7 min read · 22 Apr 2026 · SLS Engineering Team
Insulated Bearings for VFD-Driven Motors — When and Which
Stop electrical erosion in inverter-driven motors with the right insulation, hybrid ceramics or shaft grounding.
Variable-frequency drives create common-mode voltage that drives current through the motor bearings. Below ~100 kW the discharge is usually manageable; above that, fluting damage is almost certain unless something interrupts the path.
Your three engineering options
1. Insulated bearing (outer-ring coating)
Examples: SKF INSOCOAT (VL024X), FAG J20AA / J20AB, NTN E-1.
- Outer ring sprayed with 100–250 µm of aluminium oxide.
- Drop-in replacement; no shaft modification.
- Cost: ~3–4× a standard bearing.
2. Hybrid ceramic bearing
Silicon nitride (Si3N4) balls in steel rings.
- Ceramic is a natural insulator — the bearing itself stops current flow.
- Bonus: lower weight, less centrifugal stress, longer fatigue life at speed.
- Cost: ~6–10× standard.
3. Shaft grounding ring (SGR)
A conductive carbon-fibre brush that bonds the rotor to ground.
- Pairs well with one insulated bearing at the non-drive end.
- Cheap; needs periodic inspection.
SLS keeps in stock
- 6330 M/C3 VL0241 (SKF INSOCOAT)
- 6316/C3 VL0241
- 6313-2Z/C3VL0241
- Hybrid 7014 CD/HCP4A and similar spindle pairs
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