The key to understanding this phenomenon is to understand that there is one ultimate mechanism for the bearing damage, EDM from current flow due to a voltage difference between the stator and rotor, but there are multiple
sources of that voltage difference and not all of them will exist on a certain installation / motor / drive / system setup. The 4 main sources of EDM current are;
- Capacitive Coupling
- Inductive Coupling
- Frame voltage due to high ground resistance
- Externally coupled energy
Capacitive coupling is the worst, caused by the steep edges of the PWM pulses coming from the VFD transistor firing. The rapid change in voltage of the stator windings makes it act like the anode of a capacitor, with the rotor being the cathode and the air gap as the dielectric. The voltage is low, but persistent and will build up until it exceeds the dielectric of the bearing grease to where it flows through the bearings. So the power filter (dV/dt or common mode choke) works for this because it slows down the rise time of the PWM pulses, decreasing the capacitive effect.
Inductive coupling is a result of the capacitive coupling and the fact that in a PWM scheme, there is never a total zero-sum of the 3 phase voltages because of the timing of the pulses in each phase. So in some motor designs there will be an induced voltage from one end of the motor shaft to the other, exacerbating the effects of the capacitive voltage bearing damage. People found success in dealing with this by using one insulated bearing on the drive end of the motor, but the other one eventually fails anyway. Two insulated bearings can work, but the voltage will still be there, so it might end up just moving down the motor shaft to a bearing on the driven machine and do damage there.
Frame voltage issues are the result of larger motors having inadequate frame grounding connections, based on safety rules only. The larger the motor, the more critical the need for low ground resistance and that usually means larger Equipment Grounding Conductors than are called for by Codes, but when people only do the minimum, they can end up making bearing damage issues worse.
Externally coupled energy can come in the form of static electricity caused by materials the motor is being used on, like conveyor belts, some types of pumped fluids, high speed fans, welding taking place on a machine while in use and, addressing the issue of shielded cables, the Common Mode Noise that can come in on the conductors going to the motor. That's where using shielded cable can help.
The shaft grounding is really the best solution because although it doesn't deal with the SOURCE of the problems, it deals directly with the EFFECTS by giving any voltage produced between the stator and rotor a safe non-destructive path to ground. The problem is, they wear out and have to be replaced periodically. For that reason, I have been favoring having the motors built with a shaft grounding SEAL, because in those cases, the grounding aspect of it is designed to wear out at the same rate as the seal itself, so whenever you change the seal on a PM schedule, you end up replacing the shaft grounding ring at the same time.
The power line filters (dV/dt filters and Common Mode chokes) are also a good solution because they deal with the #1 cause, the capacitive coupling effects of the PWM pulses, plus they will lower the CM noise at the same time. But if there are other issues, such as an external source or a bad motor EGC that are left unattended, then you may have wasted your money. So the BEST is a filter PLUS a grounding ring in my opinion.
