Motor-compressor unit

Abstract

 A motor compressor unit comprises a hermeti­cally sealed housing (3), a vibration motor (1) with a rotationally vibrating drive shaft (6), and a symmetri­cal twin free-piston compressor (2) comprising two oppositely disposed identical free pistons (14, 15) which can be reciprocated in respective cylinders (12, 13) by the drive shaft (6). In order to minimise drift­ing away of the piston (14, 15) from the centre position each cylinder (12, 13) is formed with a duct (26, 27) connecting the respective compression space (16, 17) to the space (22) between the housing (3) and the motor-­compressor (1, 2) where the suction pressure prevails, which ducts are situated at equal distances (d) from the respective cylinder end walls (18, 19) and can be closed by the respective pistons (14, 15). As a result of this the pressure in both cylinders is built up with the same swept volume.

Description

[0001] The invention relates to a motor-compressor unit comprising a hermetically sealed housing, a vibra­tion motor having a rotationally vibrating drive shaft, and a symmetrical twin free-piston compressor comprising two oppositely disposed identical free pistons, which can be reciprocated in respective cylinders by the drive shaft to influence a respective compression space, having cylinder end walls which each bound the respect­ive compression spaces and which each comprise inlet and outlet valves.

[0002] Such a motor-compressor is known from US-A-­3,937,600.

[0003] A problem associated with motor-compressors of this type is "drifting" of the piston, i.e. the piston drifts away from its centre position, causing the piston to run asymmetrically and the efficiency to be reduced.

[0004] This problem also occurs in refrigerating machines with a single free piston as described in US-A-­4,058,387. In these machines piston drift is caused by a leakage flow of the work medium along the piston from the compression space to a buffer space and vice versa. Drifting is then mitigated by means of a system of ducts which establish a connection between the compression space and the buffer space at specific instants.

[0005] US-A-3,937,600 relates to a motor-compressor and provides a solution to preclude an excessive piston stroke as a result of a decreasing resonant frequency of the mass-spring system, which may lead to collision of the piston against the cylinder end wall (overstroke prevention). For this purpose the compressor is provided with a gas-spring cylinder at both ends. When the piston stroke becomes too large medium will flow from the surrounding to the relevant gas-spring cylinder via a duct system in an extreme position of a piston, thereby causing the stiffness of this gas spring to increase. As this results in a difference in stiffness of the two gas springs the pressures in the gas-spring cylinders are balanced in the piston centre position by temporarily interconnecting the gas-spring cylinders via ducts and the space within the double piston. The spring stiffness of the entire mass-spring system and hence the resonant frequency increases until a normal stroke length is at­tained again. This compressor is of an intricate con­struction and should be manufactured with a high accu­racy in order to achieve the desired goal.

[0006] It is an object of the invention to minimise piston drifting in a simple way in a motor-compressor unit of the type defined in the opening paragraph.

[0007] To this end the motor-compressor unit in accordance with the invention is characterized in that each cylinder is formed with a duct connecting the respective compression space to the space between the housing and the motor-compressor where the suction pressure prevails, which ducts are situated at equal distances from the respective cylinder end walls and can be closed by the respective pistons.

[0008] As a result of this symmetrical arrangement of the ducts the pressure build-up is effected at the same swept volume in both cylinders. Drift reduction is now achieved in that in the case that the piston drifts out of the centre position the piston is subjected to a resultant average gas force which is opposed to the drift direction. Therefore, no additional spring action (gas springs or mechanical springs) is needed to provide drift compensation.

[0009] The limits imposed on the location of each duct are dictated by a position of the end face of the piston when this piston occupies a centre position and by a position of the end face of the piston when this piston occupies a maximal extreme position at the end of a piston stroke.

[0010] Preferably, each duct is disposed at a location where the end face of the piston is situated when this piston has reached an extreme position at the end of a piston stroke and when the compressor has pumped a desired minimum amount of medium.

[0011] An embodiment of the invention will now be described in more detail, by way of example, with reference to the accompanying drawing. In the drawing

Figure 1 shows a motor-compressor unit, and

Figures 2 and 3 show the compressor of the unit of Figure 1 in a centre position of the piston and in a maximum extreme position respectively.

[0012] The motor-compressor unit shown in Figure 1 comprises a vibration motor 1 and a symmetrical twin free-piston compressor 2. The motor and the compressor are accommodated in a hermetically sealed housing 3. The operation of the motor-compressor is described in EP-A-­0,155,057 and may be summarized as follows. An alternat­ing current through the coils 4 of the vibration motor 1 results a rotary vibrational movement of the rotor 5 about the shaft 6. For each rotor section (5a, 5b, 5c, 5d) in the form of a sliding element the alternating magnetic field generated by the coils is superposed on the magnetic field produced by the permanent magnet 7. As result of this, the magnetic flux density in each rotor section alternately assumes a high and a low value. The coils have been wound in such a way relative to the direction of magnetisation of the permanent magnets that two diagonally opposed rotor sections (5a, 5c) experience a high magnetic flux density at the same instant, while the other two rotor sections (5b, 5d) experience a low flux density. This gives rise to a movement of the rotor sections in the air gaps 8 between the core 9 and the stator plates 10, where a high flux density exists. A reversal of the current direction will bring about a reversal of the movement of the rotor 5, thus resulting in a vibrational movement of the rotor. The compressor 2 comprises two cylinders 12, 13, in each of which one of two identical oppositely disposed free pistons 14, 15 can perform a linear reciprocating movement. The pistons are integral with one another and in their centre they are connected to an arm 11 of the vibrating rotor 5 by means of a coupling 16.

[0013] Alternately, medium is taken in, compressed and discharged by the pistons 14, 15 in the respective compression spaces 16, 17. For this purpose inlet and outlet valves 20, 21 are arranged in the cylinder end walls 19, 19. The motor-compressor is included in a refrigerating system, not shown, whose low-pressure section (suction side) is connected to a space 22 between the hermetically sealed housing 3 and the motor-­compressor 1,2. During a suction stroke medium is drawn directly from this space 22 into the compression spaces 16, 17 via inlet ports 23. During a compression stroke medium is forced to the high-pressure section (pressure side) of the refrigerating system via outlet ports 24 and a line, not shown.

[0014] In order to prevent the double piston 14, 15 from performing a reciprocating movement which is non-­symmetrical relative to the line 25 each cylinder 12, 13 is formed with a duct 26, 27, which ducts each connect the compression spaces 16 and 17 respectively to the space 22 and which are each situated at an equal dis­tance d from the respective cylinder end wall 18, 19. During each compression stroke a small amount of medium is first pressed into the space 22, where a suction pressure prevails, via a duct 26, 27, so that subse­quently the pressure build-up in each cylinder takes place with the same swept volume. This ensures that the piston 14, 15 remains correctly centred.

[0015] The limits imposed on the locations of the ducts are as follows. When the piston 14, 15 performs its minimal stroke (the piston is then in fact in the centre position), as is shown in Fig. 2, each duct is situated at the level of the end face 28, 29 of the piston. This is one limit. When the piston 14, 15 makes its maximal stroke, as illustrated in Fig. 2, each duct is situated at the level of the end face 28, 29 of the piston when this piston occupies a maximal extreme position at the end of a piston stroke. This is the other limit. Obviously, these limits are theoretical. In practice, the ducts will be situated somewhere between these limits and generally closer to the first-mentioned limit.

[0016] Satisfactory results have been obtained with a motor-compressor in which each duct 26, 27 is situated at the location of the respective end face 28, 29 of the piston when this piston has reached an extreme position at the end of a piston stroke and when the compressor has pumped a desired minimum amount of medium.

Claims

1. A motor-compressor unit comprising
- a hermetically sealed housing (3),
- a vibration motor (1) having a rotationally vibrating drive shaft (6), and
- a symmetrical twin free-piston compressor (2) comprising two oppositely disposed identical free pistons (14, 15), which can be reciprocated in respect­ive cylinders (12, 13) by the drive shaft (6) to influ­ence a respective compression space (16, 17), having cylinder end walls (18, 19) which each bound the re­spective compression spaces and which each comprise inlet and outlet valves (20, 21), characterized in that each cylinder (12, 13) is formed with a duct (26, 27) connecting the respective compression space (16, 17) to the space (22) between the housing (3) and the motor-­compressor (1, 2) where the suction pressure prevails, which ducts are situated at equal distances (d) from the respective cylinder end walls (18, 19) and can be closed by the respective pistons (14, 15).

2. A motor-compressor unit as claimed in Claim 1, characterized in that each duct (26, 27) is situated between a position of the end face (28, 29) of the piston (14, 15) when said piston occupies a centre position (Figure 2) and a position of the end face of the piston when this piston occupies a maximal extreme position at the end of a piston stroke (Figure 3).

3. A motor-compressor unit as claimed in Claim 2, characterized in that each duct (26, 27) is situated at the location of the end face (28, 29) of the piston (14, 15) when said piston has reached an extreme posi­tion at the end of a piston stroke and when the com­pressor has pumped a desired minimum amount of medium.

Drawing