Plant Services - April 2008 - (Page 55)

A 100 hp motor on your plant’s cooling-water pump just failed. Its performance had been declining for six months. You can get by for a couple of weeks with a spare unit, but it was taken oﬀ the line because it showed signs of vibrating. Now the question that lingers in the air even more pervasively than the scent of burned insulation is: Do I repair or replace the failed motor? is scenario is played out every day in the nation’s manufacturing facilities. A generation ago – before the advent of energy-eﬃcient motors – this motor decision was relatively simple and was made on the basis of the cost of the repair versus the cost of a new motor. If the repair cost wasn’t more than 57% of the cost of a new motor, the choice was to repair it. e 57% threshold was based on a combination of economical analysis and the knowledge that a rewound motor of that era would rarely last as long as a new one. It was an empirically determined maxim that most plant managers were willing to pay for a repaired motor. With the coming of energy-eﬃcient motors, repair/replace decision variables changed because it was easy to show that on long-running applications – 2,000 hours per year or more – the lion’s share of the motor’s life cycle cost lay in the amount of energy used. Energy savings with the new motors – in excess of $1,000 per year for a 200 hp motor – were so large that the price of a motor, by comparison, became insigniﬁcant. us, as motor selection became a watts-management issue, failure of every standard motor was seen as an opportunity to upgrade to an energy-eﬃcient unit that could pay for itself in energy savings. For applications such as ball mills, compressors, cooling towers, HVAC, pulp/paper mills, metal rolling lines, pumps in food processing plants and extrusion lines, this trend to upgrade continues today. Since their introduction, the ﬁrst wave of energy-eﬃcient motors has had time to run to failure. Not surprisingly, they fail for the same reasons that standard motors fail: heat, overloads, voltage spikes and mechanical problems. Now, the old question arises. Does the 57% repair/replace threshold still apply? Tradition says yes, but technology says no. Hidden beneath the hype that followed the new generation of motors was a quiet evolution in the motor repair industry. is, too, was energy focused. Old repair practices and procedures were evaluated and a few were found to degrade a motor’s eﬃciency. Of the traditional repair practices that degraded motor eﬃciency, coil burnout was the biggest oﬀender. Insulating varnish must be treated like paint to remove it, and it was not uncommon to see a motor turned on its end over a bucket of ﬂaming kerosene, the insulation surrounding the coil slowly carbonizing to ash. It was eﬀective, but this technique put the motor iron in thermal stress, with the hot end approaching 800°F or 900°F and the cooler end at less than half that. When one end was carbonized, the unit was ﬂ ipped to cook the other end. e extreme temperature gradient caused core losses in the iron and often left the slots distorted. For a motor of 50 hp or greater, the decline in eﬃciency was about 0.9% to 1.0%. Repeated rewinds made the situation even worse, dropping eﬃciency as much as 5%. 55 A .PLANTSERVICES. http://www.plantservices.com

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

Plant Services - April 2008

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