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 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 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.
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