Alexanderson alternator
An Alexanderson alternator is a
Although superseded in the early 1920s by the development of
History
Prior developments
After
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.[2] Starting with Elihu Thomson in 1889,[3][4][5][6] a series of researchers built high frequency alternators, Nikola Tesla[7][8] (1891, 15 kHz), Salomons and Pyke[8] (1891, 9 kHz), Parsons and Ewing (1892, 14 kHz.), Siemens[8] (5 kHz), B. G. Lamme[8] (1902, 10 kHz), but none was able to reach the frequencies required for radio transmission, above 20 kHz.[5]
Construction
In 1904,
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
World War I and the formation of RCA
The outbreak of
Stations
Thorn L. Mayes identified the production of ten pairs of 200 KW Alexanderson alternators, totaling 20 transmitters, in the period up to 1924:[10][11]
No. | Location | Call sign |
Wavelength (m) | Frequency (kHz) | Installed | Idled | Scrapped | Remarks |
---|---|---|---|---|---|---|---|---|
1 | New Brunswick, New Jersey, US | WII | 13,761 | 21.8 | 6/1918 | 1948 | 1953 | Replaced a 50 KW alternator installed in February 1917 |
2 | WRT | 13,274 | 22.6 | 2/1920 | 1948 | 1953 | ||
3 | Marion, Massachusetts, US | WQR | 13,423 | 22.3 | 4/1920 | 1932 | 1961 | Replaced a Marconi timed spark transmitter |
4 | WSO | 11,628 | 25.8 | 7/1922 | 1932 | 1969 | To Haiku, Hawaii in 1942 | |
5 | Bolinas, California, US | KET | 13,100 | 22.9 | 10/1920 | 1930 | 1946 | Replaced a Marconi timed spark transmitter |
6 | KET | 15,600 | 19.2 | 1921 | 1930 | 1969 | To Haiku, Hawaii in 1942 | |
7 | Radio Central, Rocky Point, New York, US | WQK | 16,484 | 18.2 | 11/1921 | 1948 | 1951 | |
8 | WSS | 15,957 | 18.8 | 1921 | 1948 | To Marion, Massachusetts 1949. Later Smithsonian Institution. | ||
9 | Kahuku, Hawaii, US | KGI | 16,120 | 18.6 | 1920 | 1930 | 1938 | |
10 | KIE | 16,667 | 18.0 | 1921 | 1930 | 1938 | ||
11 | Tuckerton, New Jersey, US | WCI | 16,304 | 18.4 | 3/1921 | 1948 | 1955 | Replaced a Goldschmidt alternator |
12 | WGG | 13,575 | 22.1 | 1922 | 1948 | 1955 | ||
13 | Caernarvon, Wales, UK | MUU | 14,111 | 21.2 | 4/1921 | 1939 | ||
14 | GLC | 9,592 | 31.3 | 1921 | 1939 | |||
15 | Varberg, Sweden | SAQ | 17,442 | 17.2 | 1924 | 1946 | 1960 | Initially 18.600 m, parallel connection |
16 | SAQ | 17,442 | 17.2 | 1924 | 1946 | Operational | Preserved at Grimeton, Sweden. | |
17 | Warsaw, Poland | AXO | 21,127 | 14.2 | 12/1923 | Seized by German army 9/1939, who destroyed the stations in 1945 | ||
18 | AXL | 18,293 | 16.4 | 1923 | ||||
19 | Pernambuco, Recife, Brazil | never | 1927 | Delivered 1924, returned to Radio Central Rocky Point in 1926 because more efficient vacuum tube transmitters were now available | ||||
20 | never | 1927 |
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:[12]
No. | Location | Call Sign |
Original Location |
Navy Operation |
Air Force Operation |
Scrapped |
---|---|---|---|---|---|---|
1 | Haiku, Hawaii | Marion, Massachusetts (WSO) | 1942-1946 | 1947-1957 | 1969 | |
2 | Bolinas, California (KET) | 1942-1946 | 1947-1957 | 1969 | ||
3 | Marion, Massachusetts | Marion, Massachusetts (WQR) | 1941-1948 | 1949-1957 | 1961 | |
4 | AFA2[13] | Radio Central (WSS) | 1949-1957 | Smithsonian | ||
5 | Tuckerton, New Jersey | Tuckerton, New Jersey (WCI) | 1942-1948 | 1955 | ||
6 | Tuckerton, New Jersey (WGG) | 1942-1948 | 1955 | |||
7 | Bolinas, California | Bolinas, California (KET) | 1942-1946 | 1946 |
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.[14] 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 Standards[15] and stored in a Smithsonian Institution warehouse.[16]
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
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
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 control
The output frequency of the transmitter is proportional to the speed of the rotor. To keep the frequency constant, the speed of the electric motor turning it was controlled with a feedback loop. In one method, a sample of the output signal is applied to a high-Q
The sets were built to operate at wavelengths of 10,500 to 24,000 meters (28.57 to 12.5 KHz). This was accomplished by three design variables. The alternators were built with 1220, 976 or 772 poles. Three gearboxes were available with ratios of 2.675, 2.973 and 3.324 and the 900 RPM driving motor was operated at slips of 4% to 20%, giving speeds of 864 to 720 RPM. Transmitters installed in Europe, operating on 50-cycle power, had a wavelength range of 12,500 to 28,800 meters due to the lower speed of the driving motor.
Performance advantages
A large Alexanderson alternator might produce 500 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 21.7 kHz. To obtain higher frequencies, higher rotor speeds were required, up to 20,000 RPM.
Along with the
The Alexanderson alternator produced "purer" continuous waves than the arc converter, whose nonsinusoidal output generated significant
Disadvantages
Because of the extremely high rotational speed compared to a conventional alternator, the Alexanderson alternator required continuous maintenance by skilled personnel. Efficient lubrication and oil or water cooling was essential for reliability which was difficult to achieve with the lubricants available at the time. In fact, early editions of the Royal Navy's "Admiralty Handbook of Wireless Telegraphy" cover this in considerable detail, mostly as an explanation as to why the navy did not use that particular technology. However, the US Navy did.
Other major problems were that changing the operating frequency was a lengthy and complicated process, and 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.
Because of the limits of the number of poles and rotational speed of a machine, the Alexanderson alternator is capable of generating transmission frequencies up to around 600kHz in the lower Medium wave band, with shortwave and higher frequencies being physically impossible.[a]
See also
- Alexanderson Day
- Grimeton Radio Station
- Goldschmidt alternator
- Joly-Arco and Bethenod-Latour radio frequency alternators
- Tonewheel
- Resolver (electrical)
Notes
- ^ Nowadays, it would be technically possible to construct an Alexanderson alternator operating at higher frequencies (for instance, an Alexanderson alternator with a 10,000-pole rotor spinning at 300,000 RPM would produce a transmission frequency of 50 MHz, into the lower portion of the VHF band), but the advances in technology required to allow a large rotor to be spun at the immensely high speeds necessary without suffering catastrophic failure did not occur until long after the Alexanderson alternator had become obsolete.
References
- ^ "Milestones:Alexanderson Radio Alternator, 1904". IEEE Global History Network. IEEE. Retrieved 29 July 2011.
- ^ "Radiation of Electric Energy" by Frederick Trouton, The Electrician (London), January 22, 1892, page 302.
- ^ "Prof. Thomson's new alternating generator". The Electrical Engineer. 11 (154). Electrical Engineer Co.: 437 April 15, 1891. Retrieved April 18, 2015.
- ^ Thomson, Elihu (September 12, 1890). "letter". The Electrician. 25. London: 529–530. Retrieved April 18, 2015.
- ^ ISBN 978-1400854608.
- ^ Fessenden, R. A. (1908). "Wireless Telephony". Annual Report of the Smithsonian Institution. Government Printing Office: 172. Retrieved April 18, 2015.
- ^ U.S. patent 447,921, Nikola Tesla "Alternating Electric Current Generator" (March 10, 1891)
- ^ a b c d Fleming, John Ambrose (1910). The principles of electric wave telegraphy and telephony, 2nd Ed. London: Longmans, Green and Co. pp. 5–10.
- S2CID 144710174.
- ^ "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/second as the speed-of-light for the calculations.
- ^ Thorn L. Mayes. "The Alexanderson 200-kW Alternator Transmitters". "Ports O' Call" Vol 4. 1975. Appendix D.
- ^ "200 Kilowatt Alexanderson Transmitters Used in U.S.A. during and after WW II" (table), Mayes (1989), page 183.
- ^ "The Alexanderson Alternator" by Jerry Proc (jproc.ca)
- ^ Mayes (1989), pages 176-177.
- ^ Mayes (1989), page 176.
- ^ Mayes (1989), quoting July 15, 1976 correspondence from "the Commanding Officer of the USCG Station Hawaii", page 180.
Further reading
- "Below 535: A Historical Review of Continuous Wave Radio Frequency Power Generators", Antique Wireless Association column edited by Frank Lotito
- David E. Fisher and Marshall J. Fisher, Tube, the Invention of Television Counterpoint, Washington D.C. USA, (1996) ISBN 1-887178-17-1
- Hammond, John Winthrop. Men and Volts, the Story of General Electric. Philadelphia & New York: J. B. Lippincott (1941), pp. 349–352, 372.
- Notes from the Navy Institute proceedings 1952 from M.G. Abernathy files.
- Letter to M.G. Abernathy from G. Warren Clark Captain USNR (Ret)
- Letter to Mr. Mayes from Lt. Francis J. Kishima Commanding Officer USCG Omega Station Hawaii
- Milestones:Yosami Radio Transmitting Station, 1929
- E. F. W. Alexanderson, U.S. patent 1,008,577 High Frequency Alternator
- N. Tesla, U.S. patent 447,921
External links
- "The World's Greatest Wireless Station" (Radio Central), The Book of Radio by Charles Taussig, pages 312-327.
- "American Marconi Station, Marion, MA" by Henry Brown
- Alexanderson Alternators at Haiku Valley, Oahu by David Jessup
- "Kahuku Marconi Wireless Station, O'ahu, Hawaii" by Jonathan H, June 20, 2007
- "Marion Mass. Alternators" (picture gallery) by David Jessup, 2012
- "Tuckerton Wireless 1912-1955" (gallery)
- "The Radio Station SAQ Grimeton" Alexanderson alternator preserved at UNESCO World Heritage Site in Grimeton, Sweden