Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~6~6g 9D-HR-13995
REFRIGERATOR DEFROST CONTROL WITH CONTROL
OF TIME INTERVAL BETWEEN DEFROST CYCLES
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Back~round of -the Invention
1. Field of the Invention
This invention relates to a control for an automatic
defrost refrigerator and in particular. to an electronically
controlled defrost system in which interva:Ls between
de~ros-t cycles are varied as a function of minimum
compressor run time and of refrigerator usage as measured
by door open time.
2. Descrip-tion of the Prior Ar-t
I-t is well known in a frost Eree refrigerator to
control the time interval between defrost cycles by means
of a motor operated electromechanical timer means coupled
across the motor operated compressor of the refrigerant
system~ Thus, in this arrangement, the compressor and
defrost timer run and shut off simultaneoulsy under
control of the refrigerator compartment thermostat
until a preset interval of compressor run time is
reached, e.g. 7-1/2 hours, as determined by the timer.
At this point, a switch on the timer removes the com-
pressor motor and substituted the defrost heaters.
Durin~ the defrost cycle, typically thirty minutes,
the timer continues to run until the defrost cycle is
completed, at which point the timer switch reverts to
the motor compressor circuit and the entire cycle as
described above is repeated U.S. Patent No. 3,890,798
to Fujimoto dated June 24, 1975 is representative of art
teaching the use of a digital timer in place of the
electrochemical timer to set the interval between defrost
cycles as a function of compressor run time.
In U.S. Patent No. 2,701,450 to Duncan dated February
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8, 1955, an electromechanical accumulator and door open
sensor are shown for the purpose of providing demand
defrost as a function of accumulated door open time.
However, a variety of environmetal conditions, as well
as structural conditions of the refrigerato:r itself, can
affect the timing of needed defrost cycles. This patent
disclosure has no provision -to take this into account
except to provide for a manual override by which the
user can accelerate or delay the timing of the initiation
of the defrost cycles. U.S. Patent No. 4,056,948 to
Goodhouse dated November 8, 1977, shows an arrangement
for modifying the accumulation of compressor run time
by a humidity controlled switch which operates to extend
the time interva:L between deErost cycles in response to
low humidity conditions in the rèfrigerator. This
compensation must be manually selected, however, and does
not take in account other factor which have an effect on
the need for defrosting.
U.S. Patent No. 2,781,641 to Foley dated February
19, 1957 discloses in Fig. 3 a defrost timing control
circuit in which the timer is operated conjointly with
the simultaneous occurrence of compressor run and door
open conditions by means of door sensor and compressor
run switches connected in series with the timer motor.
Such an arrangement, however, does not allow for the
possibility of initiating defrost independent of door
opening time as might be required during very low usage
periods or while the user is away on vacation. More-
over, there is no provision in this arrangement whereby
initiation of the defrost cycle may be delayed from a
fixed minimum time interval to a delay time which is a
measure of a fixed arnount of ac-tual door opening time.
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The serial connection of the door sensor and compressor
switches requires that both be closed to operate the timer.
Summar~ of the Invention
It :is, therefore, an object of the invention to
provide an improved automatic control of defrost timing
in a frost-free refrigerator.
It is another object of the invention to provide
automatic control of defrost timing as a function of
door opening time and compressor run time in which
com~ressor run time determines the minimum interval
between defrost cycles which may be delayed until a
fixed amount of door time is accumulated.
It is a further object oE the invention to provide an
automatic defrost control operating, in part, as a function
of door open time in which a maximum interval between
defrost is provided for vacation periods during which
few or no door openings occur.
It is a still further object of the invention to
provide an alternative form of automatic defrost control
of the type described in which the existence of an
acceptably cool level in the refrigerator compartments
operates as a prere~uisite to delayed initiation of a
defrost cycle beyond the minimum compressor run time.
These and other objects of the invention are achieved
in a self-defrosting refrigerator having at least one food
storage compartment with an access door thereto and a
refrigerant system including a motor operated compressor,
an evaporator for cooling the food storage compartments,
thermostatic means for turning the compressor on and
off to maintain a desired temperature in the food com-
partments and defrost heater means for periodically
melting frost from the evaporator. In accordance with
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the invention, there is provided the combination comprising
timer operated switch means responsive to the thermostatic
means to establish a time interval between defrost cycles
which is at least equal to a minimum predetermined amount
of compressor run tlme followed by a defrost cycle time
interval of a predetermined maximum duration. The
combination of the invention also comprises means for
accumulating total compressor run time during the entire
elapsed time between defrost cycles, and further includes
a door open sensor and means responsive to the door open
sensor for accumulating the amount of time the door is
open during the interval between defrost cycles at least
up to a predetermined amount of door-open time. The
combination of the invention still further includes
decision means for providing a first control effect
representative of the accumulation of a predetermined
compressor run time related to the minimum compressor
run time of the timer means and a second control effect
representative of when the predetermined amount of door-
open time is reached. ~'inally, the combination of theinvention includes switch means which is coupled in
electrical circuit with the timer means and is effective
in response to the decision means to interrupt operation
of the timer means in response to the sole presence
of the first control effect and to initiate or continue
operation of the timer means in response to the simulta-
neous presence of both the first and second control
effects. In this manner, e]apsed time between is held
to the minimum compressor run time during relatively
heavy usage oE the refrigerator and is extended to a
longer elapsed time during relatively light usage of the
refrigerator wherein usage is measured by door-open time.
9D-HR-13995
In a preferred form of the invention, the output
of temperature sensors in both the freezer and fresh
food sections of the refrigerator compartment are
checked when the timer means reaches the minimum
compressor run time and, if the compartment temperatures
are at normal cold levels indicative of proper
functioning without excessive build-up of frost on the
evaporator, the timer means is interrupted as described
above, assuming the requisite predetermined door-open
time has not been reached. On the other hand, if the
temperatures are warmer than normal, as would occur
with excessive frost build-up, the timer control switch
means is operated to continue the timer operation into
a defrost cycle even though the predetermined accumulation
of door-open time has not been reached. This is of
particular value when unusually high ambient humidity
conditions impose a heavier burden on the evaporator
beyond that assumed in the initial selection of the
predetermined door-open time criterion.
Brief Description of the Drawings
Fig. 1 is a partial circuit of a defrost control
system for a frost-free refrigerator constructed in
accordance with the invention and employing a micro-
computer as the decision means of the invention.
Fig. 2 is a functional block diagram useful in
explaining the operation of the circuit of Fig. 1.
Fig. 3 is a program flow diagram which may be
employed in developing a program for the microcomputer
of Fig. 1.
Detailed Descript_
Referring now to Fig. 1, a simplified schematic
diagram of a defrost control circuit is shown for a
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self-deErosting refrigerator in which a motor-compressor
unit 10 has one terminal connection to power supply L
via contact 14a of defrost timer switch 14 and ther~
mostatic control switch 16. A second terminal of
motor-compressor unit 10 is connected directly to power
supply line L2. Power lines Ll and L2 are coupled to a
power plug 20 which is adapted to be connected to a
conventional 60 hert~, 120 volt household power source.
Defrost heater means 22 has one terminal connected to
defrost timer switch contact 14b and the other terminal
connected via a bimetal defrost thermostat 24 to
power line L2. ~t appropriate times in the operation
of the self-defrosting circuit of Fig. 1, defrost timer
motor 15 advances and moves the arm of switch 14 to
bridge contacts 14b and 14c, thus connecting the one
terminal of defrost heater 22 to power line Ll via
thermostatic control switch 16 to initiate a defrost
cycle. Thermostat 24 is provided, as is well known, to
permit the defrost heater 22 to be turned off earlier
than allowed by defrost timer 12 once the frost has
been removed from the refrigerator evaporator. Ther-
mostat 16 may be of any conventional form such as a
vapor-filled bellows-activated device having a pair of
contacts 16a and 16b which close when the temperature
in the refrigerator compartment is higher than a
desired setting established by the user. Timer motor
15 is connected on one side via a normally closed relay
switch 26 to power line L2.
With the exception of the inclusion of switch 26,
-the foregoing circuit arrangement is conventional in
self~defrosting refrigerators. Timer 12 is adapted to
provide alternate refrigeration and deErost cycles
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depending on the positioning of the arm of switch 14
to either contact 14a and 14b. During the refrigeration
cycle, timer motor 15 and its associated cam arrangement
(not shown) positions the arm of switch 14 to contact
14 thus causing the motor-compressor unit 10 to cperate
whenever thermostat switch 16 is closed. Thus switch
16 cycles compressor 10 on and off to maintain a desired
temperature level in the refrigerator compartment.
Timer motor 15 is also activated at the same time as
is compressor 10 in order to establish a minimum com-
pressor run time during the refrigeration cycle as
determined by the structure of the cam arrangement of
timer 12. This run time may, Eor example, be on the
order of 7-1/2 hours depending on the structure of the
timer mechanism, although it will be appreciated that
the actual elapsed time between defrost cycles will
usually be substantially longer since the cycling of
the compressor on and off results in a compressor run time
which is only a fraction of the actual elapsed time.
Timer 12 is adapted to provide a fixed maximum
time period for the defrost cycle, typically thirty
minutes in duration. During this time, the arm of timer
switch 14 is set to contact 14b. Thermostat switch 16
remains closed since, during defrost, the interior of
the refrigerator is slightly warmer than normal. In
the event frost is fully melted before the completion
of the maximum thirty minute defrost cycle, the evaporator
temperature will then rise and thermostatic switch 24
opens to deactivate heater 22, as previously mentioned,
in order to avoid unnecessary heat up of the interior
refrigerator compartment while timer 12 finishes timing
out the portion of the defrost cycle.
9D-H~ 13995
In accordance with one aspect of the invention, there
i9 provided means for accumulating total compressor run
time during the entire elapsed time that e~ists between
defrost cycles. This includes a sensor circuit 30 connected
from timer switch contact 14a to one input of micro-
computer 32. Sensor circuit 30 may comprise any suitable
conversion circuit operative to convert the a-c signal
at its input to a digital output signal compatible for
input and storage within the memory register of micro-
computer 32 thus to accumulate the amount of time whichcompressor 10 is operated. ~ door switch 33 is coupled
to another input of microcomputer 32 via an analog-to-
digital conversion circuit 34 to provide means for
accumulating the amount of time the refrigerator door
is open. In addition, a temperature sensor 37 located
in the interior compartment of the refrigerator has
its output coupled through a suitable analog-to-digital
conversion circuit 38 to an input of microcomputer 32
to allow periodic sensing and storage of the interior
temperature of the refrigerator.
Microcomputer 32 may comprise a self-contained
integrated circuit such as a Mostek MK3872 including an
arithmetic logic circuit, appropriate memory registers
and input/output circuits as is well known in the art.
Microcomputer 32, in part, is pre-programmed to be
adapted to serve as a decision means for providing a
first control effect representative of the accumulation
of a predetermined compressor run time related to the
minimum run time established by the timer 12 and also
to provide a second control effect representative of
when a predetermined amount of accumul.ated door-open
time is reached as will be explai.ned subsequen-tly.
9D-HR-13995
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The predetermined compressor run time whieh is effeetive
to produce the first eontrol effect may be e~ual to the
minimum compressor run time of timer 12 or it may be
less than the minimum run time by a fixed amount.
A timer control switch means 26 is coupled in
electrical cireuit with timer motor 15 of timer 12 and
is operatively eontrolled by mierocomputer 33 via a
suitable driver eircuit 31 to interrupt the operation
of timer motor 15 in response to the sole oecurrenee
of the first control effect and to initiate or continue
operation of the timer motor 15 in response to the
simultaneous presence of the first and second control
effect in microcomputer 32. It will be appreciated
that interrupting operating the timer motor 15 during
the interval between defrost cycles does not effect
the refrigeration operation of the compressor 10 since
switch 14 is held with its arm bridging contaets 14a
and 14c sueh that eompressor 10 will eontinue to be
cyeled on and off by thermostatie eontrol switeh 16.
To insure that this is the ease, it may be preferred to
establish the amount of aeeumulated run time which
produces the first eontrol effeet in mieroeomputer
32 at slightly less than the full amount of minimum
eompressor run time, as previously mentioned.
Referring now to Fig. 2, the funetional operating
diagram shown therein illustrates the opera-tion of the
Fig. 1 system in extending the time interval between
defrost eyeles to thereby reduee energy usage during
periods of light usuage of the refrigerator. Initially,
it will be assumed that power is applied to the re-
frigerator with timer 12 at some unknown setting as
indicated in bloek 40. This initiates the Eunetional
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instruction applied through OR circuit 41 to block 42
that results in the normal opera~ion of timer motor 15.
Since the timer 12 is at an unknown setting, it is
necessary to establish a starting poin-t representative
of the completion of a defrost cycle from which to
begin measuring the accumulated compressor run time and
door open -time for the purpose of de-termining the start
of the next defrost cycle. It can be shown that, in
at least one exemplary form of automatic defrost refrige-
rator, the time period during which the compressorwill remain continuously off does not exceed 25 minutes
for a compressor cycling rate of 25~ which corresponds
to a very warm reErigerator setting or a low room
ambient temperature. Thus a time period slightly in
excess of this amount such as 28 minutes is selected
to indicate the existence of a completed defrost cycle.
When this occurs, as sensed in microcomputer 32 via
sensor circuit 50 as represented in functional block
43, the microcomputer will reset the accumulated com-
pressor run time and accumulated door open time memoryregisters and will begin accumulating run time and
door open time starting at time zero as represented in
functional blocks 44 and 45. When 7-1/2 hours of
accumulated run time is recorded, a functional instruc-
tion is sent via line 46 to functional block 47 to
interrupt operation of timer motor 15 (corresponding
to opening of switch 26 in Fig. 1), and also to functional
AN~ circuit 48. Assuming relatively light usage of
the refrigerator such that the amount of door open
time has not reached a total of, for example, 400
seconds, the timer motor 15 remains inactive thus
forestalling the onset of the next defrost cycle unt;:l
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9D-HR-13995
both inputs to functional AND circuit 48 occur. At this
time the output of AND circuit 48 is processed through
OR circuit 41 to reactivate timer motor 15 such that
the next defrost cycle may be commenced as determined by
the operation of timer 12. If, on the other hand, heavy
usage of the xefrigerator had resulted in an accumulation
of the 400 seconds of door open time prior to reaching
the 7-1/2 hours o:E compressor run time, it can be seen
that tlmer motor 15 would not be interrupted and timer
12 would have progressed in normal manner to initiate a
defrost cycle at the conclusion of the 7-1/2 hour eompressor
run time. Thus, it ean be seen that with this invention,
a simple and effective electronically eontrolled means
is provided whereby elapsed time between defrost eycles
is extended beyond a minimum compressor run time during
relatively light usage of the refrigerator to reduce
energy eonsumption by the self-defrosting refrigerator.
With the arrangement just deseribed, if the door
were not opened for an extended period of time, as
during vaeation time, a defrost eyele would not be
possible even though normal air leakage and heat
transfer into the refrigerator might generate sufficient
frost on the evaporator coils to require a defrost
eycle. For this purpose, a maximum time is provided,
for example, 16~1/2 hours, at the eonelusion of whieh
a defrost eyele occurs even though the requisite
aeeumulated door open time has not oeeurred. This is
represented by line 49 in Fig. 2.
The dotted portion of Fig. 2 respresents an
alternative arrangement in whieh, at the eonelusion
of the minimum eompressor run time of 7-1/2 hours,
the interruption of the timer motor that would oeeur
9D-HR-13995
during light usage (accumulated door open time less than
400 seconds) is overriden by the sensing of a warm
temperature in the refrigerator compartment which would
indicate the existence of heavy frost build-up on the
evaporator coils. This might occur, for example, if the
room ambient temperature is unusually warm if the relative
humidity of the room is unusually high such that frosting
of the coils occurs at a faster rate than suumed when
selecting the 400 second criteria for the door open time.
Thus, if a warm compartment temperature is sensed, the
"WARM" input to functional OR block 41 continues operation
of the timer motor 15 when the 7-l/2 hour compressor run
time is reached and a cold (i.e. normal) temperature
permits the timer motor to be interrupted in accordance
with the invention.
Referring now to Fig. 3, a program flow chart is
shown which may be used by those skilled in the art
to establish a set of program instructions for micro-
computer 32. It will be appreciated that the illustrated
flow chart may represent only a portion of a complete
program for microcomputer 32 by which other functions of
the refrigerator are also controlled. Thus, upon
entering the defrost control portion of the program,
inquiry determines whether the compressor is running and,
if so, instructions 62 and 64 reset the compressor off
count to zero and update the compressor on count to
the next time increment. If the compressor is not running,
instruction 61 updates the compressor off count to the
next time increment, following which inquiry 63 determines
whether the accumulated compressor off time is equal to
or greater than 28 minutes. If the answer is yes, in-
dicating the completion of a defrost cycle, instruct:ions
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65 and 67 reset the compressor run time and door open
time counts to zero in preparation for the timing of the
next ensuing interval between the just completed defrost
cycle and the next ensuing defrost cycle. If the
aacumulated compressor off time is less than 28 minutes,
or following completion of instructions 64 or 67, the
next inquiry 68 of the program determines if the door
is open. If so, instruction 69 updates the accumulated
door open count. Following this, or if the door was not
open, the program moves to instruction 70 to determine
if the minimum compressor run time of 7-1/2 hours has
been reached. If not the program moves to instruction 75
to maintain switch 26 closed (Fig.l), thereby to continue
normal operation of timer 12, following which the defrost
control program returns for completion of the main
program in microcomputer 32.
Assuming, however, that the minimum compressor run
time of 7-1/2 hours has been reached, inquiry 72 of the
program then determines if the accumulated door open
time is less than the minimum predetermined value of,
in this example, 400 seconds. An affirmative result
indicates that a light usage condition of the refriy~rator
exists which dicates that the next ensuring defrost cycle
should be delayed by interrupting the operation of the
timer 12. Before this occurs, however, inquiry 74 deter-
mines if the accumulated compressor run time is below
the maximum allowable compressor run time of 16-1/2 hours.
If not, i.e. 16~1/2 hours has been reached or exceededr
which could be the case during a vacation period, switch
26 is closed to turn on the timer motor 15 so that a
defrost cycle can be initiated even though the 400 seconds
of door open time had not been reached. On the o-ther hand,
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if the door open time has not reached 400 seconds and the
compressor run time is still below the maximum of 16-1/2
hours, the program could then move to instruction 78 to
open switch 26 thus inactivating timer motor 15. In
the example shown in Fig. 3t however, provision is made
in inquiry 76 for first determining the status of the
temperature in the refrigerator compartment. If a normal
-temperature is aensed, the program moves to instruction
78 to stop timer motor 15 since the normal temperature
indicates no abnormal build-up of frost on the evaporator
coils. Conversely, a sensed warm temperature cause the
program to revert to instruction 75 to hold closed or
re-close, as appropriate, switch 26 so that timer 12 can
move on and initiate a defrost cycle. Following initiation
of the appropriate instruction, the defrost control
program then returns to the main program.
It will be appreciated that there has been described
a simple and effective electronic defrost control for an
automatic defrost refrigerator whereby energy usage may
be minimized by delaying initiation of the defrost
cycle beyond the normal minimum compressor run time
during periods of light usage as measured by accumulated
door open time. An alternative arrangement has also
been shown which operates to override the door open
measurement when interior temperature measurment
indicates abnormal frost build-up that requires an early
initiation defrost cycle even though the door open
measurement indicates light usage of the refrigerator.
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