Understanding the motor nameplate's ratings and having a full understanding of your application can help you select the right motor for the job.
A secondary rating used to choose an electric motor is the Service Factor (SF). However, taking a closer look at what SF is and how it should be used reveals a general misunderstanding within the industry. Below we’ll take a deep dive into its definition and intended use.
What is Service Factor?
A multiplier which, when applied to rated power, indicates a permissible power loading that may be carried under the conditions specified for the service factor. To break that down into a more understandable statement, SF is the percentage of overloading that the motor can handle for short periods of time when operating within the manufacturer’s correct voltage rating. So, an SF rating of 1.15 will allow the motor to put out 15% more power when overloaded for short periods of time.
Why Have a Service Factor Rating?
Motor manufacturers design motors to be the most efficient when they run at only about 75-80% of the rated load. The calculated SF is there to make room for oversights in the predicted horsepower needed and handle the occasional overload. Additionally, it allows for times when the ambient temperature is above the rated 40°C or when the supply voltage is unbalanced. An SF enables the motor to operate below the maximum load so that it can run continuously with a cooler winding temperature at the rated load, extending the life of the motor and increasing reliability.
Can I Run at Service Factor Continuously?
No electric motor is 100% efficient, with power losses most often in the form of heat. This is most easily shown by an increase in load = an increase in current = an increase in operating temperature. And with each 10°C increase in temperature, the lifespan of the motor is cut in half. While the source of the temperature increase isn’t important, it can often be attributed to continuously running the electric motor at its SF.
Operating your motor at SF continuously could cause:
Increasingly inefficient use of energy
Insufficient starting or operating torque
Bearing failure due to overheating
Winding and winding insulation failure due to overheating
To put it most simply, operating your electric motor at the SF rated load continuously will reduce the motor's speed and efficiency and ultimately reduce the motor’s lifespan.
By misunderstanding SF as a continuous ability rather than the fail-safe for occasionaling that it is, end-users have reduced the reliability and lifespan of their electric motors and increased their operating costs. By understanding SF and the intended use of their motor, end-users will be able to better select the correct equipment for their application. Investing in a higher rated motor will increase the equipment's reliability and efficiency, helping to increase cost savings over the motor’s life.