Plant Services - February 2008 - (Page 48)

lectric motors, the most common machines in any manufacturing plant, are reliable, efficient and can drive almost any kind of load. Treat them correctly and they’ll reward you with long life. To maximize the chances of reaching that long life, put an effective preventive and predictive maintenance program in place that’s based on probable motor failure modes. Often, failure modes and effects analysis (FMEA) or reliability-centered maintenance (RCM) can serve that purpose because both technologies require intimate knowledge of how things can break down. Let’s take a closer look at electric motor failure to examine the stresses on the motor, the most common failure modes and some of the causes of those failure modes. Then, let’s look at some ways to either mitigate the failure or to detect an incipient failure. E • Insulation class • Bearing type and quality • Enclosure type • Quality of construction • Operating environment • Load characteristics The motor is part of a system that includes power supply, mounting, coupling and driven equipment. The environment includes ambient temperature, airborne contamination (moisture, corrosive fumes), ambient shock and vibration, etc. It’s a combination of stresses acting on the bearings, stator, rotor, shaft and frame that causes most motor failures. Motor stress Motor life They can last a long time, but motor death usually has nothing to do with aging. Many questions arise before you can decide when that life is over: How do you define the end of motor life? Does bearing failure constitute the end of life? Does having to rewind the stator define it? How about having to scrap the motor because it’s beyond repair? Or, is it more sensible to replace it so you can capture the energy savings that higher-efficiency motors provide? You can use standardized tables to help predict motor life (Table 1). Factors that affect motor life include: • Motor design • Materials of construction • PM/PdM effectiveness 48 Table 2 shows the major stresses acting on the five primary motor components. A number of failure mechanisms can cause the same component to fail with a common mode and failure pattern. For example, improper voltage, overloading, Table 1. Motor life by size HP Range 125 Average life (yrs) 12.9 17.1 19.4 21.8 28.5 29.3 Life range (yrs) 10 – 15 13 – 19 16 – 20 18 – 26 24 – 33 25 – 38 Source: Energy-Efficient Electric Motor Selection and Application, John C. Andreas (Marcel Decker, Inc., New York, 1982) www.PLANTSERVICES.com February 2008 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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