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

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