Plant Services - February 2008 - (Page 49)

Table 2. Motor components and stresses Type of stress Thermal Electric/dielectric Mechanical Dynamic Residual Electromagnetic Environmental X X X X Bearings X X X X Stator X X X X Rotor X X X X X X X X X X X X X X X X Shaft X Frame X Source: “Explaining Motor Failure,” Austin Bonnett and Chuck Yung, EASA. Table 3. Duty cycle limits 2-pole HP 1 5 10 15 20 50 75 100 200 250 A 15 8.1 6.2 5.4 4.8 3.4 2.9 2.6 2 1.8 C 75 83 92 100 100 145 180 220 600 1,000 A 30 16.3 12.5 10.7 9.6 6.8 5.8 5.2 4 3.7 4-pole C 38 42 46 46 55 72 90 110 300 500 A 34 18.4 14.2 12.1 10.9 7.7 6.6 5.9 4.8 4.2 6-pole C 33 37 41 44 48 64 79 97 268 440 Key: A = maximum number of starts per hour, C = minimum rest or off time in seconds between starts blocked ventilation, excessive cycling and heat all can produce the same type of winding failure. Many motor failure studies tell us that the bearings are the most common cause of failure. Next is electrical failure, followed by mechanical failure (see sidebar, “Three Representative Studies”). Bearing modes Bearing failures have many causes that can be tied to installation, cleanliness, lubrication or load. The first bearing installation reliability rule is proper selection. Type, load rating, quality grade and shielding are some of the factors. Bearing installation problems often are a question of workmanship. Proper tools, proper procedures and a clean work environment mitigate bearing failures. Inattention to cleanliness is a major cause of bearing failure. Contamination enters the bearing in a number of ways. A dirty installation environment is a prime contributor. Contamination also enters through poor practices such as not cleaning the grease fitting when applying grease. Lubrication ranks with contamination as a contributor to bearing failure. Factors to monitor include using the correct grease or oil, possible lubricant mixing that causes incompatibility problems, and extended lubrication intervals. Too much grease also can be a problem. Not allowing the bearing to relieve the excess grease through the drain plug is one February 2008 of the more common bad practices. Factors that contribute to excess bearing loading are wrong coupling type or installation; dynamic overloading (balance, misalignment); belt, sheave or insert wear; belt misalignment; incorrect belt tension; static loading; poor workmanship and defective bearing housings. Some common measures you can institute to mitigate bearing failures include visual inspection; vibration monitoring, including bearing-specific techniques (shock pulse, ultrasound and others); precision shaft alignment; belt alignment and tensioning; lubrication programs that follow best practices and proper tools; and environment and training for technicians. Because bearing failure is the most common problem for electric motors, eliminating bearing problems can prevent 25% to 80% of motor failures. Electrical modes Excessive heat in the winding breaks down the insulation, which leads to failure. A commonly accepted guideline is that every 10°C increase in operating temperature corresponds to a 50% loss in motor life. Leading standardization organizations conclude that insulation failure represented 30% of failures and, of these, 60% were caused by overheating (see “Overheating Electric Motors: One Root Cause of 49 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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