Two potentially valuable documents are available from EPRI for free (or the cost of giving up your email address);
1 - EPRI NP7502 -
Electric Motor Predictive and Preventive Maintenance Guide
2 - EPRI 1003095 -
Electric Motor Tiered Maintenance Program
The first one is the more valuable imo. It provides a simple set of recommendations (I think it's in Appendix B of the document) for electric motor maintenance. There is no distinction among criticality of motors, but you can probably consider these recommendation appropriate for high criticality motors.
The second one is supposed to help you adjust the strategy based on the factors unique to the motor, including criticality (hence the term "tiered"... more preventive maintenance and predictive monitoring for the higher tier of criticality). I don't remember exactly what is in there but as far as I recall it was a lot of "motherhood" (things that should already be obvious if you've worked around motors in a plant environment for any period of time). If anything it might be useful to enlarge your view toward the very large range of factors which could potentially be considered in selecting a maintenance strategy (beyond criticality other things like availability of spares, failure history, analysis of which time-based degradation modes might be important for that particular motor, etc)
You mentioned "replacing" the motors, that is not something we would do preventively/proactively. At our plant (clean environment), that would be more of a corrective item in response to a problem on a specific motor (although we do have a few particular families of motors in this size range 30 years old where we have now instituted preventive replacement based on the failure patterns and failure modes that we have seen within the family). Corrective maintenance is often taken in response to predictive measurements as Ed alluded. The most common corrective maintenance that we perform on electric motors is bearing replacements for problems found on vibration (or for smaller motors we might swap the whole motor instead of replacing the bearing). There is a school of thought in our industry that we should be in general pushing towards more predictive monitoring / condition-based maintenance and less preventive maintenance, although I'm not sure how much it applies to small motors since there's not a heckuva lot of preventive that we do to begin with.
For starters the biggest periodic maintenance bang for the buck on motors in this size range is imo 1: grease the bearings (assuming they are greasable) and 2: monitor vibration (assuming you have a vib monitoring program).
That's based on my experience in a clean environment. Some people in very dirty environments may find a need to do periodic external cleaning for TEFC motors in a dirty environment and a periodic disassembly for internal cleaning for ODP motors in very dirty environment (since ODP motors generally suck the external air through the motor for cooling, leaving behind traces of whatever was in the cooling air).
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(2B)+(2B)' ?
