The idea is a good idea in pricable, but the motor technology hasn't kept pace with the
ac drive systems.
You just cant take today's motors and play with the voltage and expect it to last any amount of time.
An ac motor is just not built like a dc motor.
A dc motor can take the change in voltage or is more tolerable to the up and down power ratings, where an ac will burn itself up.
So called energy saving calculators, especially for pool pumps, will not allow you to simulate turning the pump off. A pool pump only needs to run 6 to 8 hours a day. If you can't turn the pump off, of course an AC drive is going to show $2,000 a year savings. But slowing the pump down with an AC drive is not doing any good. At really low flow rates the skimmers don't skim, the vacuum won't vacuum, and the filter won't filter. Turning a standard pump off with a timer, instead of letting an AC drive run it slowly all day, WILL save thousands of dollars a year.
The one pump which you will hookup as an
AC drive pump will float with the required flow rate in the common discharge header. This will require that you have a flowmeter in the common discharge header and that flowmeter provides a signal to the speed controller of the pump with the AC drive to maintain the required flow rate. This is generally known as a cascade control where the FIC (Flow Indicator Controller) acts a Master controller which in turn provides a signal to the speed indicator controller (SIC) which is the slave controller.
more... 
