Operating NEMA Premium Motors Above Rated Load Reduces Efficiency

Authored by: John Malinowski and Jack Zedek, Baldor Electric Co.
Recently some motor manufacturers have begun to mark their motors with service factors (SFs)of 1.25 rather than 1.15, which has been used for years. Although this may indicate a more robust motor, the actual torque capability is not enhanced. Operating at SF, the motor is well past its peak efficiency and power factor (PF) peak, will use more energy for the work produced, and may experience reduced winding, bearing, and grease life. Most motors used in the U.S. are marked with a SF, which is defined by the National Electrical Manufacturers Association (NEMA) in MG 1-2011:
1.42 SF c motors: The SF of an ac motor is a multiplier which, when applied to the rated horsepower, indicates a permissible horse-power loading which may be carried under the conditions specified for the SF (see Section 14.37: Application of alternating-current motors with service factors).
Section 14.37-Application of ac motors with SFs: When a motor is operated at any SF greater than 1, it may have efficiency, PF, 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
Continuous operation above rated load raises the temperature of the motor. As a rule of thumb, for every 10 C that the temperature of an operating motor rises, its insulation life is cut in half. If we look at the 100 hp NEMA Premium efficient TEFC motor, it has a temperature rise of 60 C at rated load, and 76 C at 1.15 SF. 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 5,000 hours. When it is operated at 1.25 SF, the grease life is shortened to 1,750 hours-a 65% 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 PF 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. When it is operated at a 1.25 SF and loaded to 125 hp, it uses $64,558 annually, an increase of $14,183.
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 effciency. 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.