Re: How does a variable frequency AC drive work

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  The older mechanical variable speed coupling units have been 
replaced by hydraulic couplings akin to the fluid clutch in an automobile 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.