1. Introduction
Premature bearing failure in industrial motors powered by frequency converters (VCRs) is becoming a critical problem in modern manufacturing systems. Symptoms often manifest as increased noise levels, overheating of the bearing assembly, and eventually catastrophic raceway destruction in a characteristic pattern known as "electrical erosion" (EDM). This report analyzes this process, focusing on system interaction, including components such as the Telemecanique LD1LB030FC.
2. Overview of the component
The Telemecanique LD1LB030FC functions as a critical element in the motor control circuit. In the context of an inverter, it provides isolation and protection, but is not designed to filter the high-frequency transients that occur when switching the IGBT transistors in an inverter. An inverter-controlled motor operates at a high rate of voltage change (dV/dt), which creates a capacitive coupling between the stator winding and the rotor. This results in tension on the motor shaft relative to the housing.
3. Evidence of refusal
Technical inspection shows clear signs of EDM bearing damage:
- Visual signs: Characteristic grooves (fluting) on the inner and outer rings of the bearing, caused by micro-arc discharges.
- Vibration data: Analysis of the vibration spectrum shows high energy peaks at frequencies corresponding to the switching frequency of the drive, as well as at the frequencies of bearing defects (BPFO, BPFI).
- Shaft voltage measurements: Oscillographic measurements show shaft voltage peaks exceeding 10-15 V, which is the threshold for oil film breakdown in the bearing.
- Temperature monitoring: There is a stable excess of the operating temperature of the bearing by 15-25°C compared to similar motors operating directly from the network.
4. Investigation of the root cause
The use of the Ishikawa diagram ("fishbone" diagram) made it possible to structure the influencing factors:
- Electrical: High IF switching frequency (over 4 kHz), mismatch of cable characteristics (high parasitic capacitance).
- Mechanical: Insufficient lubrication (lubricant degradation under the action of sparks), incorrect grounding of the shaft.
- System: Lack of high-frequency filtering at the IF output, lack of stator and rotor grounding according to the high-frequency scheme (HF bonding).
5. Ranked root causes
- Common Mode Voltage: The main reason for creating capacitive current through bearings. Probability: 85%. Confirmed by measurements of shaft tension.
- Improper Grounding: High frequency currents seek the path of least resistance, which happens to be through the motor bearings rather than the ground conductor. Probability: 10%. Confirmed by the lack of proper HF-bonding between the inverter and the motor.
- Lubricant degradation: A secondary effect that increases the breakdown process. Probability: 5%. It was confirmed by the analysis of the chemical composition of the lubricant.
6. Corrective actions
To eliminate the causes, it is necessary to implement a comprehensive strategy:
- Short-term: Installation of shaft grounding rings (Shaft Grounding Rings) on the motor shaft to divert the current without passing through the bearings.
- Long term: Introduction of non-drive side (NDE) insulated bearings (ceramic or insulated rings) to break the current flow circuit.
- System: Using shielded motor cables with proper 360-degree shield grounding on both ends (for HF-grounding), and installing common-mode output chokes on the IF output to limit dV/dt.
7. Diagnostic checklist for a technician
| step | action | Tool |
|---|---|---|
| 1 | Visual inspection for the presence of metal dust | Flashlight, review |
| 2 | Measurement of shaft voltage (peak) | An oscilloscope with an isolated probe |
| 3 | Checking the motor grounding resistance | Milliommeter |
| 4 | Analysis of the level of vibration at high frequencies | Vibroanalyzer |
| 5 | Checking the integrity of the cable screen | Multimeter |
| 6 | Thermography of bearing assemblies | Thermal imager |
| 7 | Bearing Insulation Test (NDE) | Megohmmeter (with power off) |
| 8 | Assessment of the condition of the lubricant | Visual inspection / analysis |
8. Prevention strategy
To ensure system reliability, it is recommended:
- Implementation of a preventive maintenance schedule with measurement of shaft tension every 3000 hours of operation.
- Regular vibration spectrum analysis (quarterly) to detect early signs of EDM.
- Use of components tested for compatibility with the drive, which can be selected through the UNITEC-D E-Catalog, to replace critical components.
9. Conclusion
Combating bearing currents caused by IF requires a systematic approach, from physical grounding to selecting the correct bearings. An effective strategy combines condition monitoring, proper grounding installation, and the use of specialized components. For replacement bearings and protection products, visit the UNITEC-D E-Catalog.
10. Links
- EN 60034-25: Rotating electric machines. Part 25. Guide to the Design and Operation of AC Motors Powered by Inverters.
- ISO 10816-3: Assessment of machine vibration.
- Manufacturer Manuals: Telemecanique LD1LB030FC technical documentation.
- Bearing Failure Analysis Handbook: SKF/NSK technical guides.
