Finding mechanical failures in electrical rotating machinery
August 1st 2007 On-line motor monitoring looks at numerous areas of the motor's working environment however, the use of instantaneous torque signatures is quickly showing benefits in recognising mechanical problems prior to failure. A power generation plant in N. Carolina, USA found a significant problem with one of three identical submerged pumps. Through monitoring instantaneous torque at the control cabinets through current and voltage signature analysis, identification of this deteriorating submerged pump was possible. The Baker Instruments' on-line tester was used to evaluate this 1500hp, 4160V submerged pump detecting the problem.The slow turning pump (272 rpm, 2.1 meter diameter) showed an operational torque level 27% below the other two twin systems with a significantly higher level of torque ripple. Upon finding this result the pump was raised for repair. Diagnosis found that over time the bolts which attached the endbell to the pump had rusted and broken, allowing the endbell to fall approximately six meters into the water pit. The main endbell's function is to assure laminar water flow.The problem resulted in increased cavitation, which decreased the water flow into the pump. Maintenance personnel looking at the steady state torque and its signature were able to diagnose both these problems. Using the instantaneous torque signal it was possible to diagnose cavitation of this submerged pump.The signal is obtained through calculations from the low voltage side of the power and current transformers of this 4160V motor. Without the advanced monitoring equipment, the problems would have remained undetected until further and more costly problems would have developed within the three-pump system. It was only a matter of time before one or more of the pumps failed due to the added stress. After the endbell was repaired, the pump was restored into service and is working in parallel with the other two systems. After the repair, a 25% increase in productivity from this pump was recorded. Due to the 25% increase on these systems, it was calculated that the increase in productivity was worth approximately $117,120 daily. These calculations were derived from market value of kilowatt-hour and the increase in generation. On Line motor tester The Baker 'Explorer'Series can pinpoint voltage source,Variable Frequency Drive (VFD),motor or load challenges that face rotating machinery.The portable and durable Explorer is designed to accomplish fully remote monitoring from the Motor Control Centre (MCC) with aid of a plug-in device.Tests can be conducted by maintenance staff directly from the control cabinet without having to access control panel components for connections to voltage and current transformers. The Explorer identifies possible power circuit problems that degrade motor health, examines overall motor power conditions, monitors the load,motor efficiency and observes motor performance. Specifically it will uncover rotor bar problems and separate mechanical from electrical issues as well highlighting Stage 3 & Stage 4 bearing faults.Additionally it performs a start up transient analysis, giving voltage, current and torque versus time during run up to operating speed. The constant monitoring software allows motors to be monitored round the clock, with up to 47 channels being monitored at any one time. Trigger levels can be set for each channel, and when any of these are breached, the unit will record data for the duration of the breach | 
