Patent Publication Number: US-2019191960-A1

Title: Dishwasher and controlling method therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. 119 and 35 U.S.C. 365 to Korean Patent Application No. 10-2017-0177352, filed on Dec. 21, 2017, which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a dishwasher and a control method thereof, and more particularly, to a dishwasher for spraying washing water to wash dishes or cooking utensils, and a control method thereof. 
     2. Description of the Related Art 
     A dishwasher is a household appliance for washing up dirt such as food wastes on the dishes or cooking utensils (hereinafter, referred to as ‘washing object’) by high-pressure washing water sprayed from a spray arm. 
     The dishwasher generally includes a tub forming a washing chamber and a sump mounted on the bottom of the tub to store washing water. Then, the washing water is moved to the spray arm by the pumping operation of a washing pump installed inside the sump, and the washing water moved to the spray arm is sprayed at high pressure through a spray hole formed in the spray arm. Then, the washing water sprayed at a high pressure impinges against the surface of the washing object, so that the dirt on the washing object falls to the bottom of the tub. 
     Meanwhile, the dishwasher performs rinsing by spraying washing water to the washing object so that residual dirt is removed. In this rinsing process, when performing a final rinsing of spraying the heated washing water, any residual dirt should be removed as much as possible. Thus, it is required that the washing water is sprayed evenly onto the washing object. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above problems, and provides a dishwasher in which a spraying range of washing water varies during rinsing, and a control method thereof. 
     In accordance with an aspect of the present invention, a method of controlling a dishwasher includes: a water supply step of opening a water supply valve to supply washing water to the sump; and a variable spray step of spraying the washing water by at least one of the plurality of spray arms by changing the number of revolutions of the washing pump stepwise, so that the washing water can be sprayed evenly. 
     In the variable spray step, the number of revolutions of the washing pump repeatedly decreases stepwise after increasing stepwise. 
     In the variable spray step, the number of steps in which the number of revolutions of the washing pump is increased is equal to the number of steps in which the number of revolutions is decreased. 
     In the variable spray step, a period in which the number of revolutions of the washing pump is changed is constant. 
     Among the plurality of spray arms, a spray arm which sprays the washing water is formed in a bar shape and rotates, and, in the variable spray step, a period of change of the number of revolutions of the washing pump is equal to or larger than a maximum time during which the rotating spray arm rotates once. 
     Among the plurality of spray arms, a spray arm which sprays the washing water is formed in a bar shape and rotates, and in the variable spray step, a period of change of the number of revolutions of the washing pump is equal to or larger than a time during which the rotating spray arm rotates once at a minimum number of revolutions of the washing pump. 
     In the variable spray step, the washing pump has a constant increase/decrease width for each step of the number of revolutions. 
     The plurality of spray arms comprises an upper spray arm which is disposed at an intermediate portion of the tub and sprays the washing water, and in the variable spray step, the washing pump pumps the washing water to the upper spray arm. 
     The upper spray arm is formed in a bar shape and rotates, and in the variable spray step, a rotation speed of the upper spray arm is changed stepwise. 
     In the variable spray step, a spray direction of the washing water of the upper spray arm is changed stepwise. 
     The number of revolutions of the washing pump in the variable spray step is equal to or less than a maximum number of revolutions of the washing pump in the top spray step and is equal to or more than a maximum number of revolutions of the washing pump in the lower spray step. 
     The method further includes an upper spray step of maintaining the number of revolutions of the washing pump uniformly, after the variable spray step; and a drain step of stopping an operation of the washing pump and draining the washing water stored in the sump to the outside, after the upper spray step. 
     In the upper spray step, the number of revolutions of the washing pump maintains an average value of the number of revolutions which is changed in the variable spray step. 
     The method further includes a heating step of heating the washing water supplied to the sump, and, in the variable spray step, at least one of the plurality of spray arms sprays the heated washing water. 
     The water supply step, the heating step, and the variable spray step are a heat rinsing process, and the method further includes, before the heat rinsing process is performed, a rinsing step of mixing the washing water supplied from an external water source with a rinsing detergent, and spraying the mixed washing water by at least one of the plurality of spray arms and then draining. 
     In accordance with another aspect of the present invention, a dishwasher includes: a tub which accommodates a washing object; a plurality of spray arms which spray washing water into the tub and are disposed vertically; a sump which stores the washing water; a washing pump which pumps the washing water stored in the sump to at least one of the plurality of spray arms; and a controller which controls the washing pump and a heater, wherein the controller operates the heater and changes the number of revolutions of the washing motor in a stepwise manner to spray the washing water by at least one of the plurality of spray arms. 
     The controller repeatedly decreases the number of revolutions of the washing pump stepwise after increasing stepwise. 
     The controller changes the number of revolutions of the washing pump periodically. 
     The controller changes the number of revolutions of the washing pump with a constant increase/decrease width. 
     The dishwasher further includes a switching valve for selectively connecting the washing pump with at least one of the plurality of spray arms, wherein the plurality of spray arms includes an upper spray arm disposed at an intermediate portion of the tub and spraying washing water, wherein the controller controls the switching valve to connect the washing pump to the upper spray arm when the number of revolutions of the washing motor is changed stepwise. 
     The dishwasher further includes a drain pump for draining the washing water stored in the sump to the outside, wherein the controller changes the number of revolutions of the washing motor in a stepwise manner and then maintains uniformly, and stops an operation of the washing motor and then operates the drain pump. The dishwasher further includes a heater for heating the washing water sprayed by the plurality of spray arms, wherein at least one of the plurality of spray arms sprays the washing water heated by the heater. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features and advantages of the present invention will be more apparent from the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a sectional view of a dishwasher according to an embodiment of the present invention; 
         FIG. 2  is a block diagram of a dishwasher according to an embodiment of the present invention; 
         FIG. 3  is a diagram illustrating each process in a general washing course of a dishwasher according to an embodiment of the present invention; 
         FIG. 4  is a diagram illustrating a control method during the heat rinsing of a dishwasher according to an embodiment of the present invention; 
         FIG. 5  is a diagram illustrating a control of the number of revolutions of a washing pump in a control method of a dishwasher according to an embodiment of the present invention; and 
         FIG. 6  is a perspective view of an upper spray arm of a dishwasher according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are described with reference to the accompanying drawings in detail. The same reference numbers are used throughout the drawings to refer to the same or like parts. Detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. 
     Hereinafter, the present invention will be described with reference to the drawings for explaining a dishwasher and a control method thereof according to embodiments of the present invention. 
       FIG. 1  is a sectional view of a dishwasher according to an embodiment of the present invention. 
     The dishwasher  1  according to an embodiment of the present invention includes a case  11  that forms an outer appearance, a tub  12  in which a washing object is accommodated, a door  20  that is provided on the front surface of the tub  12  and opens and closes the tub  12 , a sump  100  that is disposed below the tub  12  and stores washing water, a plurality of spray arms  13 ,  14 ,  15  that spray washing water into the tub  12 , a filter  200  which filters the washing water which is sprayed from at least one of the plurality of spray arms  13 ,  14 ,  15  and collected in the sump  100 , a washing pump  150  that pumps washing water stored in the sump  100 , and a switching valve  130  which flows the washing water pumped by the washing pump  150  to at least one of the plurality of spray arms  13 ,  14 ,  15 . 
     The tub  11  is formed in a hexahedron shape having an opened front surface and forms a washing chamber  12   a  therein. A communication hole  12   c  through which the washing water flows into the sump  100  is formed in a bottom  12   b  of the tub  11 . The washing chamber  12   a  is provided with a plurality of racks  16  and  17  for receiving the washing object. The plurality of racks  16  and  17  include a lower rack  16  disposed at a lower portion of the washing chamber  12   a  and an upper rack  17  disposed at an upper portion thereof. The lower rack  16  and the upper rack  17  are vertically spaced apart and can be slid forwardly of the tub  11  to be drawn out. 
     The plurality of spray arms  13 ,  14 , and  15  are disposed in the vertical direction. The plurality of spray arms  13 ,  14 , and  15  include a lower spray arm  13  which is disposed in the lower end of the washing chamber  12   a  and sprays washing water from the lower side to the upper side toward the lower rack  16 , an upper spray arm  14  which is disposed in a middle portion of the height direction of the washing chamber  12   a  that is the upper side of the lower spray arm  13 , and sprays washing water from the lower side to the upper side toward the upper rack  17 , and a top spray arm  15  which is disposed in the upper end of the washing chamber  12   a  that is the upper side of the upper spray arm  14 , and sprays washing water from the upper side to the lower side toward the upper rack  17 . 
     The lower spray arm  13  is disposed below the lower rack  16 , the upper spray arm  14  is disposed between the upper rack  17  and the lower rack  16 , and the top spray arm  15  is disposed above the upper rack  17 . According to an embodiment, the upper spray arm  14  may spray downward toward the lower rack  16 , and spray upwardly and downwardly toward both the upper rack  17  and the lower rack  16 . 
     Each of the lower spray arm  13  and the upper spray arm  14  rotates about the center of a bar in the form of a bar. Each of the lower spray arm  13  and the upper spray arm  14  rotates by the supply of washing water. Each of the lower spray arm  13  and the upper spray arm  14  is provided with a nozzle (not shown) on the upper surface of the bar. 
     The plurality of spray arms  13 ,  14 , and  15  are supplied with the washing water from the washing pump  150  through a plurality of spray arm connecting flow paths  18 ,  19 , and  21 . The plurality of spray arm connecting flow paths  18 ,  19 , and  21  include a lower spray arm connecting flow path  18  connected to the lower spray arm  13 , an upper spray arm connecting flow path  19  connected to the upper spray arm  14 , and a top spray arm connecting flow path  21  connected to the top spray arm  15 . 
     The lower spray arm  13 , the upper spray arm  14 , and the top spray arm  15  are supplied with the washing water from the washing pump  150  through the lower spray arm connecting flow path  18 , the upper spray arm connecting flow path  19 , and the top spray arm connecting flow path  21 . 
     The sump  100  is disposed below the bottom  12   b  of the tub  12  to collect washing water. The sump  100  is connected to a water supply flow path  23  through which washing water supplied from an external water source flows. The water supply flow path  23  is provided with a water supply valve  22  for controlling washing water supplied from the external water source. When the water supply valve  22  is opened, the washing water supplied from the external water source flows into the sump  100  through the water supply flow path  23 . 
     The sump  100  is connected to a drain flow path  24  for guiding the stored washing water to the outside of the dishwasher  1 . The drain flow path  24  is provided with a drain pump  25  for draining the washing water in the sump  100  through the drain flow path  24 . The drain pump  25  includes a drain motor (not shown) for generating a rotational force. When the drain pump  25  is operated, the washing water stored in the sump  100  flows to the outside of the case  11  through the drain flow path  24 . 
     A filter  26  is installed in the communication hole  12   c  to filter the dirt from the washing water moving from the tub  12  to the sump  100 . In the filter  200 , a lower portion where mesh is formed is inserted into the sump  100 , and an opened upper portion protrudes to the upper side of the tub  12 . The washing water sprayed through the plurality of spray arms  13 ,  14 ,  15  falls to the bottom  12   b  of the tub  12  together with the dirt on the washing object. The washing water flowing on the bottom  12   b  of the tub  12  flows into the upper portion of the filter  200 , passes through the mesh of lower portion, and is stored in the sump  100  after being filtered. The level of the washing water stored in the sump  100  needs to be maintained above the lower end of the filter  200 , in order for filtration of the filter  200  to be smoothly performed. 
     The switching valve  130  selectively supplies the washing water pumped by the washing pump  150  to at least one of the lower spray arm  13 , the upper spray arm  14 , and the top spray arm  15 . The switching valve  130  selectively connects a washing water supply path  180  and at least one of the plurality of spray arm connecting flow paths  18 ,  19  and  21 . 
     The washing pump  150  supplies washing water stored in the sump  100  to at least one of the plurality of spray arms  13 ,  14 ,  15 . The washing pump  150  includes a washing motor  157  that generates a rotational force and an impeller  155  that is rotated by the washing motor  157  to pump the washing water. The washing motor  157  is a motor whose RPM can be varied, and a change of the number of revolutions of the washing pump  150  (the number of revolutions per time) means a change of the RPM of the washing motor  157 . The number of revolutions of the washing pump  150  can be maintained or the number of revolutions of the washing pump  150  can be changed during operation. The number of revolutions of the washing pump  150  is proportional to the flow rate of the washing water that is pumped by the washing pump  150 . When the number of revolutions of the washing pump  150  is changed, the spray direction of the washing water sprayed from at least one of the plurality of spray arms  13 ,  14 ,  15  connected to the washing pump  150  is changed. When the washing pump  150  is connected to at least one of the lower spray arm  13  and the upper spray arm  14 , if the number of revolutions of the washing pump  150  is changed, the rotation speed of the lower spray arm  13  and/or the upper spray arm  14  is changed. 
     The washing pump  150  is connected to the sump  100  and a water collecting path  170 . The washing pump  150  is connected to the switching valve  130  and the washing water supply path  180 . When the washing pump  150  is operated, the washing water stored in the sump  100  flows into the washing pump  150  through the water collecting path  170  and then is pumped to the switching valve  130  through the washing water supply path  180 . Since the water collecting path  170  is connected to the lateral side of the washing pump  150 , it is necessary that the level of the washing water stored in the sump  100  is maintained above a connection portion of the water collecting path  170  in order for the washing water to flow into the washing pump  150  during operation of the washing pump  150 . 
     The heater  140  is coupled to the lower side of the washing pump  150  to heat the washing water in the washing pump  150 . The heater  140  generates hot water by heating the washing water flowing in the washing pump  150  when the washing pump  150  operates. The washing water heated by the heater  140  is sprayed into the tub  12  through at least one of the plurality of spray arms  13 ,  14  and  15 . According to an embodiment, the heater  140  may be disposed within the sump  100 . 
       FIG. 2  is a block diagram of a dishwasher according to an embodiment of the present invention, and  FIG. 3  is a diagram illustrating each process in a general washing course of a dishwasher according to an embodiment of the present invention. 
     The controller  29  controls the water supply valve  22 , the washing pump  150 , the drain pump  25 , the switching valve  130 , and the heater  140  to perform washing on the washing object. The controller  29  performs each process according to the washing course selected by the user. 
     The controller  29  sequentially performs a preliminary washing  1  P 310 , a preliminary washing  2  P 320 , a preliminary washing  3  P 330 , a main washing P 340 , a rinsing P 350 , and a heat rinsing P 360 . 
     The plurality of preliminary washings P 310 , P 320 , P 330  is a process for spraying washing water on the washing object to remove dirt attached to the washing object. In each of the plurality of preliminary washings P 310 , P 320 , P 330 , the controller  29  controls the water supply valve  22  to supply the washing water from an external water source into the sump  100 . After the water supply, the controller  29  operates the washing pump  150  to pump the washing water in the sump  100  and controls the switching valve  130  to spray washing water through at least one of the plurality of spray arms  13 . The washing water sprayed through at least one of the plurality of spray arms  13 ,  14 ,  15  drops the dirt attached to the washing object onto the bottom  12   b  of the tub  12  so that the dirt can be collected by the filter  26 . After spraying the washing water, the controller  29  operates the drain pump  25  to drain the washing water in the sump  100  to the outside. 
     In the present embodiment, it is illustrated that the preliminary washing P 310 , P 320 , P 330  is performed three times. However, according to an embodiment, it may be performed various times, e.g., once or more. 
     The main washing P 340  is a process for spraying heated washing water onto the washing object so as to heat the washing object, and removing the dirt attached to the washing object. The controller  29  controls the water supply valve  22  to supply the washing water from an external water source to the sump  100 , controls the heater  140  to heat the washing water, operates the washing pump  150  to spray the heated washing water through at least one of the plurality of spray arms  13 ,  14 ,  15 , and operates the drain pump  25  to drain the washing water in the sump  100  to the outside. During the main washing P 340 , the washing detergent may be mixed with the washing water. 
     The rinsing P 350  is a process for removing residual dirt attached to the washing object. In the rinsing process P 350 , the controller  29  controls the water supply valve  22  to supply the washing water from an external water source to the sump  100 , operates the washing pump  150  to spray the washing water through at least one of the plurality of spray arms  13 ,  14 ,  15 , and operates the drain pump  25  to drain the washing water in the sump  100  to the outside. During the rinsing P 350 , the rinsing detergent may be mixed with the washing water. 
     The heat rinsing P 360  is a process for heating the washing object by spraying the heated washing water onto the washing object, and finally removing the residual dirt attached to the washing object. In the heat rinsing process P 360 , the controller  29  controls the water supply valve  22  to supply the washing water from the external water source to the sump  100 , controls the heater  140  to heat the washing water, operates the washing pump  150  to spray the heated washing water through at least one of the plurality of spray arms  13 ,  14 ,  15 , and operates the drain pump  25  to drain the washing water in the sump  100  to the outside. 
       FIG. 4  is a diagram illustrating a control method during the heat rinsing of a dishwasher according to an embodiment of the present invention. 
     The controller  29  performs water supply P 361  of the heat rinsing P 360 . The controller  29  controls the water supply valve  22  to supply the sump  100  with washing water from an external water source. When the controller  29  opens the water supply valve  22 , the washing water supplied from the external water source flows into the sump  100  through the water supply flow path  23 . The controller  29  controls the water supply valve  22  such that the level of the washing water supplied to the sump  100  at the maximum rotation speed of the washing pump  150  is equal to or higher than the water level (filterable water level) at which the filtration of the dirt can be smoothly performed in the filter  200  and the water level (washing water circulatable level) at which the washing water can be flowed by the washing pump  150 . The controller  29  closes the water supply valve  22  and terminates the water supply P 361  when the washing water is supplied to a target water level. 
     When the water supply P 361  is completed, the controller  29  performs a first top spray P 362 . In the first top spray P 362 , the controller  29  operates the heater  140  to heat the washing water. In addition, the controller  29  controls the switching valve  130  to connect the washing water supply path  180  and the top spray arm connecting flow path  21  so that the washing pump  150  is connected to the top spray arm  15 . The controller  29  operates the washing pump  150  to pump the washing water stored in the sump  100  to the top spray arm  15 . The controller  29  sprays the heated washing water through the top spray arm  15  downward toward the upper rack  17 . 
     The controller  29  gradually increases the rotation speed of the washing pump  150  until the level of the washing water supplied to the sump  100  in the first top spray P 362  is lowered to reach the filterable water level and the washing water circulatable level (the highest target rotation speed). When the number of revolutions of the washing pump  150  reaches the maximum target number of revolutions, the controller  29  maintains this number of revolutions for a predetermined time and then completes the first top spray P 362 . 
     When the first top spray P 362  is completed, the controller  29  performs a first lower spray P 363 . In the first lower spray P 363 , the controller  29  continuously operates the heater  140 , and controls the switching valve  130  to connect the washing water supply path  180  and the lower spray arm connecting flow path  18  so that the washing pump  150  is connected to the lower spray arm  13 . The controller  29  lowers the number of revolutions of the washing pump  150  to the extent that the washing water can be sprayed through the lower spray arm  13  (the minimum target number of revolutions). The controller  29  operates the washing pump  150  without stopping after the first top spray P 362  while reducing only the number of revolutions of the washing pump  150  so that the washing water stored in the sump  100  is pumped to the lower sandstone  13 . The controller  29  sprays the heated washing water through the lower spray arm  13  upwardly toward the lower rack  16 . 
     When the number of revolutions of the washing pump  150  reaches the minimum target number of revolutions, the controller  29  maintains this number of revolutions for a predetermined time and temporarily stops the operation of the washing pump  150 , operates the washing pump  150  again, and controls the number of revolutions of the washing pump  150  to be the minimum target number of revolutions. When the number of revolutions of the washing pump  150  reaches the minimum target number of revolutions, the controller  29  maintains this number of revolutions for a predetermined time and then terminates the first lower spray P 363 . 
     When the first lower spray P 363  is completed, the controller  29  performs a first variable spray P 364 . In the first variable spray P 364 , the controller  29  continuously operates the heater  140 , and controls the switching valve  130  to connect the washing water supply path  180  and the upper spray arm connecting flow path so that the washing pump  150  is connected to the upper spray arm  14 . The controller  29  changes the number of revolutions of the washing pump  150  in a stepwise manner. In the first variable spray P 364 , the controller  29  repeats the stepwise decrease of the number of revolutions of the washing pump  150  after a stepwise increase. In the first variable spray P 364 , when the controller  29  changes the number of revolutions of the washing pump  150  in a stepwise manner, the number of revolutions of the upper spray arm  14  is changed in a stepwise manner, and the spray direction of the washing water is also changed in a stepwise manner. 
     The controller  29  changes the number of revolutions of the washing pump  150  to be equal to or lower than the maximum target number of revolutions of the washing pump  150  in the first top spray P 362  and to be equal to or higher than the minimum target number of revolutions of the washing pump  150  in the first lower spray (P 363 ). The controller  29  controls the number of revolutions of the washing pump  150  to vary between the maximum number of revolutions of the washing pump  150  in the first top spray P 362  and the maximum number of revolutions of the washing pump  150  in the first lower spray P 363 . 
     A detailed description of the first variable spray P 364  will be given later with reference to  FIG. 5  and the following. 
     The controller  29  changes only the number of revolutions of the washing pump  150  and operates the washing pump  150  without stopping after the first lower spray P 363  so that the washing water stored in the sump  100  is pumped to the upper spray arm  14 . The controller  29  sprays the heated washing water through the upper spray arm  14  upwardly toward the upper rack  17 . The controller  29  changes the number of revolutions of the washing pump  150  in a stepwise manner for a predetermined time and then terminates the first variable spray P 364 . 
     When the first variable spray P 364  is completed, the controller  29  performs a second top spray P 365 . In the second top spray P 365 , similarly to the first top spray P 362 , the controller  29  continuously operates the heater  140  and controls the switching valve  130  such that the washing pump  150  is connected to the top spray arm  15  and the number of revolutions of the washing pump  150  is increased to the maximum target number of revolutions. 
     When the second top spray P 365  is completed, the controller  29  performs the second lower spray P 366 . In the second lower spray P 366 , similarly to the second lower spray P 366 , the controller  29  continuously operates the heater  140  and controls the switching valve  130  to connect the washing pump  150  to the lower spray arm  13 , and reduces the number of revolutions of the washing pump  150  to the minimum target number of revolutions. In the second lower spray P 366 , the controller  29  temporarily stops the operation of the washing pump  150  in the middle of maintaining the minimum target number of revolutions of the washing pump  150 . 
     When the second lower spray P 366  is completed, the controller  29  performs a second variable spray P 367 . In the second variable spray P 367 , similarly to the first variable spray P 364 , the controller  29  continuously operates the heater  140  and controls the switching valve  130  to connect the washing pump  150  to the upper spray arm  14 , and changes the number of revolutions of the washing pump  150  in a stepwise manner. 
     When the second variable spray P 367  is completed, the controller  29  performs an upper spray P 368 . In the upper spray P 368 , the controller  29  stops the operation of the heater  140  and terminates the heating of the washing water. The stop of the operation of the heater  140  may be accomplished at the beginning or at the end of the upper spray P 368 . In the upper spray P 368 , the controller  29  maintains the switching valve  130  to connect the washing pump  150  and the upper spray arm  14  after the second variable spray P 367 . In the upper spray P 368 , the controller  29  operates the washing pump  150  without stopping after the second variable spray P 367 , but maintains the number of revolutions of the washing pump  150  uniformly. The controller  29  controls the number of revolutions of the washing pump  150  to be the average value (or the intermediate value between the maximum number of revolutions and the minimum number of revolutions) of the number of revolutions of the washing pump  150  in the second variable spray P 367  (or in the first variable spray P 364 ) and then maintains this number of revolutions. In the upper spray P 368 , the controller  29  maintains the number of revolutions of the washing pump  150  so that the filterable water level, the washing water circulatable water level, and the washing water spray flow amount through the upper spray P 368  can be ensured, and collects dirt into the filter  200  and collects washing water into the sump  100 . The controller  29  maintains the number of revolutions of the washing pump  150  for a predetermined time, and then stops the operation of the washing pump  150  to terminate the upper spray P 368 . 
     When the upper spray P 368  is completed, the controller  29  performs a drain (P 369 ). The controller  29  operates the drain pump  25  to drain the washing water stored in the sump  100  to the outside. When the controller  29  operates the drain pump  25 , the washing water stored in the sump  100  flows to the outside of the case  11  through the drain flow path  24 . When all of the washing water stored in the sump  100  is drained, the controller  29  stops the operation of the drain pump  25  and terminates the heat rinsing P 360 . 
     In the above-described embodiment, the second top spray P 365 , the second low spray P 366 , and the second variable spray P 367  may be omitted. In this case, after the first variable spray P 364 , the upper spray P 368 , and the drain P 369  may be performed sequentially. 
     In the embodiment described above, it is illustrated that the upper spray arm  14  sprays the washing water in the steps of the first variable spray P 364  and the second variable spray P 367 . However, according to an embodiment, the lower spray arm  13  may spray the washing water. 
       FIG. 5  is a diagram illustrating a control of the number of revolutions of a washing pump in a control method of a dishwasher according to an embodiment of the present invention, and  FIG. 6  is a perspective view of an upper spray arm of a dishwasher according to an embodiment of the present invention. 
       FIG. 5  shows the number of revolutions of the washing pump  150  depending on time in the first variable spray P 364  and the second variable spray P 367 . In  FIG. 5 , x denotes a period in which the number of revolutions of the washing pump  150  is changed, and y denotes a stepwise increase/decrease width in which the number of revolutions of the cleaning pump  150  is changed. yz denotes the total increase/decrease width of the change in the number of revolutions of the washing pump  150 . z denotes the number of the increasing and decreasing steps of the number of revolutions of the washing pump  150 . 
     It is preferable that the period x of change of the number of revolutions of the washing pump  150  is equal to or larger than a maximum time during which the upper spray arm  14  rotates once. The rotation speed w of the upper spray arm  14  is changed according to the number of revolutions the washing pump  150 , and the rotation speed w of the upper spray arm  14  becomes the lowest when the number of revolutions of the washing pump  150  is the lowest. The time for one rotation of the upper spray arm  14  becomes the maximum when the rotation speed w of the upper spray arm  14  is the lowest. It is preferable that the period x of change of the number of revolutions is equal to or larger than the time of one revolution at the minimum number of revolutions of the washing pump  150 . 
     The period x of change of the number of revolutions of the washing pump  150  is equal to the period of time during which the number of revolutions of the washing pump  150  is maintained. Since the spray direction of the washing water sprayed from a nozzle  14   a  of the upper spray arm  14  is maintained uniformly when the number of revolutions of the washing pump  150  is maintained, the upper spray arm  14  is rotated at least one revolution in the same spray direction so that the washing water is uniformly sprayed on the horizontal plane. In the present embodiment, the period x of change of the number of revolutions of the washing pump  150  is 5 seconds. 
     The spray direction of the washing water may be evenly changed during a stepwise change in which the number of revolutions of the washing pump  150  is maintained uniformly and then rapidly changed and maintained uniformly again. The spray direction of the washing water is related not only to the opening direction of the nozzle  14   a  and the flow rate of washing water depending on the number of revolutions of the washing pump  150  but also to the rotation speed of the upper spray arm  14 . According to such a complex factor, the inventors of the present invention have experimentally found that the spray direction of the washing water is changed evenly to be evenly applied to the washing object, when the number of revolutions of the washing pump  150  is changed to the period x of change of the number of revolutions of the washing pump  150  in a stepwise manner in comparison with the case where the number of revolutions of the washing pump  150  is gradually changed linearly or curvedly. 
     The washing pump  150  repeats the stepwise decrease in the number of revolutions after the stepwise increase. The number z of steps of increasing the number of revolutions of the washing pump  150  is preferably equal to the number z of steps of decreasing the number of revolutions. It is preferable that the number z of steps of increasing or decreasing the number of revolutions is an even number such that the number of revolutions of the washing pump  150  is maintained for the period x of change at the average value (intermediate value between the maximum number of revolutions and the minimum number of revolutions) for the number of revolutions of the washing pump  150 . 
     It is preferable that all the increase/decrease width y of each step is constant when the number of revolutions of the washing pump  150  is increased and decreased. It is preferable that the total increase/decrease width yz of the change in the number of revolutions of the washing pump  150  is constant when the number of revolutions of the washing pump  150  is repeatedly increased or decreased in a stepwise manner. In the present embodiment, the number z of steps for increasing or decreasing the number of revolutions is 4, and the increase/decrease width y for each step is 200 RPM. Accordingly, in the present embodiment, the total increase/decrease width yz is 800 RPM. 
     The inventors of the present invention have experimentally found that when the number of revolutions of the washing pump  150  is changed un-uniformly in a stepwise manner, the water level drops sharply so that the filterable water level and the washing water circulatable water level are not ensured or the washing water is not sprayed through the nozzle  14   a . However, as described above, it has been found that when the number of revolutions of the washing pump  150  is changed uniformly in a stepwise manner by making the increase/decrease width y for each step to be equal to the number z of steps of increase, the washing water is continuously sprayed through the nozzle  14   a  while ensuring the filterable water level and the washing water circulatable water level. 
     According to the dishwasher of the present invention and the control method thereof, one or more of the following effects can be obtained. 
     First, the number of revolutions of the washing pump that pumps the washing water to the spray arm is varied stepwise so that the spray range of the washing water can be varied stepwise, thereby uniformly spraying the washing water to the washing object. 
     Second, the number of revolutions of the washing pump can be varied in the same period and in the same increase/decrease width, thereby ensuring the filterable water level and the washing water circulatable water level. 
     Third, the number of revolutions of the washing pump is changed, but the number of revolutions is maintained while the washing arm rotates at least once, so that the washing arm can rotate at least once in a certain washing water spray direction. 
     Fourth, the rinsing performance can be improved by repeatedly increasing and decreasing the number of revolutions of the washing pump in a stepwise manner between the spray pressure at which the washing water is sprayed to the washing arm and the spray pressure at which the lowest water level can be ensured. 
     Fifth, the number of revolutions of the washing pump is changed stepwise and then the number of revolutions is kept constant, thereby collecting the dirt in the filter before draining, and collecting the washing water stably in the sump. 
     Sixth, the washing water is sprayed evenly onto the washing object in a final process in which the heated washing water is sprayed, thereby completely removing the residual dirt in the washing object. 
     Hereinabove, although the present invention has been described with reference to exemplary embodiments and the accompanying drawings, the present invention is not limited thereto, but may be variously modified and altered by those skilled in the art to which the present invention pertains without departing from the spirit and scope of the present invention claimed in the following claims.