Why Motor Service Factor Matters

Why Should a Motor Have a Service Factor?

The motor service factor represents the maximum load rate at which a motor can operate continuously over the long term. It is an important parameter that reflects the motor’s continuous load-handling capability. Essentially, it provides an allowable overload margin to meet special application requirements or duty cycles. Numerically, it is defined as the ratio of the maximum permissible output power to the rated power. The service factor should not be confused with the duty type, which refers to the operating mode.

For example, for a 30 kW motor with a service factor of 1.15, the actual maximum output power is: 30 × 1.15 = 34.5 kW.

Today, let's clarify the differences between duty type and service factor with practical examples, aiming to guide both motor design and proper usage.

Common duty types include S1 (continuous duty), S2 (short-time duty), and S3–S7 (intermittent periodic duty). Less common are S8 (continuous duty with periodic speed and load variations) and S9 (duty with non-periodic variations in load and speed). When an application does not fit into any standard duty type (S1–S9)—for example, when a motor must run continuously under variable loads—the combination “S1 continuous duty + service factor” can be used to specify and constrain motor performance. Although this combination can be regarded as a special duty type, the service factor itself should not be considered a duty type. It is a comprehensive indicator, numerically equal to the maximum overload rate.

For three-phase asynchronous motors, the service factor is usually set between 1.15 and 1.25. If the factor is too low, it has little practical benefit; if too high, motor efficiency and economic performance are compromised.

Take screw air compressors as an example. The motor load fluctuates cyclically according to air demand. When the system pressure reaches the upper limit set by the user, the compressor unloads; when the pressure drops to the lower limit, the compressor reloads automatically. Motor power should be chosen so that efficiency and power factor are optimized under expected load conditions. At the same time, the motor’s shaft power must support full-load operation, allowing for long-term overload conditions. Typically, motor power under full-load conditions is selected as 1.1 times the expected economical shaft power.

If motor power were selected directly at 1.1 times the expected economical shaft power without considering a service factor, the result would be reduced efficiency and lower power factor, leading to wasted energy and higher costs. Therefore, the service factor is a key parameter for ensuring both high efficiency and economic operation of air compressors.

By properly applying and utilizing the motor's service factor, equipment such as air compressors can be kept running within reasonable and specified limits. Otherwise, the motor's initial design would inevitably deviate from actual load conditions—either failing to achieve its full performance or being unable to sustain continuous full-load operation.