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Electrical Motors - Service Factors

Service factor - SF - is a measure of periodically overload capacity at which a motor can operate without beeing damaged.

electric motor

The service factor - SF - is a measure of periodically overload capacity (Design Power) 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:

Electrical Motors - Service Factors vs. Power and Speed
Motor Power
(hp)
Service Factor - SF
Synchronous Speed (rpm)
360018001200900
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

Design Power vs. Motor Power and Service Factor can be expressed as

PD = SF P     (1)

where

PD = design power (hp, W)

SF =service factor

P = motor power (hp, W)

Example - Design Power vs. Service Factor

A 1 HP motor with a Service Factor - SF= 1.15 can operate at

PD = (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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