivered 1924, returned to Radio Central Rocky Point in 1926 because more efficient vacuum tube transmitters were then available20never1927
U.S. military use during and after World War II Beginning in 1941, seven of the twenty original 200 kW alternators were put into service by the U.S. Navy and Air Force:"200 Kilowatt Alexanderson Transmitters Used in U.S.A. during and after WW II" (table), Mayes (1989), page 183.
No.LocationCallsignOriginallocationNavyoperationAir ForceoperationScrapped1 Haiku, HawaiiMarion, Massachusetts (WSO)1942–19461947–195719692Bolinas, California (KET)1942–19461947–195719693 Marion, MassachusettsMarion, Massachusetts (WQR)1941–19481949–195719614AFA2"The Alexanderson Alternator" by Jerry Proc (jproc.ca)Radio Central (WSS)1949–1957Smithsonian5 Tuckerton, New JerseyTuckerton, New Jersey (WCI)1942–194819556Tuckerton, New Jersey (WGG)1942–194819557Bolinas, CaliforniaBolinas, California (KET)1942–19461946
During World War II the U.S. Navy recognized the need for reliable distant longwave (VLF) transmissions to the Pacific fleet. A new facility was constructed at Haiku in Hawaii, where two 200 KW Alexanderson alternators transferred from the mainland were installed. The Navy also operated an existing transmitter at Bolinas, California, again for Pacific ocean communication.Mayes (1989), pages 176–177. Both Haiku alternators were sold for salvage in 1969, possibly to Kreger Company of California.
In the late 1940s the Air Force assumed control of the Haiku and Marion, Massachusetts facilities. The Air Force found that longwave transmissions were more reliable than shortwave when sending weather information to Arctic researchers as well as bases in Greenland, Labrador, and Iceland. The two Marion transmitters were used until 1957. One was scrapped in 1961 and the other was reportedly handed over to the U.S. Bureau of StandardsMayes (1989), page 176. and stored in a Smithsonian Institution warehouse.Mayes (1989), quoting July 15, 1976 correspondence from "the Commanding Officer of the USCG Station Hawaii", page 180.
Design
The Alexanderson alternator works similarly to an AC electric generator, but generates higher-frequency current, in the very low frequency (VLF) radio frequency range. The rotor has no conductive windings or electrical connections; it consists of a solid disc of high tensile strength magnetic steel, with narrow slots cut in its circumference to create a series of narrow "teeth" that function as magnetic poles. The space between the teeth is filled with nonmagnetic material, to give the rotor a smooth surface to decrease aerodynamic drag. The rotor is turned at a high speed by an electric motor through a speed–increaser gearbox.
The machine operates by variable reluctance (similar to an electric guitar pickup), changing the magnetic flux linking two coils. The periphery of the rotor is embraced by a circular iron stator with a C-shaped cross-section, divided into narrow poles, the same number as the rotor has, carrying two sets of coils. One set of coils is energized with direct current and produces a magnetic field in the air gap in the stator, which passes axially (sideways) through the rotor.
As the rotor turns, alternately either an iron section of the disk is in the gap between each pair of stator poles, 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. Thus the magnetic flux through the stator varies sinusoidally at a rapid rate. These changes in flux induce a radio-frequency voltage in a second set of coils on the stator.
The RF collector coils are all interconnected by an output transformer, whose secondary winding is 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.
The frequency of the current generated by an Alexanderson alternator in hertz is the product of the number of rotor poles and the revolutions per second. Higher radio frequencies thus require more poles, a higher rotational speed, or both. Alexanderson alternators were used to produce radio waves in the very low frequency (VLF) range, for transcontinental wireless communication. A typical alternator with an output frequency of 100 kHz had 300 poles and rotated at 20,000 revolutions per minute (RPM) (333 revolutions per second). To produce high power, the clearance between the rotor and stator had to be kept to only 1 mm. The manufacture of precision machines rotating at such high speeds presented many new problems, and Alexanderson transmitters were bulky and very expensive.
Frequency control