Not the same thing though may be the same idea. What I have is a unit I designed as a variable frequency power source (115-125VAC) self contained that runs off line power (has own PS) and run AC motors to 100W in the range of 25-500hz. Very handy for older audio tape decks that are slightly off speed or if you slew the tape while hearing it (off speed). Mine is a proto I built 30 years ago and the production version was used to run a 3M 1" 8track commercial deck for studio use. Many of the multitrack decks had DC motor contros but not all. That variation allowed for a computer interface to alter speed off spec to find and sync timecodes on the tape. The controller also managed the deck for FF, forward, rewind, speed select and stop. Very impressive to see two decks running 14 tracks of music and this would sync both perfectly or offset as desired (for effects). That system used sinewave drive to keep noise out of the system as we found square waves worked fine but the amps and all tended to hear it.. and harmonics of it. I also have a smaller version that runs from 12V battery for smaller motors to 30watts. Most motors tolerate a squware wave source so efficientcy can be very high. The trick of running AC motors off variable frequency also is used for my antenna rotator (ham radio) so I can make large changes in azmuth with acceleration and deceleration as most rotator are AC motors and the antennas have some interta (15 and 18 foot booms). This also has a microcontroller (8749) to manage the position and motor controls. Doing it with a solidstate converter is far more efficient than motor driven alternators for frequency conversion at low power. Much quieter too. I've since seen the same thing for done for large (10hp and up) motor controller. The whole show comes from the days of doing audio consoles both analog and early digital. Obviously the method could also produce 230V AC if desired. So 50hz pulleys, no problem! Allison