Plant Services - June 2008 - (Page 46)

battery of standard online and oﬄ ine analyses, including current signature analysis, demodulated current spectrum analysis and rotor inﬂuence check (RIC), are available for motor maintenance purposes. Current signature analysis (CSA), for example, has become the standard for detecting broken rotor bars by analyzing the sidebands around line frequency. Demodulated current spectrum analysis (DCSA) enhances the ability to detect broken rotor bars, especially on twopole motors. e IEEE 43-2000 standard for insulation testing lists the polarization index (PI), a ratio of the measured resistance at 10 minutes divided by the resistance measurement at one minute. is ratio can be used for analyzing the general health of the motor’s insulation system. Plotting the resistance measurement at ﬁ ve-second intervals yields a graph called a polarization index proﬁ le (PIP). e resulting proﬁ le can then be used for additional analysis of the insulation system that can’t be obtained from the standard polarization index. 46 Power quality is a measure of the quality of the voltage and current supplying a motor or other load. By analyzing harmonics, voltage and current unbalance, over- or undervoltage and over-current conditions, a technician can determine what might be causing nuisance trips, voltage swells or sags, and other power system-related anomalies. Here are three case histories describing how these techniques were applied to minimize the total cost of motor failures. Finding broken rotor bars Routine testing revealed that a 3,500-hp, 4,160-volt, 3,590-rpm, two-pole, AC induction motor displayed a 0.7419 dB peak level of the pole pass sideband, which exceeded the alarm set point of 0.3 dB. is suggested rotor bar problems. Vibration analysis, however, indicated a healthy motor. As a result of the conﬂ icting diagnoses, we monitored the motor and trended the test results. e motor was retested periodically during the next 11 months; at that time it was removed from service. e trended data revealed that the pole pass sideband increased J .PLANTSERVICES. http://www.PLANTSERVICES.com

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Plant Services - June 2008 Contents From the Editor Letters The PS Files Up and Running Crisis Corner What Works Asset Manager Technology Toolbox Cover Story HVAC Coatings Motors Web Hunter In the Trenches Product Picks Classifieds Energy Expert

Plant Services - June 2008

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