Brief Discussion on Service Coefficient of Air Compressor Related Motor

Brief Discussion on Service Coefficient of Air Compressor Related Motor

The term motor service factor comes from the "MG1-2009 Motor and Generator Part" standard compiled and published by the National Electrical Manufacturers Association (NEMA), commonly known as the NEMA MG1 standard. This standard elaborates on the service factor (Service Factor, or S.F for short) of AC motors and the application of service factor to AC motors. The main content is: The service factor of AC motors is a multiplier. When it is multiplied by the rated power, it represents the allowable power load under the conditions specified by the service factor.

　　 Under normal use conditions, continuous operation at rated load. When the voltage and frequency maintain the values ​​specified on the nameplate, the motor can be overloaded to the rated power multiplied by the service factor on the nameplate. When the motor runs under any service factor greater than 1, its efficiency, power factor and speed can be different from those under rated load conditions, but the locked-rotor torque, current and maximum torque will remain unchanged. When the motor runs continuously under any service factor greater than 1, the expected service life will be reduced. Compared with running under rated nameplate power, insulation life and bearing life will be reduced.

In addition, the temperature rise value of motors with a service factor of 1.15 and above is given when operating under a load of the service factor: 70℃ for insulation class A; 90℃ for insulation class B; insulation class F It is 115°C for class F, and it is pointed out that special consideration should be given to bearing temperature and lubrication if F class insulation is used. The above is the motor service factor mentioned in the NEMA standard.

　　 In layman's terms, the motor service factor represents the maximum load rate that allows the motor to work continuously for a long time, and is a parameter to measure the continuous load capacity of the motor. It is to meet the overload rate of special application occasions or working mode standby. The value is the ratio of the maximum allowable output power to the rated power. The working mode is the operating mode, which should not be confused with the capacity coefficient.

　　 Such as: a motor with a power of 30kW, if the service factor is 1.15, then its actual maximum output power is 30×1.15=34.5kW.

　　 Today, we discuss the work system and service coefficients, and interpret the service coefficients through some examples, aiming to guide motor design and rational use of motors.

　　 Air compressor motor service coefficient example interpretation

　　The common working modes of motors include S1 continuous duty system, S2 short-time duty system, S3～S7 cycle duty system. Uncommon ones include S8 variable speed variable load continuous cycle duty system and S9 load and speed non-periodic change duty system. When the operation mode is not consistent with any of the standard modes S1～S9 working system, if the motor is required to run continuously under variable load, the combination of "S1 continuous working system + service coefficient" can be used to restrict and characterize the motor performance. Although the "work system + service coefficient" combination is actually a work system, it cannot be said that the service coefficient is a work system.

　　 The service factor is a comprehensive index, which is equal to the maximum overload rate in value. According to the operating characteristics of the three-phase asynchronous motor, the service coefficient is generally specified as 1.15 to 1.25, because the low service coefficient is used, and there is no substantial application efficiency; the service coefficient is high, and the motor operation economy is not ideal.

Take the screw air compressor as an example. The motor load changes periodically. It changes with the user’s demand for air volume. When the maximum working pressure set by the user is reached, the air compressor begins to unload. The air compressor automatically loads when the working pressure drops to the lowest working pressure set by the user.

　　The power selection of the motor is to make the motor generally work under the expected load with the highest efficiency and higher power factor. However, the shaft power of the motor must be able to run at full load at the same time, that is, to adapt to the actual working conditions of the customer's long-term overload operation. Generally, the selection of motor power at full load operation is 1.1 times the expected economic operating shaft power.

　　 If you directly use 1.1 times the expected economic operating shaft power to select the motor power, there will be no "service coefficient" as we call it. The consequence is that the efficiency and power factor of the motor are relatively low, resulting in waste of energy and costs. Therefore, the "service factor" is actually an important parameter to ensure the efficient and economic operation of the air compressor.

Table 1 is the measured parameters of a 2-pole 15kW, IP23, service factor 1.15 motor. 图 From the data in the table, it can be seen that the temperature rise of the stator winding, the bearing temperature, and the casing temperature have all increased significantly. Some motor manufacturers only carry out tests according to standard motors when doing type tests, and cannot accurately determine the performance indicators of the motors under other service factors.

　　The hidden rules of "motor service coefficient" in the air compressor industry

At present, the so-called "motor service coefficient" in the domestic air compressor industry has actually completely deviated from the NEMA guidelines. For example, it was originally a motor with a rated power of 110kW. The motor nameplate is written as a rated power of 90kW, and the SF mark is 1.2 or 1.25. . Those outside the industry may not be able to figure out, wouldn’t it be a loss to sell high-power motors at low power? In fact, from the paper, 110kW is 20kW larger than 90kW. When the unit input specific power is the same (the same is level 1 energy efficiency), of course, the 90kW air compressor is energy-saving.

The GB19153-2009 "Limited Values of Energy Efficiency and Energy Efficiency Grades for Positive Displacement Air Compressors", which will be formally implemented on July 1 this year, stipulates that the input power test value of rotary air compressor units should be less than the rated power of the drive motor. value. For example, the rated power of the motor is 90kW. When it is running under the rated exhaust pressure of the air compressor, the input power of the unit must not exceed 110kW. This prevents the vicious expansion of the motor service coefficient from the source. At the same time, in order to obtain a smaller unit input power (for air-cooled air compressors, cooling fan motors should also be considered), high-efficiency motors are required. Especially the variable frequency air compressor, in order to meet the volume flow rate of 40% to obtain better energy efficiency indicators, that is to say, the motor efficiency is still high at low speeds, which requires the use of permanent magnet synchronous motors or motors with the same efficiency .

　　 In short, only by making good use of and using the "service factor" of the motor can the air compressor equipment be controlled to operate within a prescribed and reasonable range. Otherwise, the initial design of the motor will inevitably deviate from the actual load conditions, and the motor performance may not be fully utilized or not capable of "full load" long-term continuous operation.