Understanding the Service Factor
of Electric Motors
The most commonly misunderstood rating on an electric motor nameplate is SF which is short for service factor. It is commonly considered an allowable overload for electric motors. This is inaccurate. Motors are not intended to run continuously in the service factor range.
Let’s look at a 100HP motor with a 1.15 SF driving a 110HP load under the assumption that it can drive a 115HP load. Motor operation under these conditions leads to many performance issues down the road, including loss of efficiency, lower operating speed and shorter service life.
Strictly speaking, the service factor was developed by the National Electrical Manufacturers Association (NEMA) with the following purposes:
- Applications where horsepower needs are subject to variations that cannot be estimated accurately.
- Increasing the service life of motor insulation, by reducing the operating temperature at rated horsepower.
- Providing tolerance for intermittent or occasional overload conditions.
- Tolerating brief ambient temperature peaks above 40°C.
- Tolerating under-voltage or voltage imbalance.
In summary, the service factor provides an operating margin for motors subject to unpredictable and brief load increases. The horsepower margin allowed by the motor service factor should never be used intentionally, not even for intermittent applications.