What is the service factor (SF) and how do you calculate it?

Friday - 28/09/2018 11:02
Determining the right load for a motor is paramount, since it improves its optimal performance and allows us to better define its service life.
One of the common problems with electric motors is that their performance decreases with loads lower than 50%. Despite overrating being the most common problem, underrating in a motor may be equally or even more damaging. Do you know exactly what is the service factor? This specification tries to measure the external aspects that affect a motor’s performance and cause malfunctions in the gear motor.

When a motor is overrated to operate continuously at most at its rated load, it is said that it has no service factor. Before defining what a service factor is, we should mention the NEMA standard, which defines it in the following manner: Service Factor is defined as a multiplier that is applied to the motor’s normal power rating to specify the load it can bear under nominal service conditions.

As per the standards, a motor’s nameplate should indicate the service factor as “SF”. The standard which defines the service factor for motors and generators is the MG-1 standard, which in turn defines powers and speeds. Likewise, for each power and speed, the MG-1 standard defines a frame number which is linked to the service factor.

What does service factor refer to in connection with a gear motor?

This specification allows for the measurement of the influence of external conditions on the speed reducer’s operation. Therefore, first and foremost, the service factor is related to the service characteristics of the machine to be driven.

Analysing this factor will enable us to know the degree of safety speed reducers operate with, and it mostly depends on the daily operation time and all the factors that measure how much a motor can be overloaded under ideal environmental conditions. The service factors can be used to determine the service life of the product based on its operation.

How do you calculate a motor’s service factor?

In order to calculate it, the motor’s power has to be multiplied by the service factor.

SF * HP

Uses or applications of the service factor (SF)

Within the uses of the service factor when referring to a speed reducer, we can mention those which allow for withstanding intermittent or occasional overloading, compensating for voltage dips or fluctuations, extending the insulation’s life by reducing the temperature, etc.

Temperature considerations

A motor that operates with a service factor of 1.15 will have a greater temperature increase than if it had no service factor, since it uses a higher current. This would imply a temperature increase ranging from 15 to 25 degrees (Celsius) when compared to the nominal load, which would translate into a decreased insulation life span.

A motor’s wear is related to four variables which are interrelated: thermal, electrical, mechanical and environmental, and is defined as the loss of its original properties. One of these variables which is associated with the service factor is thermal.

Service factor and power of a speed reducer

It is difficult for a speed reducer to operate under ideal conditions. This is basically due to the external factors acting on it. How does the service factor help us? It does so by offering a more realistic power factor.

This information considers all specific characteristics of the work to be performed, and the output –known as Design power– is the value that will specify the power required for the speed reducer in the selection tables.

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