Electric Motor Service Factor

What does the electric motor Service Factor really mean?

The nomenclature of service factor on an electric motor nameplate is designated by “S.F.”. Service factor is a multiplier, which when applied to the rated horsepower, gives the allowable horsepower loading the machine is capable of delivering without causing significant damage to the unit.

Electric motor service factor is not a direct multiplier to motor current but it is a good rule to follow since horsepower output is often proportional to current draw.For most motors, using the service factor multiplier on the nameplate to the full load current rating will provide an accurate assessment of allowable full load current.There are some exceptional cases in which this rule may not apply. Operating an older until with lower efficiency ratings or operating in conditions in which power factoris low will create less temperature increase per ampere than in modern ‘T’ frame motors will heat up dramatically when operated over full load nameplate current ratings.

The key to maximum allowable current draw is how the draw in excess of nameplate will affect core heating and the ability of the core to dissipate that heat. Electric motor service factor should never be engineered into an application. Electric motors should never be operated within their service factors for prolonged time periods. The electric motor service factor must be viewed as insurance against motor failure during periods of temporary load or power problems.

If your electric motors are operating within service factor range, find out why. Correct the problem or at the very least, know the problem. Motors designed to operate on 460 volt systems will draw slightly higher current when operated at 480 volts. Units that have been rewound many times may have sufficient core losses to account for slightly higher current draw. These factors considered should emphasize the need for a class H insulating system on all rewinds.

To learn more about electric motor service factor or for repair and replacement quotes, contact Precision Electric.