1. Introduction
At one of the heavy engineering enterprises in Ukraine, during a routine inspection of the main drive of the production line, premature failure of the cylindrical roller bearing of the AC motor was recorded. The equipment worked as part of a modernized complex under the control of the Rexroth HMS01.1N-W0030-A-07-NNNN servo controller. The operating time of the node was only 4,200 hours instead of the projected MTBF resource indicator of 40,000 hours. The nature of the destruction of the raceways and rolling elements indicated EDM (Electrical Discharge Machining) induced high-frequency in-phase voltages from the pulse width modulation (PWM) drive system.
2. Overview of the component
The Rexroth HMS01.1N-W0030-A-07-NNNN servo controller is a high-performance frequency converter (IF) designed to control industrial synchronous and asynchronous motors. It generates output power using IGBT transistors with a high pulse rate (dv/dt) that often exceeds 10 kV/μs. In this architecture, the motor bearings are at risk due to the capacitive coupling between the stator winding, the rotor and the grounded machine frame.
Operating conditions of the studied node:
- Nominal voltage of the direct current link: 700 V DC.
- Switching frequency of the inverter: 8 kHz.
- Operating temperature of the bearing shield: +75°C.
- Radial load: 12 kN.
3. Evidence of refusal
The diagnostic team conducted a vibration spectral analysis and inspected the dismantled bearing. The following symptoms were recorded:
- Vibration data: In the vibration spectrum (accelerometer with a range of up to 10 kHz), high-frequency harmonics and modulations at the switching frequency of the IF (8 kHz) in the high-frequency range (HFE) appeared. The level of vibration in the 5–10 kHz band increased 4.5 times compared to baseline values.
- Visual inspection: On the running tracks of the outer and inner rings, microscopic craters (pitting) with a density of up to 50 pieces/mm² were found. The surface of the rolling elements has acquired a matte gray shade ("wavy" wear or corrugation).
- Lubricant analysis: Metal dust (particle size less than 5 μm) and signs of thermal destruction of the base oil due to local electrical breakdowns of the lubricating film were detected in the proliter lubricant.
4. Study of root causes
The Ishikawa diagram method and failure tree analysis were used to determine the chain of events that led to the accident. The main factors are divided into four categories: electrical parameters, mechanical design, lubrication condition and grounding.
The voltage rise rate (dv/dt) creates a capacitive potential on the shaft. When the voltage on the shaft exceeds the breakdown strength of the grease wedge in the bearing (typically 10V to 30V depending on film thickness and viscosity), a microdischarge occurs. This process is repeated millions of times per hour, melting the metal on a micro level.
5. Identified root causes
According to the results of the examination, three key reasons with different probability of occurrence were identified:
- Lack of effective high-frequency equalization of potentials and current-carrying brushes (Probability: 60%). High-frequency bearing currents found their way through rolling bodies instead of special charge removal systems.
- Inconsistency of filtering parameters at the output of the IF (Probability: 25%). The absence of motor filters (dv/dt filters or sine filters) at the output of the Rexroth HMS01 led to a critical value of the rising speed of the pulse front.
- Incorrect power cable grounding geometry (Probability: 15%). The motor cable shield was connected using a "pigtail" instead of a 360-degree circular EMC clamp, which reduced the effectiveness of shielding HF interference.
- Immediate fix: Installed a carbon fiber coated shaft ground brush to safely transfer voltage from the rotor to the motor housing. Reassembly of the power cable screen was carried out in accordance with the requirements of the EN 60204-1 standard (use of EMC seals with full circular contact).
- Long-term prevention: Mounted a dv/dt filter on the output of the Rexroth servo controller to reduce the voltage rise rate below 500 V/μs according to IEC/TS 60034-25 recommendations.
- Service intervals: Inspect the condition of the shaft grounding brushes every 2,000 hours of operation.
- Condition monitoring: Implement continuous monitoring of high-frequency vibration (Envelope Analysis) to detect pitting in the early stages (degree of damage according to ISO 13373).
- Design modernization: When choosing new motors to work with Rexroth converters, require windings according to IEC 60034-18-41 standards (resistance to impulse voltages) and the use of insulated bearings (with ceramic rolling bodies or aluminum oxide ring coating).
- IEC/TS 60034-25: Rotating electrical machines — Part 25: AC electrical machines designed for the frequency converter supply.
- EN 60204-1: Safety of machinery — Electrical equipment of machines — Part 1: General requirements.
- ISO 10816-3: Mechanical vibration — Evaluation of machine vibration by measurements on non-rotating parts.
- Bosch Rexroth AG: Technical documentation for HMS01 Servo Drives, Manual DOK-INDRV*-HMS01******-PR03-EN-P.
- IEEE Std 43-2000: IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery.
6. Corrective actions
A complex of immediate and long-term measures has been developed to eliminate the identified defects and prevent repeated accidents:
7. Quick diagnostic card for field engineers
This table-checklist is designed for technical specialists who conduct an audit of drive systems directly in the workshop:
| № | Validation parameter | Method/Tool | Normative value |
|---|---|---|---|
| 1 | Voltage on the motor shaft | Oscilloscope + carbon brush-nose puller | < 15 V (peak value) |
| 2 | Grounding resistance of the inverter housing | Microohmmeter (ISO 27668) | < 0.1 Ohm |
| 3 | EMC quality of the cable connection | 360° visual inspection of clamps | Full circular contact without "pigtails" |
| 4 | High-frequency vibration of bearings | Vibroanalyzer (envelope spectrum) | Acceleration < 0.5 g in the range of 5-10 kHz |
| 5 | Lubrication condition | Drop test / analysis of wear particles | Absence of particles > 5 μm and metallic luster |
| 6 | Bearing assembly temperature | Pyrometer / thermocouple (ISO 10816) | < 85°C (depending on the insulation class) |
| 7 | The presence of filters at the IF output | Control panel inspection | Availability of dv/dt or sine filter for cable length > 15 m |
| 8 | Insulation resistance of motor windings | Megohmmeter (1000 V DC) | > 100 MΩ (according to IEEE 43) |
8. Prevention strategy
To ensure the reliability of equipment at Ukrainian industrial enterprises, it is necessary to integrate the following steps into the maintenance regulations:
9. Conclusion
Premature bearing failures due to EDM currents are a systemic problem when using modern high-speed PWM inverters such as the Rexroth HMS01. A comprehensive approach to the design of the electric drive, which includes proper shielding, filtering of the output signal and the use of devices for removing the charge from the shaft, allows you to completely eliminate electroerosive wear. Consult our catalog to select compatible components, filters and protection devices. See the complete range in the UNITEC-D E-Catalog section.
