The motor service factor represents the maximum load rate at which a motor can operate continuously for extended periods. It is a parameter that measures a motor's ability to handle continuous loads. It is an overload rate reserved for special applications or duty cycles, and is numerically the ratio of the maximum allowable output power to the rated power. The duty cycle refers to the operating mode and should not be confused with the capacity factor.
For example, a 30kW motor with a service factor of 1.15 has a maximum actual output power of 30 × 1.15 = 34.5kW .
Today, Ms. Can will explain the working system and service factor, and interpret the service factor through some examples, aiming to guide the design and rational use of motors.
Common operating modes for electric motors include S1 continuous duty, S2 short-time duty, and S3-S7 periodic duty. Less common modes include S8 continuous periodic duty with variable speed and load, and S9 non-periodic duty with varying load and speed. When the operating mode does not match any of the standard S1-S9 duty modes, and continuous operation under varying load is required, motor performance can be constrained and characterized using combinations such as "S1 continuous duty + service factor." Although the combination of "duty mode + service factor" is indeed a duty mode, the service factor itself cannot be considered a duty mode.
The service factor is a comprehensive indicator, numerically equal to the maximum overload rate. Based on the operating characteristics of three-phase asynchronous motors, the service factor is generally specified as 1.15-1.25 . Using a service factor that is too low offers no substantial practical benefit; while a service factor that is too high results in unsatisfactory motor operating economy.
Taking a screw air compressor as an example, the load on the motor changes periodically, depending on the user's demand for air volume. When the maximum working pressure set by the user is reached, the air compressor begins to unload. When the working pressure of the pipeline drops to the minimum working pressure set by the user, the air compressor automatically loads.
The selection of motor power aims to maximize the motor's efficiency and power factor when operating under the expected load. However, the motor's shaft power must also be capable of operating at full load, adapting to the customer's actual operating conditions of prolonged overload operation. Typically, the motor power selected for full-load operation is 1.1 times the expected economical operating shaft power.
If the motor power is selected directly using 1.1 times the expected economic operating shaft power, the "service factor" we mentioned is lost. The consequence is that the motor's efficiency and power factor are relatively low, resulting in waste of energy and costs. Therefore, the "service factor" is actually a crucial parameter for ensuring the efficient and economical operation of the air compressor.
Table 1 shows the measured parameters of a 2-pole 15kW motor with IP23 and a service factor of 1.15 , compiled by Ms. Can.
The data in the table shows a significant increase in stator winding temperature, bearing temperature, and casing temperature. Some motor manufacturers only conduct type tests according to standard motors, which fails to accurately determine the motor's performance under other service factor conditions.
In short, by making good use of the motor's "service factor," air compressor equipment can be controlled to operate within a specified and reasonable range. Otherwise, the initial design of the motor will inevitably deviate from the actual load conditions, and the motor's efficiency may not be fully utilized or it may be unable to withstand long-term continuous operation at "full load."
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