In section 1.42 of NEMA MG 1, service factor (SF) is defined as “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.” The NEMA (National Electrical Manufacturers Association) standard service factor for totally enclosed motors is 1.0. Some compressor manufacturers use the service factor as high as 1.35 when sizing their motors. A word of caution: going simply by printed / nameplate data of only kW of the motor may mislead to assume improved energy efficiency of an air-compressor. SF should be considered while calculating the input power of the motor / package power of a Compressor.
Caution about Service Factor (SF):
- Operation at SF load for extended periods will usually reduce the motor speed, life and efficiency
- Motors may not provide adequate starting, and pull-out torques, and incorrect starter/overload sizing is possible. This in turn affects the overall life span of the motor.
- Do not rely on the SF capability to carry the load on a continuous basis.
- The SF was established for operation at rated voltage, frequency, ambient and sea level conditions.