Patent Publication Number: US-6668566-B2

Title: System and a method of automatic defrost for a refrigeration appliance

Description:
FIELD OF THE INVENTION 
     The present invention refers to a system and a method of automatic defrost to be applied to refrigeration appliances, such as refrigerators and freezers. 
     BACKGROUND OF THE INVENTION 
     The refrigeration appliances of the natural convection type usually do not execute defrost automatically, requiring the user to promote the defrost operation manually, by turning off the refrigeration appliance (or only the compressor thereof) during the time necessary to melt all the ice accumulated on the evaporator. 
     The disadvantage of this system resides in the fact that the user has to turn off the refrigeration appliance manually. 
     In another construction of a refrigeration appliance, the thermostat of the latter is provided with a knob, for turning off the compressor, which will turn on again only when the temperature sensed by the sensing bulb of said thermostat reaches a predetermined temperature, sufficiently hot to melt the ice on the evaporator. This concept is used in most refrigeration appliances provided with a cabinet door. In this case, the user has to turn off the compressor every time a defrost operation of the evaporator is needed. Besides the disadvantage of requiring the user to initiate each defrost procedure, in this solution the refrigeration appliance loses almost all its function of promoting refrigeration during the defrost operation. The refrigeration appliances known as “cycle defrost” have another constructive variant for the defrost system. In these refrigeration appliances, the thermostat has a compressor turn on temperature, which remains fixed, regardless of the adjustment applied to the thermostat. Thus, every time the sensing bulb of the thermostat reaches a compressor turn off temperature, said compressor will turn on only when said sensing bulb detects said compressor turn on temperature. If this temperature is dimensioned to be positive and has a previously determined value, in order to allow the melting of the ice accumulated on the evaporator, the refrigeration appliance will promote defrost of the evaporator at each cycle of the compressor, whereby the user does not have to worry about executing this task. 
     Although this solution does not present the disadvantages of the previous solutions, it allows the occurrence of a great temperature variation over the food articles stored in the refrigeration appliance, which variation is much greater than in the previous cases, jeopardizing the preservation of said food articles. 
     Moreover, in load conditions or also in determined environmental conditions in the place where the refrigeration appliance is installed, the compressor may not turn off, and thereby will not promote the defrost operation. In these cases, after a certain time of refrigeration operation, there will be an excess of ice formed in the region of the evaporator that will act as a thermal insulator, impairing the refrigeration efficiency of the refrigeration appliance. On the other hand, in a normal operational condition of the refrigeration appliance, the defrost system of the latter will promote defrost at each cycle of the compressor, even when this operation is not required, causing unnecessary energy consumption, impairing the thermal recovery of the refrigeration order to count each first and second turn on conditions of the electric motor, the control unit selectively activating the thermal switch means to operate: a- in the first turn on condition and in the turn off condition, while the period of time in the first turn on condition is inferior to a respective predetermined maximum safety value and the number of cycles in the first turn on condition of the electric motor is inferior to a predetermined number; b- in the turn off condition and in the second turn on condition, when the predetermined number of cycles of the first turn on condition of the electric motor has been reached and until a subsequent second turn on condition has been reached; c- in the first turn on condition and in the turn off condition, after the second turn on condition has been reached, if the period of time in the second turn on condition is inferior to a respective predetermined maximum safety value, the control unit instructing the switch means to interrupt the energization of the electric motor when the timer indicates to the control unit a period of time of any of the first and second turn on conditions of the electric motor which is equal to the respective predetermined maximum safety values and maintaining said switch means condition until the occurrence of a subsequent second turn on condition of the electric motor, when the control unit instructs the switch means to allow the energization of the electric motor, the control unit activating the thermal switch means to operate with the other turn off condition, after the second turn on condition has been reached, if the period of time in one of the preceding turn off conditions and another turn off condition is equal or superior to a respective predetermined maximum defrost value, and to operate with the turn off condition and the first turn on condition, after the second turn on condition has been reached, if the period of time in the other preceding turn off condition is equal or inferior to a predetermined minimum defrost value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be described below, with reference to the attached drawing, in which: 
     FIG. 1 illustrates, schematically and in a block diagram, the components of the system of the present invention and their operational connections; and 
     FIG. 2 illustrates, graphically, the temperature variation in a refrigeration appliance provided with the automatic defrost system of the present invention, for a normal operation (continuous line) and for an emergency defrost operation (dashed line) of said system. 
    
    
     BEST MODE OF CARRYING OUT THE PRESENT INVENTION 
     According to the present invention, the automatic defrost system for a refrigeration appliance includes a hermetic compressor driven by an electric motor  1 , which is operatively connected to both a control unit  2  and to a thermal switch means  10  comprising a plurality of temperature sensing means defined in order to detect determined temperatures in the refrigeration appliance and to inform said temperatures to the control unit  2 . 
     According to the present invention, the automatic defrost system further comprises a timer  20 , a cycle counting means  30  and a switch means  40 , all to be described ahead and each being operatively connected to both the control unit  2  and the electric motor  1  of the compressor. 
     The control unit  2  is programmed to command a normal operational condition of refrigeration, in which it instructs the turn on and turn off of the electric motor  1  by a certain pre-established number of times, before instructing the beginning of a normal defrost regimen of this operational condition, and also to command an emergency defrost condition, when the timer  20  informs said control unit  2  about the occurrence of a turn on period of time, superior to a predetermined period of time, which represents an anomalous refrigeration condition of the refrigeration appliance. 
     According to the present invention, the thermal switch means  10  is operatively coupled to both the control unit  2  and the electric motor  1  and has: a first turn on condition, in which the electric motor  1  is activated and maintained operating at a first turn on temperature T 1 on; a second turn on condition, which activates the electric motor  1  at a second turn on temperature T 2 on higher than the first turn on temperature T 1 on; and a turn off condition, in which the electric motor  1  is turned off when a turn off temperature Toff has been reached. 
     The first turn on temperature T 1 on and the turn off temperature Toff are, for example, turn on and turn off limit temperatures of the electric motor  1 , while the refrigeration appliance is in a refrigeration operation. 
     According to the figures, the first turn on temperature T 1 on and the turn off temperature Toff are negative temperatures, while the second turn on temperature T 2 on is a positive temperature, of defrost end. 
     During the normal operation of the refrigeration appliance, at each first turn on condition, the control unit  2  instructs the timer  20  to count the period of time in this condition, which value will be compared with a reference value by the control unit  2 , so that the latter determines, as a function of the comparative result, whether to proceed or not with the current refrigeration operational regimen of the present system, and, with the period of time being at any of the first and second turn on conditions inferior to a respective predetermined maximum safety value and with the cycle counting means  20  indicating that the counted number of the first turn on conditions has reached a predetermined number, the control unit  2  instructs to turn off the electric motor  1  at the turn off condition subsequent to the turn on condition which has detected said predetermined number of counts of turn on conditions, and instructs the thermal switch means  10  to activate the second subsequent turn on temperature, indicating that the system has started a defrost regimen. 
     In the preferred solution, the control unit  2  instructs the timer  20  to measure the period of time between the beginning of the turn off condition and the subsequent second turn on condition, comparing said period of time with a respective reference period, in order to determine whether, after reaching said second turn on condition, the present system will return to the refrigeration regimen, or will operate again in a defrost regimen, as described below. 
     When said period of time is inferior to a predetermined maximum defrost value, the control unit  2  instructs the thermal switch means  10  to activate the turn off temperatures Toff and the first turn on temperature T 1 on, restarting the refrigeration operation. 
     In the illustrated construction, when the period of time between the turn off condition and the subsequent second turn on condition is equal or superior to the predetermined maximum defrost value, the control unit  2  instructs the thermal switch means  10  to operate in another turn off condition, which turns off the electric motor  1  at another turn off temperature T′off, which is lower than the turn off temperature Toff, maintaining the second turn on temperature T 2 on and the other turn off temperature T′ off activated during the time in which the timer  20  detects a period of time of the preceding other turn off condition which is equal or inferior to a predetermined minimum defrost value which, when detected, makes the thermal switch means  10  operate again at the turn off temperatures and in the first turn on condition. 
     According to the present invention, when the period of time measured by the timer  20  between a preceding turn off condition and other turn off condition and a subsequent second turn on condition is equal or inferior to the predetermined minimum defrost value and the period of time between said second turn on condition and a subsequent turn off condition is inferior to the predetermined maximum safety value, the control unit  2  instructs the switch means  40  to operate between the first turn on condition and the turn off condition, also activating the cycle counting means  30  to count each first turn on condition, until one of the turn on conditions has reached the predetermined maximum safety value, or until the predetermined number of counts of the first turn on condition has been reached. 
     When the timer detects a period of time of any of the first and second turn on conditions which is equal to the respective predetermined maximum safety values, which are for example equal, the control unit  2  instructs the switch means  40  to interrupt the energization of the electric motor  1 , maintaining this condition of the switch means  40  until the occurrence of a subsequent second turn on condition of the electric motor  1 , when the control unit  2  instructs the energization of the electric motor  1 . 
     This option of defrost operation by interrupting the energization of the electric motor  1  is adopted to avoid accumulation of the ice formed on the evaporator, due to a longer operational time of the hermetic compressor which is required, for example, by a greater demand of refrigeration from an ambient under refrigeration, said excess ice acting as a thermal insulator against the refrigeration, increasing energetic consumption, since it requires the compressor to work more, without however resulting in a better refrigeration. 
     In another constructive option, the present system further has a forced ventilation element  50 , for example a fan, which is mounted to the refrigeration appliance in order to be activated by the control unit  2  when any of the turn off conditions and another turn off condition has been reached. 
     In the construction of the present invention, while the system is operating in a normal refrigeration and defrost regimen, the turn on and turn off limit temperatures of the electric motor  1  are constantly modified in the thermal switch  10  by instruction of the control unit  2 . When the system starts the defrost operation in which the turn on and turn off limit temperatures are the second turn on temperature and the other turn off temperature, the system will operate with fixed temperatures, such as the conventional refrigeration appliances, said fixed temperatures being maintained until the thermal switch means  10  is instructed to operate in the normal refrigeration and defrost regimen.