An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high-frequency

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...

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 Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to t ...

signals by radio. It was used from about 1910 in a few "superpower" longwave

radiotelegraphy Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for t ...

stations to transmit transoceanic message traffic by

Morse code Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one ...

to similar stations all over the world. Although superceded in the early 1920s by the development of vacuum-tube transmitters, the Alexanderson alternator continued to be used until

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...

. It is on the list of IEEE Milestones as a key achievement in

electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...

History

Prior developments

After

radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (sho ...

s 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 wave A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, typically a sine wave, that for mathematical analysis is considered to be of infinite duration. It may refer to e.g. a laser or partic ...

s'' -- a 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. Starting with Elihu Thomson in 1889, a series of researchers built high frequency alternators,

Nikola Tesla Nikola Tesla ( ; ,"Tesla"
''Random House Webster's Unabridged Dictionary''. ; 1856 – 7 January 1943 ...

, 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 was able to reach the frequencies required for radio transmission, above 20 kHz.

Construction

In 1904, Reginald Fessenden contracted with

General Electric General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable ene ...

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 communication Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a trans ...

s 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. John Hays Hammond Jr. (April 13, 1888 – February 12, 1965) was an American inventor known as "The Father of Radio Control". Hammond's pioneering developments in electronic remote control are the foundation for all modern radio remote cont ...

Gloucester, Massachusetts Gloucester () is a city in Essex County, Massachusetts, in the United States. It sits on Cape Ann and is a part of Massachusetts's North Shore. The population was 29,729 at the 2020 U.S. Census. An important center of the fishing industry and a ...

and another to the American Marconi Company in

New Brunswick, New Jersey New Brunswick is a city in and the seat of government of Middlesex County, in the U.S. state of New Jersey.modulated to carry the human voice. Until the invention of vacuum-tube (valve) oscillators in 1913 such as the

Armstrong oscillator The Armstrong oscillator (also known as the Meissner oscillator) is an electronic oscillator circuit which uses an inductor and capacitor to generate an oscillation. It is the earliest oscillator circuit, invented by US engineer Edwin Armstrong ...

, the Alexanderson alternator was an important high-power

radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a tr ...

transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...

, and allowed

amplitude modulation Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to ...

radio transmission of the human voice. The last remaining operable Alexanderson alternator is at the

VLF transmitter Grimeton Grimeton Radio Station () in southern Sweden, close to Varberg in Halland, is an early longwave transatlantic wireless telegraphy station built in 1922–1924, that has been preserved as a historical site. From the 1920s through the 1940s it was ...

in Sweden and was in regular service until 1996. It continues to be operated for a few minutes on

Alexanderson Day Alexanderson Day, named after the Swedish radio engineer Ernst Fredrik Werner Alexanderson and held either on the last Sunday in June, or on the first Sunday in July, whichever comes closer to 2 July, is the day of the open house at the Swedish go ...

, 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 World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...

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

$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 Thomas Woodrow Wilson (December 28, 1856February 3, 1924) was an American politician and academic who served as the 28th president of the United States from 1913 to 1921. A member of the Democratic Party, Wilson served as the president of ...

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 The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment ...

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 20 transmitters, in the period up to 1924:

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: 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. 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 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 In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the weber ...

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 Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or ev ...

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 A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

, whose secondary winding is connected to the antenna circuit.

Modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the '' carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informat ...

or telegraph keying of the radio frequency energy was done by a magnetic amplifier, which was also used for

and voice transmissions. The

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

of the current generated by an Alexanderson alternator in

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that o ...

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

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 tuned circuit, whose

resonant frequency Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscilla ...

is slightly above the output frequency. The generator's frequency falls on the "skirt" of the LC circuit's impedance curve, where the impedance increases rapidly with frequency. The output of the LC circuit is rectified, and the resulting voltage is compared with a constant reference voltage to produce a feedback signal to control the motor speed. If the output frequency gets too high, the impedance presented by the LC circuit increases, and the amplitude of the RF signal getting through the LC circuit drops. The feedback signal to the motor drops, and the motor slows down. Thus the alternator output frequency is "locked" to the tuned circuit resonant frequency. 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 arc converter invented in 1903, the Alexanderson alternator was one of the first radio transmitters that generated

s. In contrast, the earlier

spark-gap transmitter A spark-gap transmitter is an obsolete type of transmitter, radio transmitter which generates radio waves by means of an electric spark."Radio Transmitters, Early" in Spark-gap transmitters were the first type of radio transmitter, and were the m ...

s generated a string of damped waves. These were electrically "noisy"; the energy of the transmitter was spread over a wide frequency range, so they interfered with other transmissions and operated inefficiently. With a continuous-wave transmitter, all of the energy was concentrated within a narrow frequency band, so for a given output power they could communicate over longer distances. In addition, continuous waves could be modulated with an

audio signal An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of ro ...

to carry sound. The Alexanderson alternator was one of the first transmitters to be used for AM transmission. The Alexanderson alternator produced "purer" continuous waves than the arc converter, whose nonsinusoidal output generated significant harmonics, so the alternator was preferred for long-distance telegraphy.

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 The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against Fr ...

'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 Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the dayti ...

band, with

shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...

and higher frequencies being physically impossible.

Notes

References

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

(gallery)
"The Radio Station SAQ Grimeton"
Alexanderson alternator preserved at

UNESCO The United Nations Educational, Scientific and Cultural Organization is a List of specialized agencies of the United Nations, specialized agency of the United Nations (UN) aimed at promoting world peace and security through international coope ...

World Heritage Site A World Heritage Site is a landmark or area with legal protection by an international convention administered by the United Nations Educational, Scientific and Cultural Organization (UNESCO). World Heritage Sites are designated by UNESCO for ...

in Grimeton, Sweden History of radio Radio electronics Alternators Swedish inventions