search.noResults

search.searching

note.createNoteMessage

search.noResults

search.searching

orderForm.title

orderForm.productCode
orderForm.description
orderForm.quantity
orderForm.itemPrice
orderForm.price
orderForm.totalPrice
orderForm.deliveryDetails.billingAddress
orderForm.deliveryDetails.deliveryAddress
orderForm.noItems
BEARINGS & SEALS


FEATURE SPONSOR


BEARING CURRENTS – A TICKET TO PREMATURE FAILURE


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.


GETTING DOWN TO THE DETAIL


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.


SOLUTION


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.


RELIABILITY AND PERFORMANCE


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.


66


www.windenergynetwork.co.uk


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100