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.
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.
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:
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.
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.
Underwriters Laboratories (UL) has an approved set of procedures for installing AEGIS® shaft grounding rings inside explosion proof (XP) motors. XP motors with AEGIS® rings installed according to these procedures may be used in hazardous environments, Class I: Division 1.
The following diagrams are the approved installation locations inside the XP motor.
Note: In Class I: Division 2 designs. Because shaft grounding devices must be installed inside explosion proof enclosures (as per IEEE 303) for Class I: Division 2, they may not be installed internally or externally in/on a non-XP motor.
At this time, Marathon Electric is the only electric motor manufacturer marketing XP motors with AEGIS® shaft grounding protection installed inside. They range from 3 HP (182T Frame) to 50 HP (326T Frame) and Marathon Electric will install AEGIS Rings in larger models. For more information on Marathon Hazardous Duty motors with AEGIS® Inside. For catalog numbers and pricing, see the highlighted “-P” motors on the Hazardous Duty™ pages of the 2014 catalog.
To install AEGIS® rings in explosion proof motors in a hazardous environment, a motor repair shop must first be UL approved to work on all XP motors. They must then apply for certification for the addition of adding shaft grounding to their UL binder and then demonstrate adherence to the additional set of procedures required for installing shaft grounding inside of XP motors.
Motor repair shops with UL certification to work on XP motors and certification of their ability to add shaft grounding can “rework” any size explosion proof motor and then re-nameplate the motor as explosion proof. The drawing below was created by UL to show where the ring must be installed to comply.
This video shows everything you need to install AEGIS Shaft Grounding Rings with conductive epoxy.