This is a good article from the U.S Department of Energy discussing the effects of VFDs on non-inverter duty rated motors. It discusses the insulation class recommended, in this article, “class H” is suggested. I previously had been recommending at a minimum, “class F” motor insulation. The article further discusses the use of load reactors and dV/dt filters to reduce the damaging effects upon motors, and mentions that these issues are especially high when running multiple motors on one VFD.
Typically when clients ask about running non-inverter duty rated motors with a VFD, they want to know if the motor will fail. The answer is: eventually. I inquire about the criticality of the application. Is this an application that demands constant up-time? If so, then it is wise to replace the motor when installing the VFD. Some VFD/motor manufacturers will extend their warranty if both are replaced simultaneously. If the application lends itself to some down-time, such as a multiple pump application, then run the motor until it fails, to maximize its’ initial ROI. This may be weeks, months or years later, but it will eventually fail. However, the best practice is still to be certain that the motor is rated for inverter duty to insure the longest lifespan. As the above article states, a work-around to this dilemma, rather than replacing the motor, is to install a load reactor or dV/dt filter between the VFD and the motor(s). Especially if there are multiple motors and/or the motor leads (commonly called “T” leads) are longer than about 100 feet. In these two particular applications (multiple motors or long leads), a reactor or filter is a good practice even when the motors are rated as inverter duty. Different VFD manufacturers recommend different maximum cable lengths before requiring reactors or filters. Check with the manufacturers recommendations to be certain.