"SERVICE FACTOR" — WHAT'S IT ALL ABOUT?

Most motor users are familiar with the concept of "service factor." It's been around for generations. The usual service factor for motors above 1 through 200 horsepower is 1.15 (per NEMA Standard MG1).

In NEMA Standard MG1-1.42 we find this definition: "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..." Those "conditions," include a winding temperature rise at the service factor horsepower that exceeds the rated insulation system temperature by 10° C.

Thus, at 115% of rated horsepower, a motor can be expected to run 10° hotter than the standard limit applicable to rated load. A useful rule of thumb says that 10° higher temperature cuts insulation thermal life in half. That means a motor running continuously at
the "permissible" service factor overload will have greatly reduced life.

What value, then, does a service factor have? It is not intended to simply allow an extra 15% load continuously. Rather, a service factor is meant to provide for occasional, sporadic, intermittent overload caused by variations in plant operation. These typically include process disturbances, such as random variations in the density of material being processed by industrial machinery. The service factor allows the motor to "ride through" such temporary conditions.

Many operators will also use a service factor to compensate for low or unbalanced line voltage; for harmonics in the voltage waveform; for seasonal ambient temperature swings; for temporary ventilation restriction caused by dirt buildup; or to allow for an increased power demand caused by process expansion or a higher production rate. But long-term overheating to the service factor temperature rating — 10° C above the design temperature of the insulation — will reduce winding life. If the motor is operating in a low ambient temperature (below the standard 40° C), that may not happen.

In any event, use a service factor with great care. Its existence will not increase available motor torque, nor permit more frequent or more severe starting.

If, say, a 45 hp load is to be supplied, a 40 hp motor having a 1.15 service factor will do the job. But the next larger rating (50 hp) without a service factor is normally the better choice. It offers longer life, higher torque, and probably a slightly higher efficiency at the actual load.

Richard L. Nailen, P. E