AIR-CONDITIONING APPARATUS

Abstract

An air-conditioning apparatus includes a controller that has: a first determination value obtained by multiplying by a first constant, a time constant which is a response value required until heating and cooling capacity in refrigeration circulation in a refrigeration cycle circuit is stabilized, and which is a value at a rise of a waveform representing the heating and cooling capacity calculated from a measured value; and a second determination value obtained by multiplying the time constant by a second constant smaller than the first constant. The controller increases the lowest frequency for a subsequent operation of the compressor, when in the intermittent operation, an intermittent operation time from the start to stop of the compressor is longer than the first determination value, and deceases the lowest frequency for the above subsequent operation, when in the intermittent operation, the intermittent operation time is shorter than the second determination value.

Description

TECHNICAL FIELD

The present disclosure relates to an air-conditioning apparatus using a refrigeration cycle circuit.

BACKGROUND ART

In an existing air-conditioning apparatus that performs a cooling operation, when the temperature of outdoor air is reduced and the frequency of a compressor is reduced, there is a risk that the compressor may deviate from its operation range because a condensing temperature is reduced and a discharge pressure is also reduced. In view of this point, an air-conditioning apparatus has been proposed that is capable of changing the minimum value (lowest compressor frequency) at which the compressor can operate to continue the cooling operation without deviating from a use range of the compressor (see, for example, Patent Literature 1).

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2017-53527

SUMMARY OF INVENTION

Technical Problem

However, in an air-conditioning apparatus of Patent Literature 1, in a load where a cooling capacity becomes excessive even when the compressor operates at the lowest frequency, an actual indoor temperature is at or below a target indoor temperature set in the air-conditioning apparatus, and the air-conditioning apparatus will thus eventually stop its operation. Then, in the air-conditioning apparatus of Patent Literature 1, when the indoor temperature rises, the air-conditioning apparatus is re-started. Consequently, the air-conditioning apparatus performs an intermittent operation in which the air-conditioning apparatus repeatedly stops and starts. In the intermittent operation, when the minimum frequency is too low, it takes a long time to start a refrigeration cycle circuit, and energy consumption is thus increased. By contrast, when the minimum frequency is too high, the indoor temperature reaches the target temperature or lower before the refrigeration cycle circuit is stabilized, and the air-conditioning apparatus is stopped. Consequently, the efficiency of the operation is low.

The present disclosure is applied to solve such problems as described above, and relates to an air-conditioning apparatus capable of performing an efficient operation by promptly stabilizing the refrigeration cycle circuit even in an intermittent operation.

Solution to Problem

An air-conditioning apparatus according to the embodiment of the present disclosure includes: a compressor configured to compress refrigerant; a condenser configured to condense the refrigerant discharged from the compressor; a pressure-reducing device configured to reduce a pressure of the refrigerant which flows from the condenser; an evaporator configured to evaporate the refrigerant decompressed by the pressure-reducing device; a temperature sensor configured to measure a temperature of the refrigerant in the evaporator; and a controller configured to control the compressor and the pressure-reducing device to cause a heating operation or a cooling operation to be performed, configured to cause an intermittent operation to be performed, the intermittent operation being an operation in which an operation of the compressor is stopped when a cooling load or a heating load decreases and the operation of the compressor is started when the cooling load or the heating load increases, and configured to cause, when the operation of the compressor is performed in the intermittent operation, the intermittent operation to be performed at a lowest frequency set as a lower limit frequency in a frequency range that is set for the operation of the compressor. The controller has: a first determination value obtained by multiplying by a first constant set in advance, a time constant which is a response value required until a heating and cooling capacity in circulation of the refrigerant in a refrigeration cycle circuit is stabilized, and which is a value at a rise of a waveform representing the heating and cooling capacity calculated from a value measured by the temperature sensor; and a second determination value obtained by multiplying the time constant by a second constant set in advance that is smaller than the first constant. The controller is configured to increase the lowest frequency for a subsequent operation of the compressor, when in the intermittent operation, an intermittent operation time from a start of the operation of the compressor to a stop of the operation of the compressor is longer than the first determination value, and configured to decrease the lowest frequency for the subsequent operation of the compressor, when in the intermittent operation, the intermittent operation time is shorter than the second determination value.

Advantageous Effects of Invention

In the air-conditioning apparatus according to the embodiment of the present disclosure, in the intermittent operation, when the intermittent operation time from the start of the operation of the compressor to the stop of the operation of the compressor is longer than the first determination value, the controller increases the lowest frequency for a subsequent operation of the compressor, and when the intermittent operation time is shorter than the second determination value, the controller decreases the lowest frequency for the subsequent operation of the compressor. Because the controller changes the lowest frequency of the compressor based on the intermittent operation time in the intermittent operation, the refrigeration cycle circuit can be promptly stabilized even when the intermittent operation is performed, and thus the air-conditioning apparatus can perform an efficient operation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a refrigerant circuit diagram in a cooling operation of an air-conditioning apparatus according to an embodiment.

FIG. 2 is a refrigerant circuit diagram in a heating operation of the air-conditioning apparatus according to the embodiment.

FIG. 3 is a block diagram illustrating an example of the configuration of a controller as illustrated in FIG. 1.

FIG. 4 indicates a relationship between a compressor frequency of a compressor and performance of a refrigeration cycle circuit in the air-conditioning apparatus according to the embodiment.

FIG. 5 is an explanatory view for the cooling operation of the air-conditioning apparatus according to the embodiment.

FIG. 6 is an explanatory view for the heating operation of the air-conditioning apparatus according to the embodiment.

FIG. 7 indicates operation waveforms in an intermittent operation in the case where a lowest frequency Fmin is excessively low.

FIG. 8 indicating operation waveforms in the intermittent operation in the case where the lowest frequency Fmin is excessively high.

FIG. 9 indicates operation waveforms in the intermittent operation in the case where the lowest frequency Fmin is appropriate.

FIG. 10 is a flowchart of a control flow for changing the lowest frequency Fmin in the cooling operation of the air-conditioning apparatus according to the embodiment.

FIG. 11 is a flowchart of a control flow for changing the lowest frequency Fmin in the heating operation of the air-conditioning apparatus according to the embodiment.

DESCRIPTION OF THE EMBODIMENTS

The embodiment according to the present disclosure will be described below with reference to the drawings. In each of figures that will be referred to below and include FIG. 1, components which are the same as or equivalent to those in a previous figure or previous figures are denoted by the same reference signs, and the same is true of the entire text of the following descriptions concerning the embodiment. Moreover, regarding the embodiment, components which are the same as or equivalent to those in a previous description or previous descriptions will be denoted by the same reference signs, and their descriptions may be omitted. Furthermore, configurations of components in the entire text of the specification are mere examples, and their descriptions are not limiting. In addition, regarding the embodiment, components can be partially combined, even if it is not expressly stated, unless such a combination gives rise to a problem.

Embodiment

Configuration of Air-Conditioning Apparatus 100

FIG. 1 is a refrigerant circuit diagram in a cooling operation of an air-conditioning apparatus 100 according to the embodiment. FIG. 2 is a refrigerant circuit diagram in a heating operation of the air-conditioning apparatus 100 according to the embodiment. It should be noted that solid arrows in FIG. 1 each indicate the flow of refrigerant in the cooling operation, and solid arrows in FIG. 2 each indicate the flow of refrigerant in the heating operation.

As indicated in FIGS. 1 and 2, the air-conditioning apparatus 100 according to the embodiment includes an outdoor unit 110 and an indoor unit 120. The outdoor unit 110 is installed outside an indoor space, and the indoor unit 120 is installed in the indoor space. The outdoor unit 110 and the indoor unit 120 are connected by a first extension pipe 61 and a second extension pipe 62, which form part of refrigerant pipes 65. In the embodiment, although the air-conditioning apparatus 100 including a single outdoor unit 110 and a single indoor unit 120 will be described, it is an example; that is, the air-conditioning apparatus 100 may include two or more outdoor units 110 and two or more indoor units 120.

The outdoor unit 110 includes a compressor 10, a flow switching device 20, an outdoor heat exchanger 30, a pressure-reducing device 40, and an outdoor fan 35. In addition, the outdoor unit 110 includes an outdoor temperature sensor 31.

The indoor unit 120 includes an indoor heat exchanger 50 and an indoor fan 55. In addition, the indoor unit 120 includes an indoor temperature sensor 51 and an indoor suction temperature sensor 52. It should be noted that in the air-conditioning apparatus 100 according to the embodiment, although the outdoor unit 110 includes the pressure-reducing device 40, the indoor unit 120 may include the pressure-reducing device 40 instead of the outdoor unit 110.

The air-conditioning apparatus 100 further includes a controller 70. The controller 70 controls various operations, such as the cooling operation and the heating operation in the outdoor unit 110 and the indoor unit 120. It should be noted that in the air-conditioning apparatus 100 according to the embodiment, although the outdoor unit 110 includes the controller 70, the indoor unit 120 may include the controller 70 instead of the outdoor unit 110.

In the air-conditioning apparatus 100, the compressor 10, the flow switching device 20, the outdoor heat exchanger 30, the pressure-reducing device 40, and the indoor heat exchanger 50 are connected by the refrigerant pipes 65, whereby a refrigerant circuit 60 is formed in which refrigerant circulates. In the air-conditioning apparatus 100, by a switching operation of the flow switching device 20, the flow direction of refrigerant that flows in the refrigerant circuit 60 is changed to cause the cooling operation or the heating operation to be performed.

The compressor 10 sucks low-temperature and low-pressure refrigerant, compresses the sucked refrigerant to change it into high-temperature and high-pressure refrigerant, and discharges the high-temperature and high-pressure refrigerant. The compressor 10 includes, for example, an inverter. The compressor 10 is controlled in capacity, which is a delivery amount per unit time, when its operation frequency is changed. The operation frequency of the compressor 10 is controlled by the controller 70.

The flow switching device 20 is, for example, a four-way valve. The flow switching device 20 switches the flow direction of the refrigerant between a plurality of flow directions to switch the operation of the air-conditioning apparatus 100 between the cooling operation and the heating operation of the air-conditioning apparatus 100. In the cooling operation, the state of the flow switching device 20 is switched to a state indicated by solid lines in FIG. 1 such that a discharge side of the compressor 10 is connected to the outdoor heat exchanger 30. In the heating operation, the state of the flow switching device 20 is switched to a state indicated by solid lines in FIG. 2 such that the discharge side of the compressor 10 is connected to the indoor heat exchanger 50. The switching of the flow passages by the flow switching device 20 is controlled by the controller 70.

The outdoor heat exchanger 30 causes heat exchange to be performed between outdoor air and refrigerant that flows in the outdoor heat exchanger 30. In the cooling operation, the outdoor heat exchanger 30 operates as a condenser that condenses the refrigerant by transferring heat of the refrigerant to the outdoor air. The outdoor heat exchanger 30 which operates as a condenser condenses refrigerant discharged from the compressor 10. In the heating operation, the outdoor heat exchanger 30 operates as an evaporator that evaporates the refrigerant, and by using an evaporation heat generated thereby, cools outdoor air. The outdoor heat exchanger 30 which operates as an evaporator evaporates refrigerant which is decompressed by the pressure-reducing device 40.

The outdoor fan 35 supplies outdoor air to the outdoor heat exchanger 30. Outlined arrows toward the outdoor heat exchanger 30 as indicated in FIGS. 1 and 2 each indicate the flow of air supplied by the outdoor fan 35. In the outdoor fan 35, the rotation speed of a motor (not illustrated) included in the outdoor fan 35 is controlled by the controller 70. In the air-conditioning apparatus 100, the rotation speed of the motor included in the outdoor fan 35 is controlled by the controller 70, thereby adjusting the amount of air to be supplied to the outdoor heat exchanger 30.

The pressure-reducing device 40 decompresses refrigerant that has flowed out from the condenser. The pressure-reducing device 40 is, for example, an electronic expansion valve whose opening degree of an aperture can be adjusted. The pressure-reducing device 40 is adjusted in opening degree to control the pressure of refrigerant that flows into the outdoor heat exchanger 30 or the indoor heat exchanger 50. The opening degree of the pressure-reducing device 40 is controlled by the controller 70.

The outdoor temperature sensor 31 is provided at the outdoor heat exchanger 30. When the air-conditioning apparatus 100 is in the cooling operation, the outdoor temperature sensor 31 measures a condensing temperature. When the air-conditioning apparatus 100 is in the heating operation, the outdoor temperature sensor 31 measures an evaporating temperature. That is, when the air-conditioning apparatus 100 is in the heating operation, the outdoor temperature sensor 31 measures the temperature of the refrigerant in the evaporator. The temperature measured by the outdoor temperature sensor 31 is received by the controller 70.

The indoor heat exchanger 50 causes heat exchange to be performed between indoor air and refrigerant that flows in the indoor heat exchanger 50. In the cooling operation, the indoor heat exchanger 50 operates as an evaporator that evaporates the refrigerant, and using an evaporation heat generated thereby, cools the indoor air. The indoor heat exchanger 50 which operates as an evaporator evaporates the refrigerant which is decompressed by the pressure-reducing device 40. In the heating operation, the indoor heat exchanger 50 operates as a condenser that condenses the refrigerant by transferring heat of the refrigerant to indoor air. The indoor heat exchanger 50 which operates as a condenser condenses the refrigerant discharged from the compressor 10.

The indoor fan 55 supplies indoor air to the indoor heat exchanger 50. The outlined arrows toward the indoor heat exchanger 50 as indicated in FIGS. 1 and 2 each indicate the flow of air supplied by the indoor fan 55. In the indoor fan 55, the rotation speed of a motor (not illustrated) included in the indoor fan 55 is controlled by the controller 70. In the air-conditioning apparatus 100, the rotation speed of the motor included in the indoor fan 55 is controlled by the controller 70, thereby adjusting the amount of air to be supplied to the indoor heat exchanger 50.

The indoor temperature sensor 51 is provided at the indoor heat exchanger 50. When the air-conditioning apparatus 100 is in the cooling operation, the indoor temperature sensor 51 measures an evaporating temperature. That is, when the air-conditioning apparatus 100 is in the cooling operation, the indoor temperature sensor 51 measures the temperature of the refrigerant in the evaporator. In addition, when the air-conditioning apparatus 100 is in the heating operation, the indoor temperature sensor 51 measures a condensing temperature.

The indoor suction temperature sensor 52 measures the temperature of indoor air that flows into the indoor heat exchanger 50. That is, the indoor suction temperature sensor 52 measures an indoor temperature in an air-conditioning target space in which the condenser or the evaporator is located. The indoor suction temperature sensor 52 is provided close to the indoor heat exchanger 50. The temperatures measured by the indoor temperature sensor 51 and the indoor suction temperature sensor 52 are received as data by the controller 70.

FIG. 3 is a block diagram illustrating an example of the configuration of the controller 70 as indicated in FIG. 1. The controller 70 is dedicated hardware or a central processing unit (CPU) that executes a program stored in a memory. It should be noted that the CPU is also called a central processor, a processing device, an arithmetic device, a microprocessor, a microcomputer, or a processor.

In the case where the controller 70 is dedicated hardware, the controller 70 corresponds to, for example, a single-component circuit, a composite circuit, an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or a combination of those circuits. Each of functions that are fulfilled by the controller 70 may be fulfilled by respective hardware or may be fulfilled by single hardware. The controller 70 may be configured such that some of the functions are fulfilled by dedicated hardware and other functions are fulfilled by software or firmware.

In the case where the controller 70 is the CPU, each of the functions that are fulfilled by the controller 70 is fulfilled by software, firmware, or a combination of software and firmware. The software or the firmware is written as a program and is stored in a memory. The CPU reads out and executes each of programs stored in the memory, thereby fulfilling each of the functions of the controller 70. The memory is a non-volatile or volatile semiconductor memory, such as, a random access memory (RAM), a read-only memory (ROM), a flash memory, an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM).

The controller 70 controls the compressor 10 and the pressure-reducing device 40 to cause the heating operation or the cooling operation to be performed. When causing the heating operation or the cooling operation to be performed, the controller 70 may control the rotation speeds of the outdoor fan 35 and the indoor fan 55. The controller 70 controls various operations of the air-conditioning apparatus 100, such as the cooling operation and the heating operation, and exerts control for maintaining or changing a set indoor temperature or other conditions.

When causing the operation of the compressor 10 to be performed in an intermittent operation, the controller 70 causes the intermittent operation of the compressor 10 to be performed at the minimum frequency set as the lowest frequency in a range of frequency that is set for the operation of the compressor 10. The intermittent operation is an operation in which the operation of the compressor 10 is stopped when a cooling load or a heating load is decreased to a small value and the operation of the compressor 10 is started when the cooling load or the heating load is increased to a great value.

As indicated in FIG. 3, the controller 70 includes an operation-state determination circuitry 71, a storage circuitry 72, and a timing circuitry 73. In addition, the controller 70 includes a compressor control circuitry 74, a pressure-reducing mechanism control circuitry 75, and a fan control circuitry 76.

The operation-state determination circuitry 71 makes a determination in a control flow in the cooling operation or the heating operation, which will be described later, based on operation information which indicates an operation state of the air-conditioning apparatus 100 and which is supplied from the outside, and measurement information provided by various sensors. The operation-state determination circuitry 71 receives measurement data from the outdoor temperature sensor 31, the indoor temperature sensor 51, and the indoor suction temperature sensor 52.

The storage circuitry 72 stores in advance a program, data, etc., which are required for control by the controller 70. The storage circuitry 72 stores information and other data which are necessary for the controller 70 to control devices included in the air-conditioning apparatus 100. The timing circuitry 73 is, for example, a timer or a real-time clock, and is used to obtain a current time and to measure a set time.

The compressor control circuitry 74 controls the operation frequency of the compressor 10 to control the rotation speed of the compressor 10. The pressure-reducing mechanism control circuitry 75 controls the opening degree of the pressure-reducing device 40. The fan control circuitry 76 controls the rotation speeds of the outdoor fan 35 and the indoor fan 55.

FIG. 4 is a diagram illustrating a relationship between a compressor frequency Fa of the compressor 10 and performance of a refrigeration cycle circuit in the air-conditioning apparatus 100 according to the embodiment. Characteristics of the compressor frequency Fa [Hz] of the compressor 10 will be described below with reference to FIG. 4.

In FIG. 4, (a), he horizontal axis represents the compressor frequency Fa [Hz] of the compressor 10, and the vertical axis represents the coefficient of performance (COP), which is the energy consumption efficiency. That is, FIG. 4, (a), indicates the COP for the compressor frequency Fa [Hz]. The COP is a value that indicates a cooling capacity or a heating capacity per 1 kW of energy consumption in a rated cooling operation or a rated heating operation. In the cooling operation, the COP is represented by the ratio of cooling capacity (kW) to energy consumption (kW) (cooling capacity (kW)/energy consumption). In the heating operation, the COP is represented by the ratio of heating capacity (kW) to energy consumption (kW) (heating capacity (kW)/energy consumption).

In FIG. 4, (b), the horizontal axis represents the compressor frequency Fa [Hz] of the compressor 10, and the vertical axis represents the capacity Q [kW] of the air-conditioning apparatus 100. The capacity Q [kW] of the air-conditioning apparatus 100 is a cooling capacity [kW] in the cooling operation and a heating capacity [kW] in the heating operation. That is, FIG. 4, (b), indicates the capacity Q [kW] of the air-conditioning apparatus 100 for the compressor frequency Fa [Hz].

A COP maximum frequency F1 [Hz] in FIG. 4 is the frequency of the compressor 10 at which the energy consumption efficiency increases to the maximum, and a lower limit frequency F2 [Hz] is the lower limit of the frequency at which the compressor 10 can operate. In addition, an upper limit frequency F3 [Hz] is the higher limit of the frequency at which the compressor 10 can safely operate without failure. A lowest frequency Fmin [Hz] is the frequency of the compressor 10 at which the compressor 10 starts the intermittent operation. The lowest frequency Fmin [Hz] is set as the lowest frequency in a range of frequency that is set for the operation of the compressor 10.

As described above, the lowest frequency Fmin [Hz] is the lowest frequency set in the controller 70 for the operation of the compressor 10, and is the lowest frequency in a control mode of control by the controller 70. That is, the lowest frequency Fmin [Hz] is the lower limit value of the range of frequency that is set by a user or the controller 70. On the other hand, the lower limit frequency F2 [Hz] is a frequency required for rotating the motor (not illustrated) included in the compressor 10 and is the minimum frequency required for safely operating the compressor 10 without failure. The compressor control circuitry 74 of the controller 70 operates the compressor 10 at the lowest frequency Fmin [Hz] or higher.

In the air-conditioning apparatus 100 according to the embodiment, the lowest frequency Fmin [Hz] can be changed by the compressor control circuitry 74 of the controller 70. That is, the compressor control circuitry 74 can change the position of the lowest frequency Fmin [Hz] in the horizontal axis, as indicated in FIG. 4.

As indicated in FIG. 4, (b), in the air-conditioning apparatus 100, the capacity Q increases as the compressor frequency Fa [Hz] is increased. In addition, as indicated in FIG. 4, (a), in the air-conditioning apparatus 100, regarding the COP, a further upward projecting curve is drawn as the compressor frequency Fa [Hz] is increased. As indicated in FIG. 4, (a), in the air-conditioning apparatus 100, regarding the COP, an upward projecting curve is drawn between the lower limit frequency F2 [Hz] and the upper limit frequency F3 [Hz], and the COP maximum frequency F1 [Hz] at which the energy consumption efficiency increases to the maximum is present between the lower limit frequency F2 [Hz] and the upper limit frequency F3 [Hz].

Cooling Operation

Flows of the refrigerant in operations of the air-conditioning apparatus 100 according to the embodiment will be described. First of all, the cooling operation will be described with reference to FIG. 1. In the cooling operation, the refrigerant sucked by the compressor 10 is compressed by the compressor 10 to change into high-temperature and high-pressure gas refrigerant, and the high-temperature and high-pressure gas refrigerant is then discharged therefrom. The high-temperature and high-pressure gas refrigerant discharged from the compressor 10 passes through the flow switching device 20 and flows into the outdoor heat exchanger 30 which operates as a condenser.

The refrigerant that has flowed into the outdoor heat exchanger 30 exchanges heat with outdoor air supplied by the outdoor fan 35. In the outdoor heat exchanger 30, the refrigerant transfers heat to the outdoor air and is thus condensed and liquefied, that is, is changed into liquid refrigerant. At that time, the outdoor air is heated through the heat exchange with the refrigerant. The liquid refrigerant that has flowed out from the outdoor heat exchanger 30 flows into the pressure-reducing device 40. In the pressure-reducing device 40, the refrigerant is decompressed and expanded to change into low-temperature and low-pressure two-phase gas-liquid refrigerant. The two-phase gas-liquid refrigerant that has flowed out from the pressure-reducing device 40 passes through the first extension pipe 61 which connects the outdoor unit 110 and the indoor unit 120, and flows into the indoor heat exchanger 50 which operates as an evaporator.

The refrigerant that has flowed into the indoor heat exchanger 50 exchanges heat with indoor air supplied by the indoor fan 55, and is thus evaporated and gasified. At that time, the indoor air is cooled, and cooling is thus performed in the indoor space. Then, the evaporated low-temperature and low-pressure gas refrigerant passes through the second extension pipe 62 which connects between the indoor unit 120 and the outdoor unit 110, and re-flows into the flow switching device 20. The refrigerant that has passed through the flow switching device 20 is sucked into the compressor 10, and re-compressed by the compressor 10 to change into high-temperature and high-pressure gas refrigerant, and the high-temperature and high-pressure gas refrigerant is then discharged from the compressor 10.

Heating Operation

Next, the heating operation will be described with reference to FIG. 2. In the heating operation, the refrigerant sucked into the compressor 10 is compressed by the compressor 10 to change into high-temperature and high-pressure gas refrigerant, and the high-temperature and high-pressure gas refrigerant is discharged therefrom. The high-temperature and high-pressure gas refrigerant discharged from the compressor 10 passes through the flow switching device 20 and the second extension pipe 62 which connects the outdoor unit 110 and the indoor unit 120, and flows into the indoor heat exchanger 50 which operates as a condenser.

The refrigerant that has flowed into the indoor heat exchanger 50 exchanges heat with indoor air supplied by the indoor fan 55. At the indoor heat exchanger 50, the refrigerant transfers heat to the indoor air and is thus condensed and liquefied to change into liquid refrigerant. At that time, the indoor air is heated, and heating is thus performed in the indoor space. The liquid refrigerant that has flowed out from the indoor heat exchanger 50 passes through the first extension pipe 61 which connects the indoor unit 120 and the outdoor unit 110, and flows into the pressure-reducing device 40. The liquid refrigerant that has flowed into the pressure-reducing device 40 is decompressed and expanded to change into low-temperature and low-pressure two-phase gas-liquid refrigerant. The low-temperature and low-pressure two-phase gas-liquid refrigerant that has flowed out from the pressure-reducing device 40 flows into the outdoor heat exchanger 30 which operates as an evaporator.

The refrigerant that has flowed into the outdoor heat exchanger 30 exchanges heat with outdoor air supplied by the outdoor fan 35, and is thus evaporated and gasified. At that time, the outdoor air is cooled through the heat exchange with the refrigerant. Then, the evaporated low-temperature and low-pressure gas refrigerant passes through the flow switching device 20, and is sucked into the compressor 10. The refrigerant sucked into the compressor 10 is re-compressed by the compressor 10 to change into high-temperature and high-pressure gas refrigerant, and the high-temperature and high-pressure gas refrigerant is then discharged from the compressor 10.

Intermittent Operation of Air-Conditioning Apparatus 100

The intermittent operation of the air-conditioning apparatus 100 will be described. In the case where outdoor air temperature is low in the cooling operation, the case where outdoor air temperature is high in the heating operation, or the case where the heat insulation capacity of a building is high and a thermal load is small, the air-conditioning apparatus 100 reduces the frequency of the compressor 10 provided with an inverter. Especially, when the load is small for the capacity in the case where the compressor 10 operates at the lower limit frequency F2 [Hz], the indoor temperature is reduced to a temperature lower than a set temperature, causing the air-conditioning apparatus 100 to perform the intermittent operation in which the compressor 10 is repeatedly turned on and off. The intermittent operation in the cooling operation and the intermittent operation in the heating operation in the air-conditioning apparatus 100 will be described.

Intermittent Operation in Cooling Operation

In the case where the load is small for the cooling capacity of the air-conditioning apparatus 100, the indoor temperature gradually decreases because the cooling capacity is large for the load. In order that excess cooling of the indoor space be prevented by the air-conditioning apparatus 100, the compressor control circuitry 74 of the controller 70 controls the operation of the compressor 10 with a certain range for a set temperature Tset of the air-conditioning apparatus 100. It should be noted that the set temperature Tset is a target indoor temperature, and may be stored in, for example, the storage circuitry 72 in advance, or may be stored in the storage circuitry 72 or a similar unit of the controller 70 by an operation device, such as a remote controller, which is operated by a user. In addition, the certain range may be stored in the storage circuitry 72 in advance as a set value α [degrees C] The set value α [degrees C] may be changed by the user or may be changed by the controller 70 according to the operation of the air-conditioning apparatus 100. The set value α [degrees C] is, for example, 0.5 [degrees C] or 1 [degree C].

When the indoor temperature falls below the set temperature Tset−α, the operation-state determination circuitry 71 determines that the cooling load decreases. Then, the compressor control circuitry 74 of the controller 70 stops the operation of the compressor 10 to stop the cooling operation. It should be noted that the temperature at which the controller 70 stops the cooling operation is called off-point. After that, when the indoor temperature gradually rises to exceed the set temperature Tset+α due to the stop of operation of the compressor 10, the operation-state determination circuitry 71 determines that the cooling load increases. Then, the compressor control circuitry 74 of the controller 70 starts the operation of the compressor 10 to resume the cooling operation. It should be noted that the temperature at which the controller 70 starts the cooling operation is called on-point. Because the air-conditioning apparatus 100 performs such an operation, in the case where the load is low, the intermittent operation in which the cooling operation is repeatedly stopped and started is performed.

Intermittent Operation in Heating Operation

When the load is small for the heating capacity of the air-conditioning apparatus 100, because the heating capacity is large for the load, the indoor temperature gradually increases. In the air-conditioning apparatus 100, in order to prevent excess heating of the indoor space, the compressor control circuitry 74 of the controller 70 controls the operation of the compressor 10 with a certain range α [degrees C] for the set temperature Tset of the air-conditioning apparatus 100.

When the indoor temperature rises to exceed the set temperature Tset+α, the operation-state determination circuitry 71 determines that the heating load decreases. Then, the compressor control circuitry 74 of the controller 70 stops the operation of the compressor 10 to stop the heating operation. It should be noted that the temperature at which the controller 70 stops the heating operation is called off-point. After that, when the indoor temperature gradually decreases and falls below the set temperature Tset−α due to the stop of operation of the compressor 10, the operation-state determination circuitry 71 determines that the heating load increases. Then, the compressor control circuitry 74 of the controller 70 starts the operation of the compressor 10 to resume the heating operation. It should be noted that the temperature at which the controller 70 starts the heating operation is called on-point. Because the air-conditioning apparatus 100 performs such an operation, in the case where the load is low, an intermittent operation in which the heating operation is repeatedly stopped and started is performed.

Cooling Operation of Air-Conditioning Apparatus 100

FIG. 5 is an explanatory view for the cooling operation of the air-conditioning apparatus 100 according to the embodiment. The cooling operation of the air-conditioning apparatus 100 will be described with reference to FIG. 5. The horizontal axis in FIG. 5 represents time tm. FIG. 5, (a), to FIG. 5, (d), indicate respective relationships between four indexes and the time tm.

In FIG. 5, (a), the vertical axis represents the cooling load. FIG. 5, (a), indicates the relationship between the time tm and the cooling load. In FIG. 5, (b), the vertical axis represents the indoor temperature tr [degrees C]. FIG. 5, (b) indicates the relationship between the time tm and the indoor temperature tr [degrees C]. In FIG. 5, (c), the vertical axis represents the capacity Q [kW]. FIG. 5, (c), indicates the relationship between the time tm and the capacity Q [kW]. In FIG. 5, (d), the vertical axis represents the compressor frequency Fa [Hz]. FIG. 5, (d) indicates the relationship between the time tm and the compressor frequency Fa [Hz].

As indicated in FIG. 5, the operation of the air-conditioning apparatus 100 includes operations for five time zones 1 to 5 in passage of the time tm. With reference to FIG. 5, the operations of the air-conditioning apparatus 100 in the five time zones which are time zone 1 to time zone 5 will be described.

Time Zone 1

In time zone 1, as indicated in FIG. 5, (a), the cooling load is large and the air-conditioning apparatus 100 is in a state in which the compressor 10 can continuously operate. Because the compressor control circuitry 74 of the air-conditioning apparatus 100 controls the frequency [Hz] of the compressor 10 such that the indoor temperature reaches the set temperature Tset, the frequency [Hz] changes depending on the variation of the load. More specifically, as indicated in FIG. 5, (d), the frequency [Hz] of the compressor 10 lowers with the passage of time tm. It should be noted that in time zone 1, because the operation of the compressor 10 has just started and the time tm has not passed so much since the operation of the compressor 10 started, the indoor temperature tr [degrees C] has not yet lowered, as indicated in FIG. 5, (b).

Time Zone 2

In time zone 2, the load gradually decreases as indicated in FIG. 5, (a), and the compressor 10 is continuously operated at the lower limit frequency F2 [Hz] at which the compressor 10 can operate, as indicated in FIG. 5, (d). Then, when the load is decreased to a small value for the capacity at which the operation is performed at the lower limit frequency F2 [Hz], the capacity Q is excessively large. Thus, in the cooling operation, the indoor temperature tr [degrees C] gradually decreases and reaches the off-point. When the indoor temperature tr [degrees C] reaches the off-point, the compressor 10 starts an intermittent operation from time zone 3 onward, as indicated in FIG. 5, (d). It should be noted that in FIG. 5, (d), the compressor 10 is in the stopped state at times when the frequency [Hz] of the compressor 10 is zero.

Time Zone 3

In time zone 3, the compressor 10 performs the intermittent operation at the lower limit frequency F2 [Hz]. In time zone 3, when the compressor 10 is in operation, the lowest frequency Fmin [Hz] and the lower limit frequency F2 [Hz] of the compressor 10 are equal to each other. It should be noted that when the lowest frequency Fmin [Hz] and the lower limit frequency F2 [Hz] of the compressor 10 are equal to each other, it is assumed that the lowest frequency Fmin [Hz] is a lowest frequency Fmin1 [Hz]. When the frequency at the on-point at which the compressor 10 starts the operation in the intermittent operation is higher than the lower limit frequency F2 [Hz], in some cases, an average COP in an on-off operation which is the intermittent operation may be higher than the lower limit frequency F2 [Hz], although whether it is high depends on the load. For this reason, the compressor control circuitry 74 of the controller 70 changes the lowest frequency Fmin [Hz] of the compressor 10 in the on-off operation with a method which will be described later.

Time Zone 4

A result indicated in time zone 4 is a result obtained in the case where the controller 70 increases the lowest frequency Fmin [Hz], after the compressor 10 is operated in time zone 3 and the controller 70 determines that the lowest frequency Fmin [Hz] needs to be increased. In this case, the controller 70 changes the lowest frequency Fmin [Hz] to a lowest frequency Fmin2 [Hz] that is higher than the lowest frequency Fmin1 [Hz]. The lowest frequency Fmin2 [Hz] is higher than the frequency required for the load. That is, the result indicated in time zone 4 is the result obtained in the case where the controller 70 sets the lowest frequency Fmin [Hz] to a higher frequency than requires. It should be noted that a method in which the lowest frequency Fmin [Hz] is changed by the controller 70 will be described later.

When the lowest frequency Fmin [Hz] is high for the load, the indoor temperature tr [degrees C] reaches the off-point before the refrigeration cycle circuit is stabilized, as indicated in FIG. 5, (b), and FIG. 5, (c). The stability of the refrigeration cycle circuit is indicated by a curve line in FIG. 5, (c). In FIG. 5, (c), when the capacity Q [kW] is constant with the passage of the time tm, the refrigeration cycle circuit is in a stable state. That is, the refrigeration cycle circuit is in a stable state at a straight line of a curve line that is close in straightness to a horizontal line in FIG. 5, (c).

Time Zone 5

A result indicated in time zone 5 is a result obtained in the case where the controller 70 decreases the lowest frequency Fmin [Hz] based on a result obtained when the compressor 10 is operated in time zone 4. In time zone 5, as indicated in FIG. 5, (d), the controller 70 causes the compressor 10 to operate at the lowest frequency Fmin [Hz] of the compressor 10, which is higher than the lowest frequency Fmin1 [Hz] at which the compressor 10 is operated in time zone 3. In addition, the controller 70 causes the compressor 10 to operate at the lowest frequency Fmin [Hz] of the compressor 10, which is lower than the lowest frequency Fmin2 [Hz] at which the compressor 10 is operated in time zone 4.

As indicated in FIG. 5, (c), compared with time zone 4, curve lines in time zone 5 as indicated in FIG. 5, (c), include many straight line portions that are close to horizontal lines, and thus the refrigeration cycle circuit of the air-conditioning apparatus 100 is in a more stable state, compared with time zone 4. In addition, in time zone 5, the refrigeration cycle circuit is promptly stabilized, compared with time zone 3, and the capacity in the stable state in time zone 5 is higher than the capacity in the stable state in time zone 3. As a result, the air-conditioning apparatus 100 can increase the average COP.

Heating Operation of Air-Conditioning Apparatus 100

FIG. 6 is an explanatory view for the heating operation of the air-conditioning apparatus 100 according to the embodiment. The heating operation of the air-conditioning apparatus 100 will be described with reference to FIG. 6. It should be noted that FIG. 6, (a) to FIG. 6, (d) correspond to FIG. 5. (a) to FIG. 5. (d).

Time Zone 1

In time zone 1, as indicated in FIG. 6, (a), a heating load is large and the air-conditioning apparatus 100 is in a state in which the compressor 10 can be continuously performed. Because the compressor control circuitry 74 of the air-conditioning apparatus 100 controls the frequency [Hz] of the compressor 10 such that the indoor temperature reaches the set temperature Tset, the frequency [Hz] changes according to the variation of the load. More specifically, as indicated in FIG. 6, (d), the frequency [Hz] of the compressor 10 lowers with the passage of the time tm passes. It should be noted that in time zone 1, because the operation of the compressor 10 has just started and the time tm has not passed so much since the operation of the compressor 10 started, the indoor temperature tr [degrees C] has not yet been increased as indicated in FIG. 6, (b).

Time Zone 2

In time zone 2, the load gradually decreases as indicated in FIG. 6, (a), and the compressor 10 is continuously operated at the lower limit frequency F2 [Hz] at which the compressor 10 can operate as indicated in FIG. 6, (d). Then, when the load is decreased to a low value for the capacity at which the operation is performed at the lower limit frequency F2 [Hz], the capacity Q becomes excessive, and thus, in the heating operation, the indoor temperature tr [degrees C] gradually increases and reaches the off-point of the indoor temperature tr [degrees C]. When the indoor temperature tr [degrees C] reaches the off-point, the compressor 10 starts the intermittent operation from time zone 3 onward, as indicated FIG. 6, (d).

Time Zone 3

In time zone 3, the compressor 10 performs the intermittent operation at the lower limit frequency F2 [Hz]. In time zone 3, when the compressor 10 is in operation, the lowest frequency Fmin1 [Hz] and the lower limit frequency F2 [Hz] of the compressor 10 are equal to each other. When the frequency at the on-point at which the compressor 10 starts the operation in the intermittent operation is higher than the lower limit frequency F2 [Hz], an average COP in the on-off operation, which is the intermittent operation, may be high in some cases, although whether it is high depends on the load. For this reason, the compressor control circuitry 74 of the controller 70 changes the lowest frequency Fmin [Hz] of the compressor 10 in the on-off operation with a method which will be described later.

Time Zone 4

A result indicated in time zone 4 is a result obtained in the case where the controller 70 increases the lowest frequency Fmin [Hz], after the compressor 10 is operated in time zone 3 and the controller 70 determines that the lowest frequency Fmin [Hz] needs to be increased. In this case, the controller 70 changes the lowest frequency Fmin [Hz] to the lowest frequency Fmin2 [Hz], which is higher than the lowest frequency Fmin1 [Hz]. The result obtained in time zone 4 is the result obtained in the case where the controller 70 sets the lowest frequency Fmin [Hz] to a higher frequency than requires.

When the lowest frequency Fmin [Hz] is high for the load, the indoor temperature tr [degrees C] reaches the off-point before the refrigeration cycle circuit is stabilized, as indicated in FIG. 6, (b), and FIG. 6, (c).

Time Zone 5

A result obtained in time zone 5 is a result obtained in the case where the controller 70 decreases the lowest frequency Fmin [Hz] based on the result obtained when the compressor 10 is operated in time zone 4. In time zone 5, as indicated in FIG. 6, (d), the controller 70 causes the compressor 10 to operate at the lowest frequency Fmin [Hz] of the compressor 10, which is higher than the lowest frequency Fmin1 [Hz] at which the compressor 10 is operated in time zone 3. In addition, the controller 70 causes the compressor 10 to operate at the lowest frequency Fmin [Hz] of the compressor 10, which is lower than the lowest frequency Fmin2 [Hz] at which the compressor 10 is operated in time zone 4.

As indicated in FIG. 6, (c), compared with time zone 4, curve lines in time zone 5 of FIG. 6. (c), include many straight line portions that are close in straightness to horizontal lines, and thus the refrigeration cycle circuit of the air-conditioning apparatus 100 is more stable than in time zone 4. In addition, in time zone 5, the refrigeration cycle circuit is promptly stabilized than in time zone 3, and the capacity in the stable state in time zone 5 is higher than the capacity in the stable state in time zone 3. As a result, the air-conditioning apparatus 100 can increase the average COP.

Method of Changing Lowest Frequency Fmin [Hz]

Referring to FIGS. 5 and 6, the controller 70 of the compressor 10 changes the lowest frequency Fmin [Hz]. This method of changing the lowest frequency Fmin will be described. Regarding changing of the lowest frequency Fmin [Hz], the controller 70 of the air-conditioning apparatus 100 calculates a time constant T [s] of the refrigeration cycle circuit, compares the time constant T with the intermittent operation time t of the compressor 10, and changes the compressor frequency Fa [Hz]. It should be noted that the time constant T [s] is a response value which is a value in a time period required until a heating and cooling capacity in circulation of refrigerant in the refrigeration cycle circuit is stabilized. In addition, the intermittent operation time t is an operation time in which the intermittent operation of the compressor 10 is performed. When the intermittent operation is performed, the controller 70 measures the intermittent operation time t from the start of the operation of the compressor 10 to the stop of the operation of the compressor 10. A method of determining the time constant T [s] and a method of changing the lowest frequency Fmin [Hz] will be described below.

Method of Determining Time Constant T

When the compressor 10 is started from the stopped state, the capacity Q [kW] gradually increases. In the case where the air-conditioning apparatus 100 includes the refrigeration cycle circuit, a rise of the capacity Q is indicated as a waveform of a first-order lag, as indicated in time zones 3 to 5 of FIG. 5, (c). Where circulation flow rate Gr [kg/s] is a circulation flow rate of refrigerant that flows in the refrigerant circuit 60 and refrigerant amount M [kg] is the amount of refrigerant provided in the refrigerant circuit 60 sealed, the time constant T [s] of the above waveform can be expressed by the following formula (1).

$\begin{array}{cc}T=M/\mathrm{Gr}& \left(1\right)\end{array}$

This circulation flow rate Gr [kg/s] can be expressed by a formula (2) as indicated below. In the formula (2), compressor frequency Fa [Hz] is the rotation speed of the compressor 10, Vst [m³] is the stroke volume of the compressor 10, ρs [kg/m³] is the suction density, and ηv [-] is the volumetric efficiency of the compressor 10.

$\begin{array}{cc}\mathrm{Gr}=\rho s\times \mathrm{Fa}\times \mathrm{Vst}\times \eta v& \left(2\right)\end{array}$

Where the suction density ρs [kg/m³] is a saturated gas density, the suction density ρs can be calculated from an evaporating temperature ET [degrees C], and thus the suction density ρs is indicated as a function of the evaporating temperature ET [degrees C], as expressed by the following formula (3).

$\begin{array}{cc}\rho s=f\left(\mathrm{ET}\right)& \left(3\right)\end{array}$

Therefore, in the cooling operation, the time constant T [s] can be calculated by inputting the evaporating temperature ET that is measured by the indoor temperature sensor 51 provided on the low-pressure side and the compressor frequency Fa [Hz]. In addition, in the heating operation, the time constant T [s] can be calculated by inputting the evaporating temperature ET that is measured by the outdoor temperature sensor 31 provided on the low-pressure side and the compressor frequency Fa [Hz]. That is, the controller 70 calculates the time constant T [s] at a rise of a waveform which indicates a heating and cooling capacity, from a measurement value obtained by the indoor temperature sensor 51 or the outdoor temperature sensor 31.

Method of Determining Lowest Frequency Fmin [Hz]

The operation-state determination circuitry 71 of the controller 70 determines the current compressor frequency Fa [Hz] of the compressor 10 in the air-conditioning apparatus 100. Then, the operation-state determination circuitry 71 of the controller 70 calculates the circulation flow rate Gr [kg/s], using the evaporating temperature ET [degrees C] and the compressor frequency Fa [Hz], as described above, and calculates the time constant T [s] from the circulation flow rate Gr [kg/s] and the refrigerant amount M [kg].

From the above time constant T [s], the operation-state determination circuitry 71 of the controller 70 calculates a first determination value Ta and a second determination value Tb that are expressed by formulas (4) and (5) indicated below. It should be noted that the first determination value Ta and the second determination value Tb are used to change the compressor frequency Fa [Hz] when the compressor 10 performs the intermittent operation. More specifically, the first determination value Ta and the second determination value Tb are reference values for determination whether to change the lowest frequency Fmin [Hz] in the intermittent operation of the compressor 10. It is preferable that a first constant A of formula (4) fall, for example, within the range of 5 to 7, and a second constant B of formula (5) fall, for example, within the range of 2 to 3. As indicated in formula (4), the operation-state determination circuitry 71 of the controller 70 calculates the first determination value Ta by multiplying the time constant T [s] by the first constant A set in advance. As indicated in formula (5), the operation-state determination circuitry 71 of the controller 70 calculates the second determination value Tb by multiplying the time constant T [s] by the second constant B set in advance that is smaller than the first constant A.

$\begin{array}{cc}\mathrm{Ta}=A\times T& \left(4\right)\end{array}$ $\begin{array}{cc}\mathrm{Tb}=B\times T& \left(5\right)\end{array}$

The controller 70 has the first determination value Ta obtained by multiplying the time constant T [s], which is a response value required until refrigerant circulation is stabilized in the refrigeration cycle circuit, by the first constant A set in advance, and the second determination value Tb obtained by multiplying the time constant T [s] by the second constant B set in advance, which is smaller than the first constant A.

Next, the operation-state determination circuitry 71 of the controller 70 compares the indoor temperature tr [degrees C] measured by the indoor suction temperature sensor 52 with the set temperature Tset [degrees C] stored as data in the storage circuitry 72. Then, when the operation-state determination circuitry 71 determines that the indoor temperature tr enters a state T1 or a state T2, which will be described later, the compressor control circuitry 74 tums on the compressor 10 to start the compressor 10.

It should be noted that the state T1 is a state in which the indoor temperature tr [degrees C] in the cooling operation rises to exceed a temperature obtained by adding the set value α [degrees C] to the set temperature Tset [degrees C]. In addition, the state T2 is a state in which the indoor temperature tr [degrees C] in the heating operation falls below a temperature obtained by subtracting the set value α [degrees C] from the set temperature Tset [degrees C].

When the compressor control circuitry 74 of the controller 70 starts the compressor 10, the timing circuitry 73 of the controller 70 starts measurement of the intermittent operation time t. The operation-state determination circuitry 71 uses the intermittent operation time t measured by the timing circuitry 73, in changing of the lowest frequency Fmin [Hz].

The compressor 10 continues the operation for a while after the compressor 10 is started. The operation-state determination circuitry 71 of the controller 70 compares the indoor temperature tr [degrees C] measured by the indoor suction temperature sensor 52 with the set temperature Tset [degrees C] stored as data in the storage circuitry 72. Then, when the operation-state determination circuitry 71 determines that the indoor temperature tr enters a state T3 or a state T4, which will be described later, the compressor control circuitry 74 tums off the compressor 10 to stop the compressor 10. In addition, when the operation-state determination circuitry 71 determines that the indoor temperature tr enters the state T3 or state T4, the timing circuitry 73 stops measurement of the intermittent operation time t.

It should be noted that the state T3 is a state in which the indoor temperature tr [degrees C] in the cooling operation falls below the temperature obtained by subtracting the set value α [degrees C] from the set temperature Tset [degrees C]. The state T4 is a state in which the indoor temperature tr [degrees C] in the heating operation rises to exceed the temperature obtained by adding the set value α [degrees C] to the set temperature Tset [degrees C].

FIG. 7 indicates operation waveforms in the intermittent operation in the case where the lowest frequency Fmin [Hz] is excessively low. The vertical axis in FIG. 7, (b), represents the indoor temperature tr [degrees C]. FIG. 7, (b), indicates the relationship between the time tm and the indoor temperature tr [degrees C]. The vertical axis in FIG. 7, (c), represents the capacity Q [kW]. FIG. 7, (c), indicates the relationship between the time tm and the capacity Q [kW]. The vertical axis in FIG. 7, (d) represents the compressor frequency Fa [Hz]. FIG. 7, (d), indicates the relationship between the time tm and the compressor frequency Fa [Hz].

As indicated in FIG. 7, when the lowest frequency Fmin [Hz] is excessively low as in the case where the lowest frequency Fmin [Hz] is equal to the lower limit frequency F2 [Hz], the refrigeration cycle circuit stably operates in a longer time zone, as indicated in FIG. 7, (c). The operation-state determination circuitry 71 of the controller 70 determines that the time zone in which the refrigeration cycle circuit stably operates is long, when the intermittent operation time t is longer than the first determination value Ta calculated from the time constant T (intermittent operation time t>first determination value Ta).

When the operation-state determination circuitry 71 determines that the time zone in which the refrigeration cycle circuit stably operates is long, the compressor control circuitry 74 of the controller 70 increases the lowest frequency Fmin [Hz] to promptly stabilize the refrigeration cycle circuit, and to cause the intermittent operation of the compressor 10 to be performed at a frequency at which the system performance is high. That is, when the operation-state determination circuitry 71 determines that the intermittent operation time tis longer than the first determination value Ta, the compressor control circuitry 74 increases the lowest frequency Fmin [Hz] to promptly stabilize the refrigeration cycle circuit, and to cause the intermittent operation of the compressor 10 to be performed at a frequency at which the system performance is high.

FIG. 8 indicates operation waveforms in the intermittent operation in the case where the lowest frequency Fmin [Hz] is excessively high. The vertical axis in FIG. 8, (b), represents the indoor temperature tr [degrees C]. FIG. 8, (b), indicates the relationship between the time tm and the indoor temperature tr [degrees C]. The vertical axis in FIG. 8, (c), represents the capacity Q [kW]. FIG. 8. (c), indicates the relationship between the time tm and the capacity Q [kW]. The vertical axis in FIG. 8, (d), represents the compressor frequency Fa [Hz]. FIG. 8, (d), indicates the relationship between the time tm and the compressor frequency Fa [Hz].

As indicated in FIG. 8, when the lowest frequency Fmin [Hz] is excessively high as in the case where the lowest frequency Fmin [Hz] is equal to the COP maximum frequency F1 [Hz], the operation is performed for a short time period after the refrigeration cycle circuit is stabilized, as indicated in FIG. 8, (c), and the compressor 10 stops and starts frequently, as indicated in FIG. 8, (d). The operation-state determination circuitry 71 of the controller 70 determines that the state is a state in which the operation is performed for a short time period after the refrigeration cycle circuit is stabilized and the compressor 10 stops and starts frequently, when the intermittent operation time t is shorter than the second determination value Tb calculated from the time constant T (intermittent operation time t<second determination value Tb).

When the operation-state determination circuitry 71 determines that the state is the state where the operation is performed for a short time period after the refrigeration cycle circuit is stabilized is short and the compressor 10 stops and starts frequently, the compressor control circuitry 74 of the controller 70 reduces the lowest frequency Fmin [Hz]. By reducing the lowest frequency Fmin [Hz], the compressor control circuitry 74 promptly stabilizes the refrigeration cycle circuit and causes the intermittent operation of the compressor 10 to be performed at a frequency at which the system performance is high. That is, when the operation-state determination circuitry 71 determines that the intermittent operation time t is shorter than the second determination value Tb, the compressor control circuitry 74 reduces the lowest frequency Fmin [Hz] to promptly stabilize the refrigeration cycle circuit, and to cause the intermittent operation of the compressor 10 to be performed at a frequency at which the system performance is high.

FIG. 9 indicates operation waveforms in the intermittent operation in the case where the lowest frequency Fmin [Hz] is appropriate. The vertical axis in FIG. 9, (b), represents the indoor temperature tr [degrees C]. FIG. 9. (b), indicates the relationship between the time tm and the indoor temperature tr [degrees C]. The vertical axis in FIG. 9, (c), represents the capacity Q [kW]. FIG. 9. (c), indicates the relationship between the time tm and the capacity Q [kW]. The vertical axis in FIG. 9, (d), represents the compressor frequency Fa [Hz]. FIG. 9, (d), indicates the relationship between the time tm and the compressor frequency Fa [Hz].

As described above, when the operation-state determination circuitry 71 determines that the intermittent operation time t is longer than the first determination value Ta, the controller 70 changes the lowest frequency Fmin [Hz] to a new lowest frequency Fmin [Hz] that is higher than the current lowest frequency Fmin [Hz]. That is, when the operation-state determination circuitry 71 determines that the intermittent operation time t is longer than the first determination value Ta, the controller 70 increases the lowest frequency Fmin [Hz] for a subsequent operation of the compressor 10. An increase range of the lowest frequency Fmin [Hz] is stored in the storage circuitry 72 in advance as a set increase range β. The set increase range β falls within the range of 1 to 5 Hz, but is not limited to this range.

Furthermore, when the operation-state determination circuitry 71 determines that the intermittent operation time t is shorter than the second determination value Tb, the controller 70 changes the lowest frequency Fmin [Hz] to a new lowest frequency Fmin [Hz] that is lower than the current lowest frequency Fmin [Hz]. That is, when the controller 70 determines that the intermittent operation time t is shorter than the second determination value Tb, the controller 70 reduces the lowest frequency Fmin [Hz] for the subsequent operation of the compressor 10. A decrease range of the lowest frequency Fmin [Hz] is stored in the storage circuitry 72 in advance as a set decrease range γ. The set decrease range γ falls within the range of approximately 1 to 5 Hz, but is not limited to this range.

By changing the lowest frequency Fmin [Hz] in the above manner, the controller 70 of the air-conditioning apparatus 100 determines the optimal lowest frequency Fmin [Hz] at the present moment. By changing the lowest frequency Fmin [Hz], the controller 70 determines the optimal lowest frequency Fmin [Hz] at the present moment, and causes the intermittent operation of the compressor 10 to be performed at a frequency at which the system performance is high, while promptly stabilizing the refrigeration cycle circuit, as indicated in FIG. 9.

It should be noted that in the case where t the compressor control circuitry 74 increases the lowest frequency Fmin [Hz] and as a result, the lowest frequency Fmin [Hz] rises to exceed the COP maximum frequency F1 [Hz] (lowest frequency Fmin [Hz]>COP maximum frequency F1 [Hz]), the controller 70 uses the value of the COP maximum frequency F1 [Hz] as the lowest frequency Fmin [Hz].

In addition, in the case where the compressor control circuitry 74 reduces the lowest frequency Fmin [Hz] and as a result, the lowest frequency Fmin [Hz] falls below the lower limit frequency F2 [Hz] (lowest frequency Fmin [Hz]<lower limit frequency F2 [Hz]), the controller 70 uses the value of the lower limit frequency F2 [Hz] as the lowest frequency Fmin [Hz].

By determining the lowest frequency Fmin [Hz] in the above manner, the controller 70 determines the lowest frequency Fmin [Hz] such that the lowest frequency Fmin [Hz] is higher than or equal to the lower limit frequency F2 [Hz] but lower than or equal to the COP maximum frequency F1 [Hz] (lower limit frequency F2 [Hz]≤lowest frequency Fmin [Hz]≤COP maximum frequency F1 [Hz]).

It should be noted that because the value of the COP maximum frequency F1 [Hz] in the refrigeration cycle circuit varies depending on the operation state of the refrigeration cycle circuit, the value of the COP maximum frequency F1 [Hz] may be changed using detection values obtained by the indoor temperature sensor 51 and the outdoor temperature sensor 31.

Control Flow in Cooling Operation

FIG. 10 is a flowchart of a control flow for changing the lowest frequency Fmin [Hz] in the cooling operation of the air-conditioning apparatus 100 according to the embodiment. The controller 70 of the air-conditioning apparatus 100 performs control as indicated in FIG. 10 in the cooling operation.

When the air-conditioning apparatus 100 performs the cooling operation, the indoor temperature sensor 51 measures the evaporating temperature ET [degrees C] (step S1). The evaporating temperature ET measured by the indoor temperature sensor 51 is received by the controller 70 and is used in determination by the operation-state determination circuitry 71. Next, the controller 70 of the air-conditioning apparatus 100 determines the compressor frequency Fa [Hz] of the compressor 10 (step S2). The compressor 10 is controlled by an inverter. Because the compressor 10 operates at a target frequency calculated by the CPU, the frequency indicated by the CPU is determined as the compressor frequency Fa [Hz]. The controller 70 makes a calculation based on formula (2), using the compressor frequency Fa [Hz] indicated by the CPU.

Next, the operation-state determination circuitry 71 of the controller 70 calculates the circulation flow rate Gr [kg/s] of refrigerant that flows in the refrigerant circuit 60 based on the above formula (2) (step S3).

$\begin{array}{cc}\mathrm{Gr}=\rho s\times \mathrm{Fa}\times \mathrm{Vst}\times \eta v& \left(2\right)\end{array}$

Then, the operation-state determination circuitry 71 of the controller 70 calculates the time constant T [s] based on the above formula (1) (step S4). The time constant T [s] is calculated using the circulation flow rate Gr [kg/s] of refrigerant that flows in the refrigerant circuit 60 and the amount M [kg] of refrigerant provided in the refrigerant circuit 60 sealed.

$\begin{array}{cc}T=M/\mathrm{Gr}& \left(1\right)\end{array}$

Next, the operation-state determination circuitry 71 of the controller 70 calculates the first determination value Ta based on the above formula (4) and calculates the second determination value Tb based on the above formula (5) (step S5). The first determination value Ta is calculated using the time constant T [s] and the first constant A. The first constant A falls, for example, within the range of 5 to 7. The second determination value Tb is calculated using the time constant T [s] and the second constant B. The second constant B falls, for example, within the range 2 to 3.

$\begin{array}{cc}\mathrm{Ta}=A\times T& \left(4\right)\end{array}$ $\begin{array}{cc}\mathrm{Tb}=B\times T& \left(5\right)\end{array}$

Next, the operation-state determination circuitry 71 of the controller 70 determines whether or not the indoor temperature tr [degrees C] is higher than the temperature obtained by adding the set value α [degrees C] to the set temperature Tset [degrees C] (step S6). The indoor temperature tr [degrees C] is detected by the indoor suction temperature sensor 52. When the operation-state determination circuitry 71 determines that the indoor temperature tr [degrees C] is higher than the temperature obtained by adding the set value α [degrees C] to the set temperature Tset [degrees C] (YES in step S6), the processing by the controller 70 proceeds to step S7.

When the operation-state determination circuitry 71 determines that the indoor temperature tr [degrees C] is higher than the temperature obtained by adding the set value α [degrees C] to the set temperature Tset [degrees C], it is determined that the cooling load increases, and the compressor control circuitry 74 turns on the compressor 10 to start the compressor 10 (step S7). Next, in step S7, when the compressor control circuitry 74 of the controller 70 starts the compressor 10, the timing circuitry 73 of the controller 70 starts measurement of the intermittent operation time t (step S8). That is, when the answer to the question in step S6 in the intermittent operation in the cooling operation is YES, the controller 70 starts the operation of the compressor 10 and starts measurement of the intermittent operation time t. The case where the answer to question in step S6 is YES means the case where the indoor temperature measured by the indoor suction temperature sensor 52 is higher than the temperature obtained by adding the set value α [degrees C], which is a predetermined set range, to the set temperature Tset [degrees C] set for the indoor space.

When the operation-state determination circuitry 71 determines that the indoor temperature tr [degrees C] is not higher than the temperature obtained by adding the set value α [degrees C] to the set temperature Tset [degrees C] (NO in step S6), the processing by the controller 70 proceeds to step S9. Alternatively, in step S8, when the timing circuitry 73 of the controller 70 starts measurement of the intermittent operation time t, the processing by the controller 70 proceeds to step S9.

The operation-state determination circuitry 71 of the controller 70 determines whether or not the indoor temperature tr [degrees C] is lower than the temperature obtained by subtracting the set value α [degrees C] from the set temperature Tset [degrees C] (step S9). When the operation-state determination circuitry 71 determines that the indoor temperature tr [degrees C] is lower than the temperature obtained by subtracting the set value α [degrees C] from the set temperature Tset [degrees C] (YES in step S9), the processing by the controller 70 proceeds to step S10.

When the operation-state determination circuitry 71 determines that the indoor temperature tr [degrees C] is lower than the temperature obtained by subtracting the set value α [degrees C] from the set temperature Tset [degrees C], it is determined that the cooling load is reduced, and the compressor control circuitry 74 turns off the compressor 10 to stop the compressor 10 (step S10). Next, in step S10, when the compressor control circuitry 74 of the controller 70 stops the compressor 10, the timing circuitry 73 of the controller 70 stops measurement of the intermittent operation time t (step S11). That is, when the answer to the question in step S9 is YES after the measurement of the intermittent operation time t is started, the controller 70 stops the operation of the compressor 10 and stops the measurement of the intermittent operation time t. The case where the answer to the question in step S9 is YES means the case where the indoor temperature measured by the indoor suction temperature sensor 52 is lower than the temperature obtained by subtracting the set value α [degrees C] from the set temperature Tset [degrees C].

In step S11, when the timing circuitry 73 of the controller 70 stops the measurement of the intermittent operation time t, the operation-state determination circuitry 71 of the controller 70 determines whether or not the intermittent operation time t measured by the timing circuitry 73 is longer than the first determination value Ta (step S12).

When the operation-state determination circuitry 71 determines that the intermittent operation time t is longer than the first determination value Ta (YES in step S12), the controller 70 changes the lowest frequency Fmin [Hz] to a new lowest frequency Fmin [Hz] that is higher than the current lowest frequency Fmin [Hz]. That is, when the operation-state determination circuitry 71 determines that the intermittent operation time t is longer than the first determination value Ta (YES in step S12), the compressor control circuitry 74 of the controller 70 increases the lowest frequency Fmin [Hz] for a subsequent operation of the compressor 10 (step S13). The increase range of the lowest frequency Fmin [Hz] is stored in the storage circuitry 72 in advance as the set increase range β. It should be noted that the increased lowest frequency Fmin [Hz] is the lowest frequency Fmin [Hz] of the compressor 10 which is used when the indoor temperature tr [degrees C] reaches the next on-point.

When the operation-state determination circuitry 71 determines that the intermittent operation time t is shorter than or equal to the first determination value Ta (NO in step S12), the processing by the controller 70 proceeds to step S14. Alternatively, in step S13, when the compressor control circuitry 74 increases the lowest frequency Fmin [Hz], the processing by the controller 70 proceeds to step S14.

The operation-state determination circuitry 71 of the controller 70 determines whether or not the intermittent operation time t measured by the timing circuitry 73 is shorter than the second determination value Tb (step S14).

When the operation-state determination circuitry 71 determines that the intermittent operation time t is shorter than the second determination value Tb (YES in step S14), the controller 70 changes the lowest frequency Fmin [Hz] to a new lowest frequency Fmin [Hz] that is lower than the current lowest frequency Fmin [Hz]. That is, when the operation-state determination circuitry 71 determines that the intermittent operation time t is shorter than the second determination value Tb (YES in step S14), the compressor control circuitry 74 of the controller 70 reduces the lowest frequency Fmin [Hz] for the subsequent operation of the compressor 10 (step S15). The decrease range of the lowest frequency Fmin [Hz] is stored in the storage circuitry 72 in advance as the set decrease range γ. It should be noted that the reduced lowest frequency Fmin [Hz] is the lowest frequency Fmin [Hz] of the compressor 10 which is used when the indoor temperature tr [degrees C] reaches the next on-point.

When the operation-state determination circuitry 71 determines that the intermittent operation time t is longer than or equal to the second determination value Tb (NO in step S14), the processing by the controller 70 proceeds to step S16. Alternatively, in step S15, when the compressor control circuitry 74 reduces the lowest frequency Fmin [Hz], the processing by the controller 70 proceeds to step S16.

The operation-state determination circuitry 71 of the controller 70 determines whether or not the current lowest frequency Fmin [Hz] is higher than the COP maximum frequency F1 [Hz] (step S16).

When the operation-state determination circuitry 71 determines that the current lowest frequency Fmin [Hz] is higher than the COP maximum frequency F1 [Hz] (YES in step S16), the compressor control circuitry 74 sets the lowest frequency Fmin [Hz] to the value of the COP maximum frequency F1 [Hz] (step S17). That is, when the lowest frequency Fmin [Hz] is increased, the controller 70 compares the lowest frequency Fmin [Hz] in the intermittent operation with the COP maximum frequency F1 [Hz] at which the energy consumption efficiency increases to the maximum. Then, when the lowest frequency Fmin [Hz] is higher than the COP maximum frequency F1 [Hz], the controller 70 sets the lowest frequency Fmin [Hz] to the same value as the COP maximum frequency F1 [Hz]. It should be noted that the set lowest frequency Fmin [Hz] is the lowest frequency Fmin [Hz] of the compressor 10 which is used when the indoor temperature tr [degrees C] reaches the next on-point.

When the operation-state determination circuitry 71 determines that the current lowest frequency Fmin [Hz] is lower than or equal to the COP maximum frequency F1 [Hz] (NO in step S16), the processing by the controller 70 proceeds to step S18. Alternatively, in step S17, when the compressor control circuitry 74 sets the lowest frequency Fmin [Hz] to the COP maximum frequency F1 [Hz], the processing by the controller 70 proceeds to step S18.

The operation-state determination circuitry 71 of the controller 70 determines whether or not the current lowest frequency Fmin [Hz] is lower than the lower limit frequency F2 [Hz] (step S18).

When the operation-state determination circuitry 71 determines that the current lowest frequency Fmin [Hz] is lower than the lower limit frequency F2 [Hz] (YES in step S18), the compressor control circuitry 74 sets the lowest frequency Fmin [Hz] to the lower limit frequency F2 [Hz] (step S19). That is, when the lowest frequency Fmin [Hz] is reduced, the controller 70 compares the lowest frequency Fmin [Hz] in the intermittent operation with the lower limit frequency F2 [Hz], which is the lower limit frequency required for operating the compressor 10. Then, when the lowest frequency Fmin [Hz] is lower than the lower limit frequency F2 [Hz], the controller 70 sets the lowest frequency Fmin [Hz] to the same value as the lower limit frequency F2 [Hz]. It should be noted that the set lowest frequency Fmin [Hz] is the lowest frequency Fmin [Hz] of the compressor 10 which is used when the indoor temperature tr [degrees C] reaches the next on-point.

When the operation-state determination circuitry 71 determines that the current lowest frequency Fmin [Hz] is higher than or equal to the lower limit frequency F2 [Hz] (NO in step S18), the controller 70 causes the cooling operation to be continued while keeping the lowest frequency Fmin [Hz] at the current set value. Alternatively, in step S19, when the compressor control circuitry 74 sets the lowest frequency Fmin [Hz] to the lower limit frequency F2 [Hz], the controller 70 causes the cooling operation to be continued while keeping the lowest frequency Fmin [Hz] at the lower limit frequency F2 [Hz]. Then, the air-conditioning apparatus 100 constantly performs operations of steps S1 to S19 during the cooling operation.

Control Flow in Heating Operation

FIG. 11 is a flowchart of a control flow for changing the lowest frequency Fmin [Hz] in the heating operation of the air-conditioning apparatus 100 according to the embodiment. The controller 70 of the air-conditioning apparatus 100 performs control as indicated in FIG. 11 in the heating operation.

When the air-conditioning apparatus 100 is in the heating operation, the indoor temperature sensor 51 measures the evaporating temperature ET [degrees C] (step ST1). Next, the controller 70 of the air-conditioning apparatus 100 determines the compressor frequency Fa [Hz] of the compressor 10 (step ST2).

Next, the operation-state determination circuitry 71 of the controller 70 calculates the circulation flow rate Gr [kg/s] of refrigerant that flows in the refrigerant circuit 60 based on the above formula (2) (step ST3). Then, the operation-state determination circuitry 71 of the controller 70 calculates the time constant T [s] based on the above formula (1) (step ST4).

$\begin{array}{cc}\mathrm{Gr}=\rho s\times \mathrm{Fa}\times \mathrm{Vst}\times \eta v& \left(2\right)\end{array}$ $\begin{array}{cc}T=M/\mathrm{Gr}& \left(1\right)\end{array}$

Next, the operation-state determination circuitry 71 of the controller 70 calculates the first determination value Ta based on the above formula (4) and calculates the second determination value Tb based on the above formula (5) (step ST5). The first determination value Ta is calculated using the time constant T [s] and the first constant A. The first constant A falls, for example, within the range of 5 to 7. The second determination value Tb is calculated using the time constant T [s] and the second constant B. The second constant B falls, for example, within the range of 2 to 3.

$\begin{array}{cc}\mathrm{Ta}=A\times T& \left(4\right)\end{array}$ $\begin{array}{cc}\mathrm{Tb}=B\times T& \left(5\right)\end{array}$

Next, the operation-state determination circuitry 71 of the controller 70 determines whether or not the indoor temperature tr [degrees C] is lower than the temperature obtained by subtracting the set value α [degrees C] from the set temperature Tset [degrees C] (step ST6). When the operation-state determination circuitry 71 determines that the indoor temperature tr [degrees C] is lower than the temperature obtained by subtracting the set value α [degrees C] from the set temperature Tset [degrees C] (YES in step ST6), the processing by the controller 70 proceeds to step ST7.

When the operation-state determination circuitry 71 determines that the indoor temperature tr [degrees C] is lower than the temperature obtained by subtracting the set value α [degrees C] from the set temperature Tset [degrees C], it is determined that the heating load increases, and the compressor control circuitry 74 turns on the compressor 10 to start the compressor 10 (step ST7). Next, in step ST7, when the compressor control circuitry 74 of the controller 70 starts the compressor 10, the timing circuitry 73 of the controller 70 starts measurement of the intermittent operation time t (step ST8). That is, in the intermittent operation in the heating operation, when the answer to the question in step ST6 is YES, the controller 70 starts the operation of the compressor 10 and starts measurement of the intermittent operation time t. The case where the answer to the question in step ST6 is YES means the case where the indoor temperature measured by the indoor suction temperature sensor 52 is lower than the temperature obtained by subtracting the set value α [degrees C], which is the predetermined set range, from the set temperature Tset [degrees C] set for the indoor space.

When the operation-state determination circuitry 71 determines that the indoor temperature tr [degrees C] is not lower than the temperature obtained by subtracting the set value α [degrees C] from the set temperature Tset [degrees C] (NO in step ST6), the processing by the controller 70 proceeds to step ST9. Alternatively, in step ST18, when the timing circuitry 73 of the controller 70 starts measurement of the intermittent operation time t, the processing by the controller 70 proceeds to step ST9.

The operation-state determination circuitry 71 of the controller 70 determines whether or not the indoor temperature tr [degrees C] is higher than the temperature obtained by adding the set value α [degrees C] to the set temperature Tset [degrees C] (step ST9). When the operation-state determination circuitry 71 determines that the indoor temperature tr [degrees C] is higher than the temperature obtained by adding the set value α [degrees C] to the set temperature Tset [degrees C] (YES in step ST9), the processing by the controller 70 proceeds to step ST10.

When the operation-state determination circuitry 71 determines that the indoor temperature tr [degrees C] is higher than the temperature obtained by adding the set value α [degrees C] to the set temperature Tset [degrees C], it is determined that the heating load decreases, and the compressor control circuitry 74 turns off the compressor 10 to stop the compressor 10 (step ST10). Next, in step ST10, when the compressor control circuitry 74 of the controller 70 stops the compressor 10, the timing circuitry 73 of the controller 70 stops measurement of the intermittent operation time t (step ST11). That is, when the answer to the question in step ST9 is YES after the measurement of the intermittent operation time t is started, the controller 70 stops the operation of the compressor 10 and stops the measurement of the intermittent operation time t. The case where the answer to the question in step ST9 is YES means the case where the indoor temperature measured by the indoor suction temperature sensor 52 is higher than the temperature obtained by adding the set value α [degrees C] to the set temperature Tset [degrees C].

In step ST11, when the timing circuitry 73 of the controller 70 stops the measurement of the intermittent operation time t, the operation-state determination circuitry 71 of the controller 70 determines whether or not the intermittent operation time t measured by the timing circuitry 73 is longer than the first determination value Ta (step ST12).

When the operation-state determination circuitry 71 determines that the intermittent operation time t is longer than the first determination value Ta (YES in step ST12), the controller 70 changes the set lowest frequency Fmin [Hz] to a new lowest frequency Fmin [Hz] that is higher than the current lowest frequency Fmin [Hz]. That is, when the operation-state determination circuitry 71 determines that the intermittent operation time t is longer than the first determination value Ta (YES in step ST12), the compressor control circuitry 74 of the controller 70 increases the lowest frequency Fmin [Hz] for a subsequent operation of the compressor 10 (step ST13). The increase range of the lowest frequency Fmin [Hz] is stored in the storage circuitry 72 in advance as the set increase range β. It should be noted that the increased lowest frequency Fmin [Hz] is the lowest frequency Fmin [Hz] of the compressor 10 which is used when the indoor temperature tr [degrees C] reaches the next on-point.

When the operation-state determination circuitry 71 determines that the intermittent operation time t is shorter than or equal to the first determination value Ta (NO in step ST12), the processing by the controller 70 proceeds to step ST14. Alternatively, in step ST13, after the compressor control circuitry 74 increases the lowest frequency Fmin [Hz], the processing by the controller 70 proceeds to step ST14.

The operation-state determination circuitry 71 of the controller 70 determines whether or not the intermittent operation time t measured by the timing circuitry 73 is shorter than the second determination value Tb (step ST14).

When the operation-state determination circuitry 71 determines that the intermittent operation time t is shorter than the second determination value Tb (YES in step ST14), the controller 70 changes the set lowest frequency Fmin [Hz] to a new lowest frequency Fmin [Hz] that is lower than the current lowest frequency Fmin [Hz]. That is, when the operation-state determination circuitry 71 determines that the intermittent operation time t is shorter than the second determination value Tb (YES in step ST14), the compressor control circuitry 74 of the controller 70 reduces the lowest frequency Fmin [Hz] for a subsequent operation of the compressor 10 (step ST15). The decrease range of the lowest frequency Fmin [Hz] is stored in the storage circuitry 72 in advance as the set decrease range γ. It should be noted that the reduced lowest frequency Fmin [Hz] is the lowest frequency Fmin [Hz] of the compressor 10 which is used when the indoor temperature tr [degrees C] reaches the next on-point.

When the operation-state determination circuitry 71 determines that the intermittent operation time t is longer than or equal to the second determination value Tb (NO in step ST14), the processing by the controller 70 proceeds to step ST16. Alternatively, in step ST15, when the compressor control circuitry 74 reduces the lowest frequency Fmin [Hz], the processing by the controller 70 proceeds to step ST16.

The operation-state determination circuitry 71 of the controller 70 determines whether or not the current lowest frequency Fmin [Hz] is higher than the COP maximum frequency F1 [Hz] (step ST16).

When the operation-state determination circuitry 71 determines that the lowest frequency Fmin [Hz] is higher than the COP maximum frequency F1 [Hz] (YES in step ST16), the compressor control circuitry 74 sets the lowest frequency Fmin [Hz] to the COP maximum frequency F1 [Hz] (step ST17). That is, when the lowest frequency Fmin [Hz] is increased, the controller 70 compares the lowest frequency Fmin [Hz] in the intermittent operation with the COP maximum frequency F1 [Hz] at which the energy consumption efficiency reaches the maximum. Then, when the lowest frequency Fmin [Hz] is higher than the COP maximum frequency F1 [Hz], the controller 70 sets the lowest frequency Fmin [Hz] to the same value as the COP maximum frequency F1 [Hz]. It should be noted that the set lowest frequency Fmin [Hz] is the lowest frequency Fmin [Hz] of the compressor 10 which is used when the indoor temperature tr [degrees C] reaches the next on-point.

When the operation-state determination circuitry 71 determines that the current lowest frequency Fmin [Hz] is lower than or equal to the COP maximum frequency F1 [Hz] (NO in step ST16), the processing by the controller 70 proceeds to step ST18. Alternatively, in step ST17, when the compressor control circuitry 74 sets the lowest frequency Fmin [Hz] to the COP maximum frequency F1 [Hz], the processing by the controller 70 proceeds to step ST18.

The operation-state determination circuitry 71 of the controller 70 determines whether or not the current lowest frequency Fmin [Hz] is lower than the lower limit frequency F2 [Hz] (step ST18).

When the operation-state determination circuitry 71 determines that the lowest frequency Fmin [Hz] is lower than the lower limit frequency F2 [Hz] (YES in step ST18), the compressor control circuitry 74 sets the lowest frequency Fmin [Hz] to the lower limit frequency F2 [Hz] (step ST19). That is, when the lowest frequency Fmin [Hz] is reduced, the controller 70 compares the lowest frequency Fmin [Hz] in the intermittent operation with the lower limit frequency F2 [Hz], which is the lower limit frequency required for operating the compressor 10. Then, when the lowest frequency Fmin [Hz] is lower than the lower limit frequency F2 [Hz], the controller 70 sets the lowest frequency Fmin [Hz] to the same value as the lower limit frequency F2 [Hz]. It should be noted that the set lowest frequency Fmin [Hz] is the lowest frequency Fmin [Hz] of the compressor 10 which is used when the indoor temperature tr [degrees C] reaches the next on-point.

When the operation-state determination circuitry 71 determines that the current lowest frequency Fmin [Hz] is higher than or equal to the lower limit frequency F2 [Hz] (NO in step ST18), the controller 70 continues the heating operation while keeping the lowest frequency Fmin [Hz] at the current set value. Alternatively, in step ST19, when the compressor control circuitry 74 sets the lowest frequency Fmin [Hz] to the lower limit frequency F2 [Hz], the controller 70 continues the heating operation while keeping the lowest frequency Fmin [Hz] at the lower limit frequency F2 [Hz]. Then, the air-conditioning apparatus 100 constantly performs the operations of steps ST1 to ST19 during the heating operation.

Advantages of Air-Conditioning Apparatus 100

The controller 70 of the air-conditioning apparatus 100 calculates the time constant T [s] at a rise of a waveform representing the heating and cooling capacity, from the value measured by the indoor temperature sensor 51 or the outdoor temperature sensor 31. In addition, the controller 70 calculates the first determination value Ta by multiplying the calculated time constant T [s] by the first constant A set in advance, and calculates the second determination value Tb by multiplying the calculated time constant T [s] by the second constant B set in advance, which is smaller than the first constant A. Furthermore, in the intermittent operation, the controller 70 measures the intermittent operation time t, which is the time from the start of the operation of the compressor 10 to the stop of the operation of the compressor 10. Then, when the intermittent operation time t is longer than the first determination value Ta, the controller 70 increases the lowest frequency Fmin [Hz] for a subsequent operation of the compressor 10. When the intermittent operation time tis shorter than the second determination value Tb, the controller 70 reduces the lowest frequency Fmin [Hz] for the subsequent operation of the compressor 10.

As indicated in FIG. 7, when the controller 70 determines that the intermittent operation time t is longer than the first determination value Ta, the compressor control circuitry 74 increases the lowest frequency Fmin [Hz] to promptly stabilize the refrigeration cycle circuit, and to cause the compressor 10 to perform the intermittent operation at a frequency at which the system performance is high. In addition, as indicated in FIG. 8, when the controller 70 determines that the intermittent operation time t is shorter than the second determination value Tb, the compressor control circuitry 74 reduces the lowest frequency Fmin [Hz] to promptly stabilize the refrigeration cycle circuit, and to causes the compressor 10 to perform the intermittent operation at a frequency at which the system performance is high. Therefore, since the lowest frequency Fmin [Hz] of the compressor 10 is changed by the controller 70 based on the intermittent operation time t during the intermittent operation, the air-conditioning apparatus 100 can promptly stabilize the refrigeration cycle circuit even when performs the intermittent operation, and thus can perform an efficient operation.

Furthermore, in the case where the lowest frequency Fmin [Hz] is increased, when the lowest frequency Fmin[Hz] is higher the COP maximum frequency F1 [Hz], the controller 70 sets the lowest frequency Fmin [Hz] to the same value as the COP maximum frequency F1 [Hz]. As indicated in FIG. 4, because the COP maximum frequency F1 [Hz] is the frequency at which the energy consumption efficiency increases to the maximum, the controller 70 can keep the COP at the maximum when increasing the lowest frequency Fmin [Hz].

In addition, when the lowest frequency Fmin [Hz] is decreased to fall below the lower limit frequency F2 [Hz], the controller 70 sets the lowest frequency Fmin [Hz] to the same value as the lower limit frequency F2 [Hz]. The lower limit frequency F2 [Hz] is the minimum frequency required for operating the compressor 10, and is the lower limit frequency at the time of operating the compressor 10 itself. Thus, even when the lowest frequency Fmin [Hz] is decreased, in the intermittent operation, the controller 70 can cause the compressor 10 to safely operate without failure.

Moreover, when the lowest frequency Fmin [Hz] is higher than the COP maximum frequency F1 [Hz], the controller 70 sets the lowest frequency Fmin [Hz] to the same value as the COP maximum frequency F1 [Hz]. In addition, when the lowest frequency Fmin [Hz] is lower than the lower limit frequency F2 [Hz], the controller 70 sets the lowest frequency Fmin [Hz] to the same value as the lower limit frequency F2 [Hz]. Thus, when causing the compressor 10 to operate, the controller 70 can cause the compressor to stably operate at a frequency between the lower limit frequency F2 [Hz] and the COP maximum frequency F1 [Hz].

Furthermore, in the cooling operation, when the indoor temperature is higher than the temperature obtained by adding the set value α [degrees C], which is the predetermined set range, to the set temperature Tset [degrees C] of the indoor space, the controller 70 starts the operation of the compressor 10 and starts measurement of the intermittent operation time t. After starting the measurement of the intermittent operation time t, when the indoor temperature falls below the temperature obtained by subtracting the set value α [degrees C] from the set temperature Tset [degrees C], the controller 70 stops the operation of the compressor 10 and stops the measurement of the intermittent operation time t. Thus, the air-conditioning apparatus 100 can perform the intermittent operation during the cooling operation, and can reduce occurrence of excess cooling in the indoor space.

In addition, when the indoor temperature is lower than the temperature obtained by subtracting the set value α [degrees C], which is the predetermined set range, from the set temperature Tset [degrees C] of the indoor space in the heating operation, the controller 70 starts the operation of the compressor 10 and starts measurement of the intermittent operation time t. After starting the measurement of the intermittent operation time t, when the indoor temperature rises to exceed the temperature obtained by adding the set value α [degrees C] to the set temperature Tset [degrees C], the controller 70 stops the operation of the compressor 10 and stops the measurement of the intermittent operation time t. Thus, the air-conditioning apparatus 100 can perform the intermittent operation during the heating operation, and can reduce occurrence of excess heating in the indoor space.

REFERENCE SIGNS LIST

• • 10: compressor, 20: flow switching device, 30: outdoor heat exchanger, 31: outdoor temperature sensor, 35: outdoor fan, 40: pressure-reducing device, 50: indoor heat exchanger, 51: indoor temperature sensor, 52: indoor suction temperature sensor, 55: indoor fan, 60: refrigerant circuit, 61: first extension pipe, 62: second extension pipe, 65: refrigerant pipe, 70: controller, 71: operation-state determination circuitry, 72: storage circuitry, 73: timing circuitry, 74: compressor control circuitry, 75: pressure-reducing mechanism control circuitry, 76: fan control circuitry, 100: air-conditioning apparatus, 110: outdoor unit, 120: indoor unit

Claims

1. An air-conditioning apparatus comprising: a compressor configured to compress refrigerant;a condenser configured to condense the refrigerant discharged from the compressor;a pressure-reducing device configured to reduce a pressure of the refrigerant which flows from the condenser; an evaporator configured to evaporate the refrigerant decompressed by the pressure-reducing device;a temperature sensor configured to measure a temperature of the refrigerant in the evaporator; anda controller configured to control the compressor and the pressure-reducing device to cause a heating operation or a cooling operation to be performed, configured to cause an intermittent operation to be performed, the intermittent operation being an operation in which an operation of the compressor is stopped when a cooling load or a heating load decreases and the operation of the compressor is started when the cooling load or the heating load increases, and configured to cause, when the operation of the compressor is performed in the intermittent operation, the intermittent operation to be performed at a lowest frequency set as a lower limit frequency in a frequency range that is set for the operation of the compressor,wherein the controller has a first determination value obtained by multiplying by a first constant set in advance, a time constant which is a response value which is a value in a time period required until a heating and cooling capacity in circulation of the refrigerant in a refrigeration cycle circuit is stabilized, and which is a value at a rise of a waveform representing the heating and cooling capacity calculated from a value measured by the temperature sensor, anda second determination value obtained by multiplying the time constant by a second constant set in advance that is smaller than the first constant, andwherein the controller is configured to increase the lowest frequency for a subsequent operation of the compressor, when in the intermittent operation, an intermittent operation time from a start of the operation of the compressor to a stop of the operation of the compressor is longer than the first determination value, and configured to decrease the lowest frequency for the subsequent operation of the compressor, when in the intermittent operation, the intermittent operation time is shorter than the second determination value.

2. The air-conditioning apparatus of claim 1, wherein the controller is configured to compare, when the lowest frequency is increased, the lowest frequency in the intermittent operation with a maximum frequency at which an energy consumption efficiency increases to a maximum, and configured to set, when the lowest frequency is higher than the maximum frequency, the lowest frequency to a same value as the maximum frequency.

3. The air-conditioning apparatus of claim 1, wherein the controller is configured to compare, when the lowest frequency is decreased, the lowest frequency in the intermittent operation with a lower limit frequency that is a minimum frequency required for operating the compressor, and configured to set, when the lowest frequency is lower than the lower limit frequency, the lowest frequency to a same value as the lower limit frequency.

4. The air-conditioning apparatus of claim 1, further comprising an indoor suction temperature sensor configured to measure an indoor temperature in an air-conditioning target space in which the condenser or the evaporator is located, whereinthe controller is configured to determine that the cooling load increases, start the operation of the compressor, and start measurement of the intermittent operation, when in the intermittent operation in the cooling operation, the indoor temperature measured by the indoor suction temperature sensor is higher than a temperature obtained by adding a set value, which is a predetermined set range, to a set temperature for an indoor space, andthe controller is configured to determine that the cooling load decreases, stop the operation of the compressor, and stop the measurement of the intermittent operation time, when the indoor temperature measured by the indoor suction temperature sensor is lower than a temperature obtained by subtracting the set value from the set temperature, after start of the measurement of the intermittent operation time.

5. The air-conditioning apparatus of claim 1, further comprising an indoor suction temperature sensor configured to measure an indoor temperature in an air-conditioning target space in which the condenser or the evaporator is located, whereinthe controller is configured to determine that the heating load increases, start the operation of the compressor, and start measurement of the intermittent operation time, when in the intermittent operation in the heating operation, the indoor temperature measured by the indoor suction temperature sensor is lower than a temperature obtained by subtracting a set value, which is a predetermined set range, from a set temperature for an indoor space, andthe controller is configured to determine that the heating load decreases, stop the operation of the compressor, and stop the measurement of the intermittent operation time, when the indoor temperature measured by the indoor suction temperature sensor is higher than a temperature obtained by adding the set value to the set temperature, after the start of the measurement of the intermittent operation time.