Electric motors make the industrial world go around. Found in most manufacturing facilities, electric motors are responsible for 64% of all industrial electric power used in the United States.
Why Motors Fail
Your electric motors represent a significant expense. When one fails, costs associated with that motor add up. Those costs include not only the price for repair or replacement of the failed motor but also the cost of lost production time or unplanned downtime. In fact, the cost due to lost production usually dwarfs the motor repair and replacement cost.
As vital as it is to get your motor back up and running, it is as important to find out why it failed in the first place. Discovering the cause of the failure allows you to prevent that problem from causing any future failures. Whether the investigation is done your own maintenance team or by a motor repair contractor, it is essential to learn from failures in order to improve your operations’ reliability.
Bearing failure accounts for over half of all electric motor breakdowns. So when you begin to look into why the motor failed, start by looking at the bearings.
Inspecting the Bearings
When the motor arrives for repair, cut and inspect every bearing, especially if a VFD controlled it. Inspecting the bearings provides vital information helping make the best recommendation for repair. Knowing why the bearing failed and taking steps that prevent it from happening again improves the machine's lifetime performance.
Bearing inspection is also required by AR100-2015, the recommended motor repair practices of the Electrical Apparatus Service Association (EASA). Section 2.2 on Bearings begins, “Bearings should be inspected for failure modes such as spalling, contamination, fretting, fluting, frosting, and scoring or other damage.”
We recommend the following procedure to inspect the bearings. These steps are available in the AEGIS® Handbook.
Please Note: Follow established safety precautions and use personal protective equipment including eye protection, hearing protection, face shield, gloves and protective clothing.
1. Inspect the outside and the inside of both bearings. Keep a sample of the lubricant to analyze. When examining the lubricant, look for:
- Contamination
- Signs of excessive heat
- Hardening of grease
- Abnormal coloration (blackened grease)
- Excess grease and oil escaping the bearing
2. Remove seals or shields first, then cut the outer race into halves.
3. Inspect the grease and look for any contamination in the bearing.
- Burnt Grease: Continuous electrical arcing in the motor bearings rapidly deteriorates the lubricating capability of the grease. This arcing causes bearing race damage. Arcing heats the oil component of the grease beyond its temperature capacity.
- Contamination: Besides burning the grease, arcing knocks small metal particles loose from the bearing races or balls. These loose particles become distributed in the grease. These particles are abrasive and cause the bearing to prematurely wear. Burnt bearing grease is blackened and is oftentimes contaminated with metal particles.
New bearing grease is available in many colors. The blue grease is Polyrex EM, commonly found in electric motor bearings.
4. Clean the bearing’s components using a degreaser or solvent.
5. Inspect for evidence of Electrical Discharge Machining (EDM). EDM damage is millions of microscopic electrical pits created when shaft voltage discharges through the motor’s bearings. The electrical voltage overcomes the dielectric of the bearing lubrication and instantaneously arcs through the inner race, through the rolling elements, and to the outer race. The individual pits are usually between 5 and 10 microns in diameter.
6. Frosting: Frosting appears to be a grey discolored line around all, or part, of the bearing race and evident in both the inner and outer race. Both mechanical wear and EDM causes that discoloration. Use a microscope to determine if the line is EDM or of a mechanical nature.
If the motor was operated by a VFD with no bearing protection installed, there is a high likelihood that the frosting is from EDM.
Fluting Damage: Identified by a distinctive washboard pattern. A distinctive washboard pattern identifies fluting damage, seen using the naked eye or with 10x magnification.
Take care to identify the correct electrical fluting damage using the observed damage. Sometimes, fluting gets confused with mechanical bearing damage such as brinelling or false brinelling.
Besides using these recommended practices, please refer to other bearing failure analysis experts to determine the root cause of the failure. Install new AEGIS Ring whenever replacing bearings on an inverter-driven motor.
