Motor Service Factor

“Service factor” is an interesting motor rating that you will see on many motor data tags. It simply means how much additional “work” a motor can do or how much “load” it may be placed under for short periods of time without failure or overload.

For example, the FLA or full load amps of the motor above is 10.8 amps at 115 volts.

The service factor or S.F. is 1.5, making the service factor amps 16.2 (rounded down to 16 on the motor tag) at 115v because 10.8 x 1.5 = 16.2.

Don't confuse SFA with LRA (locked rotor amps). LRA is the current the motor will draw when the rotor is stationary, such as during startup. Service factor is simply a short-term “fudge factor” that the motor has for short periods of higher than normal load.

When a motor is running above its full load amps and in the service factor range, it may function, but its operational life will be shorter. It will also generally run at lower efficiency and power factor.

In other words, only go into the “service factor” range when necessary, not as a matter of normal operation.

—Bryan

Related Tech Tips

Better Temperature Measurements
I am in the midst of testing the accuracy and repeatability of different types of airflow measurements for techs in search of the most practical methods for different applications. A commonly taught method for measuring airflow is the temperature rise method, where you use a heat source that produces a set number of BTU/h, such […]
Read more
The Two OTHER EPA violations
We all know (or should know) that venting refrigerant is a big no-no and can result in trouble from the EPA. There are many other potential violations, but two of them can easily occur if you aren't thinking ahead at the disposal of mercury and oil. Mercury is found in fairly large quantities in the […]
Read more
The Impact of Adding or Removing Water From Air
Air conditioning was about humidity control from the very start. Willis Carrier's very first air conditioning system was all about controlling the humidity with the side effect that it also could reduce the sensible temperature. Theaters caught on that this newfangled contraption could lead to big summer numbers when they installed it to keep patrons […]
Read more

2 responses to “Motor Service Factor”

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from