US3728867A

US3728867A - Defrost control system

Info

Publication number: US3728867A
Application number: US00138673A
Authority: US
Inventors: J Jarrett
Original assignee: Ranco Inc
Current assignee: Ranco Inc of Delaware; Robertshaw US Holding Corp
Priority date: 1971-04-29
Filing date: 1971-04-29
Publication date: 1973-04-24
Anticipated expiration: 1990-04-24

Abstract

A fluid flow amplifier senses increased differences in air pressure on opposite sides of the cooling unit of a forced air refrigerating system due to clogging of the cooling unit by frost, and alters the resistance in a bridge circuit type amplifier which controls one SCR in the circuit of a hysteresis synchronous timer motor which operates switching mechanism to effect a defrost cycle. The switching mechanism operates a defrost device for a predetermined period to melt frost from the cooling unit, after which it restores normal refrigeration. A resistance in the bridge circuit amplifier is located adjacent an air passage in the fluid flow amplifier to maintain the walls of the air passage above the air temperature.

Description

United States Patent [191 Jarrett [451 Apr. 24, 1973 [73] Assignee: Ranco Ohio 22 Filed: Apr. 29,1971

[21] App1.No.: 138,673

Incorporated, Columbus,

[56] References Cited UNITED STATES PATENTS 3,487,654 1/1970 Lorenz .62/l4O 3,363,429 1/1968 Weschsler... ..62/l40 3,512,372 5/1970 Kosuda v.62/15'7 Primary Examiner-Meyer Perlin AttorneyWatts, Hoffmann, Fisher & Heinke [5 7 ABSTRACT A fluid flow amplifier sens'es increased differences in air pressure on opposite sides of the cooling unit of a forced air refrigerating system due to clogging of the cooling unit by frost, and alters the resistance in a bridge circuit type amplifier which controls one SCR in the circuit of a hysteresis synchronous timer motor which operates switching mechanism to effect a defrost cycle. The switching mechanism operates a defrost device for a predetermined period to melt frost from the cooling unit, after which it restores normal refrigeration. A resistance in the bridge circuit amplifier is located adjacent an air passage in the fluid flow amplifier to maintain the walls of the air passage above the air temperature.

4 Claims, 5 Drawing Figures PATENTEDAPRM I975 3,728,867

' sum 1 OF 2 I INVENTOK -JAY H, JARRETT I BY finite, v ,%4%

- ATTORNEYS DEFROST CONTROL SYSTEM BACKGROUND OF THE INVENTION Many present-day refrigerators have separate fresh food storage and frozen food storage compartments. The frozen food compartment is maintained at temperatures well below freezing and the fresh food compartment is maintained at a temperature several degrees above freezing. The two compartments are chilled by a common evaporator of a compressor-condenser-expander type refrigerating system. The desired cooling of the compartments is accomplished by force circulating air from the two compartments into heat exchange with the evaporator and back to the compartments. Generally, thermostatically operated damper or vane means are provided for regulating the flow of chilled air from the fresh food compartment to the evaporator so as to maintain the relatively higher temperature in the compartment suitable for preserving fresh foods. The circulation of air over the evaporator surfaces results in a more or less gradual accumulation of frost on these surfaces which reduces the refrigerating effect because air flow is reduced and the frost tends to insulate the fin and tube surfaces from the air. Consequently means must be provided for periodically removing the frost from the evaporator. This defrosting operation has generally comprised melting the frost from the evaporator by heating the evaporator by electric resistance elements or by passing hot gas through the evaporator. Various methods for automatically effecting defrosting operations have been employed. One type of defrost control comprises means responsive to a change in air flow over the evaporator due to accumulation of frost on the evaporator tending to block the flow of air. One form of this type of control is disclosed in U.S. Pat. No. 2,505,201. Another form of control is shown in U.S. Pat. No. 3,487,654. The defrost system disclosed in U.S. Pat. No. 2,505,201 is expensive and is subject to inaccurate operation because it does not respond accurately to relatively light frost deposits. The system shown in U.S. Pat. No. 3,487,654 is relatively expensive and in some cases defrosting cycles may be initiated by transient conditions in the control circuit and fluid amplifier. Another disadvantage is the fact that frost may collect in the control jet passages of the fluid amplifier and interfere with proper operation of the amplifier.

THE PRESENT INVENTION One object of the present invention is the provision of a defrost control apparatus for refrigerators of the type mentioned which comprises a fluid amplifier responsive to changes in pressure of air upstream and downstream of the evaporator to alter the resistance in a bridge circuit of a solid state amplifier connected in the power circuit for the compressor motor, the output of the solid state amplifier controlling a silicon controlled rectifier connected in the power supply circuit ofa hysteresis synchronous electric timer motor, which drives a sequentially operating switch mechanism for establishing defrost producing circuits after a given period of timer motor operation, the timer operated switching mechanism comprising a switch which closes the triggering circuit for the gate of the silicon controlled rectifier for maintaining motor operation for a given defrost period, after which the switching mechanism restores normal refrigeration operation and resets the timer motor cycle.

Another object of the invention is the provision of a defrost control apparatus for the evaporator of a refrigerator of the type described comprising a fluid amplifier adapted to sense change in the pressure differential of the air stream forced through the evaporator and effect change in the resistance of a bridge circuit for a solid state amplifier which is connected in the power circuit for. the refrigerator compressor motor, the fluid amplifier including air passage means having a panel supporting a resistor for the solid state amplifier circuit arranged to heat the walls of the fluid amplifier and prevent the collection of frost.

Other objects and advantages of the invention will be apparent from the following description of a preferred form of the invention, reference being made to the accompanying drawings in which:

FIG. 1 is a schematic illustration of a two compartment refrigerator embodying the invention;

FIG. 2 is a view of the fluid amplifier structure taken substantially along line 22 of FIG. 3;

FIG. 3 is a sectional view taken along line 3-3 of FIG.

FIG. 4 is a sectional view taken along line 44 of FIG. 2; and

FIG. 5 is a plan view in elevation of the fluid amplifier assembly.

THE DISCLOSURE Referring to the drawings, the invention is shown embodied in a defrost control apparatus applied to a refrigerator 10. The refrigerator 10 is of a conventional type comprising a frozen food storage compartment 11 and a fresh food compartment 12. The compartments are refrigerated by air withdrawn from the compartments, directed over the surfaces of the chilling unit or evaporator 13 of a compressor-condenser-evaporator type refrigerating system and returned to the compartments.

The evaporator 13 is located between the compartments 11 and 12, and comprises tubing 14 formed in flat coils to which parallel metal cooling fins 15 are attached. Air from the compartments is directed across the tubing and fins and returned to the compartments by a shroud structure 16 and fan 17.

The fan 17 draws air from an intake chamber 20 and forces it through an inlet 21 of the shroud 16. Air passes over the surfaces of the evaporator 13 where it is chilled, and exits the shroud 16 through an outlet 22. The chilled air enters the freezer compartment 11 through a passage 23 formed by a wall 24 of the inlet chamber 20 and the refrigerator wall 25. Air may also enter the fresh food compartment 12 via a passage 26 formed between the wall 25 and a wall of the shroud 16. Air from the freezer compartment is returned to the inlet chamber 20 through openings 27 in a wall of the chamber. Air from the fresh food compartment 12 is returned to the inlet chamber 20 through a passage 30 formed between a wall of the shroud l6 and the wall 31 of the refrigerator. The arrows in the drawing indicate air flow.

The circulation of air through the compartment 12 is regulated by a vane or damper 32 which is preferably positioned by a suitable thermal element, not shown,

subject to the temperature in the compartment 12. The damper 32 tends to close the passage 30 as the temperature in the compartment 12 approaches that desired to be maintained and opens the passage to a greater degree as the temperature increases above that desired. Thermostatically operated dampers of the type referred to are well known and further description is unnecessary to the understanding of the present invention.

The evaporator 13 is connected in a conventional compressor-condenser-expander type refrigerating system. The compressor and condenser are not shown as they are well known in the art. Suffice to say, the compressor is driven by an electric motor 33 connected with a commercial power such as 120V AC. supply mains Ll, 2.The circuit of the compressor motor 33 is controlled by a conventional thermostatically operated switch T so as to normally operate the compressor to maintain temperatures of approximately F. to lO F., for example, in the freezer compartment 1 1.

The evaporator 13 may be heated periodically to melt frost therefrom by an electric heater 34, which is a well known expedient in the art. The heater 34 is connected in the power circuit line Ll through a double throw, timer motor operated switching mechanism 35. The opposite side of the heater 34 is connected to line L2 of the power circuit.

The switching mechanism 35 may be of any suitable construction and is illustrated as comprising a movable contact arm v36 swingable about its pivot between two

fixed contacts

37, 38. The contact arm 36 is oscillated between the two fixed contacts by a cam 40 which is engaged by a cam follower 41 on the contact arm. The cam 40 is rotated by a timer motor 42 through a drive shaft 43. The timer motor 42 is a well known hysteresis synchronous type motor whose impedance is such that it dissipates approximately the same power under half wave power conditions as it does under full wave power. One side of the motor 42 is connected with main L2 through a silicon controlled rectifier (SCR) and a diode D1 in parallel circuit with the SCR. By the circuit arrangement described, when the SCR is nonconducting, the motor 42 is energized by one-half wave voltage only and does not rotate. The half-wave voltage energization occurs during the portion of the power cycle in which L2 is positive relative to L1. When the SCR is conducting, the motor 42 will be subjected to full wave energization and will rotate at a given rate.

The SCR is rendered conductive by the impression of a firing voltage at its gate junction 44 at the beginning of each half-wave voltage in which main L1 is positive in relation to main L2. The firing voltage is applied to the gate of the SCR by the output of an amplifier circuit 45 which responds to the voltage changes in a bridge circuit comprised of two thermistors 46,47. In the form shown, both of the thermistors 46,47 have the same positive temperature coefficients of at least :6 percent per degree C. The thermistors 46,47 are of substantially the same resistance value and operate at power levels which will raise their temperatures above the ambient in which they are disposed.

The thermistors 46,47 are located in

alternate air passages

50,51 respectively, of a fluid amplifier device 52. The fluid amplifier 52 is similar to that disclosed in U.S. Pat. No. 3,487,654 and comprises a body 53 having a main air stream passage 54 arranged to direct an air stream through a throat 55 and into a diverter chamber 56. The chamber 56 is fan shaped and has a main outlet passage 57, axially aligned with the throat 55, and two alternate diverging

outlet passages

50,51 mentioned previously. The main air stream directed through the throat 55 and into the chamber 56 may be diverted towards either of the

passages

50,51 by signal air jets striking the main air stream from one side or the other. The signal air jets are provided by nozzle like passages 60,61 which are directed into opposite sides of the throat 55 and in axial alignment with one another. The nozzle 60 communicates with the upstream of passage 54 by a loop passage 62. A hand valve member 63 in the passage 62 may be set to regulate the air flow to the nozzle 60. The nozzle 61 is connected with a passage 64.

The amplifier body 53 is suitably supported in the inlet chamber 20 of the refrigerator and the air passage 54 is in communication with the area enclosed by the shroud 16 between the fan 17 and the upstream side of the evaporator 13. The

discharge passages

50,51 and 57 open into the chamber 20. The signal jet passage 64 opens into the chilled air discharge passage 23, downstream of the evaporator 13. lt will be seen that when the fan 17 is operating, the air pressure at the inlet of passage 54 will be greater than that at the outlets of

passages

50,51 and 57. The pressure of the air in jet passage 62will be higher than the pressure in the throat 55. Likewise, air pressure in the jet passage64 will be higher than the air pressure in the throat 55. Consequently, signal air jets will impinge the air stream passing through the throat 55. If the signal jets issuing from the nozzles 60,61 are equal in force the air stream discharging from throat 55 into the chamber 56 will exit the body 53 principally through the passage 57. In this situation, the thermistors 46,47 will dissipate heat at the same rate and will have the same temperatures and resistance values. Normally, the air pressure in the passage 23 and in the jet passage 64 will be lower than the air pressure at the entrance to the signal air passage 62. The differential air pressure will increase as the amount of frost collected on the evaporator increases. The signal jets issuing from the nozzles 60 and 61 can be balanced when the evaporator is clear of frost by the degree of opening of an adjusting valve 65 in the passage 62. Thus, should a'significant amount of frost collect on the surfaces of the evaporator 13, the air pressure will drop in the passage 23. This weakens the jet signal at the nozzle 61 relative to the jet signal at the nozzle 60. Consequently, the stream of air entering chamber 56 is diverted towards the outlet passage 51. This diversion further reduces whatever air flow occurred through passage 50 and the change in air flow in the

passages

50 and 51 causes a reduction in cooling rate of the thermistor 46. concomitantly, an increase in cooling rate of the thermistor 47 occurs. Accordingly, the resistance of thermistor 46 increases and the resistance of thermistor 47 decreases.

The change in resistances of the thermistors 46,47, which occur when frost accumulates on the evaporator as just described, is detected and amplified and utilized to trigger the SCR and cause operation of the timer motor 42. A simplified bridge circuit is formed by the thermistors 46,47 connected in series between mains L1 and L2, and resistor R1, potentiometer R2 and resistor R3 connected between mains L1,L2. A diode D2 provides a half-wave rectified DC to the bridge circuit, and a resistor R4 drops the voltage to a convenient level. A transistor Q1 is provided having its base connected with the junction 66 between the transistors 46,47. The emitter of the transistor is connected with the potentiometer R2 and the collector circuit of the transistor has a load resistor R5. The collector circuit is connected with the gate of the SCR, and the resistor R5 provides the proper voltage for firing the SCR when the transistor is conducting. The potentiometer R2 may be adjusted to provide the desired signal detection line at which the transistor conducts. Because the base emitter drive voltage of the transistor varies with temperature in the same manner as a diode, a diode D3 is included in the leg of the bridge circuit with resistor R3. The diode D3 thus compensates for temperature change effects on the base emitter drive voltage.

It is apparent that when the voltage at junction 66 is equal to the voltage at the emitter of the transistor, there will be no collector current. However, when the voltage at junction 66 drops, due to collection of frost on the evaporator as described previously, the voltage at the emitter is higher than that at the base and current will flow in the collector circuit during the time the voltage at the emitter is positive relative to that at the base. This current flow triggers the SCR which switches on the circuit for the timer motor 42 during the half wave voltage occurring when main L1 is positive relative to main L2. The diode D1 establishes the motor circuit during the half wave when the voltage at main L2 is positive relative to main Ll. Thus, the timer motor 42 is energized and drives the cam 40.

The cam 40 has a high portion 67 which causes the cam follower 41 to close contact arm 36 on contact 37 and thereby close the circuit for the compressor motor through the switching mechanism 35. When in this position, the switching mechanism permits normal refrigerating cycles to occur. When the cam follower engages the low portion of the cam 40, the contact arm 36' swings from contact 37 to contact 38. This movement opens the compressor motor circuit and energizes the heater for melting frost on the evaporator. Closure of contact 36 on contact 38 also completes a circuit to the gate of the SCR through a resistor R6. This circuit fires the SCR during each positive half cycle so that the SCR will continue to provide conduction for causing operation of the timer motor after initiation of the defrosting cycle.

The timer motor drives the cam 40 until the high portion engages the cam follower and causes the contact arm 36 to swing from contact 38 back to contact 37. This switch action terminates the heater operation and conditions the compressor motor circuit for normal refrigeration. The circuit for the gate of the SCR through the resistor R6 is broken and the timer motor will then stop.

The duration of the defrost cycle can be determined by the speed of the cam and the cam contour to suit the requirement of particular refrigerators.

It will be noted that the timer motor 42 will operate for several minutes before a defrost cycle is initiated. Thus, spurious signals in the amplifier circuit may occur without necessarily initiating a defrost cycle.

tegral with the wall 71 and depends from the bottom wall in alignment with and opening into the amplifier inlet 54. The sleeve 77 provides a convenient method of connecting a tube with the interior of the shroud 16, described previously. A nipple 80 is formed integral with the side wall 72 and provides convenient means to connect the passage 64 with the downstream refrigerated air passage 23. F

The member 53 is closed by a cover plate 81 which rests on the top edges of the side walls 7275 and the passage walls 76. The cover 81 is formed of a suitable dielectric material and is secured in place by nuts 82 threaded on posts 83 imbedded in the wall 71. The amplifier circuit 45 is carried on the cover 81 with the thermistors, resistors and transistor depending into the member 52. The leads for the various circuit com-- ponents extend upwardly through openings through the cover plate and are electrically connected to thin copper conductor plates. The conductor plates lie on the top surface of the cover plate and provide the proper circuit connections. More specifically, a conductor plate 84 has a terminal end 85 arranged to be conveniently connected with the line L1 through a wiring harness or the like, not shown in detail. The diode D2 is series connected with the dropping resistor R4 by a conductor plate 86. A conductor plate 87 forms the circuit connections between resistor R4, thermistor 46 and resistor R1. Resistor R1 is connected with potentiometer R2 by a conductor plate 90, and a conductor plate 91 connects one terminal of the potentiometer with diode D3. D3 is connected with R3 by a plate 92, R3 is connected to a plate 94 which has a terminal end 95 by which connection is made with line L2 through the wiring harness referred to previously. The plate 94 also connects R5 with L1. The sliding contact of the potentiometer R2 is connected with the emitter of Q1 by a plate 96. The collector of O1 is connected with the gate of the SCR through a conductor plate 97 having an end terminal 100 adjacent the

terminals

85,95 and which terminal may be connected with the gate terminal of the SCR by the wiring harness mentioned. The base of O1 is connected with the thermistors 46,47 by a conductor plate 101.

ltwill be appreciated that heat is generated by the passage of current through resistor R4. This heat warms the walls forming the fluid amplifier passages, and particularly the jet nozzles 60,61. Heat of the resistance R4 is conducted to the

conductor plates

86 and 87 which plates serve to conduct heat to the walls of the areas of the amplifier particularly critical to frost formation. The heated walls prevent the collection of moisture and possible frost in the fluid amplifier, thereby forestalling blockage of the control jets by frost and thereby assuring proper operation of the device.

I claim:

1. ln a refrigerating system comprised of an electric motor driven compressor and an evaporator, a blower for circulating airover the evaporator, electrically controlled means operative to heat said evaporator to melt frost therefrom, switching mechanism operative to initiate and terminate operation of said evaporator heating means, an electric timer motor for actuating said switching mechanism to initiate and terminate heating of said evaporator, characterized by said timer motor comprising a hysteresis synchronous motor, circuit means to energize said timer motor including an AC power source, a diode and a gate controlled silicon rectifier in parallel circuit and series connected with said timer motor in said circuit means whereby said diode and rectifier are capable of conducting alternate waves respectively of the alternating wave voltage of said AC current, and means responsive to the collection of frost on said evaporator for applying a firing voltage to the gate of said silicon controlled rectifier for rendering said silicon rectifier conductive of substantially onehalf wave of said AC current.

2. A refrigerating system as defined in claim 1 further characterized by means responsive to operation of said switching mechanism for applying a firing voltage to the gate of said silicon rectifier when said switching mechanism is operated to cause heating of said evaporator.

3. In a refrigerating system comprising an electric motor driven compressor and an evaporator, an air circulating structure for withdrawing air from a refrigerator compartment, forcing said air through the evaporator and returning to said compartment; means operative to heat said evaporator to melt frost therefrom;

- control means for said evaporator heating means including, a fluid amplifier device having a hollow body member, walls in said body member forming alternate air passages therethrough and control air passages intersecting at least one of the first mentioned air passages, means for connecting one end of said one air passage with the air stream of said circulating structure in an area immediately upstream of said evaporator and connecting the other end of said air passages with an area downstream of said evaporator in said air circulating structure, an electronic control circuit for said means to heat said evaporator and including a pair of resistors disposed in the respective alternate air passages; and means to supply electric current to said control circuit comprising a current supply circuit of high voltage relative to the operating voltage of said control circuit and a voltage dropping resistor between said supply circuit and said control circuit, said dropping resistor being disposed in said hollow body and arranged to heat said walls comprising said control air passages.

4. In a refrigerating system as set forth in claim 3 further characterized by said current supply circuit comprising an AC power circuit for said electric motor.

Claims

1. In a refrigerating system comprised of an electric motor driven compressor and an evaporator, a blower for circulating air over the evaporator, electrically controlled means operative to heat said evaporator to melt frost therefrom, switching mechanism operative to initiate and terminate operation of said evaporator heating means, an electric timer motor for actuating said switching mechanism to initiate and terminate heating of said evaporator, characterized by said timer motor comprising a hysteresis synchronous motor, circuit means to energize said timer motor including an AC power source, a diode and a gate controlled silicon rectifier in parallel circuit and series connected with said timer motor in said circuit means whereby said diode and rectifier are capable of conducting alternate waves respectively of the alternating wave voltage of said AC current, and means responsive to the collection of frost on said evaporator for applying a firing voltage to the gate of said silicon controlled rectifier for rendering said silicon rectifier conductive of substantially one-half wave of said AC current.

3. In a refrigerating system comprising an electric motor driven compressor and an evaporator, an air circulating structure for withdrawing air from a refrigerator compartment, forcing said air through the evaporator and returning to said compartment; means operative to heat said evaporator to melt frost therefrom; control means for said evaporator heating means including, a fluid amplifier device having a hollow body member, walls in said body member forming alternate air passages therethrough and control air passages intersecting at least one of the first mentioned air passages, means for connecting one end of said one air passage with the air stream of said circulating structure in an area immediately upstream of said evaporator and connecting the other end of said air passages with an area downstream of said evaporator in said air circulating structure, an electronic control circuit for said means to heat said evaporator and including a pair of resistors disposed in the respective alternate air passages; and means to supply electric current to said control circuit comprising a current supply circuit of high voltAge relative to the operating voltage of said control circuit and a voltage dropping resistor between said supply circuit and said control circuit, said dropping resistor being disposed in said hollow body and arranged to heat said walls comprising said control air passages.