Patent Publication Number: US-2017363332-A1

Title: Air-conditioning apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an air-conditioning apparatus. 
     BACKGROUND ART 
     A conventional air-conditioning apparatus detects, in a heating operation, the current value of an outdoor fan motor and the rotation speed of an outdoor fan, and determines whether to start a defrosting operation based on whether the current value of the outdoor fan motor becomes equal to or larger than a reference current value or the rotation speed of the outdoor fan decreases by a predetermined rotation speed (refer to Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2009-58222 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the air-conditioning apparatus disclosed in Patent Literature 1, the reference current value is determined in advance and cannot be changed with taken into account decrease in a fan input due to decrease in the fan rotation speed when the efficiency of the outdoor fan motor degrades by aging. This configuration prevents transition to the defrosting operation at appropriate timing in the heating operation. In other words, defrosting cannot be performed efficiently. 
     The present invention is intended to solve the above-described problem and provide an air-conditioning apparatus that performs a defrosting operation more efficiently than conventionally practiced. 
     Solution to Problem 
     An air-conditioning apparatus according to an embodiment of the present invention includes, by connecting, a compressor, an outdoor heat exchanger, an indoor heat exchanger, and a switching device, the switching device being provided closer to a discharge side of the compressor than the outdoor heat exchanger and provided closer to the discharge side of the compressor than the indoor heat exchanger. The air-conditioning apparatus includes a fan configured to deliver air toward the outdoor heat exchanger, a power unit configured to supply electric power to the fan, a fan input detector configured to detect a physical value related to the electric power supplied to the fan, and a controller configured to control the switching device to switch between a first operation in which the outdoor heat exchanger functions as an evaporator and a second operation in which the outdoor heat exchanger functions as a condenser. The first operation is switched to the second operation when the physical value detected by the fan input detector is equal to or larger than a reference value. The controller adjusts the reference value so that the reference value when refrigerant flowing through the outdoor heat exchanger has a high temperature is smaller than the reference value when the refrigerant has a low temperature. 
     Advantageous Effects of Invention 
     The air-conditioning apparatus according to an embodiment of the present invention includes the controller configured to control the switching device to switch between the first operation in which the outdoor heat exchanger functions as an evaporator and the second operation in which the outdoor heat exchanger functions as a condenser. The first operation is switched to the second operation when the physical value detected by the fan input detector is equal to or larger than the reference value. The controller adjusts the reference value so that the reference value when the refrigerant flowing through the outdoor heat exchanger has a high temperature is smaller than the reference value when the refrigerant flowing through the outdoor heat exchanger has a low temperature. With this configuration, a defrosting operation can be started at an appropriate timing while a heating operation is being performed. Thus, the defrosting operation can be performed more efficiently than has been conventionally practiced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view illustrating an air-conditioning apparatus  100  according to Embodiment 1 of the present invention. 
         FIG. 2  is a diagram illustrating change in a frosting amount and total electric power with elapsed time in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. 
         FIG. 3  is a diagram illustrating change in the frosting amount and total current value with elapsed time in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. 
         FIG. 4  is a diagram illustrating change in an electric power amount with elapsed time in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. 
         FIG. 5  is a diagram illustrating change in a total electric power amount with elapsed time in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. 
         FIG. 6  is a schematic view illustrating a state in which frost exists on an outdoor heat exchanger  3  of the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. 
         FIG. 7  is a diagram illustrating a relation between a relative humidity φ and a frost density ρ in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. 
         FIG. 8  is a diagram illustrating a relation between a refrigerant temperature and a necessary defrosting heat amount in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. 
         FIG. 9  is a diagram illustrating change in the frequency of a compressor  1  with elapsed time in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. 
         FIG. 10  is a diagram illustrating change in the frequency of the compressor  1  with elapsed time in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
     An air-conditioning apparatus  100  of the present invention will be described in detail below with reference to the drawings. The sizes of components in the drawings are in a relation different from that of their actual sizes in some cases. In the drawings, any components denoted by an identical reference sign are identical or equivalent to each other. This notation applies through the entire specification. In addition, any configuration of the components described in the entire specification is merely exemplary, and thus the present invention is not limited by the description. 
       FIG. 1  is a schematic view illustrating the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. As illustrated in  FIG. 1 , the air-conditioning apparatus  100  includes a compressor  1 , a four-way valve  2 , an outdoor heat exchanger  3 , an expansion valve  4 , and an indoor heat exchanger  5 . The compressor  1 , the four-way valve  2 , the outdoor heat exchanger  3 , the expansion valve  4 , and the indoor heat exchanger  5  are, for example, sequentially connected by pipes to form a refrigerant circuit  90 . 
     The compressor  1  is a variable capacity compressor configured to compress sucked refrigerant and discharge the refrigerant as high-temperature and high-pressure refrigerant. The four-way valve  2  is a switching device that switches a direction in which the refrigerant discharged from the compressor  1  flows, in response to, for example, execution of a heating operation or a cooling operation. The four-way valve  2  is provided closer to the discharge side of the compressor  1  than the outdoor heat exchanger  3  and provided closer to the discharge side of the compressor  1  than the indoor heat exchanger  5 .  FIG. 1  illustrates an exemplary state in which the four-way valve  2  is switched to perform a cooling operation. In  FIG. 1 , a solid line arrow indicates the flow of the refrigerant when the cooling operation is performed. In  FIG. 1 , a dashed line arrow indicates the flow of the refrigerant when a heating operation is performed. 
     The outdoor heat exchanger  3  is a heat exchanger configured to function as a condenser at the cooling operation and function as an evaporator at the heating operation. An outdoor side fan  31  is an air-sending unit configured to supply external air to the outdoor heat exchanger  3  and form airflow. The outdoor side fan  31  is, for example, an axial-flow fan or a centrifugal fan. The outdoor side fan  31  rotates when an outdoor side motor (not illustrated) is driven. Heat is exchanged between the air supplied from the outdoor side fan  31  and the refrigerant flowing inside the outdoor heat exchanger  3 . The outdoor side fan  31  is driven by a power unit (not illustrated) configured to supply electric power. 
     The expansion valve  4  is used to decompress and expand the refrigerant flowed out of the outdoor heat exchanger  3  at the cooling operation, and decompress and expand the refrigerant flowed out of the indoor heat exchanger  5  at the heating operation. 
     The indoor heat exchanger  5  is a heat exchanger configured to function as an evaporator at the cooling operation and function as a condenser at the heating operation. An indoor side fan  51  is an air-sending unit configured to supply indoor air to the indoor heat exchanger  5  and form airflow. The indoor side fan  51  is, for example, an axial-flow fan or a centrifugal fan. The indoor side fan  51  rotates when an indoor side motor (not illustrated) is driven. Heat is exchanged between the air supplied from the indoor side fan  51  and the refrigerant flowing inside the indoor heat exchanger  5 . 
     An outdoor side refrigerant temperature sensor  32  is a temperature detection unit configured to detect the temperature of the refrigerant flowing through the outdoor heat exchanger  3 . An indoor side refrigerant temperature sensor  52  is a sensor configured to detect the temperature of the refrigerant flowing through the indoor heat exchanger  5 . In the following description, a “refrigerant temperature” refers to the temperature of the refrigerant flowing inside the outdoor heat exchanger  3 . 
     A controller  80  controls the outdoor side motor to control the rotation speed of the outdoor side fan  31 , and controls the indoor side motor to control the rotation speed of the indoor side fan  51 . The controller  80  controls the outdoor side motor by changing voltage and current input to the outdoor side motor. The control of the rotation speed of the outdoor side fan  31  by the controller  80  allows control of the volume of air passing through the outdoor heat exchanger  3 . 
     A rotation speed detection unit configured to detect the rotation speed of the outdoor side fan  31  may be provided to detect the current rotation speed of the outdoor side fan  31 . Alternatively, the current rotation speed of the outdoor side fan  31  may be estimated from information on current applied to the outdoor side motor and voltage applied to the outdoor side motor. In the following description, a “fan input” refers to a physical value related to electric power supplied to the outdoor side fan  31  (the outdoor side motor configured to rotate the outdoor side fan  31 ). 
     The controller  80  controls the indoor side motor so that the outdoor side fan  31  rotates, for example, when the air-conditioning apparatus  100  starts operating. The controller  80  is, for example, hardware such as a circuit device or software executed on an arithmetic device such as a microcomputer or a CPU, which are configured to achieve this functionality. 
     The cooling operation is executed when the controller  80  switches the four-way valve  2  to cooling. The heating operation is executed when the controller  80  switches the four-way valve  2  to heating. In the following description, a “defrosting operation” refers to an operation executed when the controller  80  switches the four-way valve  2  to cooling and stops the outdoor side fan  31 . The heating operation corresponds to a “first operation” of the present invention, and the defrosting operation corresponds to a “second operation” of the present invention. 
     The following first describes, with reference to  FIG. 1 , the flow of the refrigerant when the air-conditioning apparatus  100  of the present invention executes the cooling operation. The refrigerant discharged from the compressor  1  flows into the outdoor heat exchanger  3 . Having flowed into the outdoor heat exchanger  3 , the refrigerant exchanges heat with the air supplied to the outdoor heat exchanger  3  through rotation of the outdoor side fan, and then flows out of the outdoor heat exchanger  3 . Having flowed out of the outdoor heat exchanger  3 , the refrigerant flows in the expansion valve  4  and is depressurized therein, and then flows out of the expansion valve  4  before flowing into the indoor heat exchanger  5 . Having flowed into the indoor heat exchanger  5 , the refrigerant exchanges heat with the air supplied to the indoor heat exchanger  5  through rotation of the indoor side fan, and then flows out of the indoor heat exchanger  5 . Having flowed out of the indoor heat exchanger  5 , the refrigerant flows into the compressor  1 . 
     The following describes, with reference to  FIG. 1 , the flow of the refrigerant when the air-conditioning apparatus  100  of the present invention executes the heating operation. The refrigerant discharged from the compressor  1  flows into the indoor heat exchanger  5 . Having flowed into the indoor heat exchanger  5 , the refrigerant exchanges heat with the air supplied to the indoor heat exchanger  5  through rotation of the indoor side fan, and then flows out of the indoor heat exchanger  5 . Having flowed out of the indoor heat exchanger  5 , the refrigerant flows in the expansion valve  4  and is depressurized therein, and then flows out of the expansion valve  4  before flowing into the outdoor heat exchanger  3 . Having flowed into the outdoor heat exchanger  3 , the refrigerant exchanges heat with the air supplied to the outdoor heat exchanger  3  through rotation of the outdoor side fan, and then flows out of the outdoor heat exchanger  3 . Having flowed out of the outdoor heat exchanger  3 , the refrigerant flows into the compressor  1 . 
       FIG. 2  is a diagram illustrating change in a frosting amount and total electric power with elapsed time in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention.  FIG. 3  is a diagram illustrating change in the frosting amount and total current value with elapsed time in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. 
     In  FIG. 2 , the horizontal axis represents elapsed time [min], and the vertical axis represents the frosting amount [g] and a total electric power amount [W]. In  FIG. 2 , a solid line indicates the frosting amount, and a dashed line indicates the total electric power. As illustrated in  FIG. 2 , the frosting amount increases as time elapses, and the total electric power increases as time elapses. 
     In  FIG. 3 , the horizontal axis represents elapsed time [min], and the vertical axis represents the frosting amount [g] and a total current value [A]. In  FIG. 3 , a solid line indicates the frosting amount, and a dashed line indicates the total current value. As illustrated in  FIG. 3 , the frosting amount increases as time elapses, and the total current value increases as time elapses. 
       FIG. 4  is a diagram illustrating change in an electric power amount with elapsed time in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention.  FIG. 5  is a diagram illustrating change in the total electric power amount with elapsed time in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention.  FIGS. 4 and 5  illustrate a case in which the fan input is the electric power amount, which is the product of current value applied to an outdoor fan motor and voltage value applied to the outdoor fan motor. Processing illustrated in  FIGS. 4 and 5  is performed at the heating operation. 
     First, as illustrated in  FIG. 4 , the controller  80  detects the fan input and calculates the amount of change in the fan input at each elapse of a predetermined time. Specifically, for example, when the fan input at time (t−1) is represented by W(t−1) and the fan input at time t is represented by W(t), the controller  80  calculates ΔW(t) as the difference between the fan inputs through Expression (1.1) below. 
       Δ W ( t )= W ( t )− W ( t− 1)  (1.1)
 
     Subsequently, as illustrated in  FIG. 5 , the controller  80  calculates ΔWtotal by summing ΔW(t) according to Expression (1.2) below. 
       Δ W total=ΣΔ W ( t )  (1.2)
 
     Then, the controller  80  determines whether ΔWtotal is equal to or larger than a threshold α as in Expression (1.3) below. When having determined that ΔWtotal is equal to or larger than the threshold α, the controller  80  controls the four-way valve  2  to start the defrosting operation. When having determined that ΔWtotal is smaller than the threshold α, the controller  80  continues the heating operation. 
       Δ W total≧α  (1.3)
 
     The threshold α varies with the refrigerant temperature. Specifically, for example, it is assumed that the density of frost on the outdoor heat exchanger  3  is larger at α higher refrigerant temperature, and thus the controller  80  decreases the value of a accordingly. When the value of α is decreased in this manner, ΔWtotal becomes equal to or larger than α at earlier timing and the defrosting operation is started earlier. For example, it is assumed that the density of frost on the outdoor heat exchanger  3  is smaller at a lower refrigerant temperature, and thus the controller  80  increases the value of α accordingly. When the value of α is increased in this manner, ΔWtotal becomes equal to or larger than α at later timing and start of the defrosting operation is delayed. In the above description, the fan input is the electric power, but the present invention is not limited thereto. For example, the fan input may be the current value applied to the outdoor fan motor or the voltage value applied to the outdoor fan motor. 
       FIG. 6  is a schematic view illustrating a state in which frost exists on the outdoor heat exchanger  3  of the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. As illustrated in  FIG. 6 , the frost on the outdoor heat exchanger  3  has a height Hf_total [mm], and adjacent fins  3   b  are apart from each other by a distance Fp [mm]. It is assumed that wind blows from one end of each fin  3   b  in the longitudinal direction thereof to the other end. Since frost exists on the outdoor heat exchanger  3  as illustrated in  FIG. 6 , a wind speed ua decreases, and thus heat exchange at the outdoor heat exchanger  3  is hindered as compared to a case in which no frost exists on the outdoor heat exchanger  3 . 
     In the heating operation, frost exists on a heat transfer tube  3   a  and the fins  3   b  included in the outdoor heat exchanger  3 . As the frost grows, draft resistance increases and input of the outdoor side fan  31  increases. The frost has a lower density as the heat transfer tube  3   a  and the fins  3   b  have lower temperatures. In other words, the frost density is smaller at a lower refrigerant temperature. 
     Thus, when the fins  3   b  is blocked, the amount of frost on the outdoor heat exchanger  3  differs for different frost densities. In other words, the defrosting operation needs different defrosting heat amounts for an identical blockage state of the outdoor heat exchanger  3  and an identical amount of increase in the fan input. Specifically, at a higher refrigerant temperature, a larger amount of heat is needed to melt frost on the outdoor heat exchanger  3 . 
       FIG. 7  is a diagram illustrating a relation between a relative humidity φ and a frost density ρ in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. In  FIG. 7 , the horizontal axis represents the relative humidity φ [%], and the vertical axis represents the frost density ρ [kg/m 3 ].  FIG. 7  illustrates cases with the refrigerant temperature Ts [degrees C.] at −30 degrees C. and −20 degrees C. 
     As illustrated in  FIG. 7 , the frost density ρ decreases as the relative humidity φ increases. The frost density ρ is larger when the refrigerant temperature Ts is −20 degrees C. than when the refrigerant temperature Ts is −30 degrees C. In other words, the frost density ρ increases as the refrigerant temperature Ts increases. A defrosting duration increases as the frost density ρ increases, and a larger defrosting capacity is needed as the frost density ρ increases. Thus, the defrosting duration increases as the refrigerant temperature Ts increases. 
       FIG. 8  is a diagram illustrating a relation between the refrigerant temperature and a necessary defrosting heat amount in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. As illustrated in  FIG. 8 , the necessary defrosting heat amount is proportional to the temperature of the refrigerant flowing through the refrigerant circuit  90  inside the outdoor heat exchanger  3 . 
     As illustrated in  FIG. 8 , the defrosting duration increases as the refrigerant temperature Ts increases. Specifically, for example, a minimum defrosting duration is one minute when an average refrigerant temperature is −40 degrees C. to −30 degrees C. For example, the minimum defrosting duration is three minutes when the average refrigerant temperature is −10 degrees C. to −5 degrees C. For example, the minimum defrosting duration is five minutes when the average refrigerant temperature is −5 degrees C. to 0 degrees C. 
     Although  FIG. 8  illustrates, for sake of simplicity of description, the proportional relation between the necessary defrosting heat amount and the refrigerant temperature Ts, the present invention is not limited to such a relation. The amount of increase in the necessary defrosting heat amount for increase in the refrigerant temperature Ts does not need to be constant. 
       FIG. 9  is a diagram illustrating change in the frequency of the compressor  1  with elapsed time in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention.  FIG. 10  is a diagram illustrating change in the frequency of the compressor  1  with elapsed time in the air-conditioning apparatus  100  according to Embodiment 1 of the present invention. 
     In  FIGS. 9 and 10 , the horizontal axis represents elapsed time, and the vertical axis represents the frequency of the compressor  1 . In  FIGS. 9 and 10 , a solid line indicates change in the frequency of the compressor  1  when the refrigerant temperature is relatively high, and a dashed line indicates change in the frequency of the compressor  1  when the refrigerant temperature is relatively low. 
     The defrosting operation can be performed in a shorter time at a relatively low refrigerant temperature than at a relatively high refrigerant temperature. However, efficient execution of the defrosting operation requires a time for melting frost on the outdoor heat exchanger  3  and a time for allowing melted frost to drop from the outdoor heat exchanger  3 . Thus, melted frost potentially freezes again when the duration of the defrosting operation at a relatively low refrigerant temperature is shorter than the duration of the defrosting operation at a relatively high refrigerant temperature. For this reason, in Embodiment 1, the operation is performed with identical defrosting durations at a relatively low refrigerant temperature and a relatively high refrigerant temperature and with a low frequency of the compressor  1 , which will be described below. 
     The following describes, with reference to  FIG. 9 , an example in which the frequency of the compressor  1  is changed based on the refrigerant temperature in the defrosting operation. In  FIG. 9 , Interval (a) refers to an interval in which the heating operation is executed, Interval (b) refers to an interval in which the defrosting operation is executed, and Interval (c) refers to an interval in which the heating operation is executed after the defrosting operation. 
     As illustrated in  FIG. 9 , in Interval (a), the controller  80  controls the compressor  1  so that the compressor  1  has a predetermined frequency while the four-way valve  2  is switched to heating. After the compressor  1  is operated at the predetermined frequency for a predetermined time, the controller  80  controls the compressor  1  to decrease the frequency thereof. Then, when the frequency of the compressor  1  becomes zero (t 11 ), the controller  80  switches the four-way valve  2  to cooling and starts the defrosting operation. 
     As illustrated in  FIG. 9 , in Interval (b), when the refrigerant temperature is relatively high, the controller  80  controls the compressor  1  so that the compressor  1  has a predetermined frequency fmax while the four-way valve  2  is switched to cooling. After the compressor  1  is operated at the predetermined frequency fmax for a predetermined time, the controller  80  controls the compressor  1  to decrease the frequency of the compressor  1 . Then, when the frequency of the compressor  1  becomes zero (time t 14 ), the controller  80  switches the four-way valve  2  to heating again and starts the heating operation. 
     As illustrated in  FIG. 9 , in Interval (b), when the refrigerant temperature is relatively low, the controller  80  controls the compressor  1  so that the compressor  1  has the predetermined frequency fmax while the four-way valve  2  is switched to cooling. After the compressor  1  is operated at the predetermined frequency fmax for a predetermined time (time t 12 ), the controller  80  controls the compressor  1  to decrease the frequency thereof so that the compressor  1  has a predetermined frequency f 1 . After the frequency of the compressor  1  is decreased to the predetermined frequency f 1  (time t 13 ), the controller  80  operates the compressor  1  at the predetermined frequency f 1  for a predetermined time. After the compressor  1  is operated at the predetermined frequency f 1  for the predetermined time (time t 13 ), the controller  80  controls the compressor  1  to decrease the frequency of the compressor  1 . Then, when the frequency of the compressor  1  becomes zero (time t 14 ), the controller  80  switches the four-way valve  2  to heating again and starts the heating operation. 
     As illustrated in  FIG. 9 , in Interval (c), the controller  80  controls the compressor  1  so that the frequency thereof has a predetermined frequency while the four-way valve  2  is switched to heating. 
     The following describes, with reference to  FIG. 10 , an example in which the frequency of the compressor  1  is changed based on the refrigerant temperature in the defrosting operation. In  FIG. 10 , Interval (a) refers to an interval in which the heating operation is executed, Interval (b) refers to an interval in which the defrosting operation is executed, and Interval (c) refers to an interval in which the heating operation is executed after the defrosting operation. In  FIG. 10 , change in the frequency of the compressor  1  as time elapses in Interval (a) and Interval (c) is identical to that in  FIG. 9 , and thus description thereof will be omitted. 
     As illustrated in  FIG. 10 , in Interval (b), when the refrigerant temperature is relatively high, the controller  80  controls the compressor  1  so that the compressor  1  has the predetermined frequency fmax while the four-way valve  2  is switched to cooling. After the compressor  1  is operated at the predetermined frequency fmax for a predetermined time, the controller  80  controls the compressor  1  to decrease the frequency of the compressor  1 . Then, when the frequency of the compressor  1  becomes zero (time t 24 ), the controller  80  switches the four-way valve  2  to heating again and starts the heating operation. 
     As illustrated in  FIG. 10 , in Interval (b), when the refrigerant temperature is relatively low, the controller  80  controls the compressor  1  so that the compressor  1  has a predetermined frequency f 2  while the four-way valve  2  is switched to cooling. After the compressor  1  acquires the predetermined frequency f 2  (time t 22 ) and has operated for a predetermined time (time t 23 ), the controller  80  controls the compressor  1  to decrease the frequency of the compressor  1 . Then, when the frequency of the compressor  1  becomes zero (time t 24 ), the controller  80  switches the four-way valve  2  to heating again and starts the heating operation. 
     As described above, in the air-conditioning apparatus  100  according to Embodiment 1, the compressor  1 , the outdoor heat exchanger  3 , the indoor heat exchanger  5 , and the four-way valve  2  provided closer to the discharge side of the compressor  1  than the outdoor heat exchanger  3  and provided closer to the discharge side of the compressor  1  than the indoor heat exchanger  5  are connected with each other. The air-conditioning apparatus  100  includes the fan  31  configured to deliver air toward the outdoor heat exchanger  3 , the power unit configured to supply electric power to the fan  31 , a fan input detector configured to detect a physical value related to the electric power supplied to the fan  31 , and the controller  80  configured to control the four-way valve  2  to switch between the first operation in which the outdoor heat exchanger  3  functions as an evaporator and the second operation in which the outdoor heat exchanger  3  functions as a condenser. The first operation is switched to the second operation when the physical value detected by the fan input detector is equal to or larger than a reference value. The controller  80  adjusts the reference value so that the reference value when the refrigerant flowing through the outdoor heat exchanger  3  has a high temperature is smaller than the reference value when the refrigerant has a low temperature. With this configuration, the defrosting operation can be started at appropriate timing when the heating operation is performed. Accordingly, the defrosting operation can be performed more efficiently than conventionally practiced. 
     In the air-conditioning apparatus  100  according to Embodiment 1, the compressor  1 , the outdoor heat exchanger  3 , the indoor heat exchanger  5 , and the four-way valve  2  provided closer to the discharge side of the compressor  1  than the outdoor heat exchanger  3  and provided closer to the discharge side of the compressor  1  than the indoor heat exchanger  5  are connected with each other. The air-conditioning apparatus  100  includes the fan  31  configured to deliver air toward the outdoor heat exchanger  3 , the power unit configured to supply electric power to the fan  31 , the fan input detector configured to detect a physical value related to the electric power supplied to the fan  31 , and the controller  80  configured to control the four-way valve  2  to switch between the first operation in which the outdoor heat exchanger  3  functions as an evaporator and the second operation in which the outdoor heat exchanger  3  functions as a condenser. The first operation is switched to the second operation when the physical value detected by the fan input detector is equal to or larger than a reference value. The controller  80  controls the frequency of the compressor  1  so that the frequency of the compressor  1  when the refrigerant flowing through the outdoor heat exchanger  3  has a high temperature is higher than the frequency of the compressor  1  when the refrigerant has a low temperature. With this configuration, the defrosting operation can be performed in accordance with the frosting amount more appropriately than conventionally practiced. Accordingly, the defrosting operation can be performed more efficiently than conventionally practiced. 
     Embodiment 2 
     In Embodiment 2, unlike Embodiment 1, the timing of execution of the defrosting operation is determined based on a frosting amount Mf, and the frequency of the compressor  1  in the defrosting operation is determined based on the frosting amount Mf. In Embodiment 2, any characteristic is same as that of Embodiment 1 unless otherwise stated, and any identical function and configuration will be described by using identical reference signs. 
     The frosting amount mf(t) is given based on a surface area A 0  [m 2 ], the frost density ρf [kg/m 3 ], and a frost height Hf(t) through Expression (2.1) below. 
         mf ( t )= A 0×ρ f ( t )× Hf ( t )  (2.1)
 
     Expression (2.1) below assumes that frost uniformly exists on the outdoor heat exchanger  3 . The surface area A 0  [m 2 ] is a heat exchange surface area of the outdoor heat exchanger  3 . The frost density ρf [kg/m 3 ] is the density of frost on the outdoor heat exchanger  3 , which is affected by a cooling surface temperature and a relative humidity. The frost height Hf(t) is the height of frost on the outdoor heat exchanger  3 . 
     The frosting amount Mf is given based on the frosting amount mf(t) through Expression (2.2) below. 
         Mf=Σm ( t )  (2.2)
 
     A defrosting heat amount Qf [kJ] is given based on the frosting amount Mf [kg] and a latent heat ΔH [kJ/kg] through Expression (2.3) below. 
         Qf=Mf×ΔH   (2.3)
 
     A defrosting duration Tf [sec] is given based on the defrosting heat amount Qf [kJ] and a defrosting capacity P [kW] through Expression (2.4) below. 
         Tf=Qf/P   (2.4)
 
     As described above, the controller  80  of the air-conditioning apparatus  100  according to Embodiment 2 determines the defrosting duration in accordance with the frosting amount. Accordingly, the defrosting operation can be performed more efficiently than conventionally practiced. 
     The outdoor side fan  31  corresponds to a “fan” of the present invention. 
     REFERENCE SIGNS LIST 
     1 compressor  2  four-way valve  3  outdoor heat exchanger  3   a  heat transfer tube  3   b  fin  4  expansion valve  5  indoor heat exchanger  31  outdoor side fan  32  outdoor side refrigerant temperature sensor  51  indoor side fan  52  indoor side refrigerant temperature sensor  80  controller  90  refrigerant circuit  100  air-conditioning apparatus A 0  surface area f 1 , f 2 , fmax predetermined frequency Hf frost height Mf frosting amount mf frosting amount P defrosting capacity Qf defrosting heat amount t 11 , t 12 , t 13 , t 14 , t 21 , t 22 , t 23 , t 24  time Tf the defrosting duration Ts surface temperature ua wind speed ΔH latent heat α threshold ρ frost density ρf frost density φ relative humidity