Alexanderson alternator

An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson for the generation of high frequency alternating current up to 100 kHz, for the purpose of radio communication. 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 (later to be known as Hertz), it would generate wireless energy [ [http://earlyradiohistory.us/1892alt.htm earlyradiohistory.us 1892alt.htm ] ] . Nikola Tesla developed alternators with up to 50,000 cycles per second 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.US patent|447920, "Method of Operating Arc-Lamps" (March 10, 1891) : described an alternator that produces high-frequency current for that time period, around 10,000 cycles per second .] [Leland Anderson, "Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power", Sun Publishing Company, LC 92-60482, ISBN 0-9632652-0-2]

Construction

In 1904, Reginald Fessenden contracted with General Electric for an alternator that generated a frequency of 100,000 cycles per second 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 to Reginald Fessenden at Brant Rock, Massachusetts, another to John Hays Hammond, Jr. in Gloucester, Massachusetts and the 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 sound of the human voice. Until the invention of vacuum tube (thermionic valve) oscillators in the 1920s, the Alexanderson alternator was an important high-power radio transmitter, and allowed amplitude modulation radio transmission of the human voice. The last remaining workable Alexanderson alternator is at the VLF transmitter Grimeton in Sweden.

tations

Starting in 1942 four stations were operated by US Navy: the re-established station Haiku (Hawaii) as well as stations in Bolinas (both until 1946), Marion and Tuckerton (both until 1948). 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 were scrapped in 1961 and another one was handed over to US the office OF standard. 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 so as 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 simply 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 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 always employed to maintain a stable transmit frequency; the speed regulator was designed to compensate for the effect of keying (and the subsequently varying load) upon the rotor speed.

Disadvantages

Because of the extremely high rotational speed compared to a conventional alternator, the Alexanderson alternator required continuous maintenance by highly 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 however, widely used by the U.S. Navy. Other major problems were that changing the operating frequency was a lengthy and complicated process, and that, 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 microwave being physically impossible.

ee also

* Alexanderson Day

Notes

References

* [http://www.antiquewireless.org/otb/blw535202.htm 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.
* [http://www.ieee.org/web/aboutus/history_center/alexanderson.html IEEE History Center]

Patents

* E. F. W. Alexanderson, US patent|1008577 "High Frequency Alternator"
* N. Tesla , US patent|447921

External links

* [http://earlyradiohistory.us/1920alt.htm Description of the 200 kW alternator at New Brunswick]