Introduction: The Silent Killer of Bearings
Have you ever encountered repeated, unexplained bearing failures in your electric motors? Despite proper lubrication, alignment, and load conditions, the bearings wear out prematurely?
The culprit may not be mechanical — it could be an invisible electrical threat: shaft current.
This often-overlooked phenomenon can cause catastrophic bearing damage, leading to unplanned downtime, costly repairs, and reduced motor lifespan. Understanding how shaft current forms — and how to stop it — is essential for maintaining reliable motor operation.
What Is Shaft Current?
Shaft current is an unwanted electrical current that flows through the motor shaft and its bearings. It occurs when a voltage difference — known as shaft voltage — builds up on the rotating shaft.
When this voltage finds a path to ground — typically through the bearings — current flows through them, causing progressive and often irreversible damage.
How Is Shaft Voltage Generated?
Several factors can induce voltage on a motor’s shaft. The most common sources include:
Imperfections in the motor’s magnetic circuit — such as uneven air gaps or inconsistencies in the stator/rotor laminations — create an unbalanced magnetic field. This imbalance acts like a small generator, inducing a voltage in the shaft (a process called magnetic induction).
Modern VFDs use fast-switching IGBTs that produce high-frequency common-mode voltages. These voltages capacitively couple onto the motor shaft, especially in long cable runs or unshielded installations.
This is the primary cause of shaft current in today’s industrial motors.
In some cases, static charges accumulate on the rotor due to friction from belt drives, cooling fans, or airflow. While less common, this can still generate enough voltage to discharge through the bearings.
How Shaft Current Destroys Bearings
Bearings are mechanical components — not electrical conductors. When current passes through them, even at low amperage, it causes severe damage through Electrical Discharge Machining (EDM).
The Damage Process:
Micro-Arcing: Current jumps between rolling elements (balls/rollers) and raceways.
Localized Melting: Each discharge creates extreme heat (thousands of °C), melting microscopic craters into the steel surface.
Pitting and Fluting: Over time, these pits align into rhythmic, washboard-like ridges across the raceway — a pattern known as fluting.
Progressive Failure: Fluting increases vibration, noise, and temperature. Eventually, the bearing fails catastrophically.
Visual Clue: If you see a corrugated or frosted pattern inside a failed bearing, shaft current is likely the cause.
How to Prevent Shaft Current Damage
Prevention focuses on one goal: divert or block the current before it reaches the bearings.
1. Install Shaft Grounding Rings
A cost-effective, reliable solution.
Uses conductive microfibers or brushes to make contact with the shaft.
Provides a low-impedance path to ground, bypassing the bearings entirely.
Ideal for VFD-driven motors.
2. Use Insulated Bearings
Features a ceramic coating (e.g., plasma-sprayed alumina) on the outer or inner ring.
Breaks the electrical circuit, preventing current flow through the bearing.
Often used on the non-drive end (NDE) of the motor.
3. Grounding Brushes
Simple carbon or copper brushes that contact the shaft.
Less durable than grounding rings but effective for low-speed or light-duty applications.
4. Proper Motor & Drive Installation
Use shielded motor cables and proper grounding practices.
Minimize cable length between VFD and motor.
Consider sinusoidal filters or dv/dt filters to reduce common-mode voltage.
Conclusion: Protect Your Investment
Shaft current is a silent but serious threat — especially in modern, VFD-controlled systems. While the symptoms may go unnoticed until failure occurs, the solution is both preventable and cost-effective.
By recognizing the signs — like fluting in failed bearings — and implementing protective measures such as grounding rings or insulated bearings, you can:
Extend bearing and motor life
Reduce maintenance costs
Avoid unexpected downtime
Don’t let an invisible current compromise your operations. Protect your motors. Protect your productivity.
Need high-precision bearings resistant to electrical erosion?
At Beining Intelligent Technology, we offer insulated bearings, hybrid ceramic bearings, and custom solutions designed for VFD-driven motors, robotics, and high-speed spindles.
Contact us for technical support or product recommendations.
