Web Exclusive: Proper motor sizing
April 30, 2009According to WEG Electric Motors Corp., supplier of motors, drives, controls, generators and transformers, proper motor sizing could easily save vast amounts of energy. The loss of energy efficiency owing to oversized electric pump motors is a common problem, according to WEG. And, if the motor specification far exceeds the application, it could be costing industry billions of dollars in wasted energy.
“Industry figures suggest that around 80 percent of pump motors could be the incorrect size,” said Andrew Glover, product manager for WEG. “The majority of motors are over specified by as much as 10 or 15 percent by engineers wanting to be ‘on the safe side.’ As pumps typically account for around 30 percent of an industrial country’s energy usage, this represents a serious loss of energy.
“This does not take into account the extra cost of a larger motor with all the associated equipment, drives, cabling, etc.,” Glover said. “Electric motor manufacturers are forever striving to increase energy efficiency by one or two percentage points, but incorrect specification by an application engineer can mean that is wasted effort.”
Glover explained that a single percentage point increase in energy efficiency can save the equivalent of the purchase price of an electric motor over its design life.
Under-sizing is also relatively common and should not be ignored. An electric motor can operate above its rated output to allow for temporary overloads. However, such a motor will run hotter as a result and overheating will cause damage and/or shorten its useful life.
Two points where this can directly affect the motor life are the bearings, which influence the motor reliability, and the coil insulation. Overheating degrades the insulation more rapidly and encourages discharges, which further degrade the insulation, thus shortening the motor’s life.
Specification of motor size should, therefore, include starting condition as well as running torque. Method of starting is also important. Direct on line starting methods will create high torque that also impose mechanical stresses on the pump and hydraulic components, while star delta starting delivers lower torque and current.
Including a variable speed drive or soft starter in the system specification, matched to the requirements of motor and pump, will easily overcome these problems. Variable speed drives also improve energy efficiency in the long term by matching application requirements with the correct motor speed and thus avoid energy wasted by, for example, belt drives, clutches and gears.
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