Plant Services - February 2008 - (Page 50)

RELIABILITY Motors Insulation Failure,” Ernesto J. Wiedenbrug, Baker Instrument Co., www.bakerinst.com/BakerWeb/PDF_Files/ Overheating_Electric_Motors.pdf). It’s important to focus on eliminating potential sources of overheating, including: • High ambient temperature • High altitude • Dirt buildup on cooling fins • Mechanical overload • Poor power conditioning • Excessive effective service factor • Frequent stops and starts • Environmental matters Increasing the motor horsepower in the face of recurrent failures might make the problem worse, according to Wiedenbrug. If the problem is start/stop cycling, be aware that a larger motor has more stringent duty cycle limits than a smaller motor (Table 3). Other causes of insulation breakdown include attack by dust, moisture, chemical agents and contaminated windings. Heat and other causes such as vibration or mechanical shock might induce electrical problems, including: • Shorts between conductors or coils • Insulation-to-ground faults • Air gap faults, including eccentric rotors • Rotor faults, casting voids and broken rotor bars • Overloading More resources at www.PlantServices.com/thismonth Motor repair (part 1) – “Analyzing premature winding failures – part 1” Motor repair (part 2) – “Analyzing premature winding failures – part 2” Rewind service centers – “Tracking down excellence” RCA – “Going around in circles” Shaft currents – “Bearings and electricity don’t mix” For more, search www.PlantServices.com using the keywords bearing, insulation and rotor. • Improper matching of motor to load (inertia matching) • Loose connections • Vibration/mechanical looseness The power supply also has the potential to cause motor failure. Some common problems include: • Overvoltage or undervoltage • Voltage unbalance • VFD waveforms • Voltage surges Identify and eliminate electrical problems using visual inspections, an effective cleaning program, vibration monitoring, motor current analysis, current signature analysis, power-quality monitoring and thermography. Mechanical modes Three representative studies Wilson and Smith – “Electric Motor Reliability Model”, Shaker Research Corp. RADCNTR-77-40H Based on an analysis of 3,000 fractional horsepower motor failure histories, Wilson and Smith found: • Bearing failures – 80.85% • Electrical failures – 16.55% • Mechanical failures – 2.60% These motors had grease packed, single- and double-shielded rolling element bearings of quality grade 3 and 5. The insulation was NEMA classes F and H, corresponding to IEEE class 150ºF and 180ºF. Reliability Analysis Center (RAC) – Reliability Toolkit, System Reliability Center, Rome, N.Y. A sampling of failure mode information revealed the following causes: • Bearing failure – 28% • Winding failure – 31% • Fail to run after start – 23% • Fail to start – 18% IEEE Petro-Chemical Paper PCIC-94-01 This document shows the following failure distribution: • Bearing – 51% • Stator winding – 16% (might have been voltage, water, overload, etc.) • External – 16% (environment, voltage, load – likely to recur) • Rotor bar – 5% • Shaft/coupling – 2% • Unknown – 10% (no RCFA performed) Your motors are more likely to suffer from bearing or electrical failure than mechanical failure. But don’t leave the mechanical modes out of your analysis. Primary mechanical motor problems include bad mechanical fits, mechanical unbalance, mechanical breakage, mechanical resonance and rotor deflection. The motor shaft also can exhibit problems that should be included in your root cause analysis. These include bad or worn shafts, shaft deformation, mechanical vibration, wrong coupling type, shaft misalignment and overhung loads. The motor frame isn’t immune. Look for frame warpage, broken mounts, base plate distortion, missing or deteriorated grouting, foundation deterioration, and inadequate foundation size and design. Measures you can take to ensure these problems are uncovered and eliminated include visual inspections, vibration monitoring, precision shaft alignment, belt alignment and tensioning, and proper installation. Performing an FMEA or an RCM analysis is time-consuming, but to ensure you get the maximum benefit from the time spent, you must be thorough. This list of possible failures should only be a starting point. As you do your analysis, use this list as a springboard for brainstorming unique modes that might affect your system. James W. Taylor is director of operations at Machinery Management Solutions, Inc., Clarks Hill, Ind. Contact him at Jim.taylor@ machineryhealthcare.com and (765) 366-4285. 50 www.PLANTSERVICES.com February 2008 http://www.bakerinst.com/BakerWeb/PDF_Files/Overheating_Electric_Motors.pdf http://www.PlantServices.com/thismonth http://www.bakerinst.com/BakerWeb/PDF_Files/Overheating_Electric_Motors.pdf http://www.PlantServices.com http://www.PLANTSERVICES.com

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Plant Services - February 2008 Contents From the Editor Letters The PS Files Up and Running Crisis Corner What Works Asset Manager: Why Six Sigma? Technology Toolbox: Ensuring a Calm Response Cover Story: Best Practices Awards Lubricants: A Slick Success Story Motors: Reliable Torque Compressors: Before You Call for Help Web Hunter: Exploiting Intellectual Capital In the Trenches Product Picks Classifieds Fast Facts Energy Expert: Energy, Climate and Food

Plant Services - February 2008

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