The costs of poor motor sizing
A
WEG Electric Motors product story
Edited by the Engineeringtalk editorial team
Jun 12, 2008
Incorrect specification makes a mockery of manufacturers' efforts to improve motor efficiency, argues Andrew Glover.
Energy efficiency losses through oversizing of pump electric motors by engineers are commonplace and, if the motor specification far exceeds the application, it could be costing industry billions of pounds in wasted energy.
Industry figures suggest that around 80% of pump motors could be the incorrect size.
The majority are overspecified by as much as 10 or 15% by engineers wanting to be "on the safe side".
As pumps typically account for around 30% of an industrial country's energy usage, this represents a serious loss of energy.
This takes no account of the extra cost of a larger motor with all the associated equipment, drives, cabling etc.
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.
Where a single percentage point increase in energy efficiency can save the equivalent of the purchase price of an electric motor over its design life, it seems ludicrous to waste energy through poor specification.
Undersizing is also relatively common and should not be ignored.
An electric motor can operate above its rated output thus allowing for temporary overloads.
However, the motor will run hotter as a result and if this state continues, for example, where the pump continues to stop and start in its duties or a higher specific gravity liquid is pumped than originally specified, overheating will cause damage.
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 online starting methods will create high torque that also impose mechanical stresses on the pump and hydraulic components, whereas 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.
Andrew Glover is a Product Manager for WEG Motors.
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