Electrical corrosion of motor bearings, also known as electrical pitting or bearing fluting, is caused by the passage of electrical current through the bearings. This can lead to damage that shortens the lifespan of the bearings and compromises motor performance.
Burnt bearings are a common problem encountered during the use of motors. To solve the problem of electrical corrosion of motor bearings, it is not enough to just replace the bearings. The cause must be eliminated. So what are the general causes of electrical corrosion? The author of this article gives a detailed explanation.
What are the causes of electrical corrosion of motor bearings?
1. Shaft Voltage
Cause: Electrical currents induced in the motor shaft due to electromagnetic imbalances or external sources, such as variable frequency drives (VFDs), can create voltage potential across the bearings.
Impact: If the shaft voltage exceeds the dielectric strength of the lubricant, a current path forms through the bearings, causing arcing and surface damage.
2. Variable Frequency Drives (VFDs)
Cause: VFDs, which control motor speed by modulating power, often generate high-frequency voltage spikes. These spikes can induce currents that find a path through motor bearings.
Impact: Frequent arcing creates microscopic pits on the bearing surface, leading to wear, vibration, and noise.
3. Insufficient Lubrication
Cause: Thin or degraded lubrication can reduce the insulating properties of the lubricant film between bearing components.
Impact: Electrical currents can more easily pass through, causing damage to bearing surfaces.
4. Grounding Problems
Cause: Poor or absent grounding of the motor and shaft allows stray currents to pass through bearings instead of being safely dissipated.
Impact: Bearings become a path for current, resulting in electrical erosion.
5. Electrostatic Discharge (ESD)
Cause: Static electricity can build up due to friction in certain environments, such as during belt-driven operations or in high-speed motors.
Impact: Sudden discharge through bearings causes localized pitting.
6. Motor Design
Cause: Motors with asymmetrical magnetic fields or poorly balanced windings may induce higher shaft voltages.
Impact: This increases the likelihood of electrical discharge through bearings.
7. Contaminants in Lubricant
Cause: Contaminants such as dirt, water, or metallic particles in the lubricant can reduce its insulating properties.
Impact: This can make it easier for electrical currents to arc through the bearing.
How to deal with electrical corrosion of motor bearings?
Dealing with electrical corrosion of motor bearings involves a combination of preventative measures, diagnostic practices, and repair strategies. Here’s a comprehensive approach:
1. Preventing Electrical Corrosion
(a) Install Shaft Grounding Devices
Solution: Install shaft grounding rings or brushes to safely divert electrical current away from the bearings.
Benefit: Prevents currents from using bearings as a discharge path.
(b) Use Insulated Bearings
Solution: Replace standard bearings with insulated bearings (e.g., ceramic-coated or hybrid bearings).
Benefit: Interrupts the electrical path, protecting bearings from electrical currents.
(c) Optimize Motor Grounding
Solution: Ensure proper grounding of the motor frame and VFD system.
Benefit: Safely dissipates stray currents and minimizes voltage buildup.
(d) Install Filters or Reactors
Solution: Use filters (e.g., common-mode chokes) or reactors in VFD systems.
Benefit: Reduces high-frequency voltage spikes and shaft voltages caused by VFD operation.
(e) Improve Lubrication
Solution: Use high-dielectric-strength lubricants specifically designed for motors exposed to electrical discharge.
Benefit: Creates a stronger insulating film to resist current flow through the bearing.
(f) Regularly Balance and Align the Motor
Solution: Ensure that the motor is properly balanced and aligned to reduce electromagnetic imbalances.
Benefit: Minimizes shaft voltages and stray currents.
2. Detecting and Diagnosing Electrical Corrosion
(a) Inspect Bearings
Method: Check bearings for signs of pitting, fluting, or excessive wear.
Tools: Visual inspection or advanced tools like scanning electron microscopy (SEM).
(b) Monitor Vibration and Noise
Method: Use vibration analysis or acoustic monitoring to detect bearing damage early.
Signs: Fluted bearings may produce distinct patterns in vibration signals.
(c) Measure Shaft Voltage
Method: Use a shaft voltage probe or oscilloscope to detect electrical discharges.
Frequency: Perform periodic testing, especially in VFD-driven motors.
3. Repairing Electrical Corrosion
(a) Replace Damaged Bearings
Action: Replace bearings that show significant wear, pitting, or fluting.
Recommendation: Consider switching to insulated or hybrid ceramic bearings.
(b) Re-Lubricate the Bearings
Action: Drain and refill the bearing housing with high-quality dielectric lubricant.
Frequency: Follow the manufacturer’s maintenance schedule.
(c) Repair Shaft or Housing Damage
Action: If the shaft or housing is damaged due to electrical erosion, re-machine or replace the components.
Caution: Ensure proper alignment during reassembly.
4. Monitoring and Long-Term Maintenance
Schedule Regular Inspections: Periodically inspect bearings, lubrication, and grounding systems.
Upgrade Components: If operating conditions change (e.g., higher speed or VFD upgrades), reassess and upgrade motor protection systems.
Employee Training: Train maintenance teams to identify early signs of electrical corrosion and use diagnostic tools effectively.
5. Seek Professional Support
If the problem persists or is complex, consult motor manufacturers or specialized maintenance providers to:
Conduct an in-depth analysis of the motor system.
Recommend customized solutions, such as retrofitting the motor with advanced bearing protection technologies.
By combining these methods, you can effectively address and prevent electrical corrosion of motor bearings, ensuring longer motor life and reliable operation.
Conclusion
Electrical corrosion of motor bearings is a critical issue caused primarily by shaft voltage, VFD-induced currents, insufficient lubrication, and poor grounding. If left unaddressed, it can lead to significant motor damage, increased downtime, and costly repairs. Preventative measures such as using insulated bearings, installing shaft grounding devices, optimizing grounding systems, and employing VFD filters are essential to mitigate the risk. Regular inspections, vibration analysis, and proper maintenance further enhance protection. By understanding the causes and implementing effective solutions, businesses can ensure the longevity and reliability of their motors, reducing operational disruptions and enhancing overall efficiency.
