As a generalization, synchronous motors are typically selected because a) they are needed for additional purposes such as acting as a synchronous condensor, or b) the application needs a low rpm motor and you don't want to sacfifice efficiency and/or add cost by gearing down a 4 pole asynchronous induction motor.
The second issue is made moot by using a VFD. You will be able to run at reduced speed while saving power (assuming it is a centrifugal pump) using a standard "off-the-shelf" squirrel cage induction motor.
If on the other hand, your entire facility has a poor power factor, the synchronous motor can be run at a leading power factor and correct for the other loads in the facility. In this case your motor will be acting as a synchronous condensor. Candidates for this use are mtors that run continuously for long periods while the other poor power factor loads are also running. If this pump will cycle on and off, it is probably not a good synch. condensor candidate. Don't worry about the PF of this particular motor, if you use a VFD it will maintain a .95 PF to the line without needing to use a synchronous motor.
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