Bearing currents occur in specific motor and drive installations and can lead to motor fatigue and failure. While no system is inherently immune to bearing currents, few installations experience bearing currents during normal operation. While many installations have preventive measures installed, knowing that protection is in place is sometimes outside the owner or operator’s scope. Therefore, end users must understand the symptoms and causes of bearing currents to properly identify and address this problem.
What Are Bearing Currents?
Bearing currents occur when an induced voltage on the motor shaft is high enough to overcome the breakdown voltage of the bearing lubricant. This is typically greater than 50 volts. These shaft voltages will do one of the following:
- Cause the current to flow directly from the shaft, through a bearing, and through the motor or load frame into the ground (see Figure 1)
- Cause the current to circulate from one side of the shaft, through a bearing, through the motor frame, back into the opposite bearing, and back into the shaft (see Figure 2).
Figure 1. Current flow from shaft to ground
Figure 2. Circulating current flow in a motor
Figure 3. Magnetic fields from driving current
Either current type results in fluting in the bearing race. Therefore, the bearing’s rotation is no longer smooth (see Image 1).
Image 1. Fluting in bearing race caused by bearing currents
Bearing Current Causes
Voltage in the shaft is induced by the magnetic fields generated by current flowing through the motor windings. For motors connected across the utility line (not connected to a drive), these magnetic fields are ideally balanced and yield a net voltage of zero with respect to earth on the rotor and shaft. This is because the stator windings are symmetrical. When motors have asymmetry within the windings, it can cause a net voltage different than zero, which can lead to bearing currents.
Drives can cause imbalances in the supply voltage through pulse width modulation (PWM). Motors supplied by PWM are unbalanced because the motor is supplied by pulses. This can result in a neutral point that is not equal to zero (such as a direct-current [DC] offset). The new neutral voltage is proportional to the common-mode voltage, or DC-link voltage, of the drive. Figure 4 is an example of common-mode voltage.
Figure 4. Three-phase voltage source inverter driving voltage with a common mode other than zero volts
This new neutral voltage has a frequency equal to the switching frequency of the inverter and can yield high-frequency currents in the bearing with limited amplitude and duration. Motors and drives with proper low-impedance grounding rarely exhibit bearing currents. Often, bearing currents will occur because of specific installation conditions.
How to Prevent Bearing Currents
Many methods for preventing bearing currents exist. Depending on the cause, some are better options than others. Different options include:
- Use shielded motor cables that have symmetrical “protective earth” wires. This will provide a low-impedance path for currents caused by the high-frequency, common-mode voltages to return to the drive.
- Use specially insulated bearings on the opposite drive end or an insulated load coupling to stop the flow of current.
- Use shaft grounding brushes, which can be used to dissipate voltage buildup along the shaft. This is a better option for low horsepower motors.
- Fine stranded grounding cables can be used on the motor and the drive to more effectively ground high-frequency currents.
- Filtering can be applied to the motor side of the drive to remove high-frequency currents. No universal filter exists to remedy bearing currents. Therefore, this option is expensive and other options should be considered first.
These options are highlighted in Figure 5. The red arrow displays shaft to ground currents. The green arrow displays circulating currents, and the yellow arrow displays currents from the shaft through the load and to the ground.
Figure 5. Three-phase voltage source inverter driving voltage with a common mode other than zero volts
A large grain elevator in North Dakota had a 400 horsepower motor and variable frequency drive (VFD) installed on top of it. While both the motor and VFD were grounded, the length of cable to the ground was more than 1,000 feet. Loud squealing and noises began coming from the motor bearings. This indicated fluting in the bearing race caused by bearing currents.