Service Factor

For many years it was common practice to give standard open motors a 115% service factor rating; that is, the motor would operate at a safe temperature at 15% overload. This has changed for large motors, which are closely tailored to specific applications. Large motors, as used here, include synchronous motors and all induction motors with up to 16 poles (450 rpm at 60 Hz).

New catalogs for large induction motors are based on standard motors with Class B insulation of 80°C rise by resistance, 1.0 service factor. Previously, they were 70°C rise by thermometer, 1.15 service factor.

Service factor is mentioned nowhere in the NEMA standards for large machines. There is no standard for temperature rise or other characteristics at the service factor overload. In fact, the standards explicitly state that the temperature rise tables are for motors with 1.0 service factor. Neither standard synchronous nor enclosed large induction motors have included service factor for several years.

Today, almost all large motors are designed specifically for a particular application and for a specific driven machine. In sizing the motor for the load, the horsepower is usually selected so that additional overload capacity is not required. Therefore, customers should not be required to pay for capability they do not require. With the elimination of the service factor, standard motor base prices have been reduced 4-5% to reflect the savings. Users should specify standard horsepower ratings, without service factor for these reasons:

1. All of the larger standard horsepowers are within or close to 15% steps.

2. As stated in NEMA, using the next larger horsepower avoids exceeding standard temperature rise.

3. The larger horsepower ratings provide increased pull-out torque, starting torque, and pull-up torque.

4. The practice of using 1.0 service factor induction motors would be consistent with that generally followed in selecting horsepower requirements of synchronous motors.

The common practice of using Class F insulated motors with a Class B rise at 1.0 SF in effect provides some obtainable service factor above 1.0 if the user is willing to operate the motor up to the Class F limits in response to some contingency. In many cases this provides at least 15% margin,

In NEMA size ranges, motors with service factors are still available; however, for compressor drives, it would be better if they were not. Experience with operation into the service factor rating has not been sat isfactorv

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