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Case: Motor bearing failures with Frequency inverters

Case: The motors from approx 22kw - 250kw and are double star-wound to facilitate a wide speed range. The motors and frequency inverters are matched. The bearings are not showing the usual ESD damage indications but we are experiencing short life (less than 8000 hours in some cases). The bearings on some units are insulated (80kw plus). Is it the bearings that re undersized or if there may be another issue?

Bearings may fail for many reasons, we suggest using the resources of the bearing manufacturer to tell you why the bearings are not providing expected normal life. Once they tell you the cause of the bearing failure, then you can use their corrective action plan.

Motors travel from the factory to destination several hundred to several thousands of miles. The bearings are being damaged by the vibration of the transport means. Some users will automatically replace the bearing and pack them with suitable grease for the intended application.

The grease installed at the motor manufacturer plant may not be suitable for the application at the users location.

Temperatures of the NDE and DE bearing could be significantly different requiring two different viscosities of grease for the actual operating temperatures. The viscosity of the selected grease must match the bearing clearance dimension at the bearing running temperature otherwise the bearing will fail due to poor lubrication.

Once you have a frequency inverter rated motor, If this is a an application where the shaft is ungrounded, i.e. a blower, you may need the solutions mentioned above, to isolate the bearings from bearing current flow, or giving the current a path to ground, i.e. grounding rings etc. As well if the distance of the output inverter cables to the motor is in excess of 100 meters, the terminal voltage may increase due to the Fast rise time of the frequency inverter output, cable capacitance and propagation and motor damage can result. Proper frequency inverter output cable can mitigate this issue. As you've also looked at the bearing grease, I'd also insure the mechanical system is not causing vibration or contributing to mechanical resonance. Hang in there as this issues that can cause these problems can be fairly difficult to find.

There have been occasions where of a small amount of electrical discharge through the bearing lubricant can cause additional heating and subsequent lubricant breakdown. This then precipitates a bearing failure mode that often will wipe out the evidence of the arching / pitting inside the bearing race that might point to the initial cause.

The only way to determine if this is actually happening is to analyze the motor shaft bearings BEFORE they fail. It is mentioned that some bearings are failing in less than 8000 hours of run time. I suggest proactively dismantling, cutting open and examining the motor bearings at intervals of 6,000 hours, and 10,000 hours and perhaps 15,000 hours. As was suggested, have the motor and bearing manufacturer participate in this process as well. If possible, try to select a motor location where the failures have been most frequent. Note: Convincing production management to allow this may be more difficult than actually turning the wrenches.

If this proves to be an electrical discharge issue, the remedies vary in cost and complexity. Involving the machinery and motor manufacturers may bring additional resources that can expedite a resolution.

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