Plant Services - April 2008 - (Page 56)

EfficiEncy Motors Better than duct tape Economic choice 9,000 Energyefficient range Annual service hours 6,000 Pipeline pumps Cooling water pumps Line shafts Rolling tables Speed reducers Ball mills 3,000 Standardefficient range Crushers Saws Figure 1. Tape provides both insulation and a substrate to which the varnish bonds. 0 57 58 59 60 61 62 63 64 65 66 67 Repair/replace threshold (%) Figure 2. Above is an empirical chart for choosing a new repair/replace threshold. For relatively low operational hours, the curve is relatively flat, but as hours increase above 3,000 per year, it rises exponentially. “Bucket burnout” or other direct-flame methods are no longer considered effective ways to remove damaged coils. Today burnout is done in an oven where temperature zones are controlled to within ±10°F. With temperature peaks limited to 700°F (370°C) and evenly distributed, core iron emerges both mechanically and electrically unchanged. Recently, even safer chemical techniques have shown promise to remove varnish with no danger of thermal damage. Better balance, stalwart stators Procedures for restoring the coils have changed, too. Replacements now are wound with overlapping layers of tape insulation that, by itself, has a high dielectric strength (Figure 1). In addition to insulation, this tape also provides a porous surface into which new insulating varnish is applied via vacuum/pressure impregnation (VPI). The porous tape gives teeth for adhesion of the new varnish. The materials used in rewinds have evolved toHeadline improve motor performance. VPI is not new, but because it became the standard for infusing rewound motors with varnish, new resins with extreme dielectric strength and mechanical rigidity have been developed. Today’s rewound stator shows little tendency to shift with the same current surges that would have cracked older insulation systems. The motor repair industry also owes grudging thanks to ISO Walt Dryburg is chief engineer at Integrated Power Services, Washbecause it forced a wholesale reevaluation of practices and proington, Pa. Contact him at wtdryburg@integratedps.com and (724) cedures, particularly on the metrology side of the business. To225-2900. day, the process of taking measurements and balancing rotating parts is more precise. Even on large rotors and armatures, imbalances are held to less than 0.1 mil or better. More resources at www.PlantServices.com a one-off fluke. An EASA/AEMT study put it this way: “… good practice repair methods maintain efficiency to within the range of accuracy that is possible to measure using standard industry test procedures (±0.2%) and may sometimes improve it.” This is true for both energy-efficient and standard motors. New rewind technology won’t improve a standard motor to the point that it can compete with an energy-efficient motor, but it makes rewinds of both types more attractive, particularly if new core plate is included. For energy-efficient motors in relatively good shape – they failed for electrical reasons, not mechanical or thermal reasons – energy efficiency gains of 0.5% have been reported after rewinding. Standard motors sometimes fare even better; it’s possible to see as much as a 1.5% increase in efficiency after a rewind. What the end user gets for the money is a motor that can improve the plant’s watts-management position. For this reason, today’s repair/replace threshold has shifted from 57% to the mid-60% (Figure 2). Efficiency dividend What obliterates the old 57% threshold is the startling fact that today’s rewound motor can be more efficient than it was when it was new. This isn’t always true, but where it occurs, the gain is, typically, a fraction of a percent. There’s enough history with the new rewind technology to prove that this trend is a fact, not 56 Motor decisions – “Electric motors: repair or replace” Premium-efficiency motors – “Conserving electricity with premiumefficient motors” IEEE STD 43-2000 – “Testing the insulation resistance of rotating machinery” Rewind service centers – “Tracking down excellence” Tips for efficiency – “An energy diet for electric motors” For more, search www.PlantServices.com using the keywords dielectric, insulation and motor. April 2008 www.PLANTSERVICES.com http://www.PlantServices.com http://www.PlantServices.com http://www.PLANTSERVICES.com

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Plant Services - April 2008

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