View attachment 184018
.
[/QUOTE
OP stated that the motors pass a Megger test but fail anyways in post #1. Excessive bearing currents result in pitting of the bearings that appears as a "frosred" appearance of the rolling elements (instead of shiny and polished) and there is nearly always a series of grooves called bearing fluting some place on the inner or outer race. Bearing failures allow the shaft to hit the stator ripping through the insulation and shorting the stator to the frame, causing a ground fault trip. These are very easy to detect because it fails a Megger test every time. OP however is reporting NONE of the failures failed a Megger test.
The authors also completely misunderstand bearing currents. There are 4 different common ones and they only describe one of them. The most common known one that has been around since induction motors were invented is caused by a magnetic field disortion which causes circulating currents within the motor itself (end to end). The electrical path is from the shaft where the current is inducted through the bearing on one end, through the frame, and returning back through the other bearing. As motors exceed about 1,000 HP it becomes necessary to install grounding brushes to short circuit the bearings to avoid damage. The second kind are low level currents that are caused by stray capacitance within the motor, but it's so low level that it never exceeds the bearing current limits (milliamps). The third kind are high frequency currents that couple through the air gap. The motor is insulated (mostly) and forms a capacitor. High frequency harmonics can easily pass through the motor bearings as non-circulating currents. This effect though is limited to smaller motors, generally under around 10 HP. Over about 25 HP the air gaps are just too large. This is also the only effect the article actually talks about so apparently the author is mostly familiar with small fractional HP motors. A second effect is that common mode voltages applied to the motor create currents in the bearings and this effect can be very severe with VFD's that lack any kind of output filtering. As an example two different 1200 A Allen Bradley VFD's at two different sites caused this type of bearing current with random discharges exceeding 90 A. It took quite a large common mode filter (a couple hundred pounds) to bring the bearing currents down to under the 10 A the bearings could pass without damage. Another similar effect you sometimes see with overhung fans is that static discharges for example cause the same bearing currents as common mode voltages in drives.
Reason I'm saying this is because I work for a motor shop and we get calls on bearing fluting fairly often. We actually measure it and recommend solutions based on what we're measuring. We also follow up because sometimes it takes more than one "filter" (whatever device it is) to do the job. And with grounding brushes customers often like the Aegis rings but they don't actually work very well (again measurements don't lie). All the manufacturers recommend their solution but there is no one size fits all. For instance shaft grounding (Aegis, Helwig) works great for circulating currents but does nothing for HF currents. HF currents are easily blocked by ferrite cores that Allen Bradley likes to use but useless on larger low frequency common mode currents. MH&W Cool Blue stuff doesn't block circulating currents.
