Electrical damage to bearings prevented by new coating

Paul Boughton

Schaeffler has developed a new surface coating for rolling element that insulates the bearings from damage caused by the passage of electrical current in large AC or DC motors. Schaeffler's Isotect A coating can be applied to the outer or inner ring external surfaces; alternatively, depending on the application, the bearings can be assembled with ceramic rolling elements.

In the past, bearings used in large AC or DC motors with only a few pairs of poles were prone to damage from the passage of electrical current. Today, due to the increasing use of frequency converters, this potential danger is growing. Around three per cent of all electrical machines manufactured worldwide are controlled by a frequency converter, and this figure is expected to rise to 10 per cent over the next couple of years.
Isotect A acts like a resistor and capacitor connected in parallel. With DC voltage and low-frequency AC voltage, the choice depends on the ohmic resistance of the bearing. With higher-frequency AC voltage, which is often found in frequency converters, it depends on the capacitive reactance of the bearing. To ensure good insulation, the ohmic resistance should be high and the capacitance should be as low as possible.
Depending on the operating temperature, Isotect A has a high resistance ranging from several ohms up to 10Gohm. To allow for the capacitive resistance of bearings used with a frequency converter, Schaeffler has selected not only the appropriate material for the insulating layer, but also the most suitable layer thickness, which both play a key role in ensuring optimum protection.
As capacitance is derived from 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 even higher protection, Schaeffler offers rolling-element bearings assembled with ceramic rolling elements across most of its bearing range.
Not every bearing in an electrical machine will need to be fitted with one that has electrically insulating properties. With mains-operated machines, it is often sufficient to replace only one bearing with a current-insulated bearing in order to avoid possible damage. However, when frequency converters are used, it is normally necessary to fit current-insulated bearings in multiple bearing locations.
For more information, visit www.schaeffler.co.uk

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