Alexanderson alternator

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 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. It was used from about 1910 in a few "superpower" longwave radiotelegraphy stations to transmit transoceanic message traffic by Morse code to similar stations all over the world.

Although obsolete by the early 1920s due to the development of vacuum-tube transmitters, the Alexanderson alternator continued to be used until World War 2. It is on the list of IEEE Milestones as a key achievement in electrical engineering.^[1]

History

Prior developments

After radio waves were discovered in 1887, the first generation of radio transmitter, 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 bandwidth 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 waves, 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.^[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]

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, which is either the last Sunday in June or first Sunday in July every year.

Stations

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 200 kW 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.

Design

Rotor of 200 kW alternator

Closeup of above rotor. It has 300 narrow slots cut through the rotor. The "teeth" between the slots are the magnetic poles of the machine.

The Alexanderson alternator works similarly to an AC electric generator, but generates higher-frequency current, in the radio 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.

The machine operates by variable reluctance (similar to an electric guitar pickup), changing the magnetic flux 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 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, 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 of the current generated by an Alexanderson alternator in hertz 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) (330 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 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.

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 continuous waves. In contrast, the earlier spark-gap transmitters 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 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 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 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, 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 at most capable of transmission in the lower mediumwave band, with shortwave and upper bands being physically impossible.

See also

• Alexanderson Day
• Tonewheel
• Resolver (electrical)

Notes

References

External links

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