Household refrigeration appliance with linear compressor and method for operation

Abstract

The invention relates to a household refrigeration appliance (2) with a refrigeration circuit (1, 9, 10) which can be driven by means of a linear compressor (1), wherein the linear compressor (1) has a piston (5) which is linearly movable in a cylindrical piston chamber (4) of a piston housing (3), the piston (5) has at least one permanent magnet (15) on which a force directed in the direction of movement of the piston (5) can be exerted by means of at least one motor coil (14), which can be electrically controlled by means of a control device (16) in order to specify a linear movement of the piston (5), the piston (5) forms a piston chamber (11) with the piston chamber (3), into which at least one inlet valve (7) and at least one outlet valve (8) open, which fluidically connect the piston chamber (11) with the rest of the refrigeration circuit (9, 10), wherein the piston (5) is a quasi freely movable piston (5).The invention also relates to a method for operating such a linear compressor (1).

Description

[0001] The invention relates to a household refrigeration appliance with a refrigeration circuit that can be driven by a linear compressor. The invention also relates to a method for operating the linear compressor. The invention is particularly advantageously applicable to refrigerators and / or freezers.

[0002] In modern household refrigeration appliances with a refrigerant circuit driven by a linear compressor, the piston of the linear compressor is guided by a mechanically rigid spring system, which ensures a constant resonant frequency. The design of such a spring system must also accommodate high delivery rates, which, however, negatively impacts efficiency at low delivery rates. This, in turn, results in high demands and costs regarding the necessary tolerances in the spring design and manufacturing with respect to resonance, installation position, and dimensions.

[0003] CN 105258448A discloses a method for controlling a refrigerator that uses a linear compressor, the method comprising: determining whether the refrigerator transitions from a standby state to a start-up state, and if so, controlling the linear compressor in the start-up state to operate it for a predetermined duration at a predetermined input power, wherein the predetermined input power is greater than a normal initial input power adapted to the linear compressor but less than a maximum input power of the linear compressor. This is intended to reduce noise generation during the start-up of the linear compressor.

[0004] The object of the present invention is to overcome at least some of the disadvantages of the prior art and, in particular, to provide a particularly effective way of operating a linear compressor of a household refrigeration appliance.

[0005] This problem is solved according to the features of the independent claims. Preferred embodiments can be found in particular in the dependent claims.

[0006] The task is solved by a household refrigeration appliance with a refrigeration circuit that can be driven or is driven by a linear compressor, wherein - the linear compressor has a piston that moves linearly within a cylindrical piston chamber of a piston housing, - the piston has at least one permanent magnet on which a force directed in the direction of movement of the piston can be exerted by means of at least one coil (“motor coil”), - which at least one coil can be electrically controlled by means of a control device in order to specify a linear movement of the piston, - the piston with the piston chamber forms a front partial chamber (hereinafter referred to as "piston chamber" or "working chamber" without limitation of generality), into which at least one inlet valve and at least one outlet valve open, which fluidically connect the piston chamber to the rest of the refrigeration circuit and wherein - the piston is a virtually freely moving piston.

[0007] A “quasi-free moving” piston can be understood as a piston that is not connected to the piston housing via a mechanical spring element or only via a soft spring with a small spring constant, as will be explained further below.

[0008] In this linear compressor, the use of a virtually free-moving piston advantageously results in practically resonance-free piston movement. In contrast to a rigid spring system, this enables the efficient pumping of refrigerants with high efficiency at both low mass flow rates with short piston strokes and high mass flow rates with long piston strokes. Specifically, energy losses due to spring extension and / or energy storage in the spring can be avoided, practically independent of the actual center of piston oscillation (hereinafter also referred to as the "center of oscillation").

[0009] Furthermore, friction losses can be advantageously kept low. The soft or absent spring allows the center of oscillation to be more easily shifted by an asymmetrical current flow. This ensures that the gaseous refrigerant is completely expelled, consequently keeping the dead volume small under all operating conditions. The resulting stroke is minimal, resulting in particularly low friction. In contrast, with a conventional stiff spring (with a high spring constant), the center of oscillation is largely fixed and must be designed for a high flow rate (which is rare in household appliances, but must be achievable). At low flow rates, losses are high because less aspirated gas is expelled and the stroke is unnecessarily large, resulting in a high frictional component relative to the small gas flow rate.In both cases, a small distance to the exhaust valve and thus a small dead volume can be advantageously achieved. A rigid spring system, on the other hand, does not allow the center of the piston oscillation to be shifted towards the exhaust valve in order to keep the dead volume small.

[0010] A further advantage is that the natural gas spring behavior of the gaseous refrigerant during the compression and expansion phases of a polytropic process can be effectively utilized. The center of oscillation also depends on the amount of remaining gaseous refrigerant in the piston chamber. Following the refrigerant discharge phase through the discharge valve, the residual refrigerant pressure in the piston chamber can move the piston back, optionally with assistance from an appropriate current applied to the motor coil(s). The shape of the motor coil's control current can advantageously be supported by transient thermodynamic-electrical-mechanical modeling.

[0011] Further advantages of using a soft mechanical spring or no spring at all include lower costs, a particularly compact design, and a shift of the efficiency optimum towards lower mass flow rates. The ability to achieve high mass flow rates and high loads remains. Conversely, any additional effort required for modeling and control is minimal with modern control algorithms and intelligent inverter designs.

[0012] A household refrigeration appliance can be, for example, a refrigerator, a freezer, or a combination thereof.

[0013] The refrigeration cycle utilizes the physical effect of the latent heat of vaporization. In this process, heat is extracted from the cooling chamber of a household refrigerator (which can then also be called a "compression refrigerator") and transferred to the outside. The refrigerant fluid required for the process is circulated within the closed refrigeration cycle and undergoes various states of matter. The refrigerant passes through the components of the refrigeration cycle: compressor, condenser (also called liquefier), expansion valve, internal heat exchanger, and evaporator. The compressor circulates the refrigerant. Refrigeration cycles are generally well-known and therefore will not be described in detail here.

[0014] The fact that the refrigeration cycle can be driven or is driven by means of the linear compressor includes, in particular, that the refrigerant can be conveyed or is conveyed by the compressor.

[0015] The fact that a force directed in the direction of movement of the piston can be exerted on the at least one permanent magnet includes, in particular, the fact that a force can be exerted on the at least one permanent magnet which has a force component directed in the direction of movement of the piston. This force component is suitable for moving or displacing the piston linearly within the piston chamber.

[0016] The force exerted on the at least one permanent magnet is generated by injecting an alternating current ("control current") into the at least one motor coil, which in turn generates a magnetic field in the piston chamber that exerts a magnetic force on the piston's permanent magnet – a principle that is generally known. The at least one motor coil can, for example, be located around the piston housing or embedded within the piston.

[0017] The piston can be moved or shifted within the piston chamber in a first, "forward" direction and a second, "rearward" direction by applying suitable current to the coil. In the forward direction, the piston, particularly its "front" end face, together with its "front" end region, defines the piston chamber. Moving the piston forward reduces the size of the piston chamber, while moving the piston backward increases its size.

[0018] Because the at least one inlet valve and the at least one outlet valve open into the piston chamber, the piston chamber is fluidically connected to, or fluidically integrated into, the rest of the refrigeration circuit. Gaseous refrigerant can be drawn in through the at least one inlet valve when the piston chamber enlarges, and then compressed and expelled through the at least one outlet valve when the piston chamber subsequently shrinks. In particular, the at least one inlet valve can be fluidically connected to the at least one evaporator of the refrigeration circuit, while the at least one outlet valve is fluidically connected to the at least one condenser of the refrigeration circuit.

[0019] Typically, a piston's movement cycle can be roughly divided into four phases: - A first ("compression") phase, during which the piston is moved forward, reducing the size of the compression chamber and increasing the pressure of the gaseous refrigerant within, but still below the opening pressure of the exhaust valve. Thus, both the inlet and outlet valves are closed, and the gas behaves polytropically. The compression phase primarily determines the mechanical drive power and should be utilized as fully as possible in the subsequent phase. - A second ("exhaust") phase follows the compression phase, during which the exhaust valve is open while the piston continues to move forward. This forces the refrigerant out through the exhaust valve. This exhaust phase significantly determines the efficiency of the linear compressor's operation. A third (expansion) phase follows the exhaust phase, during which the piston moves back, increasing the size of the compression chamber and reducing the pressure of the gaseous refrigerant within, but still above the opening pressure of the exhaust valve. Thus, both the inlet and outlet valves are closed, and the gas behaves polytropically. Identifying and observing this phase can be used in further training to generate an efficient operating model for an optimizing control algorithm. - a fourth (intake) phase that follows the expansion phase, in which the intake valve is open while the piston continues to move backwards.

[0020] This draws the refrigerant in through the inlet valve. This phase is important for the resulting delivery rate of the following phases.

[0021] The intake phase can be followed by a new compression phase, and so on, allowing several of the above cycles to be completed. During each cycle, the piston performs a linear oscillation centered around the center of oscillation. The mass flow rate delivered through the compression chamber can be adjusted by the piston stroke and the piston's oscillation frequency.

[0022] It is known to determine the (linear) position of the piston in the piston chamber, for example by electronic observers, measurements of e.g. a mutual inductance, external sensors, etc.

[0023] This design incorporates a connection between the piston and the piston housing via at least one flexible spring that is elastic in the direction of the piston's movement. This is advantageously simple to implement and facilitates the piston's movement into a defined position when the linear compressor is not in operation ("zero position"), without noticeably affecting the piston's movement when the motor coil(s) are energized. The spring can serve as an alternative or additional guide for the piston, although this can also be achieved through other means, such as the use of plain bearings. In particular, the flexible spring does not imprint or "fix" a resonant frequency, nor does it store energy for the compression phase.The spring constant is therefore so small that no resonance frequency needs to be considered when shaping the control current in the motor coil(s), and the piston stroke or piston position is practically freely adjustable within the piston chamber. In other words, the soft spring is advantageously practically negligible for the piston's movement characteristics. Consequently, in a further development, the design of the spring or the determination of the spring constant can be approached in such a way that the desired function of guiding the piston and / or positioning the piston at rest is achieved with the smallest possible spring constant.

[0024] The spring can be connected to the piston at a rear end of the piston, facing away from the piston chamber.

[0025] One design feature is that the piston is springless, meaning it is not attached to the piston housing by a spring. This advantageously allows for a particularly compact design and energy-saving, free positioning of the piston within the piston chamber. To move the piston to a desired position, such as a zero position or an initial position at the start of a conveying cycle, or to hold it there, a corresponding control current can be applied. The piston can, for example, be guided by a plain bearing.

[0026] It is a further development that the linear compressor is configured to prevent movement and / or positioning of the piston beyond a maximum rear dead center. This prevents, for example, the piston from hitting a stop or sliding out at the rear end of the piston chamber. This further development can include, for example, the use of a rubber element as a stop, the application of an electronic counter-voltage to the motor coil, etc.

[0027] It is a design in which the at least one motor coil can be selectively controlled in at least a first operating mode and in a second operating mode, wherein - the piston is moved around a first center of vibration or a first vibration equilibrium position in the first (“large mass flow”) operating mode with a first stroke, - the piston is moved around a second center of vibration in the second (“small mass flow”) operating mode with a second stroke, - the first center of vibration is further away from at least one exhaust valve than the second center of vibration and - the first stroke is longer than the second stroke.

[0028] This advantageously allows for a high mass flow rate of the gaseous refrigerant through the compression chamber in the first operating mode, while a lower mass flow rate can be achieved in a particularly energy-saving and efficient manner in the second operating mode. The second operating mode, in particular, takes advantage of the fact that, unlike using a rigid spring, little to no energy is required to overcome the spring force during the short-stroke movement near the outlet valve. A particularly efficient power supply to the coil in both modes can be supported by transient thermodynamic / electrical / mechanical modeling.

[0029] In the second operating mode, the small stroke also advantageously results in particularly low friction, for example by keeping the piston speed and / or piston stroke small.

[0030] The second operating mode is typically predominant in normal operation of the household refrigeration appliance, while the first operating mode is usually only used occasionally, e.g. when switching on, when leaving a refrigerator door open, when a targeted rapid reduction of the refrigerator temperature, etc.

[0031] It is a further development that the at least one motor coil can be selectively controlled in at least one additional (e.g., third, fourth, etc.) operating mode. This additional operating mode can, in particular, have a center of oscillation and a stroke between those of the first and second operating modes. In principle, the additional operating mode can be configured arbitrarily, e.g., with respect to a defined current profile.

[0032] It is a design such that the front dead center (or reversal point) of the piston is at least approximately the same for the first and second operating modes. The front dead center corresponds, in particular, to the piston's position furthest forward, i.e., towards the exhaust valve. The further forward the front dead center is located, the smaller the remaining dead volume of the piston chamber is advantageously.

[0033] One embodiment of the control unit is configured to supply at least one motor coil with a symmetrical current signal in the first operating mode. This is particularly easy to implement and advantageously supports a high mass throughput. A symmetrical current signal can be understood, in particular, as a control signal waveform in which the pulse duration of the positive and negative pulses is equal.

[0034] One configuration involves the control unit being set up to supply at least one motor coil with a current signal in the second operating mode. This signal, used for forward piston movement, imprints a higher current value on the motor coil than for backward movement. Due to the lack of spring stiffness, this asymmetrical current supply can be used to achieve a particularly smooth piston movement. This is because the aforementioned (first to fourth) phases produce different pressure forces on the piston, which can be taken into account during the current supply.The asymmetric and particularly energy-efficient excitation for generating a symmetrical (short-stroke) piston oscillation is advantageously implemented because the residual gas remaining in the piston chamber after the exhaust phase has a very small dead volume, and the current applied to the motor coil during the return stroke (backward) can be optimized for the required delivery rate during a subsequent expansion phase. In particular, it takes advantage of the fact that a significant amount of energy is required to compress the gaseous refrigerant, while comparatively little energy is needed to move the piston backward in a controlled manner. This can be advantageously achieved using asymmetric current application.

[0035] With this household refrigeration appliance, the maximum efficiency can therefore be shifted towards lower mass flows (especially cooling capacity) depending on the requirements for household applications.

[0036] This design features a control unit configured to continuously increase the drive current in the motor coil at the start of a compression phase. This means the motor current does not jump to a final current value or pulse height, but increases continuously. This allows, in particular, the limitation of an initial piston speed that would otherwise result from the lack of a stiff spring's return force. As the piston moves towards the exhaust valve and the piston chamber compresses, the pressure of the gaseous refrigerant in the piston chamber increases, exerting a growing resistance to the piston's movement until the exhaust valve opens.The increasing counterforce exerted by the gas against the piston movement is compensated by the rising values ​​of the applied current, whereby the final pulse height of the applied control current should be at least approximately reached by the transition to the exhaust phase at the latest. For the piston speed, this means that the piston can move forward during its stroke without pronounced speed peaks and thus with only minimal sliding friction against the piston skirt. It is therefore a further development that, at the beginning of a compression phase, a current is applied to the motor coil with a continuously increasing current rise until a predetermined final value (namely the maximum pulse height) is reached. In contrast, with a stiff spring, the piston speed is similar to a sinusoidal oscillation and can only be minimally altered by the current applied.

[0037] It is an embodiment in which the at least one motor coil comprises exactly one motor coil which is set up (i.e., designed and arranged) in such a way that it achieves its maximum efficiency when the piston is moved in the second operating mode.

[0038] It is a further development that at least one motor coil has a center terminal, which is used for current application when the piston is to be moved in the second operating mode. When moving in the first operating mode, the two end terminals can then be used. This further development is also particularly energy-efficient when using only one motor coil.

[0039] It is a further development that the at least one motor coil has at least two motor coils, wherein the control device is configured to control the first of these motor coils to move the piston in the first operating mode and to control the second of these motor coils to move the piston in the second operating mode.

[0040] The problem can also be solved by a method for operating a linear compressor as described above. The method can be designed analogously to the linear compressor, and vice versa, and offers the same advantages.

[0041] Thus, it is a design feature that in at least one operating mode - a “compression” control current is impressed into the motor coil(s), which moves the piston forward towards the exhaust valve during the compression and exhaust phases, thereby compressing the piston chamber and increasing the gas pressure until the exhaust valve opens; - this compression control current is switched off before reaching a front or "top" dead center of the piston, so that the piston reaches the front dead center due to its inertia, at which point the exhaust valve is closed and the remaining gaseous volume is under pressure (especially the opening pressure of the exhaust valve); - followed by an “expansion” control current being impressed into the motor coil(s), which moves the piston backwards or back to reach a rear or “bottom” dead center; - where the expansion control current is smaller than the compression control current.

[0042] This design is particularly energy-saving and efficient. It is especially advantageous to use it after the piston has already been moved back once following the start of this operating mode, in order to expand the piston chamber and draw in gaseous refrigerant through the inlet valve. In a further development, this operating mode can correspond to the second ("low mass flow") operating mode described above.

[0043] When the compression control current is impressed into the motor coil(s), the piston chamber is filled with previously drawn-in gaseous refrigerant, and both the inlet and outlet valves are closed.

[0044] If the compression control current is switched off before the piston reaches top dead center, thus deactivating the magnetic force, the piston continues to move a certain distance, but is pushed backward by the compressed gas in the piston chamber. This slows the piston down until it comes to a standstill, and the gas pressure remains at or just below the opening pressure of the exhaust valve, which then closes. The opening pressure (difference) of the exhaust valve and / or the intake valve can be in the range of a few millibars. The valves can, for example, be designed as spring-loaded valves.

[0045] The expansion control current, subsequently applied to the motor coil(s), moves the piston backwards within the piston chamber, generating a force sufficient to move the piston against friction and draw in the refrigerant. This backward movement of the piston can be assisted, at least initially, by the residual gas still under pressure in the piston chamber. Depending on the design of the linear compressor, particularly the desired stroke, this residual pressure effect is noticeable, allowing the expansion control current to be set correspondingly lower than the compression control current. For high required delivery rates, this effect can be utilized, albeit at the expense of high efficiency, to enable particularly long expansion strokes.

[0046] The application of an expansion control current that moves the piston backward to reach the rear dead center can include the piston continuing to move a certain distance backward due to mass after the expansion control current is switched off, or the piston stopping practically immediately after the expansion control current is switched off.

[0047] The properties, features and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more easily understood in connection with the following schematic description of an exemplary embodiment, which will be explained in more detail in conjunction with the drawings. Fig. Figure 1 shows a sketch of a linear compressor in a sectional side view; Fig. Figure 2 shows a plot of pressure in a piston chamber of the linear compressor against a piston stroke for a first operating mode and a second operating mode; Fig. Figure 3 shows a plot of a drive current against time for the first operating mode; Fig. Figure 4 shows a plot of a drive current against time for the second operating mode; and Fig. Figure 5 shows a plot of a modified drive current against time.

[0048] Fig. Figure 1 shows a cross-sectional sketch of a linear compressor 1 of a household refrigeration appliance 2. The linear compressor 1 has a tubular piston housing 3, the hollow cylindrical interior of which (the "piston chamber" 4) serves to movably accommodate a piston 5. A longitudinal axis L of the piston chamber 4 thus coincides with a linear direction of movement of the piston 5. The front of the piston chamber 4 is closed by a valve plate 6, which contains an inlet valve 7 and an outlet valve 8 for gaseous refrigerant. The valve plate can be part of the piston housing 3. A movement of the piston 5 towards the valve plate 6 is also referred to as "forward" movement, and a movement of the piston 5 in the opposite direction away from the valve plate 6 is also referred to as "backward" or "retract" movement.The inlet valve 7 can be fluidically connected to an evaporator 9, while the outlet valve 8 can be fluidically connected to a condenser 10. A working or piston chamber 11 of the linear compressor 1 is formed by the piston housing 3, the front face 12 of the piston, and the valve plate 6. In this case, the piston 5 is connected at its rear to the piston housing 3 by a very soft spring 13, e.g., a Bourdon tube spring. The main function of this spring 13 can be, in particular, to position the piston 5 in its rest position at a corresponding zero position.

[0049] To drive or move the piston 5, a magnetic field is generated at the location of the piston 5 by energizing at least one coil (“drive coil” 14) of the linear compressor 1. This magnetic field interacts with at least one permanent magnet 15 embedded in the piston 5. This exerts a force on the piston 5 in the direction of its movement. The magnitude and direction of the force exerted on the piston 5 can be controlled by adjusting the electrical excitation or control current I, which is applied to the drive coil 14. In this case, the control current I can be set by a control device 16 and applied via an inverter 17.

[0050] Alternatively, spring 13 can be omitted. Positioning of piston 5 can then be achieved, for example, by measuring and correcting the position of piston 5, etc.

[0051] Fig. Figure 2 shows a plot of pressure p in bar in piston chamber 11 against piston position x in mm for a first operating mode Z1 and a second operating mode Z2. The cycles for both operating modes Z1 and Z2 are qualitatively similar and, described here using the first operating mode Z1 as an example, include a point K1 at which the piston 5 has a respective maximum rear position x, which can also be described as "back dead center". The piston chamber 11 is then filled with gaseous refrigerant at the opening pressure p0.

[0052] During a compression phase Ph1, the drive coil 14 is energized such that the piston 5 is moved forward from its rear dead center (here: to the left in the direction of the x-axis), thereby reducing the size of the piston chamber 11 and the gas it contains, until the opening pressure p1 of the outlet valve 8 is reached at a point K2. With further forward movement of the piston 5, the gaseous refrigerant is forced out of the outlet valve 8 during an outlet phase Ph2 until a forward point K3 is reached, which corresponds to a front dead center or maximum piston stroke Δx. Point K3 can be reached, for example, by switching off the control current I shortly before reaching the front dead center, so that the piston 5 reaches the front dead center under its own weight. Upon reaching point K3, the outlet valve 8 closes.At this point, not all of the gaseous refrigerant has been expelled from piston chamber 11; a dead volume remains. This gas, at pressure p1, exerts a force on piston 5 in a rearward direction. Therefore, and due to the application of a control current I to the motor coil 14, which pushes piston 5 backward, piston 5 moves backward in an expansion phase Ph3. During the expansion phase Ph3, as in the compression phase Ph1, both valves 7 and 8 are closed. If, during the compression phase Ph3, the pressure p has dropped to the opening pressure p0 of the inlet valve 7 at a point K4, the inlet valve 7 opens as piston 5 continues to move backward, and the gaseous refrigerant is drawn in through the inlet valve 7 during a suction phase Ph4, until point K1 is reached again.The gaseous refrigerant in the piston chamber 11 exhibits the polytropic behavior of a gas spring during the compression phase Ph1 and the expansion phase Ph3.

[0053] This cycle can be repeated until the desired mass flow has been conveyed through the piston chamber 11. Several identical cycles result in an oscillating motion of the piston 5, which, however, due to the softness of the spring 13 or its absence, exhibits no or at least no pronounced resonance frequency.

[0054] The higher the maximum piston stroke Δx maxThe longer the distance between the rear and front dead centers (here: between points K1 and K3), the higher the mass flow rate typically conveyed in one cycle. However, a high mass flow rate is often not required to operate a household refrigerator 2, so short stroke movements of the piston 5 are sufficient and also more energy-efficient. In this case, for example, operating mode Z1 can represent a high mass flow rate, while operating mode Z2 represents a low mass flow rate or short stroke movements.

[0055] The more different the maximum piston stroke Δx max The greater the difference between operating modes Z1 and Z2, the more pronounced the equilibrium position or the center of oscillation x0 of the associated piston oscillation: the higher the maximum piston stroke Δx max The larger the value, the further the equilibrium position x0 is typically from the valve plate 6 or the exhaust valve 8.

[0056] In contrast to the case of a stiff spring, this has little to practically no energy disadvantage, since there is no pronounced resonance frequency and thus the center of oscillation x0 can be chosen virtually freely without an "energy penalty". The linear compressor 1 can therefore be operated without resonance. This also advantageously allows the piston 5 to be moved so close to the exhaust valve 8 without significant energy losses that the remaining dead volume of the piston chamber 11 at point K3 is particularly small.

[0057] Particularly in small mass flow with short piston stroke Δx, energy-efficient operation is possible because the piston 5 can be moved with low sliding friction, since the piston speed is then comparatively small.

[0058] Fig. Figure 3 shows a plot of a control current I(Z1) through the motor coil 14 against time t for the first operating mode Z1 with long piston stroke Δx, which can be used in particular for a large mass flow through the piston chamber 11.

[0059] The control current I(Z1) is symmetrically pulsed, with the positive pulses PI + , which cause a forward movement of piston 5, the same pulse duration and absolute pulse height I +,max such as the negative pulses PI., which cause a backward movement of piston 5, i.e. |I +,max | = |I -,maxThis applies. Such a pulse sequence is particularly easy to implement because the polytropic behavior of the gas in the piston chamber 11 during the compression phase Ph1 and the expansion phase Ph3 is negligible. The center x0 of the piston 5's oscillation motion corresponds to the center of the stroke, allowing virtually the entire stroke to be utilized. Furthermore, a very high delivery rate is achieved even at low delivery pressure.

[0060] Fig. Figure 4 shows a plot of a control current I(Z2) through the motor coil 14 against time t for the second operating mode Z2 with short piston stroke Δx, which can be used in particular for a small mass flow through the piston chamber 11.

[0061] The control current I(Z2) is also pulse-shaped, with the pulse width of the positive pulses being PI. + is now longer than the pulse width of the negative pulses PI -, and also shorter than the pulse width of the positive pulses PI + in the first operating mode Z1. Furthermore, the absolute pulse height of the negative pulses is PI. - smaller than the pulse height of the positive pulses PI + This system utilizes the fact that the gas-filled piston chamber 11 acts as a gas spring during the expansion phase Ph3, pushing the piston 5 backward. This effect is noticeable at small piston strokes Δx, so that a correspondingly lower control current I is required during the expansion phase Ph3 than in the first operating mode Z1. In extreme cases, the negative current pulses PI can even be omitted entirely. The asymmetry of the current supply brings the piston movement particularly close to the exhaust valve 7 at medium load, i.e., medium delivery pressure and medium delivery rate, thereby achieving a particularly small dead volume.

[0062] Similarly, in the compression phase Ph1, the pressure p in the piston chamber 11 increases more rapidly and the effect of the gas spring is more noticeable than in the first operating mode Z1. Therefore, the pulse height of the positive pulses can advantageously be set higher than in the first operating mode Z1.

[0063] Fig. Figure 5 shows a plot of a modified drive current I against time. The drive current I of the positive pulses PI + The pulse rate will no longer jump to its maximum level. +,max not set, but rises with a defined gradient up to the maximum pulse height I +,maxa current is applied, which is then maintained for part of the pulse duration in the further development shown. This modified control current I takes advantage of the fact that at the beginning of the compression phase Ph1, the pressure in the piston chamber 11 is still relatively low, and therefore a small force is sufficient to move the piston 5 forward. This has the advantage of reducing the initial speed of the piston 5, which helps to reduce friction losses.

[0064] The control current I can generally be determined, in particular with the aid of a transient thermodynamic-electrical-mechanical model.

[0065] Of course, the present invention is not limited to the embodiment shown.

[0066] In general, “ein”, “eine”, etc. can be understood to mean singular or plural, especially in the sense of “at least one” or “one or more”, etc., unless this is explicitly excluded, e.g. by the expression “exactly one”, etc.

[0067] A numerical specification can also include exactly the specified number as well as a normal tolerance range, unless this is explicitly excluded. Reference symbol list 1 linear compressor 2 household refrigeration appliances 3 Piston housings 4 piston chamber 5 pistons 6 Valve plate 7 Inlet valve 8 exhaust valve 9 evaporators 10 Capacitor 11 Piston chamber 12 Front face of the piston 13 spring 14 Drive coil 15 permanent magnet 16 Control unit 17 Inverter I Control current I+,max Maximum pulse height of the positive current pulses I -,max Maximum pulse height of the negative current pulses K1-K4 (x; p) operating points L Longitudinal extent of the piston chamber p Pressure in the piston chamber p0 Opening pressure of the inlet valve p1 Opening pressure of the outlet valve Ph1 Compression Phase Ph2 outlet phase Ph3 Expansion phase Ph4 Intake Phase PI + Positive current pulse PI - Negative current pulse t time x Position of the piston x0 vibration center Z1 First operating mode QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] CN 105258448A

[0003]

Claims

[1] Household refrigeration appliance (2) with a refrigeration circuit (1, 9, 10) which can be driven by means of a linear compressor (1), wherein - the linear compressor (1) has a piston (5) which is linearly movable in a cylindrical piston chamber (4) of a piston housing (3), - the piston (5) has at least one permanent magnet (15) on which a force directed in the direction of movement of the piston (5) can be exerted by means of at least one motor coil (14), - the at least one motor coil (14) can be electrically controlled by means of a control device (16) in order to specify a linear movement of the piston (5), - the piston (5) forms a piston chamber (11) with the piston chamber (3), into which at least one inlet valve (7) and at least one outlet valve (8) open, which fluidically connect the piston chamber (11) with the remaining refrigeration circuit (9, 10), wherein - the piston (5) is a virtually freely movable piston (5). [2] Household refrigeration appliance (2) according to claim 1, wherein the piston (5) is connected to the piston housing (3) by means of at least one soft spring (13) which is extensible in the direction of movement of the piston (5). [3] Household refrigeration appliance (2) according to claim 1, wherein the piston (5) is a spring-free piston (5). [4] Household refrigeration appliance (2) according to one of the preceding claims, wherein the linear compressor is designed to prevent movement and / or positioning of the piston beyond a maximum rear dead center. [5] Household refrigeration appliance (2) according to one of the preceding claims, wherein the at least one motor coil (14) is selectively controllable at least in a first operating mode (Z1) and in a second operating mode (Z2), wherein - the piston (5) is moved in the first operating mode (Z1) with a first stroke around a first oscillation center (x0), - the piston (5) is moved in the second operating mode (Z2) with a second stroke around a second oscillation center (x0), - the first vibration center (x0) is further away from the at least one outlet valve (8) than the second vibration center (x0) and - the first stroke is longer than the second stroke. [6] Household refrigeration appliance (2) according to claim 5, wherein a front reversal point for the first operating mode (Z1) and the second operating mode (Z2) is at least approximately the same. [7] Household refrigeration appliance (2) according to one of claims 5 to 6, wherein the control device (16) is designed to apply a symmetrical control current (I) to the at least one motor coil (14) in the first operating mode (Z1). [8] Household refrigeration appliance (2) according to one of claims 5 to 7, wherein the control device (16) is designed to apply a control current (I) to the at least one motor coil (14) in the second operating mode (Z2), which current has a higher current value (I +,max ) into the at least one motor coil (14) than for movement backwards. [9] Household refrigeration appliance (2) according to one of the preceding claims, wherein the control device (16) is designed to impress the control current (I) into the motor coil (14) in a continuously increasing manner at the beginning of a compression phase (Ph1). [10] Method for operating a linear compressor (1) in a household refrigeration appliance (2) according to one of the preceding claims, in which in at least one operating mode (Z1, Z2) - a compression drive current (PI +) is impressed into the at least one motor coil (14), which moves the piston (5) forward in the direction of the exhaust valve (8); - this compression drive current (PI + ) is switched off before reaching a front dead center of the piston (5); and - followed by an expansion control current (PI - ) is impressed into the motor coil(s), which moves the piston (5) backwards to reach a rear dead center; - where the expansion drive current (PI - ) is smaller than the compression drive current (PI + ).

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