VFD Motors

Motors that are designed for use behind VFDs offer distinct advantages over those that are not, but VFDs were actually designed to be used on any ac induction motor. The consequences of using a VFD on an older motor are totally dependent upon a number of variables;

1. Voltage. If the line voltage is 380V or higher, a VFD rated motor provides extra protection from what are called "standing wave spikes" that can damage the insulation of older motors. 230V VFDs do not create the same level of peaks in the spikes and most motor winding insulation can handle them. If you want to use a 380V or higher existing motor on a VFD however, you can purchase retrofit load filters that will mitigate this problem.
2. Operating speed. When you turn down the speed of the motor, you are also turning down the speed of the motor's cooling fan (if there is one) and thus reducing the amount of convection cooling it provided, So at very low speeds, the motor may end up burning itself up even though it is not over loaded. A standard motor is good for a turn-down ratio of maybe 4:1, whereas you can get VFD rated motors that are good to 1000:1. But if you are going to operate your motor at 50-75% speed, this is not likely to be an issue.
3. Bearing damage. VFDs can cause a static charge to build up in the rotor and it discharges across the bearings and raced. The long term effect of this is that eventually the bearings and races become etched by a process similar to EDM (Electrical Discharge Machining) and fail prematurely. Good quality VFD rated motors will come with grounding bushings that provide a safe discharge path for the rotor currents. You can also buy retrofit grounding bushings for existing motors as well.

So the bottom line is, you CAN use non-VFD rated motors with VFDs, but you may have consequences. You can mitigate those consequences in existing motors, but on small ones, the costs may outweigh the cost of replacement. What I always tell people is to consider the application, installation AND the process costs. If having an inopportune motor failure while running is going to run the risk of significant product and/or production losses, spend the money for a VFD rated motor now. If not, run the one you have until it fails and replace it with a VFD rated one next time.