1. An automatic defrost system for a refrigerator, freezer or like device, comprising a temperature sensing means and a control means.

2. The automatic defrost system as claimed in claim 1, wherein the control means comprises a microcontroller and a storage memory.

3. The automatic defrost system as claimed in claims 1 or 2, wherein the temperature sensing means comprises a thermistor or like device for sensing the temperature at atleast one location inside the refrigerator.

4. The automatic defrost system as claimed in claim 3, wherein the temperature sensing means is provided at atleast one location on the evaporator plate.

5. The automatic defrost system as claimed in any of the preceding claims, further comprising a control display for manually selecting/changing the mode of operation of the refrigerator.

6. The automatic defrost system as claimed in claim 5, wherein the mode of operation of the refrigerator comprises auto, manual, frost melt, express ice, etc.

7. A direct cool refrigerator comprising the automatic defrost system as claimed in any of the preceding claims.

8. A method of automatic defrosting of a refrigerator, wherein the defrost initiation and termination are based on compressor run-time and the temperature inside the refrigerator.

9. The method of automatic defrosting of a refrigerator as claimed in claim 8, wherein the defrost initiation takes place after the compressor run time exceeds a predefined value and the sensed temperature is lower than a preset value. 10.The method of automatic defrosting of a refrigerator as claimed in claim 8, wherein the defrost termination takes place after the defrost run time exceeds a predefined value or the sensed temperature is higher than a preset value.

11. The method of automatic defrosting of a refrigerator as claimed in any of the claims 8 to 10, wherein upon restoration of power supply after a power failure, the compressor run time is reset if the temperature sensed is higher than a preset value and remains unaffected otherwise.

FIELD OF INVENTION
The invention relates to an automatic defrost system, more particularly to an automatic defrost
system for a refrigerator, freezer or like device.
BACKGROUND OF THE INVENTION
Domestic refrigerators of the natural convection or direct cool type have an evaporator plate as
the source of cooling inside the refrigerator and freezer sections. Thus, with prolonged
operation of the refrigerator, the moisture inside the refrigerator accumulates as frost over the
cold evaporator plate. As the layer of frostlice thickens, it acts as an additional thermal
resistance for heat transfer from inside the refrigerator to the evaporator, thereby affecting the
performance of the refrigerator. Thus, it is imperative to remove this layer of frostlice from to
time maintain the energy efficiency of the refrigerator.
Direct cool refrigerators usually do not have any device for automatic defrosting of the
refrigerator. In many cases, the user has to initiate defrosting manually whenever frost build-up
is observed. The compressor is turned off while the other major components keep getting
power. A thermostat is provided to sense the temperature inside the refrigerator. Once the
temperature exceeds a preset value, the defrosting operation is ended and the compressor
starts running. Thereafter, the next defrost operation is carried out only after the user initiates
it manually.
Thus, in order to solve this problem of manual observation based defrosting wherein there are
frequent instances of excessive frost buildup, there was a need for an automatic defrosting
technology for direct cool refrigerators. One method of doing this was to provide the
refrigerator with a time based automatic defrost assembly consisting of control unit (Printed
Circuit Board with microcontroller) and thermal switch (thermostat). The compressor run time
is stored in the control unit and after a pre-defined time limit, the control unit switches off the
compressor via thermal switch to initiate the defrost operation. The defrost operation is
terminated after predefined time limit stored in the control unit.
A problem with the above mentioned method is that due to fixed defrost time sometimes it
leads to partial defrost, which causes increased frost accumulation, cooling loss and higher
energy consumption. Secondly, in case of frequent power cuts, the refrigerator starts defrost
operation after completion of the predefined time limit, without taking into consideration the
current temperature inside the refrigerator. In this scenario, the frost may have already melted
during power failure but the refrigerator continues in defrost mode for fixed time. Thus, the
temperature inside the refrigerator as well as the freezer increases leading to melting of ice in
the freezer. Thus, the foodstuffs and other items stored in the refrigerator or freezer
compartments may get spoilt. This, problem is particularly severe in countries where power
failures are very common and irregular. In cases of intermittent power supply, a situation arises
wherein no ice can be formed in the freezer and the temperature in the refrigerator
compartment remains higher than the prescribed limits.
Thus, there is a need for an automatic defrosting system for direct cool refrigerator which can
overcome the problems and limitations of the existing automatic defrosting systems mentioned
above.
SUMMARY OF THE INVENTION
The present invention provides a better solution for defrosting which is based on time as well
as temperature. The auto defrost system/ assembly provided ensures complete defrost and
curb excessive defrosting in the event of power failure. Once the power is restored, the system
first checks the evaporator plate temperature which indicates whether defrosting is completed
or not. If defrosting has already happened due to power cut, the defrost cycle is bypassed.
In accordance with an embodiment of the invention is provided an automatic defrost system
for a refrigerator which is based on time and temperature to ensure complete defrost.
In accordance with another embodiment of the invention, is provided an automatic defrosting
system for a refrigerator which allows complete defrosting of the refrigerator and prevent
excessive defrosting in the event of power failure.
In accordance with yet another embodiment of the invention, is provided an automatic defrost
system for a refrigerator which helps in saving energy and increases the energy efficiency of the
refrigerator.
In accordance with an embodiment of the invention is provided an automatic defrost system
for a refrigerator, freezer or like device, comprising a temperature sensing means and a control
means.
In accordance with another embodiment of the invention the control means of the automatic
defrost system comprises a microcontroller and a storage memory.
In accordance with yet another embodiment of the invention, the temperature sensing means
of the automatic defrost system comprises a thermistor or like device for sensing the
temperature at atleast one location inside the refrigerator.
In accordance with yet another embodiment of the invention, the temperature sensing means
of the automatic defrost system is provided at atleast one location on the evaporator plate.
In accordance with yet another embodiment of the invention, the automatic defrost system
further comprises a control display for manually selecting/changing the mode of operation of
the refrigerator.
In accordance with yet another embodiment of the invention, the automatic defrost system of
the refrigerator comprises auto, manual, frost melt, express ice, etc. modes of operation.
In accordance with yet another embodiment of the invention is provided a direct cool
refrigerator comprising the said automatic defrost system.
In accordance with an embodiment of the invention is provided a method of automatic
defrosting of a refrigerator, wherein the defrost initiation and termination are based on
compressor run-time and the temperature inside the refrigerator.
In accordance with another embodiment of the invention, in the said method of automatic
defrosting, the defrost initiation takes place after the compressor run time exceeds a
predefined value and the sensed temperature is lower than a preset value.
In accordance with yet another embodiment of the invention, in the said method of automatic
defrosting, the defrost termination takes place after the defrost run time exceeds a predefined
value or the sensed temperature is higher than a preset value.
In accordance with yet another embodiment of the invention, upon restoration of power supply
after a power failure, the compressor run time is reset if the temperature sensed is higher than
a preset value and remains unaffected otherwise.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is a schematic diagram showing some important components in a direct cool refrigerator
comprising the auto defrost system of the invention.
Fig. 2 is a flow diagram showing the compressor run and defrosting algorithm in accordance
with an embodiment of the invention.
Fig. 3 is a flow diagram showing the memory backup algorithm in accordance with an
embodiment of the invention.
It is to be noted that the above mentioned figures are with reference to a direct cool
refrigerator. However, the teachings of the invention can be readily applied to other types of
refrigerators also with or without some minor modifications.
DESCRIPTION
Discussed below are some representative embodiments of the current invention. The invention
in its broader aspects is not limited to the specific details, representative devices and methods,
and illustrative examples shown and described in this section in connection with the
embodiments and methods. The invention according to its various aspects is particularly
pointed out and distinctly claimed in the attached claims read in view of this specification, and
appropriate equivalents.
It is to be noted that, as used in the specification and the appended claims, the singular forms
"a," "an," and "the" include plural referents unless the context clearly dictates otherwise.
Fig. 1 is a schematic diagram showing some important components in a direct cool refrigerator
comprising the auto defrost system of the invention. The freezer compartment labeled (109)
has an evaporator plate (112) for supplying cooling in a direct cool refrigerator. A bulb (110) is
provided to give indication to the user about the refrigerator being on or off. Thus, when the
refrigerator has been turned off or power fails, the bulb will not glow, but it will continue to
glow otherwise. The temperature sensing means, for example a thermistor (113) senses the
temperature at a location on the evaporator plate. The frame of the freezer (111) is attached at
the front and usually has a lid to close the freezer compartment from the front.
A control means (120) mounted on the main PCB (Printed Circuit Board) is the key component
@ that controls the functioning of the various other components. It comprises a microcontroller
(102) and a storage memory (107). The compressor (103), condenser (104), capillary (105) and
evaporator (106) together operate the refrigeration cycle of the refrigerator. The compressor
(103) is connected with the microcontroller and a power source (108). The microcontroller can
switch the compressor on or off depending upon the cooling requirements.
A control display (101) connected to the microcontroller (102) may also be provided to allow
the user to select different modes of operation for the refrigerator. The user can turn the
refrigerator on or off and select the cooling level amongst a number of preset options. The user
can also select to use one of several modes like express ice, frost melt, auto defrost, child lock
etc. Based on the input from the user, the microcontroller sends signal to the compressor,
temperature sensing means, power source, etc.
The microcontroller senses the evaporator plate temperature via the temperature sensing
means, which in this case is a thermistor (113) and saves the compressor run time in its
memory (107). During normal operation, once the compressor run time reaches a preset value,
say 30 Hrs the microcontroller checks the evaporator plate temperature. If the temperature is
lower than a preset value, say 4"C, the microcontroller switches the compressor off by cutting
power supply to it, thereby initiating an automatic defrost operation. However, if the
temperature is higher than the preset value, the compressor is allowed to keep running. The
temperature condition helps determine if defrosting had occurred recently due to power failure
or due to any other factor.
In case the defrost operation has been initiated, the compressor is in off state and the frost
@
inside the refrigerator may melt naturally due to ambient heat load. The microcontroller keeps
checking the defrost run (or compressor off) time and the evaporator plate temperature. If the
evaporator plate temperature exceeds a preset value, say 6°C or the defrost run time reaches a
preset value, say 5 Hrs the defrost operation is terminated. Thereafter, the power supply to the
compressor is restored.
Fig. 2 is a flow diagram showing the compressor run and defrosting algorithm in accordance
with an embodiment of the invention. Some of the major steps and logic used by the control
means (120) have been shown in the given figure. In decision block 201, it is determined
whether the auto defrost button has been pressed for at least 3 seconds. If the microcontroller
senses that the auto defrost button was pressed for 3 or more seconds, then the control moves
directly to a block 208. In decision block 208, it is checked whether the sensor temperature
obtained via thermistor (113) is greater than 4°C.
However, if the decision block 201 determines that the auto defrost button was not pressed for
at least 3 seconds, the control moves to block 202. In decision block 202, it is checked whether
the auto defrost button is pressed. If the said button on the control display is not pressed, the
control passes to a block 203 and the refrigerator is operated continuously and continuous
power is supplied to the compressor. However, if it is determined that the auto defrost button
is pressed, the control passes to a block 204. The block 204 denotes normal operation, which
means the refrigerator will operate in auto defrost mode. The control then passes to a block
205.
In decision block 205, it is checked whether the compressor is on. If it is determined that the
compressor is on, then the control passes to a block 206. In block 206, the microcontroller
decrements the compressor run count stored in the storage memory (107). The control then
passes to a block 207. In decision block 207, it is checked whether the compressor run count
equals zero or not. If it is determined that the compressor run count does not equal zero, the
control moves back to block 205. Otherwise, the control moves to a block 208.
In decision block 208, it is checked whether the sensor temperature obtained via thermistor
(113) is greater than 4°C. If the temperature is found to be greater than 4"C, the control passes
to a block 214. In block 214 the compressor run is started from the beginning and the
compressor run count variable is set to 1800. Thereafter, the control passes back to the block
201.
However, if the evaporator plate temperature checked in decision block 208 is determined to
be less than 4"C, the control passes to a block 209. In block 209 the defrost run is started and
the defrost run count variable is set to 300. The control then passes to a block 210. In the
@
decision block 210, it is determined whether the evaporator plate temperature exceeds 6°C. If
the microcontroller determines the temperature sensed by thermistor (113) to be greater the
said value, the control passes to a block 213. However, if the temperature is not greater than
the said temperature, the control passes to a block 211. In the block 211, the microcontroller
decrements the defrost run count stored in the storage memory. The control then passes on to
a block 212.
In decision block 212, it is determined whether the defrost run count equals zero or not. If the
said condition holds true, the control passes to a block 213 where the defrost operation is
ended and the control passes on to block 214 where the compressor run is started again.
However, if it is determined that the defrost run count does not equal zero, the control moves
back to block 210, where the evaporator plate temperature is rechecked and compared with
the set value.
A block 215 determines whether a power failure has occurred or not. If the decision block 215
@ determines that there is no power failure, the control passes to a block 218. In block 218, the
compressor run count is restored from the memory storage device. Thereafter the control
passes to the decision block 207. However, if power failure is determined in the block 215, the
control passes to a block 216.
In decision block 216, it is determined whether the evaporator plate temperature is greater
than 4°C or not. If the said condition is determined to be false, the control passes to the block
218 where the compressor run count is restored from the memory storage device. However, if
the temperature is determined to be greater than the 4°C) the control passes to a block 217. In
block 217, the compressor run is started and the microcontroller sets the compressor run count
variable to 1800.
The above mentioned drawing provides a particular control logic for automatic defrosting and
operating the compressor. Other variations are possible without diverting from the scope of the
invention.
Fig. 3 is a flow diagram showing the memory backup algorithm in accordance with an
embodiment of the invention. Several schemes for operating the storage memory are known
and other variations are possible. In the given drawing, a block 301 initializes the memory backup
count to 60. The control then passes to a block 302 in which the memory backup count is
decremented and the control passes to a block 303. In the decision block 303, it is determined
whether the memory back-up count is zero or not. If the memory back-up count is determined
to be zero, the control passes to a block 304, where the compressor run count is saved to the
memory. However, if the decision block 303 determines that the memory back-up count is not
equal to zero, the control passes back to the block 302.
The refrigerator may be operated in several other modes, with some changes to the algorithm.
Some of these modes are explained in the underlying paragraphs. In case of "Frost Melt", the
defrost operation is started immediately upon selecting that mode, and any other mode
selected before it gets cancelled/unselected. No time limit is applied to the defrost cycle and
the defrost operation is continued until the evaporator plate temperature is detected to exceed
a preset value, say 6°C.
In case of "Express Ice" mode, any other mode gets cancelled and the compressor is run for a
predetermined time, say 2 hrs so that ice can be formed in the freezer. If the mode was
initiated during the defrost operation of the refrigerator, the defrost operation is restarted
upon termination of the Express Ice mode.
0 Several variations may be possible for the present invention. For instance, the temperature
sensing device can be placed at any location on the evaporator plate or at any other location
inside the refrigerator. The temperature conditions can be changed in the algorithm
accordingly. Other temperature sensing devices like RTD (Resistance Temperature Detector),
thermocouples, etc. can also be used in place of a thermistor. The storage device can also be
selected from a variety of memory backup devices like EEPROM
(Electrically Erasable Programmable Read-Only Memory), flash memory, ROM (Read-Only
Memory), RAM (Random Access Memory) etc., depending upon the rate at which the
information has to be stored, updated and retrieved.
The teachings of the invention can be readily applied to similar devices like freezers,
refrigerated shelves, etc.
We Claim:
1. An automatic defrost system for a refrigerator, freezer or like device, comprising a
temperature sensing means and a control means.
2. The automatic defrost system as claimed in claim 1, wherein the control means comprises a
microcontroller and a storage memory.
3. The automatic defrost system as claimed in claims 1 or 2, wherein the temperature sensing
means comprises a thermistor or like device for sensing the temperature at atleast one
location inside the refrigerator.
4. The automatic defrost system as claimed in claim 3, wherein the temperature sensing
means is provided at atleast one location on the evaporator plate.
5. The automatic defrost system as claimed in any of the preceding claims, further comprising
a control display for manually selecting/changing the mode of operation of the refrigerator.
6. The automatic defrost system as claimed in claim 5, wherein the mode of operation of the
refrigerator comprises auto, manual, frost melt, express ice, etc.
7. A direct cool refrigerator comprising the automatic defrost system as claimed in any of the
preceding claims.
8. A method of automatic defrosting of a refrigerator, wherein the defrost initiation and
termination are based on compressor run-time and the temperature inside the refrigerator.
9. The method of automatic defrosting of a refrigerator as claimed in claim 8, wherein the
defrost initiation takes place after the compressor run time exceeds a predefined value and
the sensed temperature is lower than a preset value.
10.The method of automatic defrosting of a refrigerator as claimed in claim 8, wherein the
defrost termination takes place after the defrost run time exceeds a predefined value or the
sensed temperature is higher than a preset value.
11. The method of automatic defrosting of a refrigerator as claimed in any of the claims 8 to 10,
wherein upon restoration of power supply after a power failure, the compressor run time is
reset if the temperature sensed is higher than a preset value and remains unaffected
otherwise.