Alexanderson alternator

From Freepedia

An Alexanderson alternator is a rotating machine for the generation of high frequency up 100 kHz. You can see the only workable Alexanderson alternator at the VLF transmitter Grimeton. The alternator was invented by Ernst Alexanderson. Until the invention of vacuum tube (thermionic valve) oscillators in the 1920's, the Alexanderson alternator was an important high-power radio transmitter, and the first device that allowed amplitude modulation radio transmission of the human voice.

1 Theory of Operation
2 See also
3 External links
4 References

Theory of Operation

The Alexanderson alternator operates by variable reluctance, 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. The second set of coils generates the radio-frequency voltage. The rotor is a laminated iron disk with holes or slots cut into its circumferance. The openings are filled with non-magnetic material so as to reduce air drag.

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 rotor has no windings or electrical connections to it.

The RF collector coils were all interconnected by an output transformer, whose secondary winding was connected to the antenna circuit. Modulation or on-off keying of the radio frequency energy was done by a magnetic amplifier, which was also used for amplitude modulation and voice transmissions.

A large Alexanderson alternator might produce 200 kW of output radio-frequency energy and would be water-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 22 kHz. To obtain higher frequencies, higher rotor speeds were required, up to 20,000 RPM.

Unlike the spark-gap transmitters also used at the time, the Alexanderson alternator produced a continuous wave output of higher purity. 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.