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| Assuming this is an ac induction motor and not a wound rotor/synchronous motor since an armature fed with another source opens up other possibilities.
First piece of information is if motor is fed by vfd or run across the line. Vfds are well known to increase shaft voltages/currents. I think this is the problem you are seeing. Though damaging the bearing in the first run is unusual. Though the bearings could have been damaged in shipping and the shaft/bearing currents were also noted on run in but did not necessarily do the initial damage. Bearing currents usually take time to do damage. Let's say the rotor was not properly blocked and movement during shipping created flat spots on the bearing.
Now, if we are talking a large amount of current that literally welded the bearings rather than cause pitting then that is not something I have seen and can't explain it from a power system stand point. The 3 phase system is isolated from the rotor/bearings by both the insulation of the windings and the air gap to the rotor. Ground fault will trip the unit off long before a bearing got literally welded. Ground fault usually set under 5 amps. So I expect you are talking about shaft voltages here.
Bearings will often be insulated to reduce current circulating from shaft, through one bearing, through frame, back through other bearing and into shaft setting up a loop. Sometimes the motor is assembled on site and the insulating method is compromised so path not interrupted. Lots of things can be a factor. A good motor tech will know about the details.
You want to start reading something like this link. Lots of info out there. But the people you need to fix the problem will be well aware of the solutions. Though a motor guy may not pay any attention to whether shielded or unshilelded cable is used between motor and vfd. But those details matter. If a vfd is involved then the issue has more possibilities.
https://www.pumpsandsystems.com/best-practices-mitigating-bearing-cu...
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Why would an electric motor ground through the shaft?
