Specifying a motor for variable-frequency supply
Insulation stress, bearing currents and cooling at low speed — the three things that shorten the life of a standard motor on a drive.
A motor supplied from a variable-frequency drive experiences conditions the sine-wave rating does not describe. Three of them account for most premature failures.
First, insulation stress. Fast switching edges and long motor cables produce voltage overshoot at the terminals that can approach twice the DC link voltage. A reinforced insulation system, or a filter at the drive, is required above roughly 500 V.
Second, bearing currents. Common-mode voltage drives current through the bearing lubricant film, eroding the raceway. An insulated non-drive-end bearing and a shaft earthing arrangement address it.
Third, cooling. A shaft-mounted fan delivers cooling proportional to speed. A machine running continuously at 30 % speed under full torque will overheat unless it has separate forced ventilation.
None of these are exotic requirements, but all three have to be stated in the enquiry — they cannot be retrofitted economically.