JP5033655B2 - Cooling device and open showcase - Google Patents

Description

  The present invention relates to a cooling device installed in a low temperature showcase, a low temperature storage, an air conditioner, etc., in particular, a cooling device that performs PID control of the rotation speed of a compressor motor based on a deviation e between a target temperature and a current temperature, and It relates to the open showcase provided.

Conventionally, for example, a cooling device employed in a low temperature showcase has a predetermined refrigerant circuit formed by connecting a compressor, a condenser, a pressure reducing device, an evaporator, and the like sequentially in a ring shape by piping, and the refrigerant circuit Is filled with a predetermined amount of refrigerant. When the compressor is operated, the refrigerant is compressed into a high-temperature and high-pressure gas state and flows into the condenser. In the condenser, the refrigerant dissipates heat and is condensed and liquefied, and then decompressed by a decompression device and supplied to the evaporator. In the evaporator, the liquid refrigerant after being depressurized evaporates, and at that time, the cooling effect is exhibited by absorbing heat from the surroundings (for example, see Patent Document 1).
JP 2004-232994 A

  Incidentally, the rotational speed of the compressor is normally controlled between the minimum rotational speed and the maximum rotational speed by the control device. That is, when the temperature in the cabinet, which is the space to be cooled in the showcase, reaches the upper limit temperature, the control device starts the compressor. And a control device controls the rotation speed of a compressor within the range of the minimum rotation speed and the maximum rotation speed which were preset based on the output of the various sensors etc. for detecting the temperature of a refrigerant | coolant, and internal temperature. Then, the compressor is stopped when the inside temperature of the showcase is lowered to the lower limit temperature. Thereby, the inside of the showcase was maintained at a predetermined cooling temperature.

  Here, the maximum number of revolutions and the minimum number of revolutions of the compressor are set in advance as described above, and the maximum number of revolutions is high in order to obtain a necessary cooling capacity at the time of pull-down when the power is turned on or after the defrosting operation is completed. As a result, the cooling efficiency during pull-down was improved.

  However, during normal operation other than pull-down, the compressor is frequently turned on and off due to the maximum rotational speed set high, increasing power consumption and maintaining the internal temperature at the target temperature with high accuracy. There was a problem that made it difficult to do.

Therefore, the compressor motor is controlled by the inverter device so that the operating frequency is PID-controlled, and the internal temperature is brought closer to the target temperature more smoothly and accurately. The PID control is executed by a PID arithmetic processing unit provided inside the control device. Specifically, the proportional (P), integral (I), and differential (D) calculations are executed from the deviation e between the target temperature and the actual internal temperature, and are reduced in proportion to the deviation e. Proportional operation for calculating the control amount in the direction, integration operation for calculating the control amount in the direction to reduce the integral value of the deviation e (the value obtained by integrating the deviation e with the target temperature in the time axis direction), and the change in the deviation e A differential operation for calculating a control amount in the direction of decreasing the inclination (differential value) is performed, and the operating frequency of the compressor motor is determined from the control amount obtained by adding these control amounts. An arithmetic expression is shown below.
Arithmetic operation amount = proportional amount + integral amount + derivative amount + fixed amount proportional amount = (current temperature-target temperature) x Kp
Integral amount = ((previous temperature−target temperature) + (current temperature−target temperature)) × Ki
Differential amount = (current temperature−previous temperature) × Kd
Fixed amount = Df
In this case, when the internal temperature approaches the target temperature and stabilizes at that temperature, the operating frequency of the compressor motor calculated from the current temperature and the target temperature is a predetermined speed lower than the minimum rotational speed. Number. Even in this case, in order to realize smooth operation of the compressor, the control device continues the operation with the compressor motor at the minimum rotation speed, and the compressor is operated when the current showcase internal temperature is lowered to the lower limit temperature. The motor was stopped.

  For this reason, even when the temperature in the storage is stable near the predetermined lower limit temperature, the compressor is continuously operated at the minimum number of revolutions, the interior is cooled to the predetermined lower limit temperature, The compressor was stopped when the internal temperature reached a predetermined lower limit temperature.

  In particular, this phenomenon occurs when the front opening of the open showcase is closed by a so-called night cover at the end of business. That is, at the time of business operation, the operating frequency of the compressor motor calculated from the difference between the internal temperature and the target temperature due to the disturbance of the air curtain formed in the front opening by opening the front opening is , Almost never less than the minimum speed. On the other hand, at the end of business, the front opening is closed by the night cover, so that the influence of disturbance is reduced and the internal temperature is stabilized near the target temperature. Therefore, even if a situation occurs where the calculated compressor operating frequency is lower than the minimum rotational speed, the compressor motor will be operated at the minimum rotational speed for a relatively long time until the internal temperature reaches the target temperature. It led to soaring running costs.

  Therefore, the present invention has been made to solve the conventional technical problems, and provides a cooling device and an open showcase that can effectively save energy when the current temperature is stabilized near a set temperature. .

  The cooling device of the present invention includes a control device that controls a compressor constituting the refrigeration cycle, and the control device performs a PID calculation based on a deviation e between the set temperature and the current temperature, By controlling the operation of the compressor between a predetermined minimum speed and a maximum speed, the current temperature is brought close to the set temperature, and if the current temperature falls to a predetermined lower limit temperature lower than the set temperature, the compressor is In addition, the compressor is stopped regardless of the current temperature when the value calculated by the PID calculation decreases to a value corresponding to a predetermined number of revolutions lower than the minimum number of revolutions of the compressor.

  The open showcase of the invention of claim 2 displays the product while cooling the product in the display chamber by circulating the cool air exchanged with the evaporator included in the refrigeration cycle in the display chamber, and opens the display chamber. A compressor that constitutes a refrigeration cycle and a control device that controls the compressor, the control device comprising a set temperature of a display room and a current temperature. A PID calculation is performed based on the deviation e, and the operation of the compressor is controlled between a predetermined minimum rotation speed and a maximum rotation speed, whereby the current temperature is brought close to the set temperature, and the current temperature is lower than the set temperature. When the temperature is lowered to the lower limit temperature of the compressor, the compressor is stopped, and when the value calculated by the PID calculation is lowered to a value corresponding to a predetermined rotational speed lower than the minimum rotational speed of the compressor, the current temperature Regardless, characterized by stopping the compressor.

  According to the present invention, in the cooling device including the control device that controls the compressor constituting the refrigeration cycle, the control device performs the PID calculation based on the deviation e between the set temperature and the current temperature, and performs a predetermined minimum By controlling the operation of the compressor between the rotation speed and the maximum rotation speed, the current temperature is brought close to the set temperature, and when the current temperature falls to a predetermined lower limit temperature lower than the set temperature, the compressor is stopped. When the calculated value by the PID calculation decreases to a value corresponding to a predetermined rotational speed lower than the minimum rotational speed of the compressor, the compressor is stopped regardless of the current temperature, so that the compressor is protected, It is possible to reduce energy consumption as a whole and realize energy saving operation.

  That is, when the current temperature approaches the set temperature and the calculated value by the PID calculation has decreased to a value corresponding to a predetermined rotation speed lower than the minimum rotation speed of the compressor, before the current temperature reaches the set temperature. Even so, the compressor is stopped as the current temperature is sufficiently lowered to the vicinity of the set temperature and stabilized. As a result, the current temperature is lowered to around the set temperature, and the operation value at the compressor minimum rotation speed higher than the calculated value is obtained even though the calculated value by the PID calculation is lower than the minimum rotation speed of the compressor. Therefore, it is possible to avoid an increase in running cost due to the continuing operation.

  According to the second aspect of the present invention, the chilled air exchanged with the evaporator included in the refrigeration cycle is circulated in the display chamber to display the product while cooling the display chamber, and the opening of the display chamber is closed. In an open showcase having a night cover, a compressor constituting a refrigeration cycle and a control device for controlling the compressor are provided, and the control device has a deviation e between the set temperature of the display room and the current temperature. The PID calculation is executed based on the above and the operation of the compressor is controlled between a predetermined minimum number of revolutions and a maximum number of revolutions to bring the current temperature closer to the set temperature, and the current temperature is lower than the set temperature. When the temperature is lowered to the temperature, the compressor is stopped, and the calculated value by the PID calculation is reduced to a value corresponding to a predetermined rotation speed lower than the minimum rotation speed of the compressor. Regardless, because stopping the compressor, while protecting the compressor, to reduce power consumption as a whole, it is possible to realize the energy-saving operation.

  For example, in the state where the opening of the display room is closed by the night cover, the influence of disturbance is small, so when the current temperature approaches the set temperature, the calculated value by the PID calculation is lower than the minimum rotation speed of the compressor. It may decrease to a value corresponding to the rotational speed. In such a case, even before the current temperature reaches the set temperature, the current temperature is sufficiently lowered to the vicinity of the set temperature and the compressor is stopped as being stable. As a result, the current temperature is lowered to around the set temperature, and the operation value at the compressor minimum rotation speed higher than the calculated value is obtained even though the calculated value by the PID calculation is lower than the minimum rotation speed of the compressor. Therefore, it is possible to avoid an increase in running cost due to the continuing operation.

  Next, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1: has shown the vertical side view of the open showcase 1 of the Example provided with the cooling device R of this invention. The open showcase 1 of an Example is a showcase which has an opening in the front surface installed in stores, such as a convenience store and a supermarket.

  The main body 2 is constituted by a heat insulating wall 3 having a substantially U-shaped cross section that opens to the front surface of the showcase 1 and side plates 4 and 4 attached to both sides thereof. A back panel 6 and a top panel 7 are disposed on the back and top surfaces of the inside of the heat insulating wall 3 with a space therebetween, and the back panel 6 and the top panel 7 and the heat insulating wall 3 are spaced from the back. A rear duct 9 extending upward is formed.

  In addition, a deck pan 10 extending forward is provided at the lower end of the back panel 6, and a display chamber 11 is formed inside the back panel 6, the top panel 7, and the deck pan 10. In the display chamber 11, shelves 12 are installed in a plurality of stages, in this embodiment, three stages up and down. A lower duct 14 communicating with the rear duct 9 and constituting a part thereof is formed below the deck pan 10.

  The upper end of the rear duct 9 communicates with the cool air discharge port 16 located at the upper edge of the front opening of the display chamber 11, and the front end of the lower duct 14 is located at the lower edge of the front opening of the display chamber 11 and is made of a plurality of slits. It communicates with the suction port 17. Further, a cool air circulation blower 19 is disposed in the lower duct 14 below the deck pan 10, and an evaporation constituting a refrigeration cycle together with a compressor 26 and a condenser 27 described later in the rear duct 9 behind the display chamber 11. A container 15 is provided vertically.

  On the other hand, a machine room 25 is formed below the heat insulating wall 3, and a compressor 26, a condenser 27, a condenser blower 28, and the like are installed in the machine room 25, as well as a power source and a control. An electrical box (not shown) that houses the substrate is also provided.

  Here, the refrigerant circuit 40 which comprises the cooling device R in a present Example is demonstrated with reference to the refrigerant circuit figure of FIG. A condenser 27 is connected to the piping 42 on the discharge side of the compressor 26. An expansion valve 44 as a pressure reducing device is connected to the outlet side of the condenser 27 via a pipe 43. The expansion valve 44 is connected to the evaporator 15, and the outlet side of the evaporator 15 is connected to the compressor 26 to constitute an annular refrigeration cycle.

  A back plate 29 made of steel plate is attached to the back of the heat insulating wall 3 with a predetermined distance from the back surface of the heat insulating wall 3, and an exhaust duct is provided between the back plate 29 and the heat insulating wall 3. 30 is configured. The lower end of the exhaust duct 30 opens and communicates with the rear portion of the machine room 25, and the upper end is open above the showcase 1. Therefore, when the condenser blower 28 is operated, the outside air sucked into the machine room 25 passes through the condenser 27 and exchanges heat, and is then blown to the compressor 26 to cool the compressor 26 by air. Then, it is discharged to the outside through the exhaust duct 30.

  A panel 31 closes the front surface of the machine room 25 so that it can be opened and closed. Reference numeral 32 denotes an evaporating dish provided in the lower part of the machine room 25, and drain water (dewed water, defrosted water, etc.) from the evaporator 15 flows in through a drain hose (not shown) and is stored.

  On the other hand, a canopy 33 is attached to the front end of the top wall 3 </ b> A of the heat insulating wall 3 so as to protrude forward over the entire width, and an illumination lamp 34 is attached to the inside of the canopy 33. Further, a holder 36 to which a night cover 35 is attached is attached to the lower surface of the canopy 33 behind the illumination lamp 34.

  The night cover 35 is wound around a torsion bar (not shown) attached to the holder 36 and is urged in the direction of being always taken up, and can be pulled out from the front end of the lower surface of the holder 36. When it is pulled out, it also has a stopper function that stops at a position halfway between full open and full close. Note that this stopper function is released by a predetermined operation of pulling downward and is wound up. As a result, the night cover 35 can be freely drawn / wound by the holder 36.

  Further, the lateral width of the night cover 35 is substantially equal to the width dimension of the front opening in the embodiment. An edge member 37 made of hard resin is attached to the front end (lower end) of the night cover 35. A handle (not shown) is formed at the center of the edge member 37. A fixing member (not shown) separately formed on the handle or the edge member 37 can be detachably attached to a locking member (not shown) formed at the upper end of the front wall 3B formed at the lower part of the heat insulating wall 3. Has been.

  Thereby, during the opening of the store, the night cover 35 is wound around the holder 36 as shown in FIG. When closing the store, the night cover 35 is pulled out from the holder 36 and lowered, and the handle or the like is locked to a locking member formed on the upper end of the front wall 3B of the heat insulating wall 3 so that the front side is outside the cold air curtain. Cover the opening. Thereby, during closing of the store, the front opening is closed with the night cover 35, whereby the cool air in the display chamber 11 can be retained in the display chamber 11, and the cooling efficiency can be improved.

  Next, the control device C in the present embodiment will be described with reference to FIG. The control device C is composed of a general-purpose microcomputer and controls the cooling device R, and includes a timer 50, a PID arithmetic processing unit 56, and a storage unit 57 as time limit means. Further, a control panel 47 having various setting switches and a display unit is connected. The various setting switches also include an LCD panel (setting means) 8 that can arbitrarily set the set temperature in the display chamber 11 as will be described in detail later. Further, on the input side of the control device C, an internal temperature sensor 48 for detecting the internal temperature, a refrigerant inlet side temperature sensor 54 for detecting the refrigerant temperature on the refrigerant inlet side of the evaporator 15, and the evaporator 15. A refrigerant outlet side temperature sensor 55 and the like for detecting the refrigerant temperature on the refrigerant outlet side are connected. Here, the in-compartment temperature sensor 48 is provided, for example, on the ceiling in the display room 11 of the showcase 1 and detects the in-compartment temperature that is the temperature in the display room 11.

  On the other hand, on the output side of the control device C, a compressor motor (DC motor) 26M that drives the compressor 26, a blower motor 19M that drives the blower 19 for circulating cold air, a blower motor 28M that drives the blower 28 for condenser, An expansion valve 44 and the like are connected. Here, the compressor motor 26M is connected via the inverter device 41, and thereby the frequency of the power source is changed, and the rotational speed of the compressor motor 26M is changed to thereby compress the refrigerant in the compressor 26. Can be changed. The blower motors 19M and 28M are connected via drive circuits 52 and 53 such as a chopper circuit, respectively, so that the rotational speed can be arbitrarily changed. And based on each output mentioned above, the control apparatus C controls the opening degree of the expansion valve 44, and controls the rotation speed of each motor 26M, 19M, and 28M.

Here, the control device C in the present embodiment performs PID control on the operation frequency of the compressor motor 26M. This PID control is executed by a PID calculation processing unit 56 provided inside the control device C. The PID calculation processing unit 56 is a temperature detected by the internal temperature sensor 48 (a space to be cooled). From the deviation e between the temperature in the display chamber 11 (hereinafter referred to as the current temperature) and the cooling target temperature set by the LCD panel 8 of the control panel 47, the proportional (P), integral (I), and differential (D) The operation is executed. Specifically, the PID arithmetic processing unit 56 calculates the control amount in a direction to decrease the current e and the target temperature in proportion to the deviation e, and the integral value of the deviation e (deviation e from the cooling target temperature e (The value obtained by integrating in the time axis direction) an integral operation for calculating a control amount in a direction to reduce, and a differential operation for calculating a control amount in a direction to reduce the slope (differential amount) of the deviation e. The operating frequency of the motor 26M of the compressor 26 is determined from the control amount obtained by adding. An arithmetic expression is shown below.
Calculation formula Control amount = Proportional amount + Integral amount + Derivative amount + Fixed amount Proportional amount = (Current temperature-Target temperature) x Kp
Integral amount = ((previous temperature−target temperature) + (current temperature−target temperature)) × Ki
Differential amount = (current temperature−previous temperature) × Kd
Fixed amount = Df
Based on the calculated control amount, the inverter device 41 controls the operating frequency of the compressor motor 26M. Here, Kp in the above equation is a proportional coefficient used for calculating the proportional quantity, Ki is an integral coefficient used for calculating the integral quantity, Kd is a differential coefficient used for calculating the differential quantity, and Df is It is a fixed amount. Each coefficient is stored in the storage unit 57 of the control device C.

  The cooling control of the showcase 1 with the above configuration will be described. The control device C detects the temperature in the display room (cooled space) 11, that is, the current temperature by the internal temperature sensor 48, and executes the cooling operation based on the current temperature and a preset target temperature. That is, when the operation of the compressor 26 is started, the high-temperature and high-pressure gas refrigerant discharged from the piping 42 on the discharge side of the compressor 26 flows out to the condenser 27. Here, since the expansion valve 44 is controlled to be opened and closed by the control device C, the sufficiently condensed and liquefied refrigerant is decompressed by the expansion valve 44 and then flows into the evaporator 15. Then, the refrigerant flowing into the evaporator 15 disposed in the back duct 9 evaporates, takes heat from the surroundings and exhibits a cooling action, and then returns to the compressor 26.

  The cool air exchanged with the evaporator 15 is discharged from the cool air discharge port 16 by a cool air circulation blower 19 disposed in the lower duct 14 communicating with the rear duct 9, and the cool air circulates in the display chamber 11. After cooling to a predetermined cooling temperature, it returns to the lower duct 14 via the cold air inlet 17 located at the lower edge of the front opening of the display chamber 11. Thereby, an air curtain by cold air is formed in the front opening of the display chamber 11, and cold air leakage and outside air intrusion in the display chamber 11 are suppressed.

  Here, specific frequency control of the compressor motor 26M will be described. In this embodiment, as described above, the PID calculation is executed based on the deviation e between the current temperature and the target temperature, and the operating frequency of the motor 26M of the compressor 26 is determined based on the control amount calculated by the PID calculation. The compressor 26 is operated between a preset minimum rotation speed (30 Hz in this embodiment) and a maximum rotation speed (80 Hz in this embodiment). As a result, the temperature in the display chamber 11 can be brought close to the target temperature with high accuracy, and overshooting, undershooting, and the accompanying hunting phenomenon can be suppressed. It becomes possible to realize temperature control with high accuracy.

  At this time, the control device C executes the OFF-Hz control in order to execute an effective energy saving operation. Hereinafter, the OFF-Hz control will be described with reference to the flowchart of FIG. With this OFF-Hz set value, the controller C first determines in step S1 whether or not the current operating frequency of the compressor motor 26M is the minimum rotation speed (lower limit value). When the operation frequency is not the minimum rotation speed, that is, when the compressor motor 26M is operated at the maximum rotation speed or the operation frequency between the minimum rotation speed and the maximum rotation speed, the process proceeds to step S5. Then, the compressor motor 26M is controlled based on the control amount calculated by the PID calculation.

  On the other hand, when the current operating frequency of the compressor motor 26M is the minimum rotation speed (here, 30 Hz) in step S1, the process proceeds to step S2. In step S2, the control device C determines whether or not the operating frequency based on the control amount currently calculated by the PID calculation is lower than the minimum rotation speed. That is, when the operating frequency based on the calculated control amount is the minimum rotational speed, the process proceeds to step S5 as described above, and the compressor motor 26M is continuously operated based on the control amount calculated by the PID calculation. Execute control.

  In step S2, when the operating frequency based on the control amount currently calculated by the PID calculation is lower than the minimum rotational speed (when it is below the minimum rotational speed), the control device C proceeds to step S3 and performs the calculation. It is determined whether or not the operation frequency based on the controlled amount is lower than a predetermined OFF-Hz set value. Here, the OFF-Hz set value is a value (in this embodiment, 25 Hz) lower by a predetermined value than the predetermined minimum rotation speed of the compressor motor 26M.

  If the operation frequency based on the calculated control amount is equal to or greater than the OFF-Hz set value, the process proceeds to step S5, and the compressor is continuously based on the control amount calculated by the PID calculation as described above. Control of the motor 26M is executed.

  On the other hand, if the operation frequency based on the calculated control amount is lower than the OFF-Hz set value, the control device C proceeds to step S4, and even if the current temperature does not reach the cooling target temperature, The operation of the compressor 26 is stopped. Thereafter, the process proceeds to step S5, and the compressor motor 26M is controlled based on the control amount calculated by the PID calculation.

  However, after the compressor 26 is stopped, the start-up of the compressor 26 is prohibited for a predetermined protection period (for example, about 5 minutes) in order to prevent a short cycle. Therefore, the control of the compressor motor 26M based on the calculated control amount in step S5 is performed after the protection period has elapsed. In this case, as the protection period elapses, the internal temperature is naturally affected by the outside air temperature. When the internal temperature detected by the internal temperature sensor 48 rises to a predetermined compressor ON point, the control device C controls the compressor motor 26M based on the control amount calculated by the PID calculation. Execute.

  Thus, in the control of the operating frequency of the compressor motor 26M according to the present invention, the current temperature detected by the internal temperature sensor 48 approaches the set temperature, and the calculated value by the PID calculation is less than the minimum rotation speed of the compressor motor 26M. If the current temperature falls to a value corresponding to a low predetermined rotational speed (here, OFF-Hz set value), the current temperature is sufficiently close to the set temperature even before the current temperature reaches the set temperature. And the compressor motor 26M is stopped as a stable one.

  As a result, the current temperature decreases to near the set temperature, and the compressor motor 26M higher than the calculated value is obtained even though the calculated value by the PID calculation is sufficiently lower than the minimum rotational speed of the compressor motor 26M. It is possible to avoid an increase in running cost due to continued operation at the minimum rotation speed. Therefore, when the internal temperature is stabilized near the set temperature while appropriately protecting the compressor 26, the compressor motor 26M is stopped, thereby reducing the overall power consumption and saving energy. Driving can be realized.

  In particular, in the open showcase 1 having a large opening on the front surface as in this embodiment, the operating frequency of the compressor motor 26M calculated by the PID calculation due to the influence of disturbance due to customer traffic, etc. when the store is opened is Almost no lower than the minimum speed. However, when the front opening is closed by the night cover 35 as described above when the store is closed, the influence of the disturbance is small and the influence of air conditioning in the store is also small. In such a case, unlike when the store is opened, when the current temperature detected by the internal temperature sensor 48 approaches the set temperature, the calculated value by the PID calculation is lower than the minimum rotation speed of the compressor motor 26M. The value may decrease to a value corresponding to the number (OFF-Hz set value).

  In such a case, even before the current temperature reaches the set temperature (compressor OFF point), the current temperature is sufficiently lowered to the vicinity of the set temperature and the compressor 26 is stopped as being stable. As a result, the current temperature is reduced to around the set temperature, and the calculated value by the PID calculation is lower than the minimum rotation speed of the compressor 26, but the minimum rotation of the compressor motor 26M that is higher than the calculated value. It is possible to avoid an increase in running cost due to continued operation by number.

  Therefore, when the internal temperature is stabilized near the set temperature while appropriately protecting the compressor 26, the compressor motor 26M is stopped, thereby reducing the overall power consumption and saving energy. Driving can be realized.

  In addition, although the present Example demonstrates the case where the cooling device R which performs the said control was provided in the open showcase 1, it is not limited to this, For example, a front opening can be opened and closed freely by a door. This is effective even in the case of a showcase that closes.

It is a vertical side surface of the open showcase of the Example provided with the cooling device of this invention. It is a refrigerant circuit diagram. It is an electrical block diagram of a control apparatus. It is a flowchart of OFF-Hz control.

Explanation of symbols

R Cooling device C Control device 1 Open showcase 11 Display room 15 Evaporator 16 Cold air outlet 17 Cold air inlet 19 Cooling air circulation blower 25 Machine room 26 Compressor 26M Compressor motor 27 Condenser 44 Expansion valve 47 Control panel 48 Warehouse Internal temperature sensor 56 PID calculation processing unit 57 Storage unit

Claims (2)

In a cooling device provided with a control device for controlling a compressor constituting a refrigeration cycle,
The control device performs a PID calculation based on a deviation e between the set temperature and the current temperature, and controls the operation of the compressor between a predetermined minimum speed and a maximum speed, thereby reducing the current temperature. When approaching the set temperature and the current temperature is lowered to a predetermined lower limit temperature lower than the set temperature, the compressor is stopped,
Cooling, wherein the compressor is stopped regardless of the current temperature when the calculated value by the PID calculation is reduced to a value corresponding to a predetermined rotational speed lower than the minimum rotational speed of the compressor. apparatus. Opening provided with a night cover for closing the opening of the display chamber while circulating the cool air exchanged with the evaporator included in the refrigeration cycle in the display chamber while cooling the product in the display chamber In the showcase,
A compressor constituting the refrigeration cycle;
A control device for controlling the compressor;
The control device performs a PID calculation based on a deviation e between the set temperature of the display room and the current temperature, and controls the operation of the compressor between a predetermined minimum speed and a maximum speed, When the current temperature is brought close to the set temperature and the current temperature falls to a predetermined lower limit temperature lower than the set temperature, the compressor is stopped,
The open is characterized in that the compressor is stopped regardless of the current temperature when the calculated value by the PID calculation is reduced to a value corresponding to a predetermined rotational speed lower than the minimum rotational speed of the compressor. Showcase.

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