A series of studies by the Institute of Electrical and Electronics Engineers, Inc. (IEEE) and the Electric Power Research Institute (EPRI) from 1983 until 1995 1, 2, 3 that covered all industries reported that bearing faults, followed by winding faults, were prevalent (see Figure 1). These studies, one performed by Advanced Energy4 and another performed by the Electrical Apparatus Service Association (EASA)5 also revealed that repaired motors, in many cases, do not last as long as they should because of poor repair practices. The repetitive failures of these motors may have resulted, in part, because of a lack of diagnostics or forensics reported to the motor owner during the repair process.
|Figure 1. Comparison of three motor reliability studies|
Repair and Communication
While many studies and surveys performed through 2013 identify that a good repair will last longer, few have identified the importance of providing the motor owner with feedback on the possible root causes of the failures. How does the owner’s maintenance department or the reliability engineers respond to a report of replaced bearings or a rewound stator? Do they respond in a way that prevents the problem from recurring?
A significant difference between the 2003 and 2013 Motor Diagnostics and Motor Health Studies (MDMH)6 was that in 2003, 56 percent of organizations reported electricians as the person responsible for electric motors, and only 2 percent were reliability engineers. In 2013, the number shifted to 43 percent reliability engineers and 26 percent electricians with a significant decrease in mechanical and general maintenance personnel. By definition, a reliability engineer requires feedback to pursue his trade, meaning that an understanding of the equipment failure—including potential root causes and corrective recommendations—becomes more critical in the repair process.
For instance, if the motor failed because the bearings were noisy, what caused them to become noisy? Were they over or under greased? Was the belt experiencing over-tension or misalignment? Was there bearing fluting because of shaft currents? This information, including recommendations, will assist the motor owner or reliability engineer in developing a plan to mitigate future problems.
For example, the bearings in a motor that is operated by VFD are fluted (see Image 1). This fluting is an indicator of shaft currents. Corrective action using a shaft brush or a shaft brush and insulated bearing should be taken. This also means that either the repair facility’s representative should be asking application questions, and/or the motor owner should be providing that application information.
|Image 1. Bearing fluting caused by shaft currents|
Rewind related failure modes must also be identified. If the winding has been single phased and that information is not communicated, the failure will likely reoccur. Identifying that the failure occurred would be important, in this instance, because an electronic overload with single-phase protection would mitigate similar failures in the future. If, on the other hand, the winding failed because it was overloaded, the reliability engineer should recommend that the overload protection is also evaluated.
Repair’s Impact on Reliability
As noted by the U.S. Department of Energy, “Of the quality assurance procedures shops used, 40 percent were repair procedure specifications, 25 percent were test specifications, and 21 percent were EASA standards. Only one of the 65 shops surveyed used any form of quality assurance testing.”7
In fact, per the same report, almost half the shops surveyed performed no winding tests during the entire repair. Of the repair shops, 81 percent also reported that they changed winding configuration in electric motors during the repair process primarily for shop preference or ease of winding (73 percent), 10 percent with the owners’ knowledge and only 4 percent for the purpose of reliability or durability. The remainder did not provide a reason.
Proper repair practices are vital to the reliability of the repaired electric motor. While the studies primarily focus on energy consumption or efficiency of the machines, the 2013 MDMH identified a solid 0 percent of interest in energy as a driver for a motor program (only 3 percent in 2003). The key driver for the motor program was reliability (75.5 percent).
|Image 2. Overloaded winding|