Plant Services - June 2008 - (Page 47)

1,420% from 0.1814 dB at a running speed of 3,591 rpm to 2.5851 dB at a running speed of 3,592 rpm during the most recent 33 months the motor was online. Test data during a three-year period showed an exponential increase in the pole pass peak levels, which typically is indicative of at least one broken rotor bar. e load variation should remain constant under normal operating and motor conditions. is motor displayed a 275% increase in the load variation, from 0.855% in to 2.345%, during the past 33 months. e current spectrum taken when the motor was pulled from service showed an increase in sideband activity around the fundamental frequency, which also indicated broken rotor bars. All test results indicated broken rotor bars. e rotor inﬂuence check also indicated a rotor anomaly. e motor was sent to a motor shop for disassembly. A visual inspection revealed that 22 of 51 rotor bars were either broken or cracked. e root cause of the broken and cracked rotor bars was badly brazed joints between the bars and end rings from a J rotor repair performed three years ago. e cost to repair the motor was $60,000, plus $30,000 in planned plant downtime. Had the motor run to failure, the cost would have been $170,000 for a new motor, plus $200,000 in unplanned downtime. In this case, the total savings were $280,000. Polarization index proﬁle We applied a polarization index proﬁ le to an induced draft fan motor. It revealed a low polarization index value and a quick initial rise time of the resistance to a relatively low overall value. ese symptoms are indicative of an insulation system containing a signiﬁcant amount of moisture. IEEE 43-2000 recommends an insulation resistance of no less than 100 megohms and a minimum polarization index value of 2.0 for this motor. Moisture around the power cable entrance caused the low overall PIP and low PI value. A follow-up polarization index proﬁ le taken after the cables and components were dried was indicative of a motor with a healthy insulation system. When insulation systems become con47 .PLANTSERVICES. http://www.PLANTSERVICES.com

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Plant Services - June 2008

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