Service Factor for Air Compressor Motors

Currently, there exists an industry term for the existence of a service factor (SF) concept for electric motors throughout the PM VSD screw air compressor industry. China’s motor IEC standard does not have the concept of “service factor”, only this statement called rated power, the rated power is greater than the shaft power, and leaves a certain margin. The “service factor” for electric motors is derived from the National Electric Motor Association (NEMA) MG1 standard. This standard interprets the service factor as a multiplier for AC motors. When multiplied by the rated power, it indicates the power load permitted to be carried under the conditions specified in the service factor. Under normal conditions of use, the motor is operated continuously at the rated load. When voltage and frequency are maintained at the values specified on the nameplate, the motor may be overloaded to the power value of the rated power multiplied by the service factor on the nameplate. Motors operated at any service factor greater than 1 may have efficiencies, power factors, and speeds different from the values at rated load conditions, but plugging torque, current, and maximum torque will remain unchanged. Continuous operation of the motor at any service factor greater than 1 will reduce the expected service life. Insulation life and bearing life will be reduced compared to operation at rated nameplate power.

1. Significance of the existence of SF

Screw compressors generally have two designs and operating modes: equal power and equal exhaust volume. Generally, the screw compressors used in power plants are those with equal displacement design. The design scheme of equal exhaust volume means that in the same type of the air end, in the case that the exhaust volume is basically kept constant, i.e., the speed of the air end is kept constant, with the increase or decrease of the exhaust pressure, only the power of the electric motor increases or decreases accordingly. In other words, in the same type of air compressor mainframe, manufacturers have to use electric motors of different powers to match the user’s demand for the variation of maximum exhaust pressure. However, the subsequent difficulty for air compressor manufacturers is the need to stock many motors of different powers, which increases procurement and management costs. The concept of service factor proposed by the U.S. NEMA precisely solves this problem for manufacturers. Its introduction can reduce the types of motors, thus reducing the cost of procurement and management and increasing the competitive advantage of air compressor manufacturers. But, on the other hand, it takes a loss of user interest because the user spends the same amount of money but buys a smaller motor.

The size of the motor load varies periodically, it varies with the user’s demand for air, and when it reaches the maximum working pressure set by the user, the air compressor begins to unload. Usually, the motor power after unloading is 30% to 45% of the full load power. When the working pressure of the pipe network drops to the minimum working pressure set by the user, the air compressor is automatically loaded. In fact, the time when it is really at full load (near maximum operating pressure) is very short. For the selection of the motor power, it is ideal to make the motor work at the highest efficiency and with a large power factor. For the above project, if the service factor is not taken into account, the manufacturer should configure the motor with a rated power of at least 272.8kW. This will not pose any problem from the configuration point of view, but the reality is that the specificity of the compressor’s operation mode results in the allocated motor operating at low efficiency and power factor for a long period of time, resulting in a waste of energy and cost.

2. The use of SF

Take a foreign project as an example to explain the concept and use of service factor. The suction volume required by the user of the instrument air compressor for this project is ≥40m³/min, and the exhaust pressure is 0.8MPa. According to this requirement, the air compressor manufacturer selected the instrument air compressor model SM250, with suction volume of 41.5m³/min, exhaust pressure of 0.8MPa, rated motor power of 250kW, and service coefficient of 1.15 for the motor. The data from the manufacturer shows that the compressor shaft power is 272.8 kW. Without considering the margin, the motor rated power should be at least greater than 272.8 kW. However, after considering the transmission efficiency of the shaft, the aging of the equipment after long-term operation and other factors, in the selection of the rated power of the motor, you need to add some margins on the basis of the shaft power, generally ranging from 5% to 15%. However, the motor configured by the manufacturer has a rated power of 250kW, which is less than the shaft power of 272.8kW. Therefore, numerically speaking, the motor power configured by the manufacturer is small. The manufacturer’s explanation is that since the motor has a service factor, 250kW x 1.15 = 287.5kW > 272.8kW, which can meet the requirements.

3. Advantages and disadvantages arising from the citation of service factors

Air compressors in power plants run continuously for long periods of time, and the operating cost (power consumption) over the life of the plant is relatively large compared to the cost of the air compressor itself. The quoted service factor can be better adapted to the special operation mode of the air compressor, so that the motor works at relatively high efficiency and high power factor, which saves electricity well and reduces the operating cost of the power plant.

The invocation of the service factor also brings certain disadvantages. First of all, it creates a situation in which “a little goes a long way”. The rated power of the motor is less than the shaft power. If the motor is operated under rated conditions for a long period of time, the motor will be overloaded for a long period of time at a higher power than the rated power, resulting in the actual service life of the motor being shorter than the expected service life, which cannot reach the service life guaranteed by the manufacturer. Secondly, for the whole plant, the actual plant power consumption rate is higher than the design value. The design value of the plant power consumption rate is calculated based on the rated power of the motor, but the actual operation of the motor power is greater than the rated power. In the end, the user spends the same amount of money but the manufacturer configures a smaller motor, which will be difficult for the user to accept.

4. Response strategy

We may not be able to eliminate the references of motor service factor in a short period of time. Thinking from the user’s point of view, we suggest that some special requirements can be added to the contract to bind the manufacturer to ensure the rated output of the air compressor and the life of the motor. Specifically, we can ask the manufacturer to provide the actual shaft power of the air compressor, and to ensure that the nameplate power of the motor is greater than the shaft power, the manufacturer needs to ensure that with the nameplate power and the power multiplied by the service factor, the motor of the air compressor can satisfy the class F insulation and class B temperature rise, and that the actual current value does not exceed the rated current value. In addition, we recommend that when purchasing an air compressor we should require the manufacturer to reflect at least the rated power, service factor, air compressor shaft power and other parameters on the motor nameplate, and ensure the accuracy of the parameters, thus facilitating an accurate understanding of the motor parameters. Read new blog.