Service Factor — What's It All About.docx

7/27/2019 Service Factor — What's It All About.docx

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" Serv ice Factor" — What 's It Al l Abou t?

Most motor users are familiar with the concept of "service factor." It's beenaround for generations. The usual service factor for motors above 1 through 200horsepower is 1.15 (per NEMA Standard MG1).

In NEMA Standard MG1-1.42 we find this definition: "The service factor of an acmotor is a multiplier which, when applied to the rated horsepower, indicates apermissible horsepower loading which may be carried under the conditionsspecified..." Those "conditions," include a winding temperature rise at the servicefactor horsepower that exceeds the rated insulation system temperature by 10°

C.

Thus, at 115% of rated horsepower, a motor can be expected to run 10° hotter than the standard limit applicable to rated load. A useful rule of thumb says that10° higher temperature cuts insulation thermal life in half. That means a motor running continuously at the "permissible" service factor overload will have greatlyreduced life.

What value, then, does a service factor have? It is not intended to simply allowan extra 15% load continuously. Rather, a service factor is meant to provide for

occasional, sporadic, intermittent overload caused by variations in plantoperation. These typically include process disturbances, such as randomvariations in the density of material being processed by industrial machinery. Theservice factor allows the motor to "ride through" such temporary conditions.

Many operators will also use a service factor to compensate for low or unbalanced line voltage; for harmonics in the voltage waveform; for seasonalambient temperature swings; for temporary ventilation restriction caused by dirtbuildup; or to allow for an increased power demand caused by processexpansion or a higher production rate. But long-term overheating to the service

factor temperature rating—

10° C above the design temperature of the insulation— will reduce winding life. If the motor is operating in a low ambient temperature(below the standard 40° C), that may not happen.

In any event, use a service factor with great care. Its existence will not increaseavailable motor torque, nor permit more frequent or more severe starting.



If, say, a 45 hp load is to be supplied, a 40 hp motor having a 1.15 service factor will do the job. But the next larger rating (50 hp) without a service factor isnormally the better choice. It offers longer life, higher torque, and probably aslightly higher efficiency at the actual load.

Richard L. Nailen, P. E

Motor Rebui ld ing

Brithinee Electric Repairs Large Motorsand Generators

Motor and Generator Repair

Electric Motor Repair and Re-winding Phone: 909-825-7971

Rewinds with inverter voltage spike resistant

insulation system



Computer-assisted winding data check and redesign

Vacuum-impregnation with 100% solids polyester resin

Electrical testing and evaluation

Testing and repair of stator core-iron laminations

Complete in-house machine and fabrication shop Dynamic balancing

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http://www.engineeringtoolbox.com/service-factor-d_735.html

Service Factor

The service factor - SF - is a measure of periodically overload capacity at

which a motor can operate without overload or damage

The service factor - SF - is a measure of periodically overload capacity at which a motor canoperate 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 lifeexpectancy 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 servicefactor load.

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Service Factor — What's It All About.docx