Chuck Yung
Senior Technical Support Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, MO
The paper "Case Studies in Failure Analysis" by Chuck Yung, presented at the EASA Convention 2019, delves into the methodologies and practical examples of root cause failure analysis (RCFA) for electric motors. Yung emphasizes the importance of understanding motor design, application, appearance of failure, and operating environment to accurately diagnose motor failures. While extensive investigations are necessary for critical motors, less critical motors can often be assessed using experience and common sense.
The paper begins with a discussion on human nature and its impact on motor failures. Yung highlights that operators may reset breakers multiple times, hoping the motor will start, which can exacerbate the damage. He advises service centers to conduct their own investigations and not rely solely on customer comments.
Case Study #1 involves a 75 hp (56 kW) motor that was flooded. The presence of mud and rust indicated water ingress. Yung recommends inspecting sump pumps, checking for obstructed drainage, and considering environmental factors like recent flooding events. He suggests adding loss-of-phase protection to sump pumps and modifying conduits to prevent future failures.
Case Study #2 examines repeated rotor failures in 400 hp (300 kW) conveyor motors used in an underground mine. The failures were traced to incorrect fluid coupling fill levels and the use of conventional oil instead of hydraulic fluid or automatic transmission fluid. Yung emphasizes the importance of educating customers about proper fluid coupling maintenance.
Case Study #3 involves a 29 kW (39 hp) compressor motor misidentified as a conventional part-winding start (PWS). The motor had a unique 2/3—1/3 PWS connection, which was incorrectly rewound using a 1/2—1/2 PWS connection, leading to higher starting current and fault codes. Yung stresses the importance of understanding special winding connections and correctly interpreting nameplate information.
Case Study #4 addresses a 600 hp (448 kW) open dripproof motor that failed due to abrasive dust. The dust sandblasted through the coil insulation, causing a ground fault. Yung recommends fully wedging the slots, inspecting coils for abrasion, and using flexible coatings to protect the windings.
Case Study #5 involves a 50 hp (37 kW) TEFC motor with vibration issues. The drive end bearing was an angular contact thrust bearing, unsuitable for horizontal motors unless there is continuous axial thrust load. Yung advises ensuring compatibility between bearings and applications.
Yung concludes by emphasizing the value of experience in failure analysis and the importance of understanding the application, enclosure, starting frequency, and ambient conditions. He encourages repair technicians to seek guidance when unsure and to recommend appropriate motor enclosures for specific environments.
Key Points Covered:
- Importance of understanding motor design, application, appearance of failure, and operating environment
- Impact of human nature on motor failures
- Case studies illustrating common motor failure scenarios
- Recommendations for preventing future failures
Key Takeaways:
- Extensive investigations are necessary for critical motors, while less critical motors can often be assessed using experience and common sense.
- Operators may exacerbate motor damage by repeatedly resetting breakers.
- Proper maintenance and understanding of fluid couplings are crucial for preventing rotor failures.
- Special winding connections must be correctly interpreted and applied.
- Abrasive dust can cause significant damage to motor windings, requiring protective measures.
- Compatibility between bearings and applications is essential to prevent vibration issues.
- Experience and understanding of various factors are key to accurate failure analysis.
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