How to make a good air compressor motor? Triggering deep reflection in the motor industry
The demand for motors in the air compressor market is relatively high, but the requirements for motors in air compressors are also high. Industry experts in the motor industry will discuss with everyone about air compressors designed for displacement and some special requirements for motors in air compressors.
Screw air compressor designed with equal displacement
A screw air compressor designed with equal displacement refers to a type of air compressor where the displacement remains relatively constant within the same main engine, i.e., the main engine speed remains constant (the same as the asynchronous motor speed). As the exhaust pressure increases or decreases, the motor power can only increase or decrease accordingly.
Usually, the main engine of an air compressor with equal displacement is slightly larger than that designed with equal power.
According to the working principle of the air compressor in this scheme, it means that within the same type of air compressor host, we need to use motors of different powers to match the user's demand for the highest exhaust pressure change. If the air compressor is designed according to this plan, the production enterprise will need to reserve many motors of different powers.
To put it another way, if we focus on the motor, that is to say, increasing the actual power of the motor to meet the needs of different exhaust pressures. In order to make the rated power of the motor the same as the rated power of the motor in the equal power design scheme, the only solution is to increase the service factor, which is currently a "hidden rule" in the screw air compressor industry.
From the perspective of screw air compressor production enterprises, considering only the motor itself, its cost has not decreased. What has decreased is the procurement and management cost (the motor specifications have been reduced), which is a temporary solution in today's low-cost competition.
Due to the absence of gear transmission, an elastic coupling is used to connect the input end of the air compressor host to the output shaft of the motor, which eliminates the problems caused by gear transmission and reduces costs. This design scheme is known as direct coupling in today's air compressor industry.
The biggest drawback of the current design scheme used in the air compressor industry is the chaotic power of the motor used in the air compressor. However, it is difficult for users to know the actual motor power and operating current of the air compressor before purchasing, which leads to insufficient capacity of the switchgear or other power supply equipment matched for the air compressor in the future, laying a great hidden danger for the safe operation of the air compressor in the future.
Requirements for motor performance in different designs
The comparison and corresponding working principles of the two designs of equal power and equal displacement can reveal a hint of the "service coefficient" of the screw air compressor. Next, let's talk about the screw air compressor and its operation mode.
Air compressors are manufacturers of compressed air, but compressed air cannot be observed for its speed like water in rivers. Compressed air reflects changes in air volume through changes in pipeline pressure. That is to say, pressure changes are a reflection of changes in gas supply and demand, pressure changes are phenomena, and gas volume changes are essence.
For air compressors, the size of the motor load varies periodically with the user's demand for air volume. When the maximum working pressure set by the user is reached, the compressor begins to unload. Typically, the motor power of a regular fixed screw air compressor after unloading is 30% -45% of its full load power. When the working pressure of the pipeline network drops to the minimum working pressure set by the user, the compressor automatically loads, and the time it truly operates at full load (close to the maximum working pressure) is very short.
In China, screw air compressors have gradually developed into the main models of the air compressor industry since the early 1990s. Their intelligent control level has greatly improved with the development of the computer and electronics industries. Due to their good intelligent control performance, they operate more safely and steadily.
In summary, regardless of which design scheme is adopted and its operating mode, for the power selection of the motor, it is the most ideal to make the motor work at the highest efficiency and high power factor. The capacity rating of a motor is a set of discrete and discontinuous values, and the motor load we determine is between these discrete values.
In the mechanical design manual, the selection of motor power is usually based on multiplying the shaft power by 1.1 times, and the rated power of the motor is not less than the product of the above two items. Using this method to select motor power does not have the so-called "service factor", but when applied to the selection of motors for screw air compressors, the consequence is that the efficiency and power factor of the motor are relatively low, resulting in energy and cost waste.
How to utilize and use the "service coefficient" of the electric motor well, and control its operation within the prescribed and reasonable range, is an urgent problem that needs to be solved in the screw air compressor industry.
At the same time, a series of problems caused by the imbalance of axial and radial forces transmitted to the rotor by the inherent compressed gas of the twin-screw air compressor host, are transmitted to some extent to the motor bearings through gearless transmission, resulting in some motor bearings frequently running at high temperatures and being damaged. This is also one of the main reasons why the motor required to drive the main engine of the twin-screw air compressor is not fully matched with the motor produced by the motor manufacturer. But we believe that with the continuous summarization of experience and lessons learned by host manufacturers and motor manufacturers, the problem will be solved.
