The service factor - SF - is a measure of periodically overload capacity at which a motor can operate without overload or damage. The NEMA (National Electrical Manufacturers Association) standard service factor for totally enclosed motors is 1.0.
A motor operating continuously at a service factor greater than 1 will have a reduced life expectancy compared to operating at at its rated nameplate horsepower.
NEMA Service Factor at Synchronous Speed (RPM) for drip proof motors:
Power
(HP) |
Synchronous Speed (RPM) |
| 3600 |
1800 |
1200 |
900 |
| 1/6, 1/4, 1/3 |
1.35 |
1.35 |
1.35 |
1.35 |
| 1/6 |
1.25 |
1.25 |
1.25 |
1.25 |
| 3/4 |
1.25 |
1.25 |
1.15 |
1.15 |
| 1 |
1.25 |
1.15 |
1.15 |
1.15 |
| 1 1/2 and up |
1.115 |
1.15 |
1.15 |
1.15 |
Example - Service Factor
A 1 HP motor with a 1.15 SF can operate at
1 HP x 1.15 = 1.15 HP
without overheating or otherwise damaging the motor if rated voltage and frequency are supplied to the motor.
Insulation life and bearings life are reduced by the service factor load. |
