The compressor market has a relatively large demand for motors, but air compressors also have high requirements for motors. Previously, we talked about screw air compressors with equal power design schemes. Today, Ms. Can will talk to you about air compressors with equal displacement design and some special requirements of air compressors for motors.
Screw air compressor with equal displacement design
A screw air compressor with constant discharge capacity design means that, within the same type of air compressor, the discharge capacity remains basically constant, that is, the speed of the air compressor main unit remains constant (the same as the speed of the asynchronous motor). As the discharge pressure increases or decreases, only the motor power increases or decreases accordingly.
Typically, the main unit of an air compressor with the same displacement is slightly larger than that of an air compressor designed for the same power.
Based on the working principle of this air compressor design, it means that within the same type of air compressor main unit, we need to use motors of different power to match the user's demand for varying maximum discharge pressure. If air compressors are designed according to this scheme, manufacturers will need to stock a large number of motors of different power.
Alternatively, one could work on the motor itself, increasing its actual power to meet varying exhaust pressure requirements. To ensure the motor's rated power matches that of a design with equivalent power, the only solution is to increase the service factor – a common practice in the screw air compressor industry.
From the perspective of screw air compressor manufacturers, considering only the motor itself, its cost has not decreased; what has decreased is the cost of procurement and management (due to the reduction in motor specifications). In today's low-cost competition environment, this is a stopgap measure.
Because there is no gear drive, the input end of the air compressor main unit and the output shaft of the motor are connected by a flexible coupling. This eliminates the problems caused by gear drive and reduces costs. This design is called a direct-drive air compressor in the current air compressor industry.
The biggest drawback of the current design scheme used in the air compressor industry is the resulting chaos in the power output of air compressor motors. Users often cannot know the actual motor power or operating current before purchasing, leading to the selection of undersized switchgear or other power supply equipment to match the air compressor. This creates significant hidden dangers for the future safe operation of the air compressor.
Different designs have different requirements for motor performance.
By comparing the two designs with equal power and equal displacement, and their corresponding working principles, we can get a glimpse of the "service factor" of screw air compressors. Next, we will talk about screw air compressors and their operating methods.
An air compressor is the producer of compressed air, but unlike water in a river, we cannot observe the flow rate of compressed air. Instead, the volume of compressed air is reflected by changes in pipeline pressure. In other words, pressure changes reflect changes in air supply and demand; pressure changes are the phenomenon, while volume changes are the essence.
For air compressors, the motor load 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. Typically, the motor power of a conventional stationary screw air compressor after unloading is 30%-45% of its full-load power. When the working pressure of the pipeline drops to the minimum working pressure set by the user, the air compressor automatically loads. The time it is truly operating at full load (close to the maximum working pressure) is very short.
In my country, screw air compressors have gradually become the main type of air compressor in the industry since the early 1990s. Their intelligent control level has been greatly improved with the development of the computer and electronics industries. Due to their better intelligent control performance, they are safer and more stable in operation.
In summary, regardless of the design scheme or operating mode, the ideal approach to motor power selection is to ensure the motor operates at its highest efficiency and with a high power factor. Motor capacity ratings are discrete and discontinuous values, while the motor load we determine falls between these discrete values. Mechanical design manuals typically select motor power by multiplying the shaft power by 1.1, and the motor's rated power must not be less than the product of these two factors. While this method doesn't provide a "service factor," its application to screw air compressor motors results in relatively low efficiency and power factor, leading to energy and cost waste.
How to effectively utilize and use the "service factor" of electric motors and control their operation within a specified and reasonable range is a problem that the screw air compressor industry urgently needs to solve.
Meanwhile, the inherent imbalance between the axial and radial forces transmitted from the compressed gas to the rotor in a twin-screw air compressor leads to a series of problems. With gearless transmission, these problems are somewhat transmitted to the motor bearings, causing some motor bearings to frequently overheat and fail. This is also one of the main reasons why the motors selected to drive twin-screw air compressors are not perfectly matched with those produced by the motor manufacturers. However, we believe that as both compressor and motor manufacturers continue to learn from their experiences, these problems will be resolved.
