When a motor is operated on a variable frequency drive, unexpected voltage can be induced on the rotor and shaft. A number of solutions have been employed in order to prevent early bearing failure in motors operated on drives.
It is an established fact that some motors operating on variable frequency drives experience early failures (less than the expected mean time between failure), even when the motor is designed per NEMA MG1 part 31 standards, which describes motors for use with variable frequency drives. Pre-mature failures can occur in as little as 500 hours, which may be less than 1% of the motor’s expected bearing life. Although the windings may be designed to perform with non sinusoidal waveforms presented by VFD’s, early failure may result from bearing damage.
When a motor is operated on a variable frequency drive, unexpected voltage can be induced on the rotor and shaft. The voltage seeks a path to ground and a current path may become established through the shaft bearings. The voltage build-up on the rotor shaft can occur due to the high dV/dT switching rate of the drive in addition to capacitive coupling between the rotor and stator. This often happens when switching rates exceed 5KHz. The capacitive coupling causes further voltage build-up on the rotor and shaft. Normally, no current flows between the rotor shaft and frame ground until the voltage level exceeds the insulating characteristic of the oil or grease film that the bearing balls or rollers ride upon. Once this lubrication layer is breached, current can flow from the rotor shaft, through the bearing to frame ground. It is the flow of current through the bearings that cause them to pit, gall or fret leading to premature failure.