- Bearing current with variable frequency drives
With the application of variable frequency (VF) speed control technology, damage due to bearing current has become increasingly problematic. Due to the presence of high frequencies, voltage can be generated on the motor shaft due to coupling through the internal capacitance of the motor. This can result in a common mode current flowing through the motor bearing.
- The impact of bearing current
When the shaft voltage exceeds the withstand value of the bearing grease insulation, the bearing will produce a destructive shaft current. This may result in a melted pit in the bearing race, which eventually creates a groove. This will increase the friction of the bearing, and shorten its life.
- Benefit of SRD
SRD Motors do not generate shaft current as their windings are symmetrically distributed. The internal magnetic field of the iron core is also symmetrical, and no voltage drop occurs at either end of the motor shaft. The motor windings are independent and so the mutual inductance is small. Additionally, there is no neutral point so there is no state of voltage imbalance. As a result bearing currents are not a problem for motors using switched reluctance technology.