The “Tattoo” of Bearing: Analyzing Electrical Erosion in VFD-Driven Motors

The “Crime Scene” – What is Fluting?

Fluting is the most typical macroscopic evidence of advanced electrical erosion. It is not ordinary wear; it is a unique and highly recognizable pattern:

• Visual Characteristics: A series of continuous, evenly spaced, dull gray ripples or grooves appear on the inner and outer raceways, perpendicular to the direction of rolling. Unlike shiny, mechanical dents, fluting has a dull, matte texture, as if it has been electrically burned.
• Tactile Characteristics: If you run a fingernail or a sharp probe across the raceway, you can clearly feel the “washboard” texture.
• Acoustic Characteristics: A bearing with this damage will often produce a unique, high-frequency noise during operation, ranging from a “hum” to a “roar.” This is distinctly different from the “clicking” or “rumbling” of mechanical damage or fatigue spalling.

This “tattoo” is the permanent scar left behind after a bearing has been repeatedly struck by electric current.

The “Motive” – Why are VFD Motors the Prime Suspects?

To understand electrical erosion, we must first understand its source: shaft current.

In traditional, across-the-line motors (connected directly to 50/60Hz power), shaft current is not a major issue due to the symmetrical nature of the power supply. The widespread adoption of Variable Frequency Drives (VFDs) has changed everything.

1. High-Frequency Switching Creates “Common Mode Voltage”: To produce a variable frequency and voltage, the power modules (IGBTs) inside a VFD switch on and off at a very high frequency (several kilohertz). This process creates an unwanted byproduct: Common Mode Voltage.
2. The Motor Shaft Becomes a “Capacitor”: This common mode voltage induces a voltage on the motor shaft via capacitive coupling between the stator and rotor. At this point, the motor shaft, bearings, and frame effectively form a capacitor, and voltage builds up on the shaft.
3. Discharge Across the Oil Film: When the shaft voltage builds high enough to overcome the dielectric strength of the bearing’s thin lubricant film (the oil film is an insulator), a tiny “lightning bolt” is created. The current arcs from the inner ring, through a rolling element, to the outer ring and into the motor frame.
4. From Micro-Craters to Macro-Fluting:

• • Each electrical discharge is like a microscopic welding event. The instantaneous high temperature (over 1000°C) melts and vaporizes a tiny piece of metal, leaving a micro-crater on the smooth raceway.
  • As the motor runs, millions of these discharges occur continuously. The countless micro-craters eventually overlap and align, forming the unique, washboard “fluting” pattern that we can see with the naked eye.

The Lineup – Differentiating Fluting from Other Suspects

In the field, a washboard pattern can sometimes be misdiagnosed as vibration or installation damage. This table helps to tell them apart:

Key Diagnostic Tip: When you see a fluting pattern, your first instinct should be to check the drive system. If the equipment is powered by a VFD, electrical erosion is the prime suspect.

The “Solution Arsenal” – How to Eliminate Electrical Erosion

Since the root cause is an electrical path through the bearing, the core of any solution is to “block” or “bypass” this path.

Solution 1: Block the Circuit – Insulated Bearings

• Principle: A thin, dense, insulating layer of aluminum oxide (Al₂O₃) is applied to the bearing’s outer or inner ring surface using a plasma-spray process. This coating acts as an “insulating jacket,” completely blocking the electrical path through the bearing.
• Types:
  
  • Coated Outer Ring (e.g., suffix VL0241): The most common type, suitable for most small to medium-sized motors.
  • Coated Inner Ring (e.g., suffix VL2071): Used for larger motors or special applications, offering superior insulation.
• When to Recommend: For VFD motors in the 30kW to 500kW range, installing a coated outer ring bearing on the non-drive end is the most reliable and cost-effective standard solution.

Solution 2: The Ultimate Block – Hybrid (Ceramic) Bearings

• Principle: These bearings use rolling elements made of a ceramic material (Silicon Nitride, Si₃N₄). Since ceramic is an excellent electrical insulator, the current is naturally blocked between the inner and outer rings.
• Advantages: Perfect insulation, plus additional benefits like higher speed capability, lower friction, and longer life.
• When to Recommend: For high-power, high-speed, or mission-critical motors, or in cases where insulated bearings do not fully solve the problem, hybrid bearings are the ultimate choice.

Solution 3: Provide a “Bypass” – Shaft Grounding Devices

• Principle: Instead of forcing the current to arc through the bearing, give it an easier path to ground. A conductive carbon or fiber brush is installed to ride on the motor shaft, safely diverting the shaft current to the motor frame.
• Advantages: Can be used on motors of all sizes and can be retrofitted onto existing equipment.
• When to Recommend:
  
  • For very large motors (>500kW), a combination approach is often best: a shaft grounding device on the drive end and an insulated bearing on the non-drive end.
  • As a retrofit solution for existing motors.

Reading the Past to Predict the Future

Analyzing the unique “tattoo” of fluting is, in essence, fixing a problem after it has occurred. But its true value lies in preparing for the future. The knowledge we gain from a piece of electrically etched steel today directly translates into the wisdom that prevents equipment failures tomorrow. With the growing prevalence of VFD technology, understanding and proactively addressing the problem of shaft currents has become an essential part of modern equipment maintenance and management.