This may be an echo of whats been said. Frequency drive motors should have ball bearings. You need a motor designed for inverter use. We have over 300 VFD motors 25 to 100 HP and we set the minimum speed at 20Hz or off. You can expect some fluting on the motor bearings; that is a static build up that discharges at the motor bearings causing premature failure. We tried the "AGIES" grounding rings with little or no success. On the up side, we have gotten really good at changing motor bearings. Using the existing motors, you will most likely get to sell them the correct motors in a few months anyway.
I have never heard of a low RPM bearing requirement on a cooling tower shaft; however, if the tower has a gear reducer low RPM's can affect bearing lubrication and an oil pump may be required to operate using a VFD at low RPM's.
The problem is not with the motor (except for the cooling issues where the motor fan is not running fast enough and an sxternal fan drives off the line will take care of this). The real problem is that the gearbox is not turning fast enough to properly lubricate the top end.
Usually the cooling tower manufacturer places the limit at 25% speed when VFDs are used. I would ask the manufacturer.
Also the problem with bearings can be eliminaated by installing insulated bearings on the ODE end and the Aegis bearing grounding on the shaft or now you can buy the CDR series INPRO seal that has the grounding brushes built in and replaces the INPRO seals in IEEE 481 motors.
Insulate the rear bearing (SKF makes insulated bearings)
Install shaft grounding
Install Inverter duty motors (with the insulated bearings and CDR not all have them unless ordered special) Your motor shop can help with this.
I have attached a picture of a bearing removed from a 250 HP 480 VAC motor in service for 4 months without insulated rear bearing and shaft grounding. THis motor was repaired by replacing bearings (rear bearing insulated SKF) and added IMPRO CDR shaft grounding brush and seal.
Evident is the washboarding caused by current discharge through the bearing.
While this can happen with an accross the line starter in a system with poor power quality it is very common with VFD drives especially with long feed lines and inadequate line and load reactors.