Electrical Motors Service Factor
Service factor - SF - is a measure of periodically overload capacity at which a motor can operate without beeing damaged.
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The service factor - SF - is a measure of periodically overload capacity at which a motor can operate without 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) | Service Factor - SF | |||
|---|---|---|---|---|
| Synchronous Speed (rpm) | ||||
| 3600 | 1800 | 1200 | 900 | |
| 1/6, 1/4, 1/3 | 1.35 | 1.35 | 1.35 | 1.35 |
| 1/2 | 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 Service Factor - SF = 1.15 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.
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