Other Motors Topics
Induced Bearing Currents
Determining the problem
There are two methods to determine if bearing currents are the cause of unexpected
motor bearing failure: measure the shaft voltage or examine the bearings.
All motors have some level of shaft voltage. Above a certain level, shaft
voltage is a failure indicator. Generally end-to-end shaft voltage should be less
than 0.5 V. Normally, voltage levels below this will not cause harmful bearing currents.
Engineers can measure the shaft voltage with any voltmeter that has an impedance of 10,000
ohms per volt or more.
When examining the bearings, look for specific types of damage.
Bearing damage results when current is broken at the contact surfaces between
rolling elements and raceways, Figure 21. The size of the damage points depends
on the magnitude of the induced voltage, the impedance of the current path,
and the bearing type. The early observable effects of this damage on the bearings
is pitting and fluting.
Figure 22 shows a series of electrical pits in a roller and in a raceway of a
spherical roller bearing.
More serious electrical damage occurs when current passes during prolonged periods
and the number of individual pits accumulates. The result is fluting, Figure 23.
Fluting in anti-friction bearing races specifically indicates the problem is bearing currents.
Fluting can occur in ball or roller bearings and develop considerable depth, producing
noise and vibration and eventual fatigue from local overstressing. Once fluting is started,
it is self-perpetuating until the bearing fails.
Figure 21 - When current is broken at the contact surface between the rolling elements and the raceways, it produces arcing damage.
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Figure 22 - Electrical pits are another indication of bearing currents. If left unchecked, this will eventually result in fluting.
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 Figure 23 - Fluting, an accumulation of pits, is a sure indicator of bearing currents. Once started, fluting is self-perpetuating until the bearing fails. |
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