Analysis and correction of VFD-induced bearing currents in electric motors

1. Introduction: Early bearing failure in frequency converter-controlled drives

The use of frequency converters (VFDs) to control the speed of electric motors offers significant advantages in terms of energy efficiency and process optimization. However, VFD-induced bearing currents, which can lead to premature wear and failure of motor bearings, represent a challenge that should not be underestimated. Typical symptoms include increased noise, excessive bearing temperatures and ultimately complete bearing failure, often well before the engine's expected lifespan. These phenomena occur in industrial applications worldwide, from pumps and fans to machine tools and conveyor systems. A detailed investigation of the causes and the implementation of preventative measures are essential to ensure operational safety and avoid unplanned downtimes.

2. Component overview: electric motor bearings and operating environment

Elektromotorlager – in der Regel Wälzlager wie Rillenkugellager oder Zylinderrollenlager gemäß DIN 625-1 bzw. DIN 5412-1 – sind kritische Komponenten, die die Rotorwelle stützen und eine reibungsarme Rotation ermöglichen. Sie sind für spezifische axiale und radiale Lasten sowie Drehzahlen ausgelegt und typischerweise für eine Lebensdauer von L₁₀ 40.000 bis 100.000 Betriebsstunden konzipiert. In einer idealen Umgebung arbeiten sie unter optimalen Schmierbedingungen und bei moderaten Temperaturen. Die Betriebstemperatur von Wälzlagern sollte laut Herstellerangaben in der Regel unter 90 °C liegen, um eine vorzeitige Alterung des Schmierstoffs zu verhindern. Frequenzumrichter erzeugen jedoch schnelle Spannungsänderungen (dV/dt) an den Motorwicklungen, die zu Gleichtaktspannungen und damit zu Ableitströmen führen können. Diese Ströme suchen den Weg geringsten Widerstands, der oft über die Motorlager verläuft.

3. Evidence of damage: Appearances of bearing damage

Technicians observe specific patterns in VFD-induced bearing damage that provide a clear diagnosis:

• Pitting and Brinelling: Microscopic craters and erosions on the raceways of bearing rings and rolling elements, caused by electrical discharges (EDM – Electrical Discharge Machining). This damage is often visible to the naked eye as dull, grayish surface discoloration.
• Fluting: A pronounced wave-like corrugation on the raceways caused by repeated electrical discharges in the same locations. This is an indicator of prolonged exposure to bearing currents and is often accompanied by audible noises (“grinding noises”).
• Frosted raceways: A general discoloration and dulling of the contact surfaces, which indicates surface heating due to the passage of current.
• Lubricant degradation: Discoloration of the lubricant, reduction in viscosity and formation of carbon particles due to the thermal effect of the discharges.
• Increased vibration: According to DIN ISO 10816-3, even small bearing currents can increase vibration values ​​in the high-frequency range before macroscopic damage becomes visible. An increase in vibration velocity of more than 3 mm/s (effective) in a Grade 2 motor (normal machines) should trigger an investigation.
• Measurements: Measurements of the shaft voltage with an oscilloscope often show peak values ​​of above 10 V, which indicates that the insulation capacity of the lubricant film has been exceeded and thus discharges. Clamp meters for bearing currents can display values ​​ranging from several hundred milliamperes to several amperes.

4. Cause analysis: Systematic investigation of bearing current phenomena

A systematic root cause analysis is required to understand the complex relationships of VFD-induced bearing currents. Methods such as the Ishikawa diagram or the 5 Why method can be used here.

Ishikawa diagram: bearing current damage

 +-------------------------------------------------+ |                BEARING POWER DAMAGE |   +---------------------------------------------------+ |   +----- VFD system -----------------------------------+ |     - Fast switching edges (dV/dt) |     - High common mode voltages |     - Insufficient filtering (dV/dt filter, sine filter) |     - Poor EMC grounding of VFD |   +----- Motor & Bearings ---------------------------------+ |     - Non-insulated bearings |     - Conventional steel bearings |     - Capacitive coupling between rotor/stator |     - Insufficient discharge mechanisms (grounding brushes) |   +----- Installation & Grounding ------------------------+ |     - Long, unshielded motor cables |     - Poor shield connection on VFD/motor |     - Missing or insufficient potential equalization lines |     - Ground paths with high impedance value |   +----- Environmental factors -----------------------------+ |     - Moisture, dust (affects insulation values) |     - Vibrations from other machines |     - Thermal overload |   +----------------------------------------------------+ 

5. Identified root causes

The primary mechanisms leading to VFD-induced bearing currents are diverse and often interrelated:

1. Common Mode Voltage: The pulse width modulated (PWM) output voltage of the VFD creates high common mode voltages. These drive currents through the windings, motor frame and bearings to ground. This is the most common cause of EDM damage to bearings.
2. Capacitive Discharge Currents: A parasitic capacitance exists between the rotor and stator of the motor. The rapid voltage changes (dV/dt) across the motor caused by the VFD charge this capacitance. If the voltage on the shaft reaches a critical threshold (typically > 10 V), the energy is suddenly discharged across the lubricating film of the bearings, which leads to spark discharges (EDM).
3. Circulating Bearing Currents: In larger motors, magnetic field asymmetries or high-frequency currents from the VFD can induce currents in the motor shaft. These currents can form a closed circuit through the bearings at one end of the motor, through the motor frame, and through the bearings at the other end.
4. High-frequency currents from the VFD: The switching frequencies of modern VFDs are typically between 2 kHz and 16 kHz and generate harmonics that can be dissipated through the bearings, even if the common mode voltage is in the nominal range.

The likelihood of these phenomena increases with the motor power, the cable length between the VFD and the motor, and the switching frequency of the VFD. Inadequate grounding of the motor and VFD system significantly exacerbates the problem.

6. Corrective measures and long-term prevention

Immediate measures:

• Bearing replacement: Replace the damaged bearing with an identical model or, if available, an electrically insulated bearing. Document the damage in accordance with DIN ISO 15242-1.
• Temporary Shaft Grounding: Install a temporary shaft grounding brush to dissipate shaft voltage until a permanent solution can be implemented.
• Measurement and Diagnosis:Immediately carry out shaft voltage and bearing current measurements to quantify the extent of the problem and verify the effectiveness of immediate measures.

Long-term prevention strategies:

Effective prevention of VFD-induced bearing currents requires a multi-layered strategy:

1. Electrically insulated bearings: Use bearings with ceramic coating on the outer ring (e.g. INSOCOAT bearings) or fully ceramic rolling bearings. These offer a high electrical resistance (typically >100 MΩ) and interrupt the current path through the bearing.
2. Shaft Grounding Brushes: Special carbon fiber brushes or metal fiber brushes provide a low resistance path for bearing currents to ground and divert them away from the shaft before they reach the bearings. The typical resistance of such a brush should be below 1 Ω.
3. VFD output filter:
   • dV/dt filter: Reduce the rate of change of voltage (dV/dt) and motor voltage peaks.
   • Sinusoidal Filter: Convert the VFD's PWM square wave voltage into a near sinusoidal voltage, dramatically reducing common mode voltages and high frequency currents.
   • Common-mode chokes: Increase the impedance to common-mode currents, forcing them to seek alternative paths or decrease.
4. Optimized grounding: Proper grounding of the motor, the VFD and the system is essential according to VDE 0100-540 and VDE 0100-444. This includes low-resistance potential equalization lines, the shielding of motor cables and the large-area connection of the cable shields to the VFD and the motor housing. The shielding should be connected at both ends to ensure effective dissipation of high-frequency interference.
5. Motor with Faraday shielding: Some motors are specially equipped with an internal Faraday shielding between the stator and rotor to reduce capacitive coupling and thus minimize the generation of bearing currents.

7. Quick diagnostic checklist for field technicians

This checklist helps technicians quickly identify potential bearing power problems:

1. Listen for unusual noises: Is there a “grinding” noise, a crackling noise, or a change in the way the engine runs?
2. Feel the bearing temperature: Is the bearing housing unusually hot (e.g. > 80 °C at normal ambient temperature)?
3. Visual Inspection: Are there signs of lubricant leakage or discoloration?
4. Shaft voltage measurement: Measure the tension between the shaft and the housing. Are there peak values ​​ over 10 V?
5. Vibration analysis: Check the vibration data in the high frequency range ( > 500 Hz). Are there elevated values ​​that indicate bearing damage (DIN ISO 10816-3)?
6. Check VFD settings: Which switching frequency is set? Is a dV/dt filter or sine filter active?
7. Check ground connections: Are all ground cables (motor, VFD, system) connected properly and do they have a low resistance (VDE 0100-540)?
8. Check motor cables: Have shielded cables been used? Are the umbrellas positioned correctly at both ends?
9. Identify storage type: Are they standard warehouses or already isolated warehouses?
10. Operating Hours and Maintenance History: Does the actual bearing life match the expected MTBF? When was the last lubrication?

8. Prevention strategy and condition monitoring

A comprehensive prevention strategy integrates proven maintenance practices with modern condition monitoring systems:

• Regular maintenance: Compliance with manufacturer specifications for lubrication and bearing replacement. For VFD-controlled motors, the intervals may need to be shortened.
• Condition Monitoring:
  • Vibration monitoring: Continuous monitoring of vibrations in accordance with DIN ISO 20816-1, especially in the high frequency range, can detect early signs of bearing damage caused by EDM. Exceeding limit values ​​(e.g. Class B according to DIN ISO 10816-3 reached or exceeded) requires immediate action.
  • Temperature monitoring: Sensors for monitoring the storage temperature. A sudden increase of >10 K above the basic value is a warning signal.
  • Current signature analysis: Analysis of motor currents for characteristic frequencies that may indicate bearing problems.
  • Shaft current measurement: Periodic or continuous measurement of shaft voltage and shaft current for early detection.
• Design optimization: When purchasing new drive systems, priority should be given to specifying motors with integrated bearing insulation or shaft grounding brushes.
• Staff training: Raising awareness of maintenance technicians and engineers about the problem of VFD-induced bearing currents and their diagnosis.

9. Conclusion

VFD-induced bearing currents pose a serious threat to the reliability of electric motors in industrial applications. A thorough understanding of the physical causes, precise diagnosis based on clear damage patterns and measured values, and the implementation of specific protective measures are essential. Through the targeted use of insulated bearings, shaft grounding brushes and VFD output filters as well as careful grounding, the service life of the motor bearings can be significantly extended and system availability increased. A proactive strategy is crucial for reliable and efficient maintenance of drive systems that meet the requirements of DIN standards.

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10. References

• DIN ISO 10816-3: Mechanical vibrations - Measurement and evaluation of vibrations of machines on non-rotating parts - Part 3: Industrial machines with nominal powers over 15 kW and nominal speeds between 120 min⁻¹ and 15,000 min⁻¹ when measuring at the installation site.
• DIN ISO 15242-1: Rolling bearings – measuring methods for vibrations – Part 1: Basic terms.
• VDE 0100-540: Installation of low-voltage systems - Part 5-54: Selection and installation of electrical equipment - earthing systems, protective conductors and potential equalization conductors.
• VDE 0100-444: Setting up low-voltage systems - Part 4-44: Protective measures - Protection against interference voltages and electromagnetic interference.
• DIN EN 61800-3: Variable-speed electric drives - Part 3: EMC requirements and special test methods.
• Manufacturer documentation for frequency converters and electric motors (e.g. Siemens, ABB, SKF).
• Practical manual for storage technology (e.g. FAG, SKF).