Using EMI Diagnostics for Condition-Based Maintenance of Rotating Machines
Maintenance planning and scheduling need to be effective and efficient as the resource allocation has to be managed masterfully. However, power plants and industrial sites have a complex production process, where rotating machines are abundant, with a wide variety of designs and functions. The production process usually requires generators and motors for different applications like circulating water pumps, condensate pumps, air compressors, fans and other components. If implemented well, the maintenance program will reduce maintenance activities and costs, and will increase equipment availability and reliability.
The challenge is to strategically allocate resources. The age of the rotating machine is a factor, but it does not necessarily reflect the actual condition. There are several failure mechanisms, and the deterioration depends on numerous factors. The best way to optimize maintenance resources is through Condition-Based Maintenance (CBM). It is also ideal to determine the condition of rotating machines while in service to avoid interference with operations. Electromagnetic Interference (EMI) Diagnostics meets these needs.
An Overview of EMI Diagnostics
Electrical and mechanical defects produce EMI signals that are collected when the rotating machine is in service. Each rotating machine has a unique EMI signature that consists of electromagnetic activity across different frequencies. These frequencies are analyzed individually to determine if the origin is an internal defect and to assess its severity. The test procedure consists of placing a split core radio frequency transformer around the neutral, power conduit or a frame ground (inherently safe grounds). The RFCT is installed and removed while the machine is energized and there is no special clearance needed for this process. The testing requires approximately one hour per asset and relevant findings can be preliminarily assessed immediately after testing. EMI diagnostics help to rank assets based on their condition. The analysis provides guidance on the units that require attention, their condition, and the recommended actions. The table below shows an example of the condition assessment provided with EMI diagnostics.
Several industries (nuclear, fossil, chemical, etc.) use EMI Diagnostics to prepare for maintenance activities. The condition assessment only requires one test. Trending is not necessary, but it can be used when previous data is available to determine if repairs were successful or if new defects have developed. Depending on the type of asset and the data analysis, a recommended testing interval can be provided.
Detect problems and ensure repairs are effective without service interruption
Recently, a customer was concerned that old motors needed rewinding and decided to rely on EMI testing to assess their condition. The EMI testing was performed by Doble Engineering Field Services on 15 motors. Four required minor maintenance and one showed insulation problems (partial discharge) in the motor leads. After an internal inspection based on this finding, it was discovered that the terminations were not properly insulated, which can lead to phase-to-ground or phase-to-phase fault. After repairs, EMI testing was again performed on the motor. The figure below shows the EMI signatures before and after repairs. It is observed that the partial discharge activity (in red) was successfully eliminated by the corrective action.
In this case, because all 15 motors had been evaluated using EMI, the maintenance could be focused only on the problem component. The maintenance personnel had the opportunity to prepare the work schedule, tools, and materials for the repairs. There was no need for additional testing nor inspections of other parts of the motor. The problem was detected at the first test and subsequent testing confirmed that it was repaired, and no additional maintenance was needed. Moreover, EMI showed that there was no need to spend resources on unnecessary rewinds based on age. Due to the benefits from the diagnostics, more motors have since been evaluated at this site and their condition is now monitored through regular EMI testing.
Conclusion
EMI Diagnostic testing is a powerful tool for quickly determining the condition of rotating machines without a service interruption. Power plants and industrial sites can incorporate this testing into their CBM programs to maximize the use of resources during outages and focus repair efforts to efficiently plan and deploy asset maintenance activities.
ADDITIONAL INFORMATION:
- Products: Doble Spark P3
- Services: EMI Diagnostic Testing
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