A VFD-driven general purpose motor can overheat if it is run too slowly. (Motors can get hot if they’re run slower than their rated speed.) Since most general purpose motors cool themselves with shaft-mounted fans, slow speeds mean less cooling. If the motor overheats, bearing and insulation life will be reduced. Therefore there are minimum speed requirements for all motors.
General purpose motors can be run with VFDs in many applications; however inverter-duty motors are designed to handle much lower speeds without overheating and they are capable of withstanding higher voltage spikes without their insulation failing. With the increased performance comes an increase in cost. This additional cost can be worth it if you need greater performance.
VFD motors are a sales gimmick! I always ask the question, "boy what do you think we used before there were VFD severe duty rated motors, because VFDs were around before the motors." In constant torque applications, such as PD pumps, they are important. But in the case of centrifugal pumps and fans, this is a complete waste of time and money. That is the reason constant torque were not ever recommended 30 years ago when we first started using them. I put my first VFD in service in 1988, and a 30 hp VFD was the size of a refrigerator. Now they are the size of a brief case. In the case of variable torque applications, the heat generated is dependent on flow (if a fan or a centrifugal pump), but constant torque is a different story. Most that power demand is still there. So if you want to spend extra money on severe duty motors on variable torque applications, go for it!!!