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Coating protects motor bearings from current damage

04 September, 2009

Schaeffler has developed a surface coating to insulate rolling element bearings used in large motors from the damage caused by the passage of electrical current. The coating, called Isotect A, is an insulating layer that can be applied to the outer or inner ring external surfaces.

The coating acts like a resistor and capacitor connected in parallel. To ensure good insulation, the resistance needs to be as high and the capacitance as low as possible. Depending on the operating temperature, the coating has a resistance that ranges from a few ohms to 10GΩ.

Because the capacitance depends on the surface area of the protective layer and its thickness, the coating is applied to either the inner or outer ring external surfaces, depending on the application. If operating conditions require an even higher level of  protection, ceramic rolling elements can be incorporated into the bearings.

Research by Schaeffler and others has shown that current densities of less than 0.1A/mm2 do not represent a danger to bearings. However, when current densities reach 1A/mm2 or higher, they can often lead to raceway damage.

This damage takes two main forms:
♦  welding beads or craters that represent the early onset of raceway destruction – identifying them requires a detailed examination of structural changes in the bearing rolling surfaces; and
♦  fluting (shown above), which takes the form of a washboard pattern across the raceway and represents the final stage of the destruction of the raceway.

Not all bearings in an electrical machine need current-insulating properties, says Schaeffler. With mains-operated machines, it is often enough to use just one current-insulated bearing to avoid possible current damage. However, when frequency converters are used, it is normally necessary to fit current-insulated bearings in several bearing locations.

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