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Alexanderson Alternator in the Grimeton VLF transmitter.

An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high frequency alternating current up to 100 kHz, 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 modulation (sound) by radio, and was used for a short period through the 1920s in the first AM radio transmitters, until it was replaced by vacuum tube transmitters. 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 (in more-familiar terms, if run fast enough and with enough poles), it would generate high-frequency 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.

Alexanderson 200-kW motor-alternator set installed at the US Navy's New Brunswick, NJ station, 1920.

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 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, the first Sunday in July every year.

Stations

Radio-Station	Callsign	Wavelength (m)	Frequency (kHz)	Power (kW)	Installation	Decommissioning	Scrapping	Remarks
New Brunswick, NJ, USA	WII	13,761	21.8		1918	1948	1953	Initially 50 kW alternator
	WRT	13,274	22.6		1920	1948	1953
Marion, MA, USA	WQR	13,423	22.3		1920	1932
	WSO	11,623	25.8		1922	1932		Haiku, HI after 1942
	AFA2	11,623	25.8		1949	1959		Smithsonian after 1960
Bolinas, CA, USA	KET	13,100	22.9		1920	1930	1946
	KET	15,600	19.2		1921	1930		Haiku after 1942
Radio Central, Rocky Point, NY, USA	WQK	16,484	18.1		1921	1948	1951
	WSS	15,957	18.8		1921	1948		Marion 1949-1959 (callsign AFA2), now at Smithsonian
Kahuku, HI, USA	KGI	16,120	18.6		1920	1930	1938
	KIE	16,667	18		1921	1930	1938
Haiku, HI		13,423	22.3		1943	1958
Tuckerton, NJ, USA	WCI	16,304	18.4		1921	1948	1955	Initially Goldschmidt alternator
	WGG	13,575	22.1		1922	1948	1955
Caernarvon, Wales, UK	MUU	14,111	21.2		1921		1939
	GLC	9,592	31.3		1921		1939
Warsaw, Poland	AXO	21,127	14.2		1923			Destroyed in World War II
	AXL	18,293	16.4		1923			Destroyed in World War II
Grimeton, Sweden	SAQ	17,442	17.2		1924			Initially 18.600 m, Operational, Preserved. An UNESCO World Heritage Site.
					1924	1960	1960	In parallel connection
Monte Grande, Buenos Aires, Argentina	LPZ	16,700	18	500	1924	1931		Current status
	LPZ	8,350	36	500	1924	1931
Pernambuco, Recife, Brazil					never			Delivered 1924, returned to Radio Central after 1946
					never			Delivered 1924, returned to Radio Central after 1946

US Navy stations

Starting in 1942 four stations were operated by US Navy: the station at Haiku, Hawaii until 1958, Bolinas until 1946, Marion, and Tuckerton (both until 1948). Two alternators were shipped to Hawaii in 1942, one each from Marion, MA and Bolinas, CA. Haiku received one. The other went to Guam but returned to Haiku after World War 2. Haiku began operation of the first 200KW alternator in 1943. The second alternator went into operation at Haiku in 1949. Both alternators were sold for salvage in 1969, possibly to Kreger Company of California. The Marion station was transferred in 1949 to the US Air Force and used until 1957 for the transmission of weather forecasts to the arctic as well as for the Basen to Greenland, Labrador, and Iceland. One of the alternators was scrapped in 1961 and another one was handed over to the US office of standard^[forrás?], it now resides in a Smithsonian Institution warehouse. The two machines in Brazil were never used because of organizational problems there. They were returned to Radio Central after 1946.

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 circumference. The openings are filled with non-magnetic material 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.

The radio frequency emitted by an Alexanderson alternator in hertz is the product of the number of stator pole pairs and the revolutions per second. Higher radio frequencies thus require more pole pairs, a higher rotational speed, or both.

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.

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 employed to maintain a stable transmit frequency; the speed regulator was designed to compensate for the effect of keying (and the subsequently varying load) on the rotor speed.

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, 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 "The Navy" did not use that particular technology. However, in the US Navy it was.

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 at most capable of transmission in the lower mediumwave band, with shortwave and upper bands being physically impossible.

See also

• Alexanderson Day
• Tonewheel – same principle, also amplitude modulated (in the Hammond organ) or with permanent-magnet excitation as speed sensor
• Resolver (electrical) – reversed roles: the pole frequency amplitude-modulates the higher excitation frequency

References

• 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, published 1941. Citation: Alexanderson alternator pages 351, 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

Patents

• E. F. W. Alexanderson, U.S. Patent 1008577 High Frequency Alternator
• N. Tesla, U.S. Patent 447921

External links

• Description of the 200 kW alternator at New Brunswick
• Alexanderson Alternators at Haiku Valley Oahu
• Alexanderson Alternators at Marion, Massachusetts