In a previous post, we explored how shaft currents occur in motors, especially when Variable Frequency Drives (VFDs) are involved. Now, let’s turn our focus to what you can do about it.
This post will help maintenance professionals and equipment designers understand the different types of bearing protection available, when to use them, and how to layer methods for maximum reliability.
Top Bearing Protection Methods to Prevent Shaft Current Damage
Shaft currents can lead to bearing pitting, fluting, premature wear, and costly downtime. Below are six proven methods of bearing protection, each with its strengths, ideal applications, and maintenance needs.
1. Shaft Grounding Rings for Motor Bearing Protection
Shaft grounding rings are one of the most widely used protection methods. These circular bands are lined with thousands of conductive microfibers that brush against the motor shaft, creating a continuous, low-resistance path to ground and safely diverting currents away from the bearings.
When to Use Shaft Grounding Rings:
- Ideal for low voltage motors up to 500HP (There are special designs for larger motors)
- Relatively clean, non-corrosive environments, although encapsulated SGRs are available
- Where minimal maintenance is preferred
Popular Brands:
- AEGIS® SGR Rings by Electro Static Technology (widely used, available in solid or split rings)
Maintenance Considerations:
- Low maintenance, easy to retrofit
- Can be internal or external
- Inspect annually or every 3-6 months in challenging environments
2. Shaft Grounding Brushes for High-Vibration and Contaminated Environments
Shaft grounding brushes are spring-loaded carbon or composite brushes that provide a grounding path for shaft voltage.
When to Use Shaft Grounding Brushes:
- Larger motors over 500HP
- Higher voltage motors 690 V and up
- In dirty, wet, or oily environments
- When dealing with very high shaft voltages or low-frequency currents
Popular Brands:
- Morgan Advanced Materials: Offering high-performance grounding brushes for large motors and harsh environments
- Helwig Carbon Products: noted for customizable brush holders and durable silver graphite options
Maintenance Considerations:
- Inspect every 3-6 months under normal conditions
- Replace every 12-24 months
- More suitable for dirty or harsh environments than microfiber rings
3. Insulated Bearings or Bearing Housings for Electrical Isolation and Bearing Longevity
Insulated bearings are a type of motor bearing designed with an electrical barrier that isolates them from the motor frame, blocking shaft current and preventing damage. Insulated bearing housings are a cost-effective way of isolating the outer race of the bearing from the motor frame with pre-coated motor end brackets.
Insulated Bearings: When Should They Be Used?
- On motors 100 horsepower and above
- Dirty, high vibration environments
- High-speed motors, typically considered above 3,600 RPM
- Use in conjunction with shaft grounding ring or brush
When Should Insulated Bearings Be Used on Both Ends of Motor?
- In larger motors (Typically 200HP or larger)
- In medium-voltage motors (Typically 690 V or higher)
- In dirty or corrosive environments where shaft grounding brush or ring contact may be intermittent
- Critical applications
- Shaft grounding device should still be used to avoid floating currents building up with no path to ground
Types of Insulated Bearings:
- Coated Bearings (Plasma/Spray Coated Outer Races) – cost-effective, common in inverter-duty motors
- Hybrid Bearings (Ceramic balls + Steel races) – excellent for high-frequency currents and large/critical motors
- Ceramic Bearings (All-ceramic or ceramic-coated races) – maximum insulation, costly but best for high-reliability systems especially high-speed
- Insulated Housings – isolate endbells or bearing housings; often standard on large motors where bearing modifications are difficult.
Popular Brands:
- SKF is a popular brand for insulated bearings with over 100 years of experience.
Maintenance Considerations:
- Low maintenance electrically
- Normal bearing maintenance
- Visually inspect whenever motor is reconditioned
4. CoolBLUE Inductive Absorbers for High-Frequency Shaft Currents
CoolBLUE inductive absorbers are passive components made from nanocrystalline material that reduce high-frequency shaft currents by filtering noise on motor cables.
When to use CoolBLUE Inductive Absorbers:
- Ideal for short to moderate cable lengths where high-frequency common mode noise is present
- Effective for filtering high-frequency currents before they reach the motor
- Best used in conjunction with shaft grounding devices for full protection
- Can use stand-alone with smaller motors and non-critical applications
Popular Brands:
- Magnetec provides CoolBLUE Inductive Absorbers
Maintenance Considerations:
- Maintenance-free
- Must be sized correctly for cable and motor current
5. Reactors and Filters to Reduce Voltage Spikes and Electrical Noise
Reactors and filters are devices installed between the drive and motor that smooth voltage spikes and reduce electrical noise. Reactors, which are simple inductors, reduce voltage spikes and electrical noise, while filters go a step further by smoothing the waveform on long cable runs. Together, they help limit shaft current issues and protect both the motor and insulation.
When to use Reactors & Filters:
Line Reactors (Input or Load Reactors):
- Power is "dirty" (voltage transients, spikes, harmonics)
- There are long motor leads, typically over 50-100 feet
- To reduce voltage rise time (dV/dt) and suppress reflections
dV/dt Filters
- Employed when very long cable runs (over 300 feet) cause excessive voltage overshoot
- Provide better voltage waveform conditioning than standard reactors
- Important in sensitive applications where motor insulation life is critical
Popular Brands:
- TCI is one of the most popular line reactor brands
Maintenance Considerations:
- Minimal Maintenance
- Add bulk to installation
6. Labyrinth Seals with Integrated Grounding for Bearings in Harsh Environments
Labyrinth seals and integrated grounding are built-in motor features designed to protect bearings. Labyrinth seals use a maze-like design to keep out contaminants, while integrated grounding provides a direct path for shaft currents to safely discharge to ground.
When to use Labyrinth Seals with Integrated Grounding:
- Where motor must meet IEEE-841 specification
- Dirty or humid environments where sealing and grounding are both critical
Popular Brands:
- AEGIS offers labyrinth seal with integrated grounding options
Maintenance Considerations:
- Maintenance-free
- Inspect annually to confirm grounding contact is clean and intact
When and How to Combine Bearing Protection Methods for Maximum Reliability
Using more than one bearing protection method can often
provide the most reliable protection against shaft currents, especially in
large motors or challenging environments. Some methods are complementary,
working well together to block and divert currents, while others are alternatives,
meaning you generally choose one or the other depending on the application.
Complementary Methods
- SGR + NDE Insulated Bearing: Provides both diversion and blocking of circulating currents. Use in motors 100 horsepower and up.
SGR + Insulated Bearings (both ends): Provides both diversion and blocking of circulating currents. This combination is common in motors 200 HP+ especially in critical applications or systems with long cable runs. - SGR + CoolBLUE: Shields the motor from high-frequency currents, particularly useful in noisy VFD setups or where there are longer motor leads.
- SGR + Grounding Brush: Offers redundancy, adding an extra layer of protection in harsh operating environments.
Alternative Options
Sometimes methods serve a similar purpose, so you generally select one over the other based on your motor and environment:
• Shaft Grounding Ring vs Grounding Brush:
- Shaft grounding rings may be preferred in cleaner environments, where there isn't easy access to the motor shaft, or where the desire is to avoid maintenance
- Brushes may be better in dirtier environments where the brushes are accessible as well as for larger (500 HP+) and higher voltage (690 V+) machines.
• CoolBLUE vs Reactors & Filters:
- CoolBlue may be more effective mitigating shaft currents
- Consider filters with longer cable runs and where there is a need to smooth waveforms
Best Grounding Configurations for Belt-Drive vs Direct-Drive Applications
Direct Drive:
Here is what to do:
- Follow earlier guidance in this post to determine combination of shaft grounding protection and possibly insulated bearings
- Insulated couplings or non-conductive coupling inserts can be used when driven equipment is very sensitive or motor grounding is not effective
Belt Drive:
That’s why there should be extra consideration given to shaft-current protection in belt drive systems:
- For smaller belt-drive motors, drive end shaft grounding is usually enough
- It is recommended to use a NDE insulated bearing and DE shaft grounding device in motors 100 horsepower and up
- Never use two insulated bearings with no grounding device in a belt drive motor as the belts and
bearings will completely electrically isolate the shaft giving the currents no place to go.
Final Thoughts on Bearing Protection for Shaft Currents
Shaft current mitigation isn’t one-size-fits-all. Understanding your motor's operating environment, load connection, and power setup is key to choosing the right protection. When in doubt, consult your ESR representative and we'll help you find the best option for your specific situation.