jraef
Member
the take away from this should be that you cannot "ASSume" anything about a motor being run at above base frequency, the ONLY people that can give you a "safe" number is the motor mfr. Bearings may not be rated for higher speeds, cooling can be an issue, machine balancing can be CRITICAL (I've seen a 500HP Toshiba motor come to pieces from being run at 100Hz, not pretty). But as you have observed, some motor suppliers ARE testing their designs above base frequency. There is just no universal answer other than to not go above stated base speed unless specifically documented as being OK.
Running at 120Hz was not likely the ONLY factor in the demise of your motor, but running at 120Hz when not properly DESIGNED to do so may have been. the biggest mistake I see is in that people fail to familiarize themselves with the concept and that torque drops as speed increases. So what happens is that the motor runs at MUCH higher slip, pulls more current, but at the same time cannot cool itself effectively. So the motor burns up before the OL protection can kick in.