Its taken a long time for the industry to discover this failure mode.
It results from high frequency currents induced by square pulses used by digital controllers.
Quoting from the Summary at the end of the article:
"Virtually all inverter-driven motors are susceptible to bearing damage, but for too long, the importance of shaft grounding to protect motor bearings has been ignored or underestimated.
Current NEMA standards dealing with the problem of shaft voltages are incomplete or not specific enough to help. They do not acknowledge the combination of capacitively induced shaft voltages and high-frequency circulating currents from VFDs. Furthermore, they fail to take into account recent advances in technology."
The full article has detailed guidance on how to prevent or minimize this and warns that most variable speed AC induction motors are not yet protected by design. I believe that the risk with a permanent magnet rotor is smaller than with a squirrel cage induction motor ... and the PM motor is more efficient while weighing less, so it is a better choice for battery power in a boat.
Other members (with more experience than I) may be able to offer a band pass filter design that could sit between the controller and the motor to further extend motor life without degrading system efficiency.
For full details, Google the article title:
"NEMA Standards Are Giving Motor Bearings the Shaft"
OR Copy the entire address below into your browser:
http://hpac.com/motors-drives/nema-standards-are-giving-motor-bearings-shaft?NL=HPAC-02&Issue=HPAC-02_20130821_HPAC-02_319&YM_RID=jcelrock%40hotmail.com&YM_MID=1417661&sfvc4enews=42
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