Operating NEMA Premium motors above rated load cuts efficiency
The operating load of any motor depends on a number of factors and it can have a major impact on its efficiency
Recently some motor manufacturers have begun to mark their motors with service factors (SFs) of 1.25 rather than the usual 1.15 number used for years. Although this may indicate one is receiving a more robust motor, the actual torque capability of the motor is not enhanced.
Operating at SF, the motor is also well past its peak efficiency and power factor (PF) peak and will use more energy for the work produced, and may experience reduced winding, bearing, and grease life.
Most motors used in North America are marked with a SF, which is defined by the National Electrical Manufacturers Association in MG 1-2011 as follows:
1.42 SF—ac motors: The SF of an AC motor is a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor (see Section 14.37: Application of alternating-current motors with service factors).
Section 14.37—Application of alternating-current motors with SFs:
When a motor is operated at any SF greater than 1, it may have efficiency, power factor, and speed different from those at rated load, but the locked-rotor torque and current and breakdown torque will remain unchanged.
A motor operating continuously at any SF greater than 1 will have a reduced life expectancy compared to operating at its rated nameplate horsepower. Insulation life and bearing life are reduced by the SF load.
Reduced winding, bearing, and grease life
Operation of a motor continuously above rated load raises the temperature rise of the motor. As a rule of thumb, for every 10 C hotter the motor operates, its insulation life is cut in half. If we look at a 100 hp NEMA Premium efficient TEFC motor, it has a temp rise of 60 C at rated load and 76 C at 1.15 SF, a 16 C difference. This means that the insulation life is cut by 250% over the motor operated at rated load.
The higher temperature operation also takes its toll on bearing and grease life. At rated load, the 100 hp motor would have a grease life of 5000 hours. When it is operated at 1.25 SF, the grease life is shortened to 1750 hours-a 285% reduction in grease life.
Operation above rated load reduces efficiency
The 100 hp motor discussed above has a nominal efficiency of 95.4% and a power factor of 82.6%. Operating this motor at a 1.25 SF reduces efficiency to 94.9% and PF to 87.4%. At rated load and operating continuously at $0.075/kWh, this same 100 hp motor uses $51,375 in electricity annually (685,000 kWh). When it is operated at a 1.25 SF and loaded to 125 hp, it uses $64,558 annually (852,680 kWh), an increase of $13,183.
Ideally the maximum efficiency for an ac induction motor is at about 80% of its rating, not above full load. Some process industries have adopted the 80% point as a best practice. A 150 hp motor operated at 80% load uses $61,138 and 815,173 kWh.
Some compressor companies also size motors to operate at SF, calling a motor that operates at rated load "oversized." Users should request a right-sized motor for best efficiency. The additional purchase cost of the larger motor will be easily offset by the lifecycle costs of a smaller motor operating in its SF. The purchase price of a motor is only about 2% of its lifecycle cost.
John Malinowski is general product manager and Jack Zedek is senior design engineer for Baldor Electric Company.
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