The working mode and service factor of the motor

The motor service factor indicates the maximum load rate that the motor can work continuously for a long time, and it is a parameter to measure the continuous load working ability of the motor. It is the overload rate reserved for special applications or working modes, and its value is the ratio of the maximum allowable output power to the rated power. The working mode is the operating mode, and should not be confused with the capacity factor. For example, for a 30KW motor with a service factor of 1.15, its actual maximum output power is 30×1.15=34.5KW.

Today, we will review the working mode and service factor, and interpret the service factor through some examples, aiming to guide motor design and rational use of motors.

Common working modes of motors include S1 continuous working mode, S2 short-time working mode, S3～S7 cyclic working mode, and uncommon ones include S8 variable speed and load continuous cyclic working mode and S9 non-periodic variation of load and speed working mode. When the operating mode does not match any of the standard modes S1～S9, such as requiring the motor to run continuously under variable load, a combination of "S1 continuous working mode + service factor" can be used to constrain and characterize the motor performance.

The service factor is a comprehensive indicator, and its value is equal to the maximum overload rate. According to the operating characteristics of three-phase asynchronous motors, the service factor is generally specified as 1.15-1.25, because using a lower service factor does not have substantial application efficiency, and using a higher service factor will result in less than ideal operating economy for the motor.

Taking a screw air compressor as an example, the load of the motor varies periodically, depending on the user's demand for air volume. When the air compressor reaches the highest working pressure set by the user, it starts unloading, and when the working pressure of the pipeline drops to the lowest working pressure set by the user, the air compressor loads automatically. 

The selection of the motor power is to ensure that the motor operates with high efficiency and power factor under the expected load, while also being able to run at full load for a long time to adapt to the actual working conditions of long-term overload by the customer. Usually, when the motor is running at full load, the selected motor power is the expected economic operating shaft power multiplied by 1.1.

If the motor power is selected directly as 1.1 times the expected economic operating shaft power without the "service factor", the consequence is that the motor's efficiency and power factor will be relatively low, resulting in energy and cost waste. Therefore, the "service factor" is actually an important parameter to ensure the high-efficiency and economic operation of air compressors.

In conclusion, by utilizing and properly using the "service factor" of the motor, air compressor equipment can be controlled to operate within the specified and reasonable range. Otherwise, the initial design of the motor will inevitably deviate from the actual load conditions, and the motor efficiency may not be fully utilized or may not be able to handle long-term continuous operation at "full load".