Electrically insulated bearings for electric motors

Electric Motor bearing, also known as motor bearing, is a kind of special bearing specially applied to motor. The bearing used in the motor is a part supporting the shaft, which can guide the rotation of the shaft, and also can bear the idle parts on the shaft. The concept of the bearing is very broad. There are four types of bearings commonly used in motor, namely rolling bearing, sliding bearing, joint bearing and oil bearing. The most common motor bearing is rolling bearing, that is the bearing with rolling element. All the motor bearings will produce strong current in operation, which will cause corrosion to the bearings, thus affecting the service life of the bearings. The design and application of electrically insulated bearings solve this problem well.

Electrically Insulated bearings can prevent the effect of stray current. Insulated bearing combines bearing function and insulation performance in the solution, so it is more cost-effective than other insulation methods.

With a unique coating process, a layer of aluminum oxide is coated on the surface of the inner or outer ring of the bearing, so that it has the insulation ability and is not affected by high temperature, moisture and chemicals. The performance and running accuracy of the bearing are the same as that of the standard uninsulated bearing, and no additional installation measures are required.

According to the specific application of the electrically insulated bearings, they can be many kinds of bearing design, electrically insulated ball bearing, insulated roller bearings, insulated bearing with inner ring coated, insulated bearing with outer ring coated, as well as rolling element coated design.

There are some of the top bearing manufacturer can produce electrically insulated bearings above mentioned.

1. SKF electrically insulated bearings

INSOCOAT bearings with coated outer ring Bearings with an electrically insulating coating on the external surfaces of the outer ring are the most common INSOCOAT bearings. They are identified by the suffix VL0241. Outer ring coated INSOCOAT bearings are recommended for medium size motors, that use 6215, 6313 size bearings and larger. For applications where smaller bearings are used, SKF recommends hybrid deep groove ball bearings.

INSOCOAT bearings with coated inner ring Bearings with an electrically insulating coating on the external surfaces of the inner ring provide enhanced protection against electric current damage. The enhanced protection results from the increased impedance due to the smaller coated surface area. Bearings with a coated inner ring are identified by the suffix VL2071 and are recommended for larger size motors (typically from bearing sizes 6226, 6324 sizes and larger), or other applications where the bearings risk being subjected to high shaft voltages.

VL0241 Electrical resistance: min. 50 MW, breakdown voltage: max. operating voltage 1 000 V DC.

VL0246 Electrical resistance: > 150 MW, breakdown voltage: max. operating voltage 3 000 V DC.

VL2071 Electrical resistance: min. 50 MW, breakdown voltage: max. operating voltage 1 000 V DC.

VL2074 Electrical resistance: > 150 MW, breakdown voltage: max. operating voltage 2 000 V DC

2.FAG Current insulating bearing

The current-insulating bearings include all rolling bearings that give insulation against current passage. Coated bearings, in which either the inner ring or outer ring has a ceramic coating, are current-insulated bearings. The ceramic layer gives insulation against current passage. Hybrid bearings in which the rolling elements are made from ceramic also have a current-insulating effect. In this case, the rolling elements prevent current passage.

The coatings are subjected to 100% quality inspection and ensure a dielectric breakdown strength of at least:

■ J20AB as insulation up to 1000 VDC

■ J20AA, J20C as insulation up to 3 000 VDC

■ J20B as insulation up to 500 VDC.

Below this voltage, the insulating layer allows only very small flows of current through the bearing. In principle, a current-insulating bearing can be integrated as a parallel circuit between resistance and capacitance, the Ohm resistance should be as high as possible and the capacitance as low as possible.

3. NTN  insulated Coated bearing

Electrical pitting often occurs in bearings used in electrical equipment, such as motors and power generators, as a result of current leakage. An electrical current present near a bearing can flow to the inside of the bearing, causing sparks that damage the rolling contact surface, ultimately leading to bearing failure. This electrical pitting and can be prevented with coated bearings or ceramic hybrid bearings.

NTN’s MEGAOHM™ Series of insulated bearings has been specifically designed to counteract electrical pitting. These bearings are available in both ceramic and resin versions that are suited to a variety of applications. Coating the outer ring of the bearing provides a barrier (capable of resisting at least 100 Megaohms at 500 VDC) against any stray currents that would pass through the bearing.

MEGAOHM™ Series bearings are featured as OEM equipment on wind turbine generators, traction motors for railway cars, and variable speed electric motors in wastewater treatment facilities.


For especially problematic applications, NTN offers ceramic hybrid bearings. Using conventional steel inner and outer rings, silicon nitride rolling elements provide excellent electrical insulation while reducing rotating mass and improving heat dissipation.

4. NKE electrically insulated rolling bearings

NKE electrically insulated rolling bearings reliably prevent current discharge. 

NKE provides bearings with oxide ceramic insulating layers on the bearing ring. Applied with plasma technology, the insulation has a guaranteed breakdown resistance of at least 1000V AC or DC. Two variants are available:

  • SQ77: Insulation on the outer ring
  • SQ77E: Insulation on the inner ring

Product range

SQ77 (insulated outer ring)

  • Cylindrical roller bearings 
    • Design variants: NJ, NU, NUP
    • Dimension series: 210-230, 310-330
    • Cages: brass and polyamide
    • Radial clearance groups: C0, C3, C4
  • Deep groove ball bearings 
    • Dimension series: 6212-6226; 6312-6326
    • Cages: brass
    • Radial clearance groups: C3, C4
  • Other types and SQ77E (insulated inner ring) upon request

5. CoatMaster lectrically insulated bearings

CoatMaster, as the Top Brand of China Bearing manufacturer, focus on reserch & develop Electrically Insulated Bearing, it can provide various types of insulated bearings

  1. Electrically insulated deep groove ball bearing
  2. Electrically insulated angular contact ball bearing
  3. Electrically insulated cylindrical roller bearing
  4. Electrically insulated tapered roller bearing
  5. Electrically insulated spherical roller bearing
  6. Electrically insulated spherical roller thrust bearing
  7. Electrically insulated linear motion bearing
  8. Inner or outer ring insulated bearing with oxide coating
  9. Hybrid bearing with electric insulating ceramic rolling element

Electrically insulated Bearing is mainly used for motor products. Its use depends on the power supply status of the motor. There is no high-frequency voltage and current component in the normal power supply network. Generally, insulated bearing is not used only for large high-voltage motor, such as rules above 400kW type motor. But now the frequency conversion power supply, because the power quality contains a lot of high-frequency components, resulting in the increase of motor shaft current, so this problem should be considered in the standardization of more than 400kW, and even the smallest motor should also be installed with insulated bearings according to the use conditions, such as electric vehicles, electric rail locomotives, etc. the main motor used is generally about 1000kW, so the electric insulated bearings must be used, or it should be used the insulates the structure to keep the shaft current from flowing.

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