The VFD has to be the same or a greater rating than the motor it is connected to. Otherwise, the on-board overload protection device won't accomodate the motor full-load-current.

VFDs are rated on AMPERAGE. Find out the full load rated current of your motor and select a VFD that has at least that much current rating or marginally more.

VFDs are usually purchased in categories that cover many motor sizes. The manufacturer builds in the design factor, so it is not a concern for end user.

The minimum information to have when purchasing a VFD to work with an electric motor is the motor's operating voltage and output power rating. It is also good to have the full load amperage (FLA) and speed (RPM) of the motor on-hand in case the manufacturer prefers to pre-program the VFD to work properly with your motor as a courtesy.

Examples of AllenBradley PowerFlex Drive Ratings:

PowerFlex4 = 0.25-5.0 hp (0.2-3.7kW) at 120, 240, or 480 VAC

PowerFlex40 = 0.5-15.0 hp (0.4-11kW) at 120, 240, 480, or 600 VAC

PowerFlex400 = 3-50 hp (2.2-37.3 kW) at 200-240 VAC or 3-350 hp (2.2-261 kW) at 380-480 VAC