Experimental study on feasibility of single side spring linear oil free silent air compressor

The feasibility of a single spring linear oil-free silent air compressor is a schematic diagram of the principle of a linear compressor with a single spring. When the oil-free silent air compressor is at rest, the initial position of the piston is arranged at the upper dead center, and the spring connected with the piston is in a free length state. During operation, under the action of gas load, the piston is offset from the initial position (upper dead center) to the center of motion, and runs reciprocally between the upper dead center and the lower dead center.

For a single-side spring linear oil-free silent air compressor, when the design stroke amplitude of the linear compressor is determined to be X, the distance between the initial position of the piston and the center of motion 4X is equal to the design stroke amplitude of the piston, that is, 4X=X, and the offset is generated by the action of pneumatic physical load.

2.2 Design and development of the prototype According to the design exhaust volume and design conditions of the linear oil-free silent air compressor, the piston stroke and diameter were optimized to determine the design stroke, the stiffness of the resonant spring and the mass of the moving parts were determined according to the results of the aerodynamic force linearization under the design conditions, and then the linear compressor structure was designed.

Structural diagram of a prototype of a single-side spring linear compressor developed for design. The compressor prototype mainly includes: linear motor, cylinder, piston, resonant spring, suction valve, exhaust valve and other components composed of inner stator, outer stator, permanent magnet actuator and excitation coil. The resonant spring is arranged between the motor and the compressor body by a cylindrical helical compression spring.

The prototype of moving magnetic single-side spring linear compressor is designed and developed. The main parameters are shown in Table 1. The feasibility verification of the prototype shows that under the condition of constant frequency and constant exhaust pressure, within a certain voltage range, the distance from the center of motion to the top dead center of the single-side spring linear compressor remains constant with the increase of voltage. This is different from the distance (4X) from the center of motion to the top dead center of the single-side spring linear compressor in the previous structural form analysis, which is generated by the action of pneumatic force load. That is, the exhaust pressure is unchanged, and the distance from the center of motion to the top dead center (4X) is also unchanged. When the voltage continues to increase, the compressor's moving center to the top dead center distance (4X) will surge, this is because the severe cylinder impact caused the compressor stroke asymmetry, so that the moving center to the top dead center distance (4X) there is a large increase.

In the air compression test of constant exhaust pressure, it is found that when the exhaust pressure is lower than the design exhaust pressure, the compressor stroke does not reach the design stroke, the phenomenon of cylinder collision will occur. As shown in (b), before the occurrence of serious cylinder collision phenomenon, under the same exhaust pressure, the compressor stroke presents a linear increase with the increase of voltage. In this linear zone, there will be a slight cylinder collision phenomenon, but because we adopt the exhaust structure design similar to the bacteria valve, even if there is a slight cylinder collision, it will not cause the compressor operating parameters to be disturbed; Under different exhaust pressure, the higher the exhaust pressure value, the greater the voltage required to reach the upper dead center position.

Therefore, in the control of single-side spring linear oil-free silent air compressor, the voltage value can be adjusted to make the piston reach the upper dead point in the process of movement, and ensure that the compressor does not impact the cylinder.

(b) and (c) respectively show that in the case of fixed frequency and fixed exhaust pressure, within a certain voltage region, the compressor stroke and power increase linearly with the increase of voltage, when the voltage exceeds this region, the piston stroke and power have a surge phenomenon, which we believe is a serious compressor cylinder collision phenomenon. The operation parameters of the compressor are unstable.

3.3 For air compression with different resonant spring stiffness, under the same exhaust pressure, the linear compressor with four sets of springs as the resonant spring group has a smaller stroke than the compressor with two sets of springs, that is, the larger the stiffness of the metal resonant spring of the single-sided linear compressor, the smaller its stroke; When the compressor reaches the same stroke, the corresponding exhaust pressure of the compressor with large metal resonance spring stiffness will be higher. This is consistent with the analysis of the structural form of the prototype that the offset (distance from the center of motion to the top dead center) of the single-side spring linear compressor is generated by the action of the air force load.

As shown in (b), under a certain valve opening, the exhaust pressure of the single-side spring linear compressor increases with the increase of voltage.

As shown in (c), the power consumption per unit stroke of the single-side spring linear compressor will obviously increase first, then decrease and then increase with the voltage change. Because when setting the valve opening, with the increase of voltage, the compressor exhaust pressure increases, and the corresponding stroke increases, resulting in an increase in the amount of compressed air and an increase in copper loss. These factors increase the power consumption of compressed gas per unit stroke. When the voltage continues to increase, the resonance unit stiffness composed of the equivalent stiffness of the compressed gas and the metal spring makes the natural frequency of the compressor close to the power supply frequency, and the compressor approaches the resonance state, and the compressor efficiency is close to the peak, so the compressed gas power consumption per unit stroke decreases obviously. When the voltage continues to increase, the exhaust pressure continues to increase, the compressor is far away from the resonance region, and its power consumption will increase again due to the increase in the amount of compressed gas, copper loss and other factors.

It can also be seen in (c) that the compressor with 2 sets of metal springs as the resonant spring set has a higher voltage value than the compressor with 4 sets of metal springs. Because the compressor is close to the resonance state, the equivalent spring stiffness of the compressed gas and the resonance unit stiffness composed of the metal spring make the natural frequency of the compressor close to the power supply frequency, and the corresponding voltage value is also high. Therefore, the voltage required for the single-spring compressor with two sets of metal springs as the resonant unit is higher when it resonates.

The variation law of compressor exhaust pressure, piston stroke, power and power consumption per unit stroke with power supply frequency.

(d) The frequency conversion performance curves of single-side spring linear compressors under fixed valve opening respectively.

As shown in the one-side spring linear compressor frequency conversion performance curve (c), the compressor power decreases with the increase of the power supply frequency, because the exhaust pressure decreases and the compressor stroke decreases (a) and (b), the exhaust pressure and the compressor stroke decrease with the increase of the power supply frequency, because the compressor exhaust pressure is low. It does not reach the ideal matching region of the design time mass and the resonance spring stiffness, so the natural frequency of the compressor is low, and with the increase of the power supply frequency, the compressor is far away from the resonance region, resulting in the reduction of exhaust pressure and stroke. In addition, at the same frequency, the exhaust pressure can be increased by increasing the voltage, and the corresponding compressor piston stroke will also increase, because the center of movement of the compressor is far away from the top dead center position with the increase of the exhaust pressure.

The combined effect results in reduced power consumption of compressed gas.

As shown in (d), the unit power consumption of the compressor increases with the increase of frequency. When the power supply frequency is low, it is close to the natural frequency of the compressor, and the compressor efficiency is high. When the power supply frequency is increased, the compressor is far away from the resonance area, and the efficiency is reduced, resulting in an increase in the unit power consumption of the compressor. In the case of the same frequency, increasing the voltage causes the exhaust pressure to increase, so that the unit power consumption of the compressor also increases.

Under the fixed valve opening, the frequency conversion performance experiment of the single-spring linear compressor shows that the compressor works near its natural frequency and has a higher efficiency. Therefore, in order to obtain higher frequency characteristics in the design of the single-spring linear compressor, the ratio of the power supply frequency and natural frequency of the compressor should be designed to be equal to about 1, usually slightly greater than 1. Since the distance between the initial position of the piston and the moving center of the single-spring linear compressor is equal to the design stroke amplitude of the piston, that is, 4X =X, the spring stiffness value selected for the single-spring linear compressor is small. In order to obtain better frequency characteristics of the single-spring linear compressor, the mass of the moving parts of the compressor will also be reduced when the power supply frequency is certain. From the perspective of product material saving, the single-side spring linear compressor uses compressed gas as a gas spring to reduce the amount of resonant spring, while reducing the mass of moving parts of the compressor, reducing the movement inertia of the compressor, and achieving the purpose of lightweight linear compressor.

4 Conclusion Based on the development of a new type of moving-magnetic single-side spring linear compressor prototype, the feasibility of air compression experiment of the prototype is studied. The following conclusions are drawn: Through the air compression experiment of the prototype, the feasibility of the single-side spring linear compressor is verified, and it has the advantages of using compressed gas as a gas spring to reduce the amount of resonant spring and reduce the mass of moving parts of the compressor, so as to realize the light weight of the linear compressor. The prototype experiment shows that when the compressor exhaust pressure condition changes, the voltage value can be adjusted to ensure that the compressor piston runs at the upper dead center position.