Get involved with Insulated bearings

We usually get involved with variable speed drives and there the insulated bearings are a mixed blessing. Wehave shown in some thread that a flash-over in the oil film leaves the outer ring at a new potential that is either positive or negative. The next PWM edge takes the rotor and inner ring to a new potential (opposite the earlier) and the potential difference between outer ring and inner ring gets higher than it would/could be if the bearing weren't insulated. The result is an EDM event with higher voltage involved, but lower capacitance (rotor's capacitance in series with bearing insulation capacitance.

In many machines, you find that both rotor-frame and bearing insulation capacitance is in the 5 - 10 nF order of magnitude. If both are equal, the resulting capacitance is half the uninsulated capacitance. And if outer ring takes on a potential equal to, say, 10 V while the rotor goes to -10 V when the flash-over occurs, you will then have twice the energy (voltage squared and capacitance halved) discharged. And, since energy dissipates in the steel, you get more damage in a bearing with insulated outer ring.

If the bearing doesn't have an insulated outer ring (type 'Insocoat') but if the bearing is insulated using other methods, the capacitance can be as low as 500 pF - 1 nF and that reduces damage so life will usually not be affected.

Linqing Nine Star Bearing Science & Technology Co., Ltd            

(Insulated bearing, Hybrid ceramic bearing and  Brass cage DGBB)

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