VFDs work by rectifying ...

VFDs work by rectifying power to DC and then inverting to a frequency set by the controller. This will run the pump at the speed dependent on that frequency and the number of poles. There are a lot of advantages to this the principal being energy saving as the power is drawn for the operation required rather than controlling by losing head over control valves. It allows for a more general pump selection and to cover a sometimes wider range of flows and heads. It is possible to supersynchronise that is the drive to a higher speed than the grid power supply. Pumps are generally built for operation of 60 Hz power and so this should not be a problem.

Points to watch are that at low speeds the motor may need additional cooling  as the fan is turning slower. This is generally achieved by selecting a higher motor frame size.

Also some pumps especially borehole pumps have a relatively high minimum speed to ensure that bearings operate correctly.

Finally the use of inverters can cause harmonics interference at the mains point of coupling. This can be controlled by the use of more sophisticated inverters. more pulses, or through harmonics filters. Generally a before and after installation survey is needed for major installations.