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.
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 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.
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