The only central vac I have experience with is a kit that a friend bought to be installed in a house that he subdivided into apartments. The kit would draw vacuum against closed ports, and at a certain vacuum level, the motor would shut off. Of course, the second someone hooked up the hose and turned the valve on for the vacuum, the motor would start again. When you're done vacuuming, you just turn the hose valve back off and disconnect the hose, and the motor will continue running for a second to pull vacuum against the valve again, until the sensor tells it that the system is closed, and then it would shut off. I'm not sure what kind of energy it used, though.
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