Rotating electrical machines with rotors supported by bearings are subject to premature failure of these bearings due to rapid and insidious damage caused by stray electrical currents

David McGaughey, Key Account Manager – Renewable Energy at SKF explains the problem and offers some field proven solutions… The widespread use of high frequency, pulse-width modulated (PWM) power electronic converters in wind turbines has placed additional stresses on generator bearings due to the presence of a common-mode voltage which may drive stray rotor currents via the bearings into the generator structure. If no attempt is made to mitigate the effects of these stray currents, the dielectric strength of the bearing lubricant will be overcome, resulting in damage to the generator rotor bearings caused by electrostatic discharge machining – essentially a process of metal transfer between the bearing surfaces.


When the insulating properties of the lubricant are compromised and an electric current passes through the bearing – flowing between inner and outer rings via the rolling elements – fluting or pitting of the bearing raceways and on the rolling element surfaces can occur. Indeed, with high frequency PWM converters the stray current discharges are so frequent that pitting can quickly cover the entire bearing race, leading to what’s known as ‘frosting’. An early symptom of damage resulting

from stray bearing currents is increased noise and vibration due to fluting; but by the time this is detected, the damage may already be in an advanced state. A higher bearing running temperature is another indicator that all is not well. Moreover, frequent electrical discharges passing via the lubricant will cause it to rapidly oxidise, severely affecting its lubrication performance. Premature bearing failure is the ultimate outcome of this scenario and with it come the heavy costs associated with a non- productive turbine installation and the expense of getting a repair crew to site to replace the bearing.


The solution is not to rely on low dielectric strength lubricant as the primary source of internal insulation, but to modify the design of the bearing – particularly its materials of construction – so that the bearing inner and outer rings are electrically isolated one from the other. SKF has led the field in developing highly effective insulated bearing solutions for the electrical machinery market and has been working closely with the wind turbine industry to develop rolling bearing technologies suitable for today’s multi- megawatt variable speed generators. These include the electrically insulating SKF XL hybrid range and the innovative INSOCOAT bearing range. Both of these high performance solutions help reduce total life costs, while eliminating incidences of stray bearing current damage and the ensuing costs of loss production and repair. Choosing between these two technologies depends on the potential severity and cause of any possible stray electric current and the size of the bearing.


Designed and developed for large wind turbine generators (including the new multi-megawatt installations), SKF XL hybrid deep groove ball bearings ensure total inner to outer ring insulation while providing the high reliability and superior performance demanded by the sector. SKF XL hybrid bearings feature a novel design comprising rings constructed from bearing steel and insulating rolling elements made from bearing grade, high hardness, low density silicon nitride. The lower density of silicon nitride, its lower coefficient of friction, high hardness and tolerance of poor lubrication conditions, all combine to provide a bearing that not only provides effective insulation against stray electrical currents, but also runs faster and longer even under the most difficult operating conditions.

INSOCOAT deep groove ball bearing VL2071 PRODUCT RANGE

Meanwhile, the bearings range – available in single row, deep groove ball and single row, cylindrical roller configurations – comprises a standard bearing design but with the external surfaces of its inner or outer ring plasma-sprayed with an aluminium oxide to form a coating. The coating is sealed with a resin to protect against the conductive effects of high humidity and moisture ingress. Outer ring coated INSOCOAT bearings

are commonly used for medium-size machines. They are suitable for all types of housings and can be applied to bearings with an outside diameter greater than 80mm. Versions that have an electrically insulating coating on the external surfaces of the inner ring provide enhanced protection for larger machines or those applications where the bearings risk being subjected to high rotor voltages. The inner ring coating can be applied to bearings with a bore diameter greater than 70mm.


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