Pump Motor Bearing Currents
Variable Frequency Drives save vast amounts of energy, but as with any technology, it’s gifts come with challenges.
Motor failure due to bearing current is on the rise. While bearing current issues are rare, even in motors using VFDs, the cost of bearing failure is high. There are many strategies to prevent bearing currents, but which make most sense?
Causes of Pump Motor Bearing Current
Understanding where bearing currents come from can tell us how to overcome them.
Bearing current is simply electricity transmitted from a motor’s shaft, through the bearings and to the casing.
All motors utilize a magnetic field to make the motor to spin. In theory, when the motor is connected directly to utility power, these magnetic fields are ideally balanced and induce no current to the shaft. While motors with across-the-line start can see bearing currents, they are more common with pumps using VFDs. Pulse Wave Modulation (PWM) used by VFDs to vary the frequency of the power and therefore the speed of the motor, can cause imbalance in the magnetic fields. This imbalance can induce current into the motor shaft. The current searches for ground and many times the path of least resistance flows through the bearings.
Currents can flow from the shaft through the bearings and to ground, or to the opposite bearing and back into the shaft, and can cause bearing damage in both circuits.
Fluting in the bearing race caused by bearing currents.
Bearing damage occurs when the level of the current is high enough to overcome the insulating capabilities of the bearing lubricant. The effects, therefore, can vary depending on the properties of the lubricant used and the maintenance of the pump motor’s lubrication.
Not all motors used with VFDs will see bearing current. Not all motors that see bearing current will result in damaged bearings. However, the costs associated with bearing failure are high enough that is makes sense to protect any bearings on motors used with VFDs.