What You Need to Know about "Service Factor"

Motor Talk

Motor “service factor” is probably the most misunderstood value on a motor nameplate.  To most it seems quite simple; if it is a 1.0 you can run a motor to 100% load, if it is 1.15, then it can be loaded to 115% of its rated load.  That’s simple enough, right?  Wrong!  It is not that simple.

First let’s look at what the National Electrical Manufacturers Association (NEMA) has to say about service factor:  (NEMA MG-1)

In order to get optimum performance and full longevity form your motors, it is important that you fully understand service factor.

The service factor of an AC motor is a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor (see 14.37).

14.37.1 General
A general-purpose alternating-current motor or any alternating-current motor having a service factor in accordance with 12.52 is suitable for continuous operation at rated load under the usual service conditions given in 14.2. When the voltage and frequency are maintained at the value specified on the nameplate, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate.

When the motor is operated at any service factor greater than 1, it may have efficiency, power factor, and speed different from those at rated load, but the locked-rotor torque and current and breakdown torque will remain unchanged.

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.

Now service factor should be abundantly clear, right?  Wrong again!  If we delve further into NEMA MG-1, we find stipulations for exceeding service factor of 1.0:

  1. To accommodate inaccuracy in predicting intermittent system horsepower needs.
  2. To lengthen insulation life by lowering the winding temperature at rated load.
  3. To handle intermittent or occasional overloads.
  4. To allow occasionally for ambient above 40°C.
  5. To compensate for low or unbalanced supply voltages.

NEMA’s reference to “intermittent” is also a major point of confusion.  How do I define intermittent?  The best advice here is to provide temperature monitoring of the motors that are running overloaded within service factor. If they approach or exceed insulation design temperatures then the load should be reduced.

NEMA does add some cautions when discussing the service factor:

  1. Operation at service factor load for extended periods will reduce the motor speed, life and efficiency.
  2. Motors may not provide adequate starting and pull-out torques, and incorrect starter/overload sizing is possible. This in turn affects the overall life span of the motor.
  3. Do not rely on the service factor capability to carry the load on a continuous basis.
  4. The service factor was established for operation at rated voltage, frequency, ambient and sea level conditions.

In order to get optimum performance and full longevity form your motors, it is important that you fully understand service factor.

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