Small refrigerators such as household refrigerators and freezers are energy-consuming households in the civilian sector. Most of the energy is consumed in refrigeration compressors. Therefore, improving compressor efficiency is the most important technical means to achieve energy saving in small refrigeration units.
Linear compressors are considered to be one of the main development directions of small-scale refrigeration compressor energy-saving technologies because of their high efficiency, small size, compact structure and convenient control.
For the specific use environment of small refrigeration equipment, the applicable linear compressor should have the characteristics of small size, simple structure, high efficiency, high reliability, easy installation, convenient control and low cost. Based on these requirements, this paper compares and analyzes the structural design of small linear refrigeration compressors.
1 Linear motor drive analysis Linear motor drive is the most important part of linear compressor. In theory, any linear motor can be the driver for a linear compressor. The characteristics of various linear motors are different, and it is necessary to select the type that best suits the refrigerator.
Linear motors can be divided into linear motors and linear actuators according to their working principles and usage angles. The linear synchronous oscillation motor can automatically generate high-frequency reciprocating linear motion by using electromagnetic force and spring resonance principle, which can directly push the reciprocating motion of the mover, has large output, low loss and high efficiency, and most meets the requirements of the linear compressor driver. Linear synchronous motor oscillation name and works are common and linear synchronous motor relatively similar, but the structure is simpler than linear synchronous motor, linear synchronous motor has performance advantages while avoiding Fund Project: 863 project (2007AA05Z258) Author: Zhang Liqin (1984-), male, doctoral student.
The linear synchronous motor has the disadvantages of complicated structure, and it is convenient to adjust the displacement by adjusting the voltage of the motor terminal during operation, which is convenient for control. In summary, the linear synchronous oscillator motor is ideal as a linear compressor driver for the operation requirements of small linear refrigerant compressor drives.
According to the type of movable body, linear synchronous oscillation motors can be divided into three types, namely, moving iron type, moving coil type and moving magnetic type, or can be divided into moving iron type, moving coil type, moving magnetic type and moving magnet type. Four types.
It is a schematic diagram of the structure of four types of motors.
The mover of the moving iron linear motor uses a core material, and the magnetic field is generated by the exciting coil. Such motors can generate large driving forces and compression ratios, but multiphase windings must be used to produce reciprocating linear motion, which is bulky and the motion of the mover in the air gap is unstable. The moving coil type is composed of a coil while being energized by another coil or provided by a fixed permanent magnet member. The advantage of this kind of motor is that the piston stroke control is easy, there is no radial force and torque on the mover, there is no axial force when there is no load, and there is no hysteresis loss. However, the driving force is relatively small, and the lead problem is not easy to solve. The moving magnetic mover is composed of a permanent magnet material, and the excitation is performed by the coil. Some studies have shown that the utilization rate and the mass of the mover are reduced, which is beneficial to the design of the corresponding resonant spring. The thrust is larger and the efficiency is higher. However, the principle is relatively complicated. The system design needs to consider the nonlinear magnetic conductance and hysteresis loss of the permanent magnet. The influence of factors such as eddy current loss. The main difference between the moving magnetic type and the moving magnet type is that the former mover is a simple permanent magnet or a permanent magnet plus a non-conductive bracket. The latter mover is a permanent magnet plus a moving iron core, but the basic working principle is the same, so sometimes Divide these two structures into one. A common feature of the moving coil type and the moving magnet type is that the winding coil can be driven by a single phase to reciprocate the moving motion of the mover, and thus can be greatly reduced in size. Since the moving magnet type has a central axis of magnetic conduction, it is also necessary to use a multi-phase winding to function. From this point of view, it is similar to the moving iron type.
(c) Dynamic magnetic (d) moving magnet type four synchronous linear oscillation motor structure diagram In different working environments, the appropriate motor form can be selected according to needs. Therefore, according to the application requirements of the small refrigeration device and the advantages and disadvantages of the above four linear synchronous oscillation motors, the moving magnetic linear oscillation motor is selected as the driver.
2 Piston Layout Structure Analysis Due to the inherent characteristics of the linear compressor linear actuator, that is, the reciprocating oscillating motion in the linear direction, and the motion state on both sides is completely symmetrical, the linear compressor can easily realize the double cylinder opposing and the double side piston compression. Therefore, whether to choose single-side piston compression or double-side piston compression is a problem to be considered in the design of linear compressors.
A comparison of the two layouts is shown. The advantage of single-sided compression is that the structure is simpler, and the double-sided compression requires a larger size, which is disadvantageous for installation in a narrow space, and the intake and exhaust system is correspondingly complicated. However, the double-sided pistons run more smoothly, because if it is unilaterally compressed, the vibration is affected by the gas force and the other half is not subjected to the gas force during half of the full stroke, and the two sides are affected. The structure of the layout can eliminate this adverse effect, and the effect of reducing the instability and noise caused by vibration is very obvious (see). In addition, the double-sided compression can lower the frequency under the requirement of the same exhaust volume, and because the length of the mover is relatively long, it is also easier to use the bearing to position the mover.
The choice of which layout to use is closely related to which type of linear drive is used. In the above analysis of various linear oscillating actuators, it is mentioned that some of the driver designs use a moving iron or moving magnet design, in which case the compressor has a solid shaft and is more suitable for use on both sides. Layout; if the meter uses a moving coil or moving magnetic single-phase linear oscillator motor, and the double-sided layout will completely offset the size and structure advantages of the single-phase motor, so use a single-sided layout at this time. More optimized.
The above theoretically compares the advantages and disadvantages of single and double side compression. However, if the compressor power is small and there is no particularly strict restriction on the installation space, the difference between the single and double side layout effects on the performance of the whole machine is not particularly obvious. Therefore, a scheme in which two structures are simultaneously fabricated is used, and a comparative study is conducted.
3 gas path structure analysis In the traditional compressor gas path structure, the suction and exhaust valve is located on the same side of the cylinder, and the valve plates of different structures are combined to form a passage for suction and exhaust. However, this design is not suitable for the line and the compressor, because in this structure, the suction and exhaust valve piece is hidden inside the entire valve plate, the valve plate itself does not have elasticity, and the piston of the ordinary rotary compressor is stroked. Due to the strict restriction of the transmission mechanism, the collision cylinder does not occur; and the piston stroke of the linear compressor is not limited by rigidity, so once the piston moves beyond the expected stroke, the valve plate will collide rigidly, causing strong noise and even zero. Parts are damaged. In order to avoid the above problem, one solution is to design a disc spring support, an exhaust valve body made of organic material, so that the entire exhaust valve portion is elastic, thereby avoiding a rigid collision and at the same time having more than a reed valve. With a large flow area, this valve minimizes over-compression losses. In addition, the suction valve is transferred to the top of the hollow piston so that both the suction valve and the suction passage are in the piston, so that the flow resistance and the suction heating loss are minimized. This airway layout has certain advantages, but only if the structure does not have a solid shaft. Therefore, it is also possible to further change the spring into an elastic hollow member, such as a bellows, etc., and at the same time, the suction and exhaust valve body does not have a rigid collision between the suction and exhaust valve body and the piston. This method has applied for a patent for invention. See the two suction and exhaust structures.
The choice of the suction and exhaust structure is also related to which type of linear drive is used. If the dynamic magnet type is used, it is difficult to use the hollow piston air intake in the presence of a solid shaft; correspondingly, the moving coil type or the moving magnetic type does not have this problem, but instead uses a hollow piston air intake to the structure. Simplification is also more beneficial. It is planned to make these two gas path structures at the same time and carry out comparative analysis.
4 Sealing Lubrication Structure Scheme Analysis Traditionally, the piston chamber seal is sealed by a piston ring and a lubricating oil film. In addition, the part of the outer surface of the piston that is in contact with the inner wall of the piston chamber may be coated with a coating material (such as Teflon, etc.), and the effect is outstanding, and is more suitable for the gas path structure scheme of inhaling the piston head. There is also a special magnetic fluid material that has also been used for the sealing of the piston chamber of the compressor. From the perspective of the gas path structure, the coating scheme is more suitable.
In theory, the linear motor-driven linear compressor can completely achieve oil-free lubrication because it eliminates the inefficient mechanical transmission mechanism and the direct contact between the mover and the stator, but this requires extremely high assembly accuracy. Guaranteed, and because of the increasing use of permanent magnet components in linear motors, the lateral forces acting on the mover shaft generally affect the performance of linear motors, so small linear compressors still require oil lubrication. However, due to the withdrawal of the rotary motion mechanism, it is necessary to research and develop a separate oil supply device. The oil supply device has essentially the same function as a small pump, so it can be designed using the principles of various types of pumps. For example, a plunger type oil supply device or an oil supply device using a diaphragm pump principle, see.
The above two oil supply device schemes will also be experimentally verified separately.
Diaphragm pump type oil supply device structure schematic resonance component scheme analysis The current resonant components in the linear compressor linear motor driver are mainly two kinds of column springs and leaf springs. In the linear oscillating motor, the resonating member is used to limit the adjustment of the movement of the mover on the one hand, and to adjust the elastic coefficient of the entire vibration system on the other hand. Column springs are easier to design, calculate, and produce than plate springs, and have a wide range of applications.
Therefore, column springs have certain advantages in terms of process and cost. However, the leaf spring is small in size, and the axial stiffness and radial stiffness are superior to the column spring. Studies have shown that the use of leaf springs can greatly improve the performance and stability of the compressor. Want to use the leaf spring, the current problem is that the plate spring is used in a narrow range, different motors have to redesign the spring shape, and the design workload is very large; the more prominent contradiction is as a sensitive technology, developed countries The design method and performance analysis method of the plate flexible spring that it masters are strictly blocked by technology, so the application is difficult. It is therefore planned to use a suitable column spring as the resonating part of the compressor.
6 Conclusion According to the requirements of the refrigerator, after comparing the linear compressor drive of the linear compressor and the different schemes of the piston layout, gas path, sealing, lubrication and resonance components, it is considered that the linear compressor driven by the moving magnetic linear synchronous oscillation motor is reasonable. The design can achieve a more optimized effect between efficiency and size cost, so this type of motor is chosen as the drive for the linear compressor, with a surface coated piston and a column spring as a resonating member. On this basis, two sets of piston layout, gas path structure and lubrication scheme were determined, and experimental research was carried out separately.
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