WO2008016227A1

WO2008016227A1 - Method for controlling start-up operation of air conditioner

Info

Publication number: WO2008016227A1
Application number: PCT/KR2007/003439
Authority: WO
Inventors: Kil Hong Song
Original assignee: Daewoo Electronics Corporation
Priority date: 2006-08-04
Filing date: 2007-07-16
Publication date: 2008-02-07
Also published as: CN101495815A; KR100766177B1; EP2047180A1; US20080028780A1

Abstract

A method for controlling a start-up operation of an air conditioner having a compressor and an indoor and an outdoor fan includes the steps of: (a) calculating indoor cooling/heating load; (b) calculating a reference operating frequency of the compressor; (c) driving the indoor and the outdoor fan during a first time period to thereby adjust pressure equilibrium between an indoor and an outdoor pressure; (d) driving the compressor during a preset time period while increasing an operating frequency of the compressor; and (e) setting the operating frequency of the compressor to the reference operating frequency and driving the compressor. The operating frequency of the compressor is increased in a stepwise manner during the preset time period and kept to be lower than the reference operation frequency throughout the preset time period.

Description

METHOD FOR CONTROLLING START-UP OPERATION OF

AIR CONDITIONER

Technical Field

[1] The present invention relates to a method for controlling the operation of an air conditioner; and, more particularly, to a method for controlling a start-up operation of a compressor in an air conditioner that adjusts the operating capacity of the compressor based on a difference between an indoor temperature and a preset temperature.

[2]

Background Art

[3] As known well in the art, a typical air conditioner has a structure illustrated in Fig. 1.

[4] Referring to Fig. 1, the typical air conditioner is largely divided into an outdoor unit

110 and an indoor unit 120. The outdoor unit 110 includes a compressor 111, a four- way valve 112, an outdoor heat exchanger 113, an electronic expansion valve (EEV) 114, an accumulator 115 and an outdoor fan 116. The indoor unit 120 includes an indoor heat exchanger 121 and an indoor fan 123.

[5] During a cooling operation of the typical air conditioner with the above-described configuration, a high-temperature and high-pressure gaseous refrigerant compressed in the compressor 111 is introduced, via the four- way valve 112, into the outdoor heat exchanger 113 that functions as a condenser. This high-pressure gaseous refrigerant undergoes heat exchange, through the outdoor heat exchanger 113, with outdoor air, whose temperature is lower than the refrigerant temperature, to be condensed to a high pressure state. Here, the outdoor fan 116 is driven by an outdoor fan motor (not shown), and serves to forcibly ventilate the outdoor air.

[6] As the high-pressure condensed gaseous refrigerant passes through the EEV 114, it turns to low-temperature and low-pressure liquid refrigerant by throttling, and is conveyed to the indoor heat exchanger 121 of the indoor unit 120. Here, the indoor fan 123 is driven by an indoor fan motor (not shown), and serves to forcibly ventilate the indoor air.

[7] Next, the refrigerant in a liquid state is evaporated through heat exchange with indoor air at the indoor heat exchanger 121 which functions as an evaporator. After evaporation, the low-temperature and low-pressure gaseous refrigerant flows back to the outdoor unit 110 along a circulation line, in which it passes through the four- way valve 112 and is introduced again into the compressor 111 via the accumulator 115. Here, the accumulator 115 is utilized to change the refrigerant that is introduced into the compressor 111 into dry saturated vapor. [8] Further, during a heating operation of the typical air conditioner with the above- described configuration, the refrigerant flow direction at the four- way valve 112 is reversed, so the refrigerant flows in opposite direction from the refrigerant flow during the cooling operation set forth above. At this time, since the indoor heat exchanger 121 functions as a condenser differently from the cooling operation, warm air is circulated again into the indoor environment by the indoor fan 123. That is, the refrigerant flow during the heating operation of the air conditioner follows the circulation line: for example, "the compressor 111 → the four- way valve 112 → the indoor heat exchanger 121 → the EEV 114 → the outdoor heat exchanger 113 — > the four- way valve 112 → the accumulator 115 → the compressor 111".

[9] Meanwhile, in such an air conditioner that adjusts the operating capacity of the compressor based on a difference between an indoor temperature and a preset temperature by using the refrigerant circulation system described above, if the compressor is driven at a target frequency from the beginning of the start-up, it may be damaged and may produce vibration and noises because of an excessive operating load thereon.

[10] One of conventional ways to solve the above problems is to drive the air conditioner while changing the operating frequency of the compressor and the opening level of the EEV in two stages based on the start-up operation algorithm of a compressor having variable rotational frequency.

[11] To be specific, as shown in Fig. 2, the compressor is running at two different frequencies, i.e., a first and a second start-up frequency A and B. Also, in correspondence thereto, the EEV is running at two different opening levels, i.e., a first and a second start-up opening level a and b of the EEV.

[12] Specifically, the EEV is opened by the first start-up opening level a during the first start-up frequency A period, while the EEV is opened by the second start-up opening level b during the second star-up frequency B period. Then, the compressor and the EEV are running at a frequency and an opening level, which are suitably set based on a heat load depending on a specific temperature set by a user.

[13] In the above-described conventional method for driving the air conditioner while changing the operating frequency of the compressor and the opening level of the EEV in two stages, however, there still exist fundamental problems. For example, if a load on the compressor rapidly increases due to a sudden elevation in an indoor and/or an outdoor temperature, or if a refrigerant leaks within an air conditioner, a discharge temperature of the compressor increases excessively along with an increase in pressure. As a result of the excessive temperature increase, the compressor may be damaged or a discharge pipe may crack.

[14] In addition, since the compressor, by its nature, functions as a vibration source throughout the operation of the air conditioner, an excessive increase in pressure due to the above reasons becomes another factor for fatigue-induced damage of the discharge pipe. [15]

Disclosure of Invention

Technical Problem [16] It is, therefore, an object of the present invention to provide a method for controlling the operation of an air conditioner, which is capable of realizing optimal compressor start-up conditions by establishing an equilibrium state, through driving an indoor and/ or an outdoor fan, between an indoor pressure and an outdoor pressure prior to running the compressor in response to an operation start signal, and then adjusting an operating frequency of the compressor in a gradual manner. [17]

Technical Solution [18] In accordance with one aspect of the present invention, there is provided a method for controlling a start-up operation of an air conditioner having a compressor and an indoor and an outdoor fan, the method including the steps of: [19] (a) calculating indoor cooling/heating load based on an indoor temperature, an outdoor temperature and a difference between the indoor temperature and a target temperature; [20] (b) calculating a reference operating frequency of the compressor by using the calculated indoor cooling/heating load; [21] (c) driving the indoor and the outdoor fan during a first time period to thereby adjust pressure equilibrium between an indoor and an outdoor pressure, when an operation start signal of the air conditioner is inputted; [22] (d) driving the compressor during a preset time period while increasing in a stepwise manner an operating frequency of the compressor, wherein the operating frequency of the compressor is kept to be lower than the reference operation frequency throughout the preset time period; and [23] (e) setting the operating frequency of the compressor to the reference operating frequency and driving the compressor. [24] In accordance with another aspect of the present invention, there is provided a method for controlling a start-up operation of an air conditioner having a compressor and an indoor and an outdoor fan, the method including the steps of: [25] (a) calculating indoor cooling/heating load based on an indoor temperature, an outdoor temperature and a difference between the indoor temperature and a target temperature; [26] (b) calculating a reference operating frequency of the compressor by using the calculated indoor cooling/heating load;

[27] (c) driving the indoor and the outdoor fan during a first time period to thereby adjust pressure equilibrium between an indoor and an outdoor pressure, when an operation start signal of the air conditioner is inputted;

[28] (d) setting an operating frequency of the compressor to an initial operating frequency less than the reference operating frequency and driving the compressor during a second time period;

[29] (e) setting the operating frequency of the compressor to a median between the reference operating frequency and the current operating frequency and driving the compressor during a third time period;

[30] (f) repeating the step (e) by N times where N is a preset value and an integer greater than 1 ; and

[31] (g) setting the operating frequency of the compressor to the reference operating frequency and driving the compressor.

[32]

Advantageous Effects

[33] As described above, unlike a conventional method which drives a compressor while changing an operating frequency of a compressor and an opening level of an EEV in two stages, the present invention controls a start-up operation of a compressor in the following manner. When a power is on, a reference operating frequency is calculated based on a difference between an indoor temperature and a preset target temperature and calibration coefficients, and then, when an operation start signal is inputted, an equilibrium state between an indoor and an outdoor pressure is established by driving an indoor and an outdoor fan. After that, an operating frequency of the compressor is increased in a stepwise manner up to the calculated reference operating frequency. Accordingly, by decreasing an increase rate of a discharge pressure of the compressor, optimal compressor operation conditions can be realized, and thus the existing problems that the compressor may be damaged or a discharge pipe may crack due to an excessive increase in discharge temperature and pressure of the compressor can be prevented efficiently.

[34]

Brief Description of the Drawings

[35] The above and other objects and features of the present invention will become apparent from the following description of embodiments, given in conjunction with the accompanying drawings, in which:

[36] Fig. 1 shows an overall structural view of a typical air conditioner system; [37] Fig. 2 provides a timing chart for explaining a conventional method for controlling a start-up operation of an air conditioner in two intervals;

[38] Fig. 3 illustrates a block diagram of an operation control device of an air conditioner suitable for applying thereto an operation control method of an air conditioner in accordance with an embodiment of the present invention; and

[39] Figs. 4 and 5 are flowcharts illustrating the operation control method of the air conditioner in accordance with the present invention.

[40]

Best Mode for Carrying Out the Invention

[41] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[42] As will be described below, unlike the conventional method which operates the air conditioner while changing the operating frequency of the compressor and the opening level of the EEV in two stages, the present invention controls a start-up operation of a compressor in a manner that a reference operating frequency is calculated based on a difference between an indoor temperature and a preset temperature and each of calibration coefficients when a power is inputted. Thereafter, an operating frequency of the compressor is increased in a stepwise manner to a calculated reference operating frequency after an equilibrium state between an indoor pressure and an outdoor pressure is first established by driving indoor and outdoor fans when an operation start signal is inputted. By employing such technique, it is easier to accomplish the object of the invention.

[43] Fig. 3 illustrates a block diagram of an operation control device of an air conditioner suitable for applying thereto an operation control method of an air conditioner in accordance with the present invention. The operation control device includes an indoor temperature sensor 302, an outdoor temperature sensor 304, an manipulation block 306, a control block 308, a memory block 309, an EEV driving block 310, an indoor fan driving block 312, an outdoor fan driving block 314, and a compressor driving block 316.

[44] Referring to Fig. 3, the indoor temperature sensor 302 is installed at, e.g., a specific position of an indoor unit 120 shown in Fig. 1, to measure an indoor temperature. The measured indoor temperature is then forwarded to the control block 308. Similarly, the outdoor temperature sensor 304 is installed at, e.g., a specific position of an outdoor unit 110 shown in Fig. 1, to measure an outdoor temperature. The measured outdoor temperature is then delivered to the control block 308.

[45] The manipulation block 306 has a plurality of manipulation keys that are arranged for allowing a user to input various operation information such as power-on, operation mode (cooling operation mode, heating operation mode, etc.), target temperature, target air volume and the like. Various operation information received from the user is transferred to the control block 308.

[46] The control block 308 includes, e.g., a microprocessor and the like, to carry out the overall operation control of the air conditioner, and determines calibration coefficients for the indoor temperature, for the outdoor temperature and for a difference between the indoor temperature and the target temperature. For this, in the memory block 309, calibration coefficients for the indoor temperature, for the outdoor temperature and for a difference between the indoor temperature and the target temperature are pre-stored in the form of tables.

[47] Further, the control block 308 calculates indoor cooling/heating load based on a preset cooling capacity, a preset heating capacity and each of the calibration coefficients determined, and also calculates a reference operating frequency of the compressor based on the calculated indoor cooling/heating load. The control block 308 adaptively controls the start-up operation of the compressor in accordance with the calculated reference operating frequency. Here, the cooling and the heating capacity are fixed values depending on the capacity of the indoor unit.

[48] Furthermore, for the operation control of the air conditioner according to the present invention, the control block 308 adaptively (i.e., in a stepwise manner or gradually) controls operations of the indoor fan, the outdoor fan, the EEV, the compressor and so on. More details on these procedures will be provided later with reference to Figs. 4 and 5.

[49] Next, the EEV driving block 310 controls an opening level of the EEV 114 shown in

Fig. 1 in response to an opening level control signal provided from the control block 308. The indoor fan driving block 312 controls the operation of the indoor fan 123 shown in Fig. 1 in response to an indoor fan driving control signal from the control block 308. The outdoor fan driving block 314 controls the operation of an outdoor fan 116 shown in Fig. 1 in response to an outdoor fan driving control signal from the control block 308. Lastly, the compressor driving block 316 controls an operation of the compressor 111 at a preset operating frequency in response to a compressor driving control signal from the control bock 308.

[50] Now, a stepwise procedure for the operation control of an air conditioner in accordance with the present invention, using the operation control device of the air conditioner having the above-described configuration, will be described.

[51] Figs. 4 and 5 are flowcharts illustrating an operation control method of an air conditioner in accordance with an embodiment of the present invention.

[52] Referring to Figs. 4 and 5, when a power-on signal is inputted by a user during a standby power mode of an air conditioner, i.e., if a power-on signal is inputted from the manipulation block 306 (steps S402 and S404), the indoor temperature sensor 302 measures an indoor temperature (indoor air temperature) T and provides it to the c ai ontrol block 308, while the outdoor temperature sensor 304 measures an outdoor temperature (outdoor air temperature) T and forwards it to the control block 308 (step ao

S406).

[53] In response to this, the control block 308 determines calibration coefficients FT for ai the indoor temperature, FT for the outdoor temperature and FdT for a difference dT ao between the indoor temperature and the target temperature (user set temperature), with reference to tables of calibration coefficients stored in the memory block 309 (step S408). Here, each of the calibration coefficients thus determined is used for adjusting the operating frequency of the compressor.

[54] For the purpose, the memory block 309 stores calibration coefficients in the form of tables. For example, those calibration coefficients may be defined as in Tables 1 to 3.

[55] [56] Table 1 [Table 1]

Cooling Mode Heating mode

FT_a FT_a

T_al ≤16 0.80 30< T_a 0.85

16< T_ai <18 0.85 25< T_al <30 0.90

18< T_ai <23 0.90 23< T_al <25 0.95

23< T₃₁ <24 0.95 18< T_al <23

24< T_a 1.00 1.1

[57] [58] Table 2 [Table 2]

Cooling Mode Heating mode τ_ao FT_ao τ_ao FT_ao

T_ao <23 0.75 19< T_ao 0.8

23< T_ao ≤24 0.80 16≤ T_ao <19 0.85

24< T_ao <29 0.85 13< T_ao <16 0.9

29< T_ao <30 0.90 10< T_ao <13 0.95

30< T_ao <39 1.00 5< T_ao <10 1

39< T_ao 0.93 -l≤ T_ao <5 1.03

- T_ao <-l 1.15

[59] [60] Table 3 [Table 3]

[61] [62] Table 1 shows an example list of calibration coefficients for indoor temperatures for use in adjusting the compressor's operating frequency in both cooling and heating operation modes, and Table 2 represents an example list of calibration coefficients for outdoor temperatures for use in adjusting the compressor's operating frequency in both cooling and heating operation modes. Table 3 depicts an example list of calibration coefficients for differences between the indoor temperatures and target temperatures in both cooling and heating operation modes.

[63] Subsequently, the control block 308 calculates indoor cooling/heating load Q by Equation 1 based on the preset cooling capacity Q , the preset heating capacity Q , the calibration coefficient FT for indoor temperature, the calibration coefficient FT for ai ao outdoor temperature, and the calibration coefficient FdT for a difference in temperature (step S410).

[64] [65] MathFigure 1 [Math.l] x FT_a x FT₀₀ x FdT (Cooling mode)

x FT_ω x FT_ao x FdT (Heating mode)

[66] [67] Next, the control block 308 calculates a reference operating frequency F b for operation of the compressor by using Equation 2, based on the indoor cooling/heating load Q obtained from Equation 1 (step S412). As shown in Equation 2, F is a function of Q.

[68] [69] MathFigure 2 [Math.2]

F_b = F(O) [70]

[71] Normally, the operating frequency of the compressor in the air conditioner approximately ranges from 30 Hz to 70 Hz for a cooling operation, and from 30 Hz to 95 Hz for a heating operation. Therefore, if the air conditioner is in cooling mode, the control block 308 calculates the reference operation frequency of the compressor within the range from about 30 Hz to 70 Hz. Similarly, if the air conditioner is in heating mode, the control block 308 calculates the reference operation frequency of the compressor within the range of about 30 Hz to 95 Hz.

[72] In other words, when power is on, the reference operating frequency for operation of the compressor is calculated through the above-described procedure.

[73] After calculating the reference operating frequency, the control block 308 checks whether an operation start signal, i.e., an operation start signal based on user manipulation (or an advance setting) is received from the manipulation block 306 (step S414). In result of checking in step S414, if the operation start signal is inputted, the control block 308 changes an opening level of the EEV 114. After that, the control block 308 changes a flow path of a refrigerant to be proper for the operation conditions by using the four- way valve 112, and at the same time runs the indoor and the outdoor fan 123 and 116 for a preset period of time (steps S416 and S418). At this time, the operating frequency of the compressor 111 is 0 Hz. In step S418, reference symbol tl denotes a first preset time period (e.g., 1 min), and reference symbol nl denotes an elapsed time peirod.

[74] The reason for operating the indoor and the outdoor fan 123 and 116 for the first preset time period without driving the compressor immediately on receiving the operation start signal is to adjust indoor/outdoor pressure equilibrium prior to driving of the compressor 111.

[75] Next, in result of checking in step S418, if the first preset time period tl has lapsed, the control block 308 sets the operating frequency of the compressor 111 to a first preset frequency ml (e.g., 30 Hz) less than the reference operating frequency, and drives the compressor 111. The operation of the compressor 111 at the first preset frequency ml continues for a second preset time period (e.g., 1 min) (steps S420 and S422). In step S422, reference symbol t2 denotes the second preset time period during which the compressor 111 is driven at the first set frequency ml, and reference symbol n2 denotes an elapsed time period.

[76] In result of checking in step S422, if the second preset time period t2 has lapsed, the control block 308 sets the operating frequency of the compressor 111 to a second set frequency m2 which is, for example, between the calculated reference operating frequency and the first preset frequency ml, and drives the compressor 111 at the second preset frequency m2. The operation of the compressor 111 at the second preset frequency m2 continues for a third preset time period (e.g., 1 min) (steps S424 and S426). In step S426, reference symbol t3 denotes a third preset time period during which the compressor 111 is driven at the second preset frequency m2, and reference symbol n3 denotes an elapsed time period. For a smooth operation control for the compressor 111, the second preset frequency m2 is preferably set to, but not limited to, an intermediate value between the calculated reference operating frequency and the first preset frequency ml.

[77] In result of checking in step S426, if the third preset time period t3 has lapsed, the control block 308 sets the operating frequency of the compressor 111 to a third preset frequency m3 which is, for example, between the calculated reference operating frequency and the second preset frequency m2, and drives the compressor 111 at the third preset frequency m3. The operation of the compressor 111 at the third preset frequency m3 continues for a fourth preset time period (e.g., 1 min) (steps S428 and S430). In step S430, reference symbol t4 denotes a fourth preset time period during which the compressor 111 is driven at the third preset frequency m3, and reference symbol n4 denotes an elapsed time period. For a smooth operation control for the compressor 111, the third preset frequency m3 is preferably set to, but not limited to, an intermediate value between the calculated reference operating frequency and the second preset frequency m2.

[78] Lastly, in result of checking in step S430, if the fourth preset time period t4 has lapsed, the control block 308 ends the start-up operation of the compressor 111 by driving the compressor 111 at the calculated reference operating frequency (step S432).

[79] In accordance with the air conditioner operation control method of the present invention, optimal compressor driving conditions can be realized by executing a startup control of the compressor such that the operating frequency of the compressor increases in a stepwise manner at specific intervals until it reaches a target operating frequency (reference operating frequency). Accordingly, discharge pressure of the compressor increases slowly, and thus such a problem as oil coking in the compressor due to a rapid increase in the discharge temperature following the increase in the pressure can be prevented more efficiently.

[80] In addition, since the start-up of the compressor is controlled to increase in a stepwise manner the compressor operating frequency at specific intervals up to a target operating frequency, an increase in operating current may be suppressed to attain a more accurate control of RPM of a compressor motor, thereby realizing the primary objective of the invention, i.e., an optimal control of high-efficiency cooling/heating system employing the rotational frequency controlled compressor.

[81] Meanwhile, in accordance with the embodiment of the present invention, although the start-up of the compressor is controlled by increasing in a stepwise manner the operating frequency of the compressor up to the reference operating frequency through four stages (0 Hz and the first to the third preset frequency), the embodiment is only for illustrative purposes, and the present invention is not limited thereto. If needed or depending on application, the operating frequency of the compressor may be classified into and controlled through more than four stages (e.g., five, six, seven, eight stages and so on), and it is apparent that the start-up operation of the compressor can be controlled even more smoothly through the use of the above scheme.

[82] Furthermore, in accordance with the embodiment of the present invention, although the compressor is driven evenly for one minute at each of the four stages of the operating frequency, the embodiment is only for illustrative purposes, and the present invention is not limited thereto. It is apparent that the time period can be increased or decreased in consideration of various factors, such as, surrounding environment of the compressor. It is also noted that the running time of the compressor at each stage can be set differently whenever needed or depending on application.

[83] Besides, in accordance with the embodiment of the present invention, although calibration coefficients for an indoor temperature, for an outdoor temperature and for a difference in temperature (i.e., a difference between the indoor temperature and a target temperature) are read out from the pre-stored tables, the embodiment is only for illustrative purposes, and the present invention is not limited thereto. It is apparent that the calibration coefficients may be calculated in real time mode, instead of being pre- stored in the tables.

[84] While the invention has been shown and described with respect to the embodiments, it will be understood by those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.

Claims

[1] A method for controlling a start-up operation of an air conditioner having a compressor and an indoor and an outdoor fan, the method comprising the steps of:

(a) calculating indoor cooling/heating load based on an indoor temperature, an outdoor temperature and a difference between the indoor temperature and a target temperature;

(b) calculating a reference operating frequency of the compressor by using the calculated indoor cooling/heating load;

(c) driving the indoor and the outdoor fan during a first time period to thereby adjust pressure equilibrium between an indoor and an outdoor pressure, when an operation start signal of the air conditioner is inputted;

(d) driving the compressor during a preset time period while increasing in a stepwise manner an operating frequency of the compressor, wherein the operating frequency of the compressor is kept to be lower than the reference operation frequency throughout the preset time period; and

(e) setting the operating frequency of the compressor to the reference operating frequency and driving the compressor.

[2] The method of claim 1, wherein the step (a) includes:

(al) measuring the indoor and the outdoor temperature and calculating the difference between the measured indoor temperature and the target temperature; (a2) determining calibration coefficients for the indoor temperature, for the outdoor temperature and for the temperature difference, respectively, based on preset calibration coefficients; and

(a3) calculating the indoor cooling/heating load by using a preset cooling capacity, a preset heating capacity, and the determined calibration coefficients.

[3] The method of claim 1, wherein the operating frequency of the compressor is set to 0 Hz before driving the indoor and the outdoor fan in step (c).

[4] The method of claim 1, wherein the step (d) includes:

(dl) setting the operating frequency of the compressor to a first frequency less than the reference operating frequency and driving the compressor during a second time period;

(d2) setting the operating frequency of the compressor to a second frequency in a range between the reference operating frequency and the first frequency and driving the compressor during a third time period; and

(d3) setting the operating frequency of the compressor to a third frequency in a range between the reference operating frequency and the second frequency and driving the compressor during a fourth time period.

[5] The method of claim 4, wherein the second frequency is a median between the reference operating frequency and the first frequency.

[6] The method of claim 4, wherein the third frequency is a median between the reference operating frequency and the second frequency.

[7] The method of claim 4, wherein each of the first to fourth time period has an identical time duration.

[8] A method for controlling a start-up operation of an air conditioner having a compressor and an indoor and an outdoor fan, the method comprising the steps of:

(d) setting an operating frequency of the compressor to an initial operating frequency less than the reference operating frequency and driving the compressor during a second time period;

(e) setting the operating frequency of the compressor to a median between the reference operating frequency and the current operating frequency and driving the compressor during a third time period;

(f) repeating the step (e) by N times where N is a preset value and an integer greater than 1 ; and

(g) setting the operating frequency of the compressor to the reference operating frequency and driving the compressor.

[9] The method of claim 8, wherein the step (a) includes:

[10] The method of claim 8, wherein the operating frequency of the compressor is set to 0 Hz before driving the indoor and the outdoor fan in step (c). [11] The method of claim 8, wherein each of the first to third time period has an identical time duration.