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Category: Motors

Testing Bearing Currents and Shaft Voltage

By definition, bearing current is the flow of current from the shaft, through the bearing, to the motor frame. the shaft voltage may allow current to pass through the bearing in the form of an arc. Electrical arcs damage the bearing by roughening (fluting) it, which causes the bearing to fail prematurely.

In order to test for current passing through the bearing you need to insert some type of sensor between the shaft and the bearing, or between the bearing and the frame. The problem with this is that it's not feasible to do on-site.

Unsuccessful Ways to Measure Bearing Current

Rogowski Coils are often used with variable frequency drives to measure the total Common Mode (CM) current going to the motor. The relationship between bearing current and CM current is complicated. Bearing current is only a small portion of the total CM current that may be passing through the bearings. You can not simply say that X% of the total CM current goes through the bearings; there are too many variables. There is also a sensitivity issue with Rogowski Coils. Standard coils for power frequency currents are too slow, and faster coils can overshoot. This can give measurement readings 300-400% too high.

To some extent, bearing currents can be measured with discharge detector pens. These pens detect bursts of radio static that bearing currents produce. Measurements done outside of a lab struggle to give accurate readings. In electrically noisy environments, meaning there are many motors and drives near, these pens can give erratic results. The pens are also sensitive to distance from the bearing. If you detect no discharges, you may be holding the wand too far from the bearing. If you do detect discharges they might be real, or it might be radio noise from other sources. You can’t be sure without a second type of measurement for confirmation.

How to Successfully Measure Bearing Current

All current is driven by voltage. You can measure the voltage across the bearing by measuring the voltage difference between the motor shaft and frame, a.k.a. the shaft voltage. You can indirectly measure bearing current by looking for sudden collapses in shaft voltage. These indicate a discharge (arc) through the bearings.

You can measure shaft voltage with an oscilloscope like the AEGIS Shaft Voltage Tester. Multimeters don’t work because shaft voltage changes too quickly. With a scope, you can both see the shaft voltage level, and see when a bearing current occurs.

In summary, bearing currents cannot feasibly be measured directly. There are practical problems with measuring the current itself, and it’s hard to reliably detect the radio signals it produces. Using an oscilloscope to measure shaft voltage allows the user to determine if there is any bearing current.

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