AC Motor bearing life

[SolarSteve]

I am curious if anyone has any info on how the AC motor is designed in the Ford Focus Electric. I will refer to this article published by SAE International (http://www.sae.org/mags/sve/9916) in protecting motor bearings from pitting due to induced voltages on the motor shaft which take the path of least resistance to ground. The motor is powered from an inverter at some frequency say 12Khz at large currents. From my Electrical Engineering background these induced voltages can be quite high => e=Ldi/dt. These induced voltages can cause arcing through the grease in the bearings, causing pitted bearing surfaces and eventual failure. I have seen some AC motors employing an AEGIS grounding ring to shunt these high voltages to ground. This one moving part may be what kills the motor!

 

[mountaineer]

This is a very well-known issue, especially with certain inverter types and waveforms, and has been for over a decade. A capable powertrain supplier (I think it was Magna in this case) will know all about it and engineer it out. Inverter-drive motors are used by the millions worldwide under very demanding industrial and automotive conditions, and I'd be astounded if Ford and its suppliers had the bearing-pitting problem.