Reliability assurance of large electric motors in a process plant

Process plants like integrated steel plants use thousands of electric motors as prime movers of process equipment. The reliability of motors, especially large motors in the range of several hundreds of kilowatt capacity is vital as breakdown of these motors leads to breakdown of major facilities which these drive and cause large production losses. The maintenance of these large motors is often planned along with the driven equipment. Due consideration is not given to the requirements of large electric motors while planning their maintenance. Therefore, large motors either tend to get over maintained or are under maintained. To overcome this situation the authors propose to adopt a three pronged approach for meeting the maintenance needs of such motors in this paper. First the large motor is considered as a system composed of four sub-systems such as, electromagnetic system, power transmission system, ventilation system and support system. These sub-systems are treated individually and then their results are combined to obtain the results for the large motor. Second approaches such as FPD, FMEA, HAZOP and FTA are simultaneously applied to these sub-systems to identify their predominant failure modes. Third an appropriate condition indicator is identified for each of these failure modes and the failures are mitigated by using the ‘Early Warning’ signal received while monitoring the condition parameters of interest. The operating context is also considered while monitoring condition indicators of interest. These signals are used to decide on the maintenance task and time. The proposed approach can also be used to review the maintenance requirements of the equipment to provide Reliability Assurance.