1. Introduction
Reliability of modern industrial equipment in Ukraine is often reduced due to premature failure of rolling element bearings in motors driven by variable frequency drives (VFD). In the foundries and machine-building plants of our country, reliability engineers encounter a typical symptom: catastrophic failure of bearings occurs after 3 000 – 8 000 hours of operation, despite the nominal MTBF (mean time between failures) of the electric motor being 40 000 hours. A typical trigger for investigation is the appearance of high-frequency whining in the bearing housing area, followed by a sharp increase in vibration at rotational frequencies and a temperature peak above +90°C on the outer ring.
2. Component Overview
In critical nodes of machine tool equipment and heavy drives, high-precision support elements are often used, including linear guides and precision units based on components of class FIBRO 2487.12.00350.025. These parts operate as part of kinematic chains, where the presence of stray electric currents leads to damage not only to the motor bearings but also to the transmission on related mechanical units.
Bearings of electric motors and adjacent reducers function under the following nominal conditions:
- Working temperature: from -20°C to +110°C (oil limit value).
- Radial and axial loads according to the calculation ISO 281.
- Rotational speeds: up to 4 500 rpm for medium sizes.
- Artificial environment with high voltage gradient $du/dt$ from pulse width modulation (PWM) of modern IGBT inverters.
3. Failure Evidence
During disassembly of the unit on the machine or on the inspection stand, technical personnel note characteristic defects:
- Electrical Discharge Machining (EDM): Rolling contact tracks covered with microscopic craters (pitting) of diameter 5–15 micrometers, which are formed due to electrical breakdown of the lubricating film.
- Fretting corrosion and "brown coffee" lubricant mixture: the grease completely loses its properties, turning into an abrasive paste consisting of iron oxides and burnt additive complex.
- Vibration spectrum: peaks at non-synchronous frequencies of bearing defects (BPFO, BPFI) are accompanied by high level of high-frequency noise (HFE) in the range from 5 kHz to 40 kHz.
4. Root Cause Analysis
The event chain was determined using the fault tree analysis method (Fault Tree Analysis). Rapid voltage oscillation of the inverter output ($du/dt \gt 5$ kV/µs) creates a capacitive coupling between the stator winding and the rotor. As a result, a voltage (shaft voltage) appears on the shaft. When this voltage exceeds the dielectric strength of the lubricant film in the bearing (usually less than 15–30 V), a micro discharge occurs.
5. Defined Root Causes
Systematization of factors leading to failure appears as follows:
- Absence of high-frequency shielding of the motor power cable (Probability: 45%). Use of unsymmetrical cables without a common symmetric copper or aluminum shield increases common-mode currents.
- Insufficient or high resistance engine housing and VFD enclosure grounding (Probability: 30%). High frequency currents seek the shortest path to the grounding loop through the bearing mechanism.
- Absence of discharge brushes from the shaft or insulated bearings (Probability: 25%). Installation of standard steel bearings without conductive brushes in PWM-inverter conditions guarantees electrical erosion.
6. Corrective Actions
To eliminate the detected defects, a combination of immediate and long-term measures is applied:
- Operative maintenance: immediate replacement of the damaged bearing to meet requirements ISO 15243 (classification of bearing damage). Installation of a bearing with ceramic rolling elements (hybrid bearing) or mounting of a brush unit with Aegis.
- Cable Tract Modernization: replacement of non-screened power cables with symmetric cables type 2XSLCHJ according to the standards series EN 50141 and EN 60204-1.
- Output Filtering VFD: installation of dV/dt filters or sinusoidal filters at the output of the frequency converter to reduce the voltage rise rate.
7. Quick Diagnostic Checklist for Technical Personnel
This checklist is intended for use by technicians on a tablet directly in the production workshop:
- [ ] Measure the voltage on the shaft using an oscilloscope with a capacitive probe (maximum allowable value: $\lt 1.5$ V peak-to-peak).
- [ ] Check the resistance between the engine bracket and the general grounding circuit (must be $\lt 0.1$ Ω).
- [ ] Conduct vibration analysis using the impact pulse method (SPM) to assess the condition of the lubricating film.
- [ ] Visually inspect cable connections for correct screen connection at 360 degrees in EMC seals.
- [ ] Check the lubrication condition: take a sample of the grease for the presence of metal powder and darkening.
- [ ] Check the carrier frequency settings (switching frequency) in the VFD parameters (reducing the frequency may decrease the capacitive current).
- [ ] Carry out an inspection with a thermal imager to detect overheating of bearing housings ($T \gt +75^\circ ext{C}$).
- [ ] Check the presence and integrity of bonding jumpers on couplings and bearing units.
8. Prevention Strategy
Prevention of recurring failures requires strict adherence to maintenance schedules according to ISO 13374 (machine condition monitoring). It is recommended to introduce the following intervals:
- Monthly vibration spectrum control with emphasis on high-frequency harmonics.
- Quarterly inspection of grounding loop resistance and brush node condition, voltage removal from the shaft.
- Use of original components and high-quality replacement parts. For selection of certified elements and industrial components, please refer to UNITEC-D E-Catalog.
9. Conclusion
Bearing currents induced by VFDs are a systemic problem of modern electric drives, which requires a comprehensive engineering approach. Implementation of shielded cables, hybrid bearings, and shaft voltage monitoring systems allows completely eliminating the risk of premature failure of equipment and achieving the projected reliability figures. For searching the required precise parts and equivalents from leading manufacturers, use UNITEC-D E-Catalog.
10. References
- DSTU EN 60034-17:2016 — Electric Motors. Part 17. Guidance for the application of asynchronous motors supplied from frequency converters.
- ISO 15243:2017 — Rolling bearings — Damage and failures — Terms, characteristics and causes.
- ISO 281:2007 — Rolling bearings — Dynamic load ratings and rating life.
- EN 50141 / EN 60204-1 — Machine Safety. Electrical Equipment of Machines.
- Technical Handbook on Shaft Voltages and Bearing Currents in VFD-fed AC Motors, ABB Engineering Guide.
