Although there are hundreds of types of rotating equipment(pumps, blowers, gearboxes), the majority of them are all driven by the electric motor. Even with such an abundance, trying to figure what is wrong with a motor can baffle so many operators & maintenance techs.
As with any troubleshooting technique, first you need to know what was the motor doing yesterday(or last week/month/year) that it is not doing now. Increased vibration, noise, temperatures, or decreased output, noise, speed, all point to a different direction. One thing I've found, most of the problems associated with motors are caused by "external" forces. Piping misalignment, coupling wear, loose hold-down bolts, deteriorated mounting, even lack of lubrication.
Motors don't usually just "go out of balance" and start vibrating. Dirt & grime may build up on the fan blades, but it does so evenly. When the dirt falls off one of the fins, then you could have imbalance. A blown fuse on a 3 phase line could make the motor run very rough & noisy, but it still might run for days. Another quarter turn of a hold-down bolt could make the difference between alarm level and acceptable vibrations! Our checklist for troubleshooting mandates an overall inspection of the installation first. Too many times I've been called in to check a noisy or vibrating motor only to find the coupling failed. If they would have simply removed the coupling guard they could have saved the expense of a service trip.
Bearings can fail in motors, but it is usually after 10 or more years of operation. Shaft misalignment can cut that time in half. Machine resonance can also shorten the life of bearings, but is seldom due to resonance in the motor. The resonance is in the complete motor/pump, baseplate & piping assembly. More new equipment startups are now calling for "bump tests" to insure the motor operation is not exciting the natural frequencies of the installation.
Bearings are the only wear item in a standard electric motor. Replacement of ball bearings in NEMA frame motors can often be completed on site in less than 1 day. However, special care needs to be taken to verify the motor end bells(bearing housings), are free of wear and/or damage. And if the motor is in a very dirty environment, or has been subjected to excessive greasing, the motor windings may have been compromised. In these cases, sending the motor to a shop is the better call.
Monitoring your equipment daily is the primary fundamental for PM. When you walk by it and you notice it is behaving differently, or sounds different, you need to determine if the change is detrimental to the operation. Vibration data collection and analysis is used to monitor the operational changes before they are seen or heard. Investing in PM, will save you money!
