The strategy and frequency for testing is a subjective judgement on where you get the most bang for your buck. It also depends on criticality of the motor, availability of spare, known degradation factors, etc. More critical machines should generally be monitored more frequently.
For grease lubricated bearings, a greasing strategy is needed. Not applicable for small machines. Essential for larger and higher speed for example above D*N = 200,000 rpm*cm. Frequency of greasing - look at any bearing manufacturer's website.
For oil lubricated beairngs, periodic check of oil level.
Vibration monitoring is one of the best predictive maintenance tools imo. We do it quarterly.
Periodic insulation and other electrical testing depends on your preference. More important for large higher voltage motors. For our largest most critical motors we have on-line partial dischrge monitoring, we do periodic (4.5 year) dc step voltage testing, 1.5 year insulation resistance testing, boroscopic internal inspection at 4.5 year interval, refurbish somewhere around 15 years. Also current signature analysis to detect rotor bar degradation.
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