Insulated bearings for inverter motors

Written by: Tom Austin-Morgan | Published:
The increasing number of reports is probably due to the implementation of one of the more recent ...

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NSK has developed a range of ceramic-coated bearings for use in general industrial machinery applications. These insulated bearings have been designed to help support the growing adoption of inverter-controlled motors. All common machinery parts, including pumps, compressors and fans, are said to benefit from this new development.

Although the use of motors with inverter speed control is expanding, there has been an increase in reports that bearings used for such applications are prone to a specific type of damage called electric corrosion. When an electrical current passes along the contact point of bearing races and rolling elements during bearing rotation, sparks are generated in the thin lubricant film, causing the surface to be fused locally into an irregular shape.

One way to prevent electric corrosion is to use non-conductive ceramic balls in the bearings. However, this can have a negative effect on productivity. Another option is to deploy an insulated housing in the motor, but this requires special parts and assembly processes. To overcome these problems, NSK has developed a ceramic-coated bearing that is insulating, highly productive and compatible with all standard inverter-controlled motors.

An aluminium ceramic material is used to coat the exterior surface of the outer ring, and by combining optimised additives, insulation performance is improved typically by a factor of around 10. In addition, the coating provides excellent heat dissipation, while durability is also improved as a result of a three-fold improvement in film impact resistance due to enhanced bearing elements.

NSK is offering ceramic-coated variants in deep groove ball bearing and cylindrical roller bearing ranges, with bore diameters from 50 to 110mm.

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NSK Europe Ltd
The increasing number of reports is probably due to the implementation of one of the more recent EU/EC directives on energy. It requires inverters to be used with a large range of motors whether they are speed controlled, soft started or neither. It affects a large proportion of motors used in buildings, both in bearing and winding design. Because inverters also send harmonics back into the distribution mains and even into the utilities mains as well as forward into the motor, the overall losses in correction devices added to the inherent losses in the inverter severely affect the overall energy saving imagined to be obtained through inverters. They can be negative where speed control of a fan or pump cannot match the characteristics of the hydraulic load, e.g. variable flow rate at near constant pressure. Consequently the overall cost benefit cannot always be achieved either..

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