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Motor service factor

The service factor, abbreviated as SF, is a measure of the periodic overload capacity of an electric motor defined by the NEMA MG 1 standard. Its standard value for totally enclosed fan-cooled (TEFC) motors is 1,0, indicating that the motor can deliver exactly its nameplate rated power without additional margin for continuous overloads. A service factor greater than 1,0, such as 1,15, means that the motor can temporarily operate at a power up to 15 % above the rated power without immediate damage, provided that rated voltage and frequency are maintained.

Service factors for open drip‑proof motors

Section titled “Service factors for open drip‑proof motors”

Open drip‑proof motors, according to NEMA, have service factors that depend on the rated power and synchronous speed. The following table shows the values for powers from 1/6 hp to 1 1/2 hp and above, where the smallest motors reach a maximum SF of 1,35.

Motor power [hp / kW] Service factor – SF at synchronous speed
Synchronous speed [rpm] 3600
1/6 hp / 0,12 kW 1,35
1/4 hp / 0,19 kW 1,35
1/3 hp / 0,25 kW 1,35
1/2 hp / 0,37 kW 1,25
3/4 hp / 0,56 kW 1,25
1 hp / 0,75 kW 1,25
1 1/2 hp and above / ≥1,12 kW 1,15

Service factor in totally enclosed motors (TEFC)

Section titled “Service factor in totally enclosed motors (TEFC)”

NEMA standard establishes a fixed service factor of 1,0 for all totally enclosed fan-cooled (TEFC) motors, regardless of their power or speed. This means that these motors do not have continuous overload capacity and must be sized for the exact power required by the load.

Design power calculation

Section titled “Design power calculation”

The design power PD that a motor can deliver under temporary overload conditions is obtained by multiplying the service factor by the nameplate rated power. The explicit formula is:

PD = SF × P

Variable Symbol Unit Description
Design power PD hp or kW Maximum permitted power in continuous service with overload
Service factor SF dimensionless Multiplier factor according to NEMA for the motor type
Rated power P hp or kW Mechanical power at the shaft indicated on the motor nameplate

For example, a 1 hp / 0,75 kW motor with SF = 1,15 can operate intermittently at a design power of 1,15 hp / 0,86 kW without overheating, provided that voltage and frequency are rated. Continuous maintenance of this overload is not recommended.

Consequences of operation above the service factor

Section titled “Consequences of operation above the service factor”

Starting or permanently operating with loads that exceed the design power shortens the motor’s useful life, especially that of the insulation and bearings. The operating temperature rises approximately 10 °C / 18 °F for each 10 % sustained overload, which accelerates the aging of the varnishes and reduces dielectric strength. Although the service factor provides a safety margin, its continuous use reduces life expectancy according to the classic Arrhenius rule: insulation life is halved for every 10 °C / 18 °F of permanent increase in operating temperature.

Application recommendations

Section titled “Application recommendations”

Correct motor selection requires that the design power PD covers 100 % of the expected maximum demand, leaving the service factor as a reserve for occasional peaks. Motors with SF = 1,0 must completely avoid any sustained overload, as they lack thermal margin. In installations where the load varies predictably, motors with SF = 1,15 can be used for heavy starts or brief fluctuations, but never as a substitute for a motor with higher rated power.

Frequently asked questions (FAQ)

Section titled “Frequently asked questions (FAQ)”

What does a service factor of 1.15 mean in an electric motor?

Section titled “What does a service factor of 1.15 mean in an electric motor?”

It means that the motor can be temporarily overloaded up to 15 % above its rated power, delivering 1,15 times the nameplate power without immediate risk of deterioration, provided that voltage and frequency are rated.

Is it advisable to operate a motor continuously at its maximum service factor?

Section titled “Is it advisable to operate a motor continuously at its maximum service factor?”

No. Continuous operation with overload, even within the declared SF, raises the operating temperature and reduces the insulation life. NEMA warns that a motor operating permanently at SF > 1,0 will have a lower life expectancy than one operated at its rated power.

How does the service factor affect motor life?

Section titled “How does the service factor affect motor life?”

Each additional 10 °C / 18 °F in winding temperature, caused by frequent overloads, halves the life of the insulation system. Therefore, abusing the service factor leads to premature failures.

Do all motors have a service factor greater than 1.0?

Section titled “Do all motors have a service factor greater than 1.0?”

No. Totally enclosed (TEFC) motors defined by NEMA have a standard SF of 1,0, meaning they do not allow continuous overload. Only certain open drip‑proof motors have service factors of 1,15, 1,25, or 1,35 according to their size and speed.

What is the difference between service factor and power factor?

Section titled “What is the difference between service factor and power factor?”

The service factor indicates the mechanical overload capacity of the motor, while the power factor measures how efficiently the electric current is converted into useful work. A motor can have a high service factor and, simultaneously, a low power factor.

How is the design power calculated from the service factor?

Section titled “How is the design power calculated from the service factor?”

The design power is obtained by multiplying the nameplate rated power by the service factor: PD = SF × P. For example, a 2 hp / 1,5 kW motor with SF = 1,25 can deliver 2,5 hp / 1,9 kW intermittently.

Sources Consulted

Section titled “Sources Consulted”