Cooling device and method for operating a cooling device

Abstract

Cooling unit (11) with a refrigeration circuit operated by means of a linear compressor (1) and with a control unit (10) configured to set an operating frequency (f w ) of a piston (3) of the linear compressor (1) driven by means of at least one drive magnet (9) equipped with at least one drive coil (9A) such that, starting from an initial operating frequency (f w,init ) a phase shift (Δφ) between an electrical voltage (U) applied to the linear compressor (1) ~ ) and an electric current (I) flowing through the linear compressor (1) meas ) is determined and the operating frequency (f w ) is gradually reduced until a predetermined target phase shift (Δφ) is reached. target ), which are significantly smaller in magnitude than the phase shift at the initial operating frequency (f w,init) is, at least approximately, achieved. The invention also relates to methods for operating a corresponding cooling device (11). The invention is particularly advantageously applicable to household cooling devices.

Description

[0001] The invention relates to a refrigeration device with a refrigeration circuit operated by a linear compressor and with a control or regulation system configured to regulate the operating frequency of a piston of the linear compressor. The invention also relates to a method for operating a refrigeration device equipped with a refrigeration circuit operated by a linear compressor, in which the operating frequency of a piston of the linear compressor is regulated. The invention is particularly advantageously applicable to household refrigeration devices.

[0002] Cooling devices are known whose refrigeration circuit uses a linear compressor as the drive. To achieve low energy consumption, the operating frequency of the piston is adjusted so that a predetermined resonance frequency of the oscillating system is matched as accurately as possible. The linear compressor is thus operated at a constant, predetermined operating frequency. Implementing this method in production requires that every linear compressor of the same type exhibit the predetermined resonance frequency as accurately as possible, which poses a major challenge for production and assembly because tolerances must be maintained within strict limits. Furthermore, the resonance frequency can change depending on load and temperature.Consequently, it often happens that the actual resonance frequency deviates from the predetermined resonance frequency, resulting in increased energy losses during operation of the linear compressor.

[0003] It is the object of the present invention to at least partially overcome the disadvantages of the prior art and in particular to provide a simple, reliable and energy-saving way of operating a linear compressor of a refrigeration device.

[0004] This object is achieved according to the features of the independent claims. Preferred embodiments can be found in particular in the dependent claims.

[0005] The object is achieved by a cooling device with a refrigeration circuit operated by means of a linear compressor and with a control system which is designed to control an oscillation or working frequency of a piston of the linear compressor driven by means of at least one electromagnet with at least one coil (“drive coil”) in such a way that - starting from an initial operating frequency, a phase shift is determined between an electrical voltage applied to the linear compressor and an electrical current flowing through the linear compressor and - the operating frequency is gradually reduced until a predetermined target phase shift, which is noticeably smaller in magnitude than the phase shift at the initial operating frequency, is at least approximately achieved.

[0006] This method offers the advantage that the operating frequency can be automatically adjusted to the actual resonance frequency of the piston or the associated vibration system, or close to it, thus minimizing the power consumption of the linear compressor. This avoids the aforementioned problems of the prior art during compressor operation. In particular, a change in the actual resonance frequency of the piston or the associated vibration system due to changing boundary conditions such as temperature or load can be easily taken into account.

[0007] For example, in addition to the linear compressor, the refrigeration circuit may comprise at least one evaporator, at least one expansion medium and at least one condenser in a basically known manner.

[0008] The piston is mounted in particular in a cylindrical piston housing of the linear compressor so that it can be moved longitudinally and is articulated by at least one elastic spring element. The piston, which is equipped with at least one permanent magnet, is typically driven by the magnetic field of the at least one drive magnet supplied with an alternating electrical voltage, thereby causing it to oscillate. Depending on the direction of movement, a working volume defined by the piston housing and an end face of the piston is increased or reduced. If the working volume is increased, refrigerant or working fluid is sucked into the working volume at an inlet from a refrigeration circuit connected to the working volume. If the working volume is then reduced again, the working fluid is forced into the refrigeration circuit at an outlet. The amplitude of the alternating voltage determines the stroke of the piston, which in turn determines the mass flow of the working fluid in the refrigeration circuit.In the steady state, the operating or oscillation frequency of the piston is determined by the voltage frequency of the AC signal. The efficiency of the linear compressor depends on the voltage frequency and thus the operating frequency. The basic operation of a linear compressor in a refrigeration circuit of a refrigeration device is well known and will therefore not be discussed further here. The linear compressor can also be considered a linear motor, with the piston acting as the rotor or "mover."

[0009] The fact that the vibration or working frequency of the piston is controlled is based on the fact that the working frequency is not set to a predetermined, pre-calculated value, but is automatically set or adjusted using at least one feedback variable.

[0010] The phase shift is particularly less than zero, since the current follows the voltage. The initial operating frequency is sensibly chosen such that its distance from the (target) operating frequency at which the target phase shift is at least approximately expected is large, in particular so large that its phase shift allows a gradual approach to the operating frequency at the target phase shift under all practically occurring operating conditions of the linear compressor. The initial operating frequency can be determined experimentally, for example.

[0011] The fact that the target phase shift is significantly smaller in magnitude than the initial phase shift takes into account the effect that the phase shift remains essentially constant at a significant distance from the resonant frequency (e.g., at approximately -90°), but decreases in magnitude from this level in a comparatively narrow range around the resonant frequency (i.e., to values ​​> -90°). The fact that the specified target phase shift is significantly smaller in magnitude than the phase shift at the initial operating frequency can, in particular, mean that the corresponding (target) operating frequency corresponds to or is close to the resonant frequency.

[0012] The initial operating frequency is therefore above the resonance frequency. In contrast to an approach from lower operating frequencies, this has the advantage that at high operating frequencies the piston stroke is small even at very high voltage (current) amplitudes. This is advantageous for a reliable start if the control system does not know the current position or deflection of the piston at the beginning. The second advantage lies in the phase shift between voltage and current, which at high initial operating frequencies has a value of approximately -90° or a magnitude of |-90°| = 90°. If the operating frequency is then reduced, it initially drops rather slowly, then drops significantly near or at resonance. In particular, the gradient is maximum at the resonance point. In this way, it can be reliably assumed that the operating frequency is in the resonance range.In contrast, if low operating frequencies are assumed, at least two extreme values ​​of the slope must be passed through before resonance is reached. Approaching from an initial operating frequency above the resonance frequency is therefore a particularly reliable and simple way to identify the resonance point.

[0013] The phase shift can therefore be understood as the phase shift as such or a value derived from it, such as the slope of the phase shift.

[0014] The operating frequency can be shifted with a constant or variable step size. For example, if the phase shift in an i-th step is still above the target phase shift and in an (i+1)-th step the phase shift is already below the target phase shift, the operating frequency can be set to the step whose phase shift is closest to the target phase shift. However, it is also possible that the target phase shift is achieved almost exactly.

[0015] In one embodiment, the operating frequency is reduced with a constant first step size Δf > 0 until the predetermined target phase shift is approximately reached, and then the operating frequency is shifted with a constant, smaller second step size Δf' with Δf > Δf' > 0 (e.g., with Δf = Δf / 2, Δf / 5, or Δf / 10) until the predetermined target phase shift is at least approximately reached. This advantageously achieves a particularly good approximation to the predetermined target phase shift. The shift with the smaller step size Δf can be performed in both directions (i.e., to higher or lower frequencies). This utilizes the fact that the phase shift Δφ, starting from higher frequencies with Δφ of approximately -90°, decreases continuously in magnitude in the range between approximately -90° < Δφ < approximately 0°, or increases continuously in a sign-sensitive manner. Therefore, if the phase shift Δφ is to be reduced in magnitude (e.g.from -90° to -45°), the operating frequency must be reduced in the range -90° < Δφ < 0°. Conversely, if the phase shift Δφ is to be increased, the operating frequency must also be increased.

[0016] In general, an approximation method, particularly with a variable step size, can be used to approximate the operating frequency from the initial operating frequency to a lower target operating frequency at which the target phase shift is present. It is particularly advantageous that the phase shift of 0° is not reached.

[0017] In one embodiment, the specified target phase shift lies in a range of between 30° and 60°, e.g. between -60° and -30°. This is particularly advantageous for enabling particularly powerful or efficient operation of the linear compressor. This takes advantage of the fact that for maximum cooling capacity the operating frequency is best located at the resonance frequency, which corresponds to a target phase shift of 45° associated with the resonance frequency. However, the power consumed is then also at its highest. More efficient operation for a certain cooling capacity with lower cooling capacity but even lower power or energy consumption is not at the resonance frequency, but slightly off it. It has been shown that the phase shift range for particularly efficient operation lies between 30° and 60° outside the resonance frequency.The entire target phase shift range between -60° and -30° thus encompasses the possibility of adjustment – ​​depending on requirements – to operation at the resonance frequency for maximum cooling performance or to operation with sufficiently high cooling performance but particularly efficient operation. The desired target phase shift can be determined, for example, experimentally or through simulations.

[0018] Alternatively or additionally, the resonance frequency can be determined by determining when the slope of the phase shift reaches a maximum, i.e., when the target phase shift reaches the value d(Δφ) / dt = 0 for the first time. Accordingly, for particularly efficient operation, a target phase shift with a specific slope between zero and the maximum can be specified.

[0019] One design is such that the specified target phase shift is approximately -45° or approximately 45° in magnitude. This is particularly easy to implement because it can theoretically be assumed that the resonance frequency is then met, and efficient operation of the linear compressor is achieved without any additional effort.

[0020] In one embodiment, the initial operating frequency is selected such that the corresponding phase shift is approximately 90° in magnitude. This provides the advantage that a reduction of the phase shift to a lower target value (e.g., between 30° and 60° in magnitude) can be carried out particularly reliably. In a further development, the initial operating frequency is advantageously so far removed from the target phase shift or the resonant frequency that the phase shifts for several frequency shifts are approximately -90° or approximately 90° in magnitude.

[0021] The control therefore particularly includes setting the initial operating frequency to a value significantly above the (actual or assumed) resonant frequency, and then determining the phase shift. This is typically approximately 90°. Subsequently, the operating frequency is set to a new, smaller value, and the phase shift is determined again. If the phase shift does not reach or fall below the target phase shift, the operating frequency is reduced again, then the phase shift is determined, and so on. These frequency reductions are repeated until the phase shift reaches or falls below the target phase shift. The operating frequency is then set to the value whose phase shift is closest to, or has been reached, the target phase shift. For example, the range of the target phase shift can be a frequency range [f res - 3 Hz; f res+ 3 Hz], while the initial working frequency is at f res + 60 Hz. The step size Δf can, for example, be Δf = 1 Hz.

[0022] In one embodiment, the control system is configured to calculate a target voltage signal or a target AC voltage for the at least one drive coil of the at least one drive magnet from the currently set operating frequency and a target piston stroke. This is advantageously particularly easy to implement and enables simple implementation of a variation of the piston's operating frequency. The target AC voltage can be applied as a sinusoidal voltage signal or as a pulsed, e.g., pulse-width-modulated, voltage signal.

[0023] In one embodiment, the control system is designed to calculate the phase shift for a set operating frequency from the target voltage for the linear compressor or the at least one drive coil and a measured actual current consumption of the linear compressor or the at least one drive coil. This offers the advantage of simple implementation with precise determination of the phase shift at the set operating frequency. This takes advantage of the fact that the target voltage can be implemented very precisely, i.e. the target value and actual value differ only slightly or practically not at all. Therefore, a voltmeter for the linear compressor, in particular for its drive coil(s), can be dispensed with, and only an ammeter for the current flowing through the linear compressor, in particular its drive coil(s), is required.

[0024] One embodiment is that the control system is configured to monitor the phase shift and, if the phase shift deviates noticeably from the specified target phase shift, the operating frequency is adjusted so that the specified target phase shift is again achieved. The control system therefore recalculates the phase shift, particularly at regular intervals, and checks whether it still corresponds to the target phase shift. If this is the case, the operating frequency is maintained. If this is not the case, the operating frequency is adjusted so that the target phase shift is again achieved.

[0025] In one embodiment, the cooling device is a refrigerator, a freezer, or a combination thereof. The cooling device is, in particular, a household cooling device.

[0026] The object is also achieved by a method for operating a refrigeration device equipped with a refrigeration circuit operated by a linear compressor, in which an operating frequency of a piston of the linear compressor driven by at least one drive magnet equipped with at least one drive coil is controlled such that, starting from an initial operating frequency, a phase shift between an electrical voltage applied to the linear compressor and an electrical current flowing through the linear compressor is determined, and the operating frequency is gradually reduced until a predetermined target phase shift, which is noticeably smaller than the phase shift at the initial operating frequency, is at least approximately achieved. The method can be designed analogously to the refrigeration device, and vice versa, and has the same advantages.

[0027] The above-described properties, features and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more clearly understood in connection with the following schematic description of an embodiment, which is explained in more detail in connection with the drawings. Fig. 1 shows a sectional side view of a sketch of a possible linear compressor; Fig. Figure 2 shows a plot of a phase shift of a linear compressor installed in a refrigerator against the operating frequency; and Fig. 3 shows a control for regulating the operating frequency.

[0028] Fig. Figure 1 shows a sectional side view of a sketch of a possible linear compressor 1. The linear compressor 1 has a piston 3 which is provided with permanent magnets 3A and which is linearly displaceable in a housing 2, as indicated by the double arrow. The piston 3 is hinged to the housing 2 via a spring element 4 and the front side of which defines a working volume 5. A unidirectional inlet 6 opens into the working volume 5, through which the working medium 7 can flow into the working volume 5, and a unidirectional outlet 8 opens through which the working medium 7 can flow out of the working volume 5. The linear compressor 1 also has an electromagnet or drive magnet 9 arranged outside the housing 2. The drive magnet 9 has a drive coil 9A, which is typically operated on the basis of an alternating voltage U ~operated, and a stator 9B made of magnetic material, e.g., sheet steel segments. and then exert a magnetic force on the permanent magnets 3A, whereby the piston 3 is excited to oscillate linearly in the housing 2. The oscillating piston 3 elastically expands the spring element 4 when moving forward or in the direction against the spring element 4 and reduces the working volume 5, so that the working medium 7 is pressurized and flows out through the outlet 8 against a final pressure. An outflow through the inlet 6 is prevented, e.g., by a one-way valve. In the opposite direction, backward or in the direction of the spring element 4, the piston 3 elastically compresses the spring element 4 and increases the working volume 5, so that a negative pressure is created there. The negative pressure, in turn, causes the working medium 7 to flow in through the inlet 6 against an intake pressure. An inflow through the outlet 8 is prevented, e.g.,through a one-way valve. The inlet 6 and the outlet 8 can, for example, both be arranged on a front-end valve plate 2A.

[0029] The vibration system comprising at least the piston 3 and the spring element 4 has a certain resonance frequency f res At constant alternating voltage U ~ a uniform back and forth movement of the piston 3 occurs with a certain piston stroke A and a certain vibration or working frequency f w So far, the resonance frequency f res for a specific linear compressor 1 type-related in advance (e.g. by solving a system of equations or a simulation program) and the alternating voltage U ~ directly adjusted so that the operating frequency f w the type-related precalculated resonance frequency f res meets.

[0030] Fig. Figure 2 shows a possible embodiment of the method according to the invention based on a schematic plot of a phase shift Δφ in degrees between an applied electrical voltage and the current flow generated thereby against an associated operating frequency f w a linear compressor installed in a cooling device 1.

[0031] First - for example when switching on the cooling device - the working frequency f w to an initial value f w,init whose phase shift Δφ is noticeably larger than a target phase shift Δφ target . The operating frequency f w is particularly noticeably larger than the resonance frequency f res For example, the initial operating frequency f w,init be at 100 Hz or more, while the resonance frequency f res for example, at 50 Hz. The phase shift Δφ at the initial operating frequency f w,initand in this case, for example, also in a wide frequency range until relatively shortly before reaching the resonance frequency f res lies on a plateau of approximately - 90°.

[0032] The working frequency f w gradually reduced (e.g. with a step size of 1 Hz) until the corresponding phase shift Δφ reaches the target phase shift Δφ target has decreased. The target phase shift Δφ target is a certain phase shift in a range Δφ target = [- 60°; - 30°], in which the resonance frequency f res typically associated phase shift Δφ res = - 45°.

[0033] After the target phase shift Δφ target is reached, the phase shift Δφ can be further monitored and the corresponding operating frequency f w in case of deviation from the target phase shift Δφ targetadjusted accordingly until the target operating frequency f w,target ,where the target phase shift Δφ target is present, is reached again. This adjustment can be continued continuously. For a simple adjustment, the fact that the phase shift Δφ in the range of the resonance frequency f res with decreasing operating frequency f w continuously increases or decreases in magnitude. If the phase shift Δφ is therefore sign-sensitively above or below the target phase shift Δφ target , the operating frequency f w However, if the phase shift Δφ is sign-sensitively below or above the target phase shift Δφ target , the operating frequency f w It is advantageous that the operating frequency f wdoes not enter a frequency range where the phase shift Δφ reaches zero or the operating frequency f w becomes even smaller.

[0034] Fig. 3 shows a control 10 for controlling the working frequency f w of the linear compressor 1, which, like the linear compressor 1, can be a component of a cooling device 11, e.g. a household refrigerator.

[0035] The control 10 is supplied with a desired piston stroke A, for example via a control device of the cooling device 11. ref for piston 3. The target piston stroke A ref is used together with a specific or desired operating frequency f w a first module (“amplitude control module”) 12, which then generates the corresponding or “correct” target AC voltage U ∼,ref for the drive coil 9A. The nominal AC voltage U ∼,refis fed to a second module (pulse width or “PWM module”) 13, which generates the desired AC voltage U ∼,ref corresponding current pulses are generated to energize the drive coil 9A.

[0036] The target alternating voltage U ~,ref and the current I measured by a current sensor 14 meas is fed to a third module (“phase shift calculation module”) 15, which calculates the current phase shift Δφ between voltage U ~,ref and current I meas calculated.

[0037] The phase shift Δφ is fed to a fourth module (“frequency control algorithm”) 16, which determines a specific operating frequency f w to the amplitude control module 12 and checks on the basis of the returned phase shift Δφ whether this is on the specified target phase shift Δφ target The frequency control algorithm 16 can then be used analogously to the Fig. 2 described processes, the working frequency fw maintain or change it step by step. In one variant, the frequency control algorithm 16 is thus configured, in particular programmed, to carry out the method.

[0038] Of course, the present invention is not limited to the embodiment shown.

[0039] In general, “a”, “an”, etc. can be understood as a singular or a plural, in particular 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.

[0040] A numerical value may also include the exact number stated as well as a usual tolerance range, as long as this is not explicitly excluded. List of reference symbols 1 linear compressor 2 housings 2A valve plate 3 pistons 3A permanent magnet 4 spring element 5 Working volume 6 Entrance 7 Working medium 8 Outlet 9 Drive magnet 9A drive coil 9B Stator 10 Regulation 11 Cooling device 12 Amplitude control module 13 PWM module 14 Current sensor 15 Performance calculation module 16 Frequency control algorithm A piston stroke A ref Target piston stroke f res Resonance frequency f w Working frequency f w,init Initial operating frequency f w,target Target working frequency I meas Actual current U ~ alternating voltage U ~,ref Target AC voltage Δφ phase shift Δφ res Phase shift at resonance Δφ target Target phase shift

Claims

[1] Cooling device (11) with a refrigeration circuit operated by means of a linear compressor (1) and with a control (10) which is designed to set an operating frequency (f w ) of a piston (3) of the linear compressor (1) driven by at least one drive magnet (9) equipped with at least one drive coil (9A) in such a way that - starting from an initial working frequency (f w,init ) a phase shift (Δφ) between an electrical voltage (U ~ ) and an electric current (I meas ) and - the operating frequency (f w ) is gradually reduced until a predetermined target phase shift (Δφ target ), which is noticeably smaller in magnitude than the phase shift at the initial operating frequency (f w,init ) is at least approximately achieved. [2] Cooling device (11) according to claim 1, wherein the operating frequency (f w ) is reduced with a first step size until the specified target phase shift (Δφ target ) is approximately reached, and subsequently the working frequency (f w ) with a smaller second step size until the specified target phase shift (Δφ target ) is at least approximately achieved. [3] Cooling device (11) according to one of the preceding claims, wherein the predetermined target phase shift (Δφ target ) lies in a range between 30° and 60°. [4] Cooling device (11) according to claim 3, wherein the predetermined target phase shift (Δφ target ) is approximately 45° in magnitude. [5] Cooling device (11) according to one of the preceding claims, wherein the initial operating frequency (f w,init ) is selected such that the corresponding phase shift (Δφ) is approximately 90°. [6] Cooling device (11) according to one of the preceding claims, wherein the control (10) is designed to determine from the set operating frequency (f w ) and a target piston stroke (A ref ) a nominal alternating voltage (U ~ ) for the at least one drive coil (9A) of the linear compressor (1). [7] Cooling device (11) according to one of the preceding claims, wherein the control (10) is designed to provide a set operating frequency (f w ) the phase shift (Δφ) from the target voltage (U ~ ) and a measured actual power consumption (I meas ) to calculate. [8] Cooling device (11) according to one of the preceding claims, wherein the control (10) is designed to monitor the phase shift (Δφ) and, when the phase shift (Δφ) deviates noticeably from the predetermined target phase shift (Δφ target ), the operating frequency (f w) is adjusted so that the specified target phase shift (Δφ target ) is reached again. [9] Cooling device (11) according to one of the preceding claims, wherein the cooling device (11) is a refrigerator, a freezer or a combination thereof. [10] Method for operating a cooling device (11) equipped with a refrigeration circuit operated by means of a linear compressor (1), in which an operating frequency (f w ) of a piston (3) of the linear compressor (1) driven by at least one drive magnet (9) equipped with at least one drive coil (9A) is controlled so that - starting from an initial working frequency (f w,init ) a phase shift (Δφ) between an electrical voltage (U ~ ) and an electric current (I meas ) and - the operating frequency (fw ) is gradually reduced until a predetermined target phase shift (Δφ target ), which is noticeably smaller in magnitude than the phase shift at the initial operating frequency (f w,init ) is at least approximately achieved.

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