What is the Motor Service Factor Rating (S.F.)?

Service Factor Allows Safe Overloading of Electric Motors

Occasionally, it is necessary to load an electric motor beyond its rated capacity. Manufacturers specify a motor service factor (SF) for this purpose, as a percentage of overloading. As such, the motor can safely handle this overloading for short periods when operating normally within the specified voltage and frequency tolerances. Practically, SF allows the operator some leeway when estimating horsepower needs and the actual running horsepower requirements. The service factor appears on the nameplate of the motor only for an SF greater than 1.0.

Motor Data

Sample Three Phase Motor Specifications with Listed Service Factor

NEMA Definition of Service Factor

According to the National Electrical Manufacturers Association (NEMA), SF is a multiplier applicable to the rated horsepower and it indicates an allowable horsepower loading. The operator may carry out the SF multiplication, under the specified conditions for the service factor, at the rated voltage and frequency of the motor.

For instance, a 10 HP motor with a standard SF of 1.5 can provide 11.5 HP when required for short-term. Fractional horsepower motors typically have higher service factors, sometimes going up to as much as 1.50SF.

The Canadian Electrical Code

Motor service factor specified under the Canadian Electrical Code (CEC) permits continuous loading at rated voltage and frequency. This is at variance with the NEMA definition that reserves SF only for short periods to ensure reliability.

Importance of Service Factor

Operators need to predict horsepower requirements for motors used for intermittent loads. The service factor offers them a safe means to accommodate inaccuracies in their prediction.

Operating the motor always at its limits makes it prone to overheating. Using the service factor to operate the motor below its rated maximum allows it to run continuously with a lower winding temperature—leading to a longer life and better reliability.

The motor can handle occasional or intermittent overloads within the limits calculated with the service factor without damage to insulation due to overheat.

The service factor allows the operator to run the motor occasionally to its safe operating limits even when the ambient is above 40°C.

Even when the supply voltage is lower than specified or it is unbalanced, the motor can be run safely, provided the operator utilizes the safety factor to compensate.

The mechanical capability and life of a motor depends on its bearings. The grease life reduces by 0.5 for every 15°C rise in temperature. When a motor is run into its service factor, it operates hotter and the grease in the bearings gradually dries out, reducing their life and the mechanical capability of the motor.

Motors with service factor of 1.15 have a stronger shaft to take care of the additional continuous shaft load. This is an important issue for large motors.

Precautions When Using Service Factor

Using service factor requires following some precautions. According to NEMA, it is not advisable to run a motor at service factor loads for extended periods. Doing so will reduce the efficiency, life, and speed of the motor.

Additionally, when operating for extended periods under service factor load, the motor may fail to provide adequate starting and pullout torques. The starter/overload sizing will possibly be incorrect, leading to a reduction in the overall life span of the motor.

The service factor capability must not be relied upon to carry load continuously. The service factor is applicable for operation at rated voltages, frequencies, ambient and sea level conditions.

When operating with a Variable Frequency Drive (VFD), the harmonic content of the drive’s output can add up to 30% additional heating to the motor system. Therefore, as a rule of thumb, when using a 1.15SF motor with a VFD, good practice is to treat it as a 1.0SF motor.

Motors and Service Factor

Broadly, open motors have a service factor of 1.15, whereas totally closed motors come with a service factor of 1.0. Although this is traditionally true for totally enclosed fan cooled (TEFC) motors, manufacturers are now offering these motors with an SF of 1.15. Likewise, although most motors meant for use in hazardous locations are made with an SF of 1.0, some specialized units are available with SF of 1.15, suitable for Class I applications.

Manufacturers rarely design their systems for continuous operation at the maximum level. For continuous operation, sizing a motor to operate below the service factor percentage is considered good practice. An electrical load-monitoring test is the usual method for establishing the efficiency of a running motor. The motor service factor offers a margin of safety to increase the reliability of the system.

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