Tom Bishop, P.E.
EASA Senior Technical Support Specialist 

Two of the most important expecta­tions informed end users (customers) have of service centers are consistent best practice repairs and root cause failure analysis (RCFA). Following best practices helps ensure reliable repairs and maintenance of motor efficiency; and RCFA identifies the cause to help prevent a recurrence of the failure. 

Service centers can meet these expectations by performing repairs in conformance with ANSI/EASA AR100 Recommended Practice for Repair of Rotating Electrical Apparatus; and by providing RCFA in accordance with a proven methodology such as that in the EASA Root Cause Failure Analysis seminar reference manual. 

Our focus in this article will be on the importance of performing root cause failure analysis.

Incomplete failure analysis
One way to appreciate the value of RCFA is to consider what might happen if it is not performed. As an example, consider a 200 hp (150 kW) air compressor motor that has had a winding failure. Initial inspection reveals an overheating condition throughout the winding (see Figure 1). Based on the only evidence im­mediately available, the service center concludes that the failure is probably due to an overload condition. Since there are many compressor applica­tions that operate in the service factor range of the motor rating, the service center conclusion is logical. In this case the service center and end user may rely on the increased insulation class rating of the rewind, such as a Class H (180° C) in place of a Class B (130° C) system, to compensate for the assumed over-current condition due to overload.  

However, this incomplete failure analysis approach essentially jumped to a conclusion. The primary question that should have been asked after overload was considered as the cause of the winding failure is: What else could have caused this?  By applying the RCFA methodology given in the EASA seminar resource manual, the analysis would examine the failure mode, pattern, appearance, applica­tion, and maintenance history associ­ated with the failed motor. Further, this analysis process would not only consider the motor, but the motor system. System elements include the motor power supply, foundation, method of connection to driven equip­ment, and the driven equipment (see Figure 2).  

Now let us examine the failure anal­ysis scenario when RCFA is utilized. The investigator examines the motor and system, objectively and methodi­cally identifying and assessing a multi­tude of potential causes. Questions such as:  “What if  “(any possible scenario)” happened?” and “What else could have caused this?” are frequently asked and fully investigated. In our compressor motor winding failure example above, one of the other possibilities that would have been identified through RCFA is reduced ventilation, which would in­crease the motor operating temperature.  

Cost of misdiagnosed failure
In fact, this example is based on an actual motor failure that was due to the compressor enclosure cooling fan op­erating in the opposite of the intended direction. The result was reduced air­flow and an ambient temperature more than 40° F (22° C) above normal (see Figure 3). Because of the misdiagnosis of overload as the cause of failure, the motor continued to fail. Compound­ing the issue, the load current was not measured. A thorough RCFA identifies each possible cause, then objectively looks further to narrow down these possibilities to identify the most prob­able cause(s) of failure. Had the RCFA process been followed, the actual oper­ating current would have been found to have not exceeded the nameplate current (see Figure 4). Following more than a half dozen rewinds and even a redesign to a higher power rating, a root cause failure analysis was finally performed – by a service center other than the original rebuilder. The subse­quent RCFA process determined that the enclosure cooling fan rotation was incorrect. Following the change in cool­ing fan rotation, the compressor motor has operated for more than a decade without a winding failure.

Don’t damage your reputation
The ironic lesson learned for the service center that initially repaired the compressor motor is that there are times when RCFA should be used, even if the end user is not willing or ready to absorb the cost and possible extended downtime associated with the analysis. Further, consider the damage to the reputation of that first service center following the discovery of the true root cause by another service center. If your firm was a part of this story, would you rather be the service center that essen­tially kept on repairing the motor or the service center that provided a solution by applying RCFA?

Categories: Troubleshooting & failure analysis

Tags: Root cause failure analysis

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