And... “Service Factor” means nothing outside of North America. So a paper from SEW
EUROdrive is not the place to look for that kind of information. IEC (European and Asian) designed motors are all the equivalent of what we would call “1.0 Service Factor” compared to NEMA designed motors, as well as being, for the most part, Class 10 which means they cannot handle an overload for the same amount of time.
In my opinion, understanding SF requires understanding how it is typically used.
Scenarios 1: Let’s say you want to build a machine powered by an AC induction motor. You know the torque you need and the speed you want, but it calculated out to be 8.47HP continuously. If you are the end user and you want the maximum life out of this machine, you select a 10HP motor, because the next size down is 7-1/2HP and that is too small. But if you are an ORM selling this machine and you need to be competitive against other lower cost suppliers, you select a 7-1/2HP that has a 1.15 SF, because it can be used at 8.625HP, albeit with an increased current draw, lower efficiency and a reduced lifespan. Years ago the NEMA design specs used to quantify some of these issues in terms of time frames, but that was removed and now they just vaguely refer to the use of the SF as “reducing life”. Here is the wording:
A motor operating continuously at any service factor greater than 1.0 will have a reduced life expectancy as compared to operating at its rated nameplate horsepower. Insulation life and bearing life can be reduced by the service factor load.
So for the OEM, so long as it outlasts their warranty, they use it. The end user is who bears the cost of the reduced life by having to replace it sooner.
Scenario 2: the machine needs 6HP most of the time, but occasionally there is a change in the load and the motor is loaded down to 8.47HP for a few minutes, then returns to normal for a while longer, enough time for it to safely dissipate that extra heat. A common example is an air compressor that runs continuously but an unloader valve cycles on and off as the demand for air changes with shop use. When the unloader closes, the compressor loads the motor more heavily until the pressure is satisfied and the unloader opens again, venting the compression chamber to atmosphere. In that case, the 7-1/2HP motor makes sense.
