October 13,2026
The core reason bearings require insulation or Electrical Resistance Required is the presence of shaft currents. When current passes through the internal contact surfaces of a bearing, it creates electrical erosion pits and fusion marks on raceways and rolling elements, and accelerates grease degradation.
Variable Frequency Drive (VFD) Motors (Primary Scenario)
Motors using variable frequency drives are the most prominent scenario for shaft current problems. PWM technology generates high-frequency common-mode voltage, which couples through the motor's internal parasitic capacitance and induces shaft voltage between the shaft and housing. When the voltage exceeds the lubricating film threshold, the film breaks down and discharges, forming electrical discharge machining (EDM) currents. The risk increases significantly with VFD usage.
Rail Transit Traction Motors
Traction motors in railway vehicles use VFD drives extensively, making shaft current problems particularly severe. Insulated bearings are the primary mitigation measure adopted by the rail transit industry.
Wind Turbine Generators
Generators in wind power systems also face shaft current risks, and insulated bearings are widely used to protect generator bearings.
New Energy Vehicle Drive Motors
Motors in electric vehicles and electric drive systems operate at high speeds, and the high-frequency shaft currents generated by inverters pose a threat to bearings.
Other Industrial Motors and Generators
Large high-voltage motors, industrial drive systems, HVAC motors, pumps, and fans may all require insulated bearings whenever VFD drives or induced voltage risks exist.
Ceramic-Coated Insulated Bearings
An aluminum oxide (Al₂O₃) insulating layer is applied to the inner or outer ring surface through plasma spraying, typically 100–200μm thick. The coating can achieve a minimum resistance of 200–250MΩ and withstand voltages up to 3,000V DC. Dimensions are identical to standard bearings and are directly interchangeable. This is the most widely used type of insulated bearing, suitable for medium-sized motors.
Hybrid Ceramic Bearings
These use bearing steel rings combined with silicon nitride (Si₃N₄) ceramic rolling elements. Ceramic material itself is non-conductive, and current cannot form a circuit through the rolling elements. Compared to coated bearings, hybrid bearings provide reliable protection under high-frequency AC voltage and high temperatures. They are particularly suited for high-speed applications, as the lighter weight of ceramic balls allows speeds approximately 20% higher than all-steel bearings. Recommended for small motors with outer diameters under 120mm and high-frequency inverter motors.
Full Ceramic Bearings
Inner and outer rings and rolling elements are all made of ceramic materials, offering the best insulation performance. Suitable for scenarios with extremely high insulation requirements and smaller dimensions, but at a higher cost.
Resin-Insulated Bearings
These use insulating resin coatings such as PPS (polyphenylene sulfide) and are relatively low-cost, primarily used in railway locomotives. The disadvantage is low thermal conductivity, which can lead to high temperatures inside the bearing.
Selection Considerations
When choosing an insulation solution, the voltage type must be considered: under DC or low-frequency AC voltage, the ohmic resistance of the coating plays a decisive role, making ceramic-coated bearings a good choice; under high-frequency AC voltage, the capacitive impedance of the bearing is more critical, giving hybrid ceramic bearings a clearer advantage. Additionally, the performance of insulated bearings is affected by humidity and temperature. Under high-humidity conditions, coating insulation performance may degrade, and products with sealed pores should be selected.
If your equipment requires electrical resistance required bearings but standard bearings cannot meet the requirements, please feel free to contact us. We can provide customized solutions tailored to your specific needs.
