Abstract:
A clothing dryer includes a drum to receive clothing, a first heater, a second heater having a capacity smaller than a capacity of the first heater, a temperature sensor to sense temperature of air exhausted from the drum and a dryness sensor to sense dryness of the clothing received in the drum. The control method of the clothing dryer includes simultaneously turning on the first heater and the second heater; alternately turning on and off at least one of the first heater and the second heater when a value of the temperature sensed by the temperature sensor reaches a predetermined temperature; and maintaining one of the first heater and the second heater in a turned-on state when a value of the dryness sensed by the dryness sensor reaches a predetermined dryness.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 12/926,261 filed on Nov. 4, 2010, which claims the benefit of Korean Patent Application No. 10-2010-0000939, filed on Jan. 6, 2010 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    Embodiments relate to a clothing dryer and a control method thereof that reduces power consumption without increasing drying time. 
         [0004]    2. Description of the Related Art 
         [0005]    Generally, a clothing dryer is an apparatus that supplies hot air heated by a hot air heater into a drum having clothing to be dried placed therein during the rotation of the drum to dry the clothing. The clothing dryer may be classified as an exhaust type clothing dryer in which high-temperature and high-humidity air, having passed through the drum, is discharged out of the clothing dryer or a condensing type clothing dryer in which moisture is removed from high-temperature and high-humidity air, having passed through the drum, and the air is supplied into the drum. 
         [0006]    In the clothing dryer, the hot air heater to heat air is turned on/off based on dryness of the clothing, internal humidity of the drum, temperature of air (exhaust air) discharged from the outlet of the drum after drying the clothing, etc. However, the hot air heater is controlled to maintain a predetermined temperature period until the drying is completed, with the result that excessive energy is consumed in a second half of a drying period, thereby lowering energy efficiency. 
       SUMMARY  
       [0007]    It is an aspect to provide a clothing dryer and a control method thereof that adjusts a heater capacity based on a humidity change amount (a drying rate) in a second half of a drying period to reduce power consumption. 
         [0008]    It is another aspect to provide a clothing dryer and a control method thereof that adjusts a heater capacity in a period in which internal humidity of a drum is suddenly decreased to reduce power consumption. 
         [0009]    Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention. 
         [0010]    In accordance with one aspect, a clothing dryer includes a drum to receive clothing, a plurality of hot air heaters to supply hot air into the drum, a humidity sensor to sense internal humidity of the drum, and a controller to calculate a humidity change amount in the drum, to compare the calculated humidity change amount with a predetermined change amount, and to control one of the hot air heaters to be operated to adjust a heater capacity in a second half of a drying period when the humidity change amount is greater than the predetermined change amount. 
         [0011]    The hot air heaters may include first and second hot air heaters having different capacities. 
         [0012]    One of the hot air heaters may be one having the relatively small capacity selected from the first and second hot air heaters. 
         [0013]    The controller may control both the first and second hot air heaters to be operated when the humidity change amount is not greater than the predetermined change amount. 
         [0014]    The clothing dryer may further include a temperature sensor to sense exhaust temperature at an outlet of the drum, and the controller may control the first and second hot air heaters to be selectively operated based on the exhaust temperature when the humidity change amount is not greater than the predetermined change amount. 
         [0015]    The controller may control both the first and second hot air heaters to be turned on/off based on the exhaust temperature. 
         [0016]    The controller may control one of the first and second hot air heaters to be turned on and the other of the first and second hot air heaters to be turned on/off based on the exhaust temperature. 
         [0017]    In accordance with another aspect, a clothing dryer includes a drum to receive clothing, first and second hot air heaters, having different capacities, to supply hot air into the drum, a humidity sensor to sense internal humidity of the drum, and a controller to control one of the first and second hot air heaters to be operated to adjust a heater capacity in a second half of a drying period when the internal humidity of the drum has reached a predetermined humidity range. 
         [0018]    The predetermined humidity range may be a range in which the internal humidity of the drum starts to suddenly decrease. 
         [0019]    The controller may control both the first and second hot air heaters to be operated when the internal humidity of the drum has not reached the predetermined humidity range. 
         [0020]    The clothing dryer may further include a temperature sensor to sense exhaust temperature at an outlet of the drum, and the controller may control the first and second hot air heaters to be selectively operated based on the exhaust temperature when the internal humidity of the drum has not reached the predetermined humidity range. 
         [0021]    In accordance with a further aspect, a control method of a clothing dryer including a drum and a plurality of hot air heaters to supply hot air into the drum includes determining whether a drying course has been selected, turning on/off the hot air heaters to perform a drying operation upon determining that the drying course has been selected, sensing changes in internal humidity of the drum during the drying operation, calculating a humidity change amount in the drum, and comparing the calculated humidity change amount with a predetermined change amount and controlling one of the hot air heaters to be turned on to adjust a heater capacity in a second half of a drying period when the humidity change amount is greater than the predetermined change amount. 
         [0022]    The control method may further include sensing exhaust temperature at an outlet of the drum and controlling the first and second hot air heaters to be selectively turned on/off based on the exhaust temperature when the humidity change amount is not greater than the predetermined change amount. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    These and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
           [0024]      FIG. 1  is a sectional view illustrating the structure of a clothing dryer according to an embodiment; 
           [0025]      FIG. 2  is a control configuration diagram of the clothing dryer according to the embodiment; 
           [0026]      FIG. 3  is a graph illustrating a first profile of heater control based on internal humidity of a drum and exhaust temperature during a drying operation of the clothing dryer according to the embodiment; 
           [0027]      FIG. 4  is a graph illustrating a second profile of heater control based on internal humidity of the drum and exhaust temperature during a drying operation of the clothing dryer according to the embodiment; 
           [0028]      FIG. 5  is a graph illustrating a third profile of heater control based on internal humidity of the drum and exhaust temperature during a drying operation of the clothing dryer according to the embodiment; and 
           [0029]      FIG. 6  is a flow chart illustrating a control method of the clothing dryer according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    Reference will now be made in detail to the embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0031]      FIG. 1  is a sectional view illustrating the structure of a clothing dryer  10  according to an embodiment. 
         [0032]    Referring to  FIG. 1 , the clothing dryer  10  includes a housing  20  formed approximately in a hexahedral shape, the housing  20  having through holes  25  formed at the rear thereof to suction external air, a cylindrical drum  30  mounted in the housing  20 , the cylindrical drum  30  being open at the front and rear thereof, a front bracket  31  and a rear bracket  33  to slidably support the inner circumference of the drum  30  at the front and rear ends thereof, respectively, and slide pads  40  mounted between the drum  30  and the front bracket  31  and between the drum  30  and the rear bracket  33  to assist smooth rotation of the drum  30 . 
         [0033]    The front bracket  31 , mounted at the front of the drum  30 , is open at the center thereof such that clothing is introduced into the drum  30  through the opening, which is closed by door  50 . At the lower part of the drum  30  is formed an exhaust port  60  through which air containing moisture evaporated from the clothing is discharged to the outside. 
         [0034]    Inside the portion of the drum  30  adjacent to the exhaust port  60  is mounted a humidity sensor  61  to sense internal humidity of the drum  30 . The humidity sensor  61  contacts clothing rotated according to the rotation of the drum  30  to generate an electric signal according to the amount of moisture contained in the clothing, based on which dryness of the clothing is determined. 
         [0035]    Below the drum  30  are mounted an exhaust duct  70 , a blowing fan  73  and an exhaust pipe  76  to discharge air discharged from the exhaust port  60  out of the clothing dryer  10 . In the exhaust duct  70  is mounted a filter  71  to filter impurities, such as dust or lint, contained in the air discharged from the exhaust port  60 . The exhaust duct  70  guides the air discharged from the exhaust port  60  to the lower part of the housing  20 . The exhaust duct  70  is connected to the blowing fan  73  to generate force to induce flow of air in the clothing dryer  10 . Air discharged by the blowing fan  73  is discharged to the outside through the exhaust pipe  76  having one end connected to a fan casing  79  of the blowing fan  73  and the other end extending out of the housing  20 . In the exhaust pipe  76  is mounted an exhaust temperature sensor  77  to sense temperature of the air discharged from the outlet of the drum  30  after drying clothing, i.e., exhaust temperature. 
         [0036]    Below the drum  30  is mounted a motor  80  to simultaneously drive the blowing fan  73  and the drum  30 . The motor  80  has a drive shaft extending back and forth. One end of the drive shaft is connected to the blowing fan  73 , and the other end of the drive shaft is connected to a pulley  83  to drive the drum  30 . The pulley  83  and the drum  30  are connected to each other via a belt  86  such that the drum  30  is rotated when the motor  80  is driven. 
         [0037]    At the upper part of the rear bracket  33 , mounted at the rear of the drum  30 , are formed intake ports  34  to suction hot air. At the rear of the rear bracket  33  is mounted an intake duct  35  to guide the hot air to the intake ports  34 . The intake duct  35  extends rearward from below the drum  30  and is bent upward to communicate with the intake ports  34 . At the inlet of the intake duct  35  are mounted first and second hot air heaters  36  and  38  to heat air suctioned from the interior of the housing  20 . At the outlet of the intake duct  35  is mounted a heater temperature sensor  39  to sense temperature of the air having passed through the first and second hot air heaters  36  and  38 . 
         [0038]    The first and second hot air heaters  36  and  38  have different power capacities. For example, when a total power capacity (M+N: 100%) is 5.3 kW, the first hot air heater  36  has a relatively large power capacity M of approximately 3.7 kW (70%), and the second hot air heater  38  has a relatively small power capacity N of approximately 1.6 kW (30%). The power capacities of the first and second hot air heaters  36  and  38  may not have a ratio of 70:30. Various ratios of the power capacities may be provided to reduce power consumption to more than a predetermined level (approximately 5%) while securing a predetermined drying time (approximately 25 minutes but less than 30 minutes). Generally, the second hot air heater  38  has a variable power capacity N of approximately 1 to 2 kW. 
         [0039]    In this embodiment, the clothing dryer is of an exhaust type although the clothing dryer may be of a condensing type. 
         [0040]      FIG. 2  is a control configuration diagram of the clothing dryer according to the embodiment of the present invention. The clothing dryer includes an input unit  100 , a controller  110  and a drive unit  120 . 
         [0041]    The input unit  100  allows a user to input operation information, such as a drying course, drying time, and an operation command, to the controller  110 . 
         [0042]    The controller  110  is a microprocessor to control overall operations of the clothing dryer according to the operation information input from the input unit  100 . The controller  110  calculates a humidity change amount (a humidity change rate per predetermined time: a drying rate) based on internal humidity of the drum  30  sensed by the humidity sensor  61  and divides a drying period into several periods based on the calculated humidity change amount (the drying rate) to control a drying operation. 
         [0043]    That is, the controller  110  divides the drying period into a transient drying period, a constant rate drying period and a falling rate drying period based on the humidity change amount (the drying rate). 
         [0044]    The transient drying period is an initial drying period in which clothing starts to be dried. In the transient drying period, temperature of air in the drum  30 , the clothing and the surroundings of the clothing is increased such that hot air heated by the first and second hot air heaters  36  and  38  easily evaporates moisture from the surface of the clothing. 
         [0045]    The constant rate drying period is a period in which main drying is performed. In the constant rate drying period, the hot air heated by the first and second hot air heaters  36  and  38  is used to dry the moisture contained in the clothing. Therefore, the temperature of the air at the outlet of the drum  30 , i.e., the temperature of the exhaust air, has a constant temperature distribution. 
         [0046]    The falling rate drying period is a period corresponding to a second half of a drying period in which drying is ended. In the falling rate drying period, less moisture is contained in the clothing than in the constant rate drying period, with the result that the hot air heated by the first and second hot air heaters  36  and  38  is used to dry the air in the drum  30 . Therefore, the temperature of the air at the outlet of the drum  30 , i.e., the temperature of exhaust air, has a suddenly rising temperature distribution. 
         [0047]    Also, the controller  110  controls only the second hot air heater  38  having the relatively small capacity to be operated, such that the internal temperature of the drum  30  is not excessively increased, at a point where the internal humidity of the drum  30  is suddenly decreased in the falling rate drying period corresponding to the second half of the drying period. More specifically, upon determining that the humidity change amount in the drum  30  is greater than a predetermined change amount as the result of the comparison therebetween, the controller  110  determines that the period at that time is a quasi-falling rate drying period in which the internal humidity of the drum  30  starts to suddenly decrease and controls only the second hot air heater  38  having the relatively small capacity to be operated. When the second hot air heater  38  alone is operated, the clothing containing little moisture is sufficiently dried, and the internal temperature of the drum  30  is maintained at a predetermined level or less. Consequently, the first and second hot air heaters  36  and  38  may not be frequently turned on/off. The operation of the second hot air heater  38  alone is continued until the drying is ended. 
         [0048]    The drive unit  120  operates the first and second hot air heaters  36  and  38  and the motor  80  according to a drive control signal from the controller  110 . 
         [0049]    Relations between the internal humidity of the drum  30  and the control of the first and second hot air heaters  36  and  38  in the divided drying periods are shown in  FIGS. 3 to 5 . 
         [0050]      FIG. 3  is a graph illustrating a first profile of heater control based on internal humidity of the drum and exhaust temperature during a drying operation of the clothing dryer according to the embodiment of the present invention. 
         [0051]    As shown in  FIG. 3 , the controller  110  controls both the first and second hot air heaters  36  and  38  to be operated with the maximum heater capacity (M+N: 100%) in the transient drying period (approximately 5 minutes or less) in which clothing starts to be dried to increase temperature of air in the drum  30 , the clothing and the surroundings of the clothing such that the clothing dryer  10  reaches an optimized drying state within a short time. 
         [0052]    Subsequently, the controller  110  controls the first hot air heater  36  having the relatively large capacity M to be turned on/off according to the temperature of the exhaust air, i.e., exhaust temperature having a control temperature period set to uniformly maintain the exhaust air, and the second hot air heater  38  having the relatively small capacity N to be continuously on in the constant rate drying period in which main drying is performed such that the moisture contained in the clothing is removed within a short time. At this time, the humidity change amount (the drying rate) is uniformly maintained. 
         [0053]    When the moisture contained in the clothing in the drum  30  is almost completely removed as the result of the drying operation in the constant rate drying period, the temperature of the air discharged from the outlet of the drum  30  after drying the clothing, i.e., the exhaust temperature, is suddenly increased, with the result that the internal humidity of the drum  30  is suddenly decreased. At this time, the humidity change amount is also decreased. Upon determining that the calculated humidity change amount is greater than the predetermined change amount as the result of the comparison therebetween, the controller  110  determines the period at that time is the quasi-falling rate drying period and controls the first hot air heater  36  to be stopped. Upon determining that any control temperature period is the quasi-falling rate drying period, the controller  110  controls the first hot air heater  36  having the relatively large capacity M to be stopped. 
         [0054]    When the heater control is performed, such that the first hot air heater  36  having the relatively large capacity M is stopped and the second hot air heater  38  having the relative small capacity N is operated, before the quasi-falling rate drying period, drying time exceeds  30  minutes, and, when the heater control is performed, such that the second hot air heater  38  alone is operated, after the quasi-falling rate drying period, a power consumption reduction rate is less than 5%, which is confirmed through experiments as shown in Table 1 below. 
         [0055]    Therefore, when the heater control is performed, such that the first hot air heater  36  having the relatively large capacity M is stopped and the second hot air heater  38  having the relative small capacity N is operated, in the quasi-falling rate drying period, power consumption is reduced without great increase of drying time, as confirmed by Table 1 below. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Power 
                   
               
               
                   
                 Yearly power 
                   
                 consumption 
               
               
                 Control 
                 consumption 
                 Drying 
                 reduction 
                 Drying time 
               
               
                 time (min) 
                 (kWh/year) 
                 time (min) 
                 rate (%) 
                 increment (%) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 15 
                 854.0 
                 38 
                 8.7 
                 53.8 
               
               
                 17 
                 856.0 
                 33.9 
                 8.4 
                 37.2 
               
               
                 18 
                 857.7 
                 31.9 
                 8.3 
                 29.1 
               
               
                 20 
                 867.1 
                 28.9 
                 7.3 
                 17.0 
               
               
                 22 
                 882.3 
                 26 
                 5.6 
                 5.3 
               
               
                 23 
                 896.7 
                 25.5 
                 4.1 
                 3.2 
               
               
                   
               
               
                 * Fundamental yearly power consumption: 935, drying time: 24.7 
               
             
          
         
       
     
         [0056]    In the state in which the first hot air heater  36  is stopped in the quasi-falling rate drying period, the controller  110  controls the second hot air heater  38  alone to be operated in the falling rate drying period (from approximately 22 minutes to the point of time at which drying is ended) corresponding to the second half of the drying period. Consequently, the drying of the clothing containing little moisture is completed using the relatively small heater capacity N, with the result that the internal temperature of the drum  30  is prevented from being excessively increased. 
         [0057]      FIG. 4  is a graph illustrating a second profile of heater control based on internal humidity of the drum and exhaust temperature during a drying operation of the clothing dryer according to the embodiment of the present invention. A repetitive description of the same portion as in  FIG. 3  is omitted. 
         [0058]    As shown in  FIG. 4 , the controller  110  controls both the first and second hot air heaters  36  and  38  to be operated with the maximum heater capacity (M+N: 100%) in the transient drying period such that the clothing dryer  10  reaches an optimized drying state within a short time. 
         [0059]    Subsequently, the controller  110  controls both the first and second hot air heaters  36  and  38  to be turned on/off according to the temperature of the exhaust air, i.e., exhaust temperature having a control temperature period set to uniformly maintain the exhaust air, in the constant rate drying period in which main drying is performed such that the moisture contained in the clothing is removed within a short time. At this time, the humidity change amount (the drying rate) is uniformly maintained. 
         [0060]    When the moisture contained in the clothing in the drum  30  is almost completely removed as the result of the drying operation in the constant rate drying period, the humidity change amount is suddenly decreased. Upon determining that the humidity change amount is greater than the predetermined change amount, the controller  110  determines that the period at that time is the quasi-falling rate drying period and controls the first hot air heater  36  to be stopped. 
         [0061]    When the heater control is performed, such that the first hot air heater  36  having the relatively large capacity M is stopped and the second hot air heater  38  having the relative small capacity N is operated, in the quasi-falling rate drying period, power consumption is reduced without great increase of drying time, as confirmed by Table 1 above. 
         [0062]      FIG. 5  is a graph illustrating a third profile of heater control based on internal humidity of the drum and exhaust temperature during a drying operation of the clothing dryer according to the embodiment of the present invention. A repetitive description of the same portion as in  FIGS. 3 and 4  is omitted. 
         [0063]    As shown in  FIG. 5 , the controller  110  controls both the first and second hot air heaters  36  and  38  to be operated with the maximum heater capacity (M+N: 100%) in the transient drying period such that the clothing dryer  10  reaches an optimized drying state within a short time. 
         [0064]    Subsequently, the controller  110  controls both the first and second hot air heaters  36  and  38  to be operated with the maximum heater capacity (M+N: 100%) in the constant rate drying period in which main drying is performed such that the moisture contained in the clothing is removed within a short time. At this time, the humidity change amount (the drying rate) is uniformly maintained. 
         [0065]    When the moisture contained in the clothing in the drum  30  is almost removed as the result of the drying operation in the constant rate drying period, the humidity change amount is suddenly decreased. Upon determining that the humidity change amount is greater than the predetermined change amount, the controller  110  determines that the period at that time is the quasi-falling rate drying period and controls the first hot air heater  36  to be stopped. 
         [0066]    When the heater control is performed, such that the first hot air heater  36  having the relatively large capacity M is stopped and the second hot air heater  38  having the relative small capacity N is operated, in the quasi-falling rate drying period, power consumption is reduced without great increase of drying time, as confirmed by Table 1 above. 
         [0067]    Hereinafter, the operation of the clothing dryer according to the embodiment will be described. 
         [0068]      FIG. 6  is a flow chart illustrating a control method of the clothing dryer according to an embodiment. 
         [0069]    When a user puts clothing to be dried in the drum  30  and selects a drying course according to kinds of the clothing ( 200 ), operation information selected by the user is input to the controller  110  through the input unit  100 . 
         [0070]    The controller  110  controls drying to be performed according to the operation information input through the input unit  100 . First, the controller determines whether an operation command has been input ( 202 ). 
         [0071]    Upon determining at Operation  202  that the operation command has been input, the controller  110  drives the motor  80  through the drive unit  120  and controls the first and second hot air heaters  36  and  38  to be operated according to the control profiles shown in  FIGS. 3 to 5  ( 204 ). 
         [0072]    The drum  30  is rotated to rotate the clothing in the drum  30  according to the driving of the motor  80 . The blowing fan  73  is also rotated according to the driving of the motor  80 . As a result, air starts to flow in the clothing dryer  10 . 
         [0073]    At this time, the first and second hot air heaters  36  and  38  are operated to heat the air flowing in the clothing dryer  10  into hot air according to the control profiles shown in  FIGS. 3 to 5 . The hot air is supplied into the drum  30  through the intake duct  35 . The hot air supplied into the drum  30  contacts the clothing rotated in the drum  30  while being repeatedly raised and lowered to dry the clothing through a drying operation to evaporate moisture contained in the clothing. 
         [0074]    Internal humidity of the drum  30  is decreased by the drying operation. At this time, the internal humidity of the drum  30  is sensed by the humidity sensor  61 . The sensed internal humidity of the drum  30  is input to the controller  110  ( 206 ). 
         [0075]    The controller  110  calculates a humidity change amount based on the internal humidity of the drum  30  sensed by the humidity sensor  61  ( 208 ). 
         [0076]    In the transient drying period in which the clothing starts to be dried, heat energy generated from the first and second hot air heaters  36  and  38  is used to increase temperature of air in the drum  30 , the clothing and the surroundings of the clothing although both the first and second hot air heaters  36  and  38  are operated with the maximum heater capacity (100%), with the result that a drying rate is small. 
         [0077]    On the other hand, in the constant rate drying period in which main drying is performed, the heat energy generated from the first and second hot air heaters  36  and  38  is used to evaporate moisture contained in the clothing, with the result that the drying rate is uniform. 
         [0078]    In this way, the heat energy generated from the first and second hot air heaters  36  and  38  is used to evaporate the moisture contained in the clothing in the constant rate drying period, with the result that the temperature of exhaust air having passed through the drum  30  is not suddenly increased by latent heat, and the humidity change amount is not also suddenly decreased. 
         [0079]    When the moisture contained in the clothing is almost removed, however, the temperature of the exhaust air having passed through the drum  30  is suddenly increased, and the humidity change amount is also suddenly decreased. 
         [0080]    Consequently, the controller  110  compares the calculated humidity change amount with a predetermined change amount (a humidity change amount in a predetermined percentage zone in which internal humidity of the drum is suddenly decreased) ( 210 ). Upon determining that the humidity change amount is not greater than the predetermined change amount, the procedure returns to Operation  204  where subsequent operations are performed. 
         [0081]    Upon determining at Operation  210  that the humidity change amount is greater than the predetermined change amount, the controller determines that the period at that time is the quasi-falling rate drying period in which the internal humidity of the drum  30  is suddenly decreased and stops the operation of the first hot air heater  36  through the drive unit  120  ( 212 ). 
         [0082]    Consequently, the second hot air heater  38  alone is operated in a subsequent drying period. Since little moisture is present in the clothing at this time, the drying operation is performed with even the small capacity of the second hot air heater  38 , with the result that power consumption is reduced. Also, the first and second hot air heaters  36  and  38  are not frequently turned on/off, with the result that the internal temperature of the drum  30  is uniformly maintained, thereby improving drying quality of the clothing. 
         [0083]    Subsequently, the controller  110  determines whether a drying completion condition has been satisfied ( 214 ). Upon determining that the drying completion condition has not been satisfied, the procedure returns to Operation  210  where subsequent operations are performed. 
         [0084]    Upon determining at Operation  214  that that the drying completion condition has been satisfied, the controller  110  stops the operation of the second hot air heater  38  through the drive unit  120  to complete the drying operation ( 216 ). The drying completion condition may be set as follows. The humidity sensor  61  contacts the clothing rotated according to the rotation of the drum  30  to generate an electric signal according to the amount of moisture contained in the clothing, based on which dryness of the clothing is determined. 
         [0085]    In this embodiment, the determination of the quasi-falling rate drying period using the humidity change amount in the drum  30  has been described as an example. However, embodiments of the present invention are not limited thereto. For example, the determination of the quasi-falling rate drying period using an increase rate in temperature of air discharged from the outlet of the drum  30  after drying the clothing, i.e., exhaust air may achieve the same effects. 
         [0086]    As is apparent from the above description, the operation of the heaters having different capacities is controlled to adjust heater capacity based on the humidity change amount (the drying rate), thereby reducing power consumption. 
         [0087]    Also, only the heater having the relatively small capacity is operated in the period in which the internal humidity of the drum is suddenly decreased, thereby reducing power consumption without great increase of drying time. 
         [0088]    Although a few embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.