Service Factor

The service factor - SF - is a measure of periodically overload capacity at which a motor can operate without overload or damage

electric motor

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)
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.

Related Topics

  • Electrical - Amps and electrical wiring, AWG - wire gauge, electrical formulas, motors and units

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