|MadSci Network: Engineering|
In simple terms, an AC motor drives a variable speed coupling which, in turn, drives an AC generator. The frequency output of the generator is controlled either manually or automatically by varying the degree of coupling. The older mechanical variable speed coupling units have been replaced by hydraulic couplings akin to the fluid clutch in an automobile transmission with the degree of coupling controlled by the amount of oil in the coupling. More oil, higher speed, etc. The real trick is in the design of the motor and generator; they are not designed to operate at the same speed. Usually, a four pole, 1,800 RPM motor will be used to drive a six pole, 1,200 RPM generator. With this setup, the generator can produce a higher frequency output than the 60 hertz, line input that drives the motor.
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