Alexanderson alternator - AbsoluteAstronomy.com

An Alexanderson alternator is a rotating machine

Alternator

An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.Most alternators use a rotating magnetic field but linear alternators are occasionally used...

invented by Ernst Alexanderson

Ernst Alexanderson

Ernst Frederick Werner Alexanderson was a Swedish-American electrical engineer, who was a pioneer in radio and television development.-Background:...

in 1904 for the generation of high frequency alternating current

Alternating current

In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

up to 100 kHz, for use as a radio transmitter

Transmitter

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

. It was one of the first devices capable of generating the continuous radio waves

Continuous wave

A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...

needed for transmission of amplitude modulation

Amplitude modulation

Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...

(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

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

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

Electrical engineering

Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...

Prior developments

In 1891, Frederick Thomas Trouton

Frederick Thomas Trouton

Frederick Thomas Trouton FRS was an Irish physicist known for Trouton's Rule and experiments to detect the Earth's motion through the luminiferous aether.- Life and work :...

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

Wireless

Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...

energy. Nikola Tesla

Nikola Tesla

Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...

developed alternators with up to 50,000 hertz output. A forerunner to the Alexanderson alternator, his devices, by early 1896, produced continuous

Continuous wave

frequencies that were in the longwave

Longwave

In radio, longwave refers to parts of radio spectrum with relatively long wavelengths. The term is a historic one dating from the early 20th century, when the radio spectrum was considered to consist of long, medium and short wavelengths...

radio frequency range of the VLF

Very low frequency

225px|thumb|right|A VLF receiving antenna at [[Palmer Station]], Antarctica, operated by Stanford UniversityVery low frequency or VLF refers to radio frequencies in the range of 3 kHz to 30 kHz. Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals...

and LF

Low frequency

Low frequency or low freq or LF refers to radio frequencies in the range of 30 kHz–300 kHz. In Europe, and parts of Northern Africa and of Asia, part of the LF spectrum is used for AM broadcasting as the longwave band. In the western hemisphere, its main use is for aircraft beacon,...

bands.

Construction

In 1904, Reginald Fessenden

Reginald Fessenden

Reginald Aubrey Fessenden , a naturalized American citizen born in Canada, was an inventor who performed pioneering experiments in radio, including early—and possibly the first—radio transmissions of voice and music...

contracted with General Electric

General Electric

General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...

for an alternator that generated a frequency of 100,000 hertz for continuous wave radio. The alternator was designed by Ernst Alexanderson

Ernst Alexanderson

Ernst Frederick Werner Alexanderson was a Swedish-American electrical engineer, who was a pioneer in radio and television development.-Background:...

. 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

Reginald Fessenden

at Brant Rock, Massachusetts, another to John Hays Hammond, Jr.

John Hays Hammond, Jr.

John Hays Hammond, Jr. was an American inventor known as "The Father of Radio Control" and son of mining engineer John Hays Hammond, Sr..-Biography:...

in Gloucester, Massachusetts

Gloucester, Massachusetts

Gloucester is a city on Cape Ann in Essex County, Massachusetts, in the United States. It is part of Massachusetts' North Shore. The population was 28,789 at the 2010 U.S. Census...

and another to the American Marconi Company in New Brunswick, New Jersey

New Brunswick, New Jersey

New Brunswick is a city in Middlesex County, New Jersey, USA. It is the county seat and the home of Rutgers University. The city is located on the Northeast Corridor rail line, southwest of Manhattan, on the southern bank of the Raritan River. At the 2010 United States Census, the population of...

.

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

Amplitude modulation

to carry the human voice. Until the invention of vacuum tube

Vacuum tube

(valve) oscillator

Electronic oscillator

An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...

s in 1913 such as the Armstrong oscillator

Armstrong oscillator

The Armstrong oscillator is named after its inventor, the electrical engineer Edwin Armstrong. It is sometimes called a tickler oscillator because the feedback needed to produce oscillations is provided using a tickler coil via magnetic coupling between coil L and coil T...

, the Alexanderson alternator was an important high-power radio

Radio

Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

transmitter

Transmitter

, and allowed amplitude modulation

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
New Brunswick, NJ, USA	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
Bolinas, CA, USA	KET	15,600	19.2		1921	1930		Haiku after 1942
Radio Central, Rocky Point, NY, USA	WQK	16,484	18.1		1921	1948	1951
Radio Central, Rocky Point, NY, USA	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
Kahuku, HI, USA	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
Tuckerton, NJ, USA	WGG	13,575	22.1		1922	1948	1955
Caernarvon, Wales, UK	MUU	14,111	21.2		1921		1939
Caernarvon, Wales, UK	GLC	9,592	31.3		1921		1939
Warsaw, Poland	AXO	21,127	14.2		1923			Destroyed in World War II
Warsaw, Poland	AXL	18,293	16.4		1923			Destroyed in World War II
Grimeton Grimeton VLF transmitter The Grimeton VLF transmitter is a VLF transmission facility at Grimeton close to Varberg, Sweden. It has the only workable machine transmitter in the world and is classified as World Heritage Site.... , Sweden	SAQ	17,442	17.2		1924			Initially 18.600 m, Operational, Preserved. An UNESCO UNESCO The United Nations Educational, Scientific and Cultural Organization is a specialized agency of the United Nations... World Heritage Site World Heritage Site A UNESCO World Heritage Site is a place that is listed by the UNESCO as of special cultural or physical significance... .
					1924	1960	1960	In parallel connection
Monte Grande, Buenos Aires, Argentina	LPZ	16,700	18	500	1924	1931		Current status
Monte Grande, Buenos Aires, Argentina	LPZ	8,350	36	500	1924	1931		Current status
Pernambuco, Recife, Brazil					never			Delivered 1924, returned to Radio Central after 1946
Pernambuco, Recife, Brazil					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, 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

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

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

Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

and produces a magnetic field in the air gap of the stator. The second set of coils generates the radio-frequency voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

. The rotor is a laminated iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

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

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

, whose secondary winding was connected to the antenna circuit. Modulation

Modulation

In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

or telegraph keying

Telegraphy

Telegraphy is the long-distance transmission of messages via some form of signalling technology. Telegraphy requires messages to be converted to a code which is known to both sender and receiver...

of the radio frequency energy was done by a magnetic amplifier

Magnetic amplifier

The magnetic amplifier is an electromagnetic device for amplifying electrical signals. The magnetic amplifier was invented early in the 20th century, and was used as an alternative to vacuum tube amplifiers where robustness and high current capacity were required...

, which was also used for amplitude modulation

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 200 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 transmitter

Spark-gap transmitter

A spark-gap transmitter is a device for generating radio frequency electromagnetic waves using a spark gap.These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio and the first demonstrations of practical radio were carried out using them...

s and arc converter

Arc converter

The arc converter, sometimes called the arc transmitter or Poulsen arc after its inventor Danish engineer Valdemar Poulsen, is a device that used an electric arc to convert direct current electricity into radio frequency alternating current...

s also used at the time, the Alexanderson alternator produced a continuous wave

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. The technology was widely used by the US Navy.

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.

Patents

E. F. W. Alexanderson, High Frequency Alternator
N. Tesla ,

External links

Alexanderson Alternators at Marion, Massachusetts

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