US2166602A

US2166602A - Refrigeration control system

Info

Publication number: US2166602A
Application number: US196445A
Authority: US
Inventors: William L Mcgrath
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1938-03-17
Filing date: 1938-03-17
Publication date: 1939-07-18
Anticipated expiration: 1956-07-18

Description

July 18, 939- w. L. MCGRATH REFRIGERATION CONTROL SYSTEM Filed March l'7, 1958 lNvENToR WilllumLQMGnml'h ATTORNEY Patented July 1s, 1939 RFRIGEBATION CONTROL SYSTEM William L. McGrath, St. Paul, Minn., assignor to Minneapolis-Honeywell Regulator Company',

Minneapolis, Minn., a corporation of Dcla- Appucaudn Myth 17, 193s,4 serial No, 196,445

15 claims.

This invention relates to control systems for a refrierating apparatus and more particularly to control systems for -accomplishing periodic de- !roting of the refrigerating apparatus.

' 'Pheprime object of this invention is to provide a control-system for a refrigerating appa-A 'ratus wherein the refrigerating apparatus is started and stopped to maintain desired conditions, wherein means are provided for preventing 1o stirring or the refrigemung apparatus 'until de' frosting of the evaporator hasoccurred, and

Figure 2 is a diagrammatic illustration of an] other form of this invention.

- Referring now'to Figure 1 a refrigerating appaiatus is'generally designated at 9 for maintaining vdesired conditions such as desired temperature conditions in a space generally designated atfI-l. The space I3 may be a walk-in box, a

storage compartment, a room, or any other device in Iwhich it is desired to maintain predetermined conditionsl such as temperature. humidity, etc. An evaporator II is located in the space I for g5 cooling the same and `expanded refrigerant is withdrawn from the evaporator II through a s'uctionor low pressure pipe I2 4by a compressor i3 operated by a motor I4. The compressor I3 compresses the expanded refrigerant and deo livers this compressed refrigerant in'to a condenser I5 wherein it is liquefied. The liquidre- -frigerant iiows from the condenser into a receiver I6 and then through a liquid line i1 into the evaporator Ii under the control of an expansion valve l3.- For purposes of illustration the expansion valve I9 is shown to be a thermostatic expansion valve having a capillary tube I9 and a blb 23 containing a volatile fluid located adjacent the suction line I2 at the discharge side of the evaporator II.

`The operation of the compressor I3, that is, the starting and stopping of the compressor is controlled -by a starter lor relay generally designated at 22 and this relay may comprise a relay coil 23 foi-:operating switch arms 24, 25. and 26 with'respectv to

contacts

21, 28, and 29, respectively. Upon energization of the relay coil 23 the switch arms 24 and 25 are moved into engagement with the -contacts -21 and 28 and the switch arm 26 momentarily engages the contact 29. Upon deenergization of the relay coil 23, the

switch arms

24, 25, and 26 are moved to the right by means of springs gravity or other means, not shown, to cause switch arms24, 25 to disengage contacts 21 and 23. During right-hand movement of the switch arm 26 thev contact is not engaged since a piece of insulating material 30 prevents this, hence the contact 29 is only momentarily en- 'gaged by the switch arm 26 when the relay coil 23 is energized.

'I'he starter or relay 22 is controlled primarily by a temperature responsive controller generally designated at 32 which responds to changes in the temperature within the space or box I 0. This controller may comprise a bellows-33 containing a volatile iluid forl operating a lever 34 against the action of a spring 35. The lever 34 in turn operates a switch 36 having electrodes 31 and 38. Upon an increase in space or' box temperature the bellows 33 expands to cause the mercury to bridge electrodes 31 and 36 and upon a decrease in temperature the electrodes 31 and 38 are unbridged. By suitably adjusting the tension in the spring 35 the temperature setting of the controller 32 may be varied at will. For purposes of illustration it is assumed that the mercury switch 36 is closed when the temperature in the box rises to 40 and is opened when the temperature decreases to 38. l

'I'he relay or starter is also controlled by a suction pressure controller generally designated at 43 and this controller may comprise a bellows 4I -connected by a pipe 42 to the suction line I2. The bellows 4I operates a lever 43 against the action of a tension spring 44. The lever 43 operl ates a mercury switch 45 having electrodes 46 and 41. Upon an increase in suction pressure the bellows 4I expands to cause the mercury to bridge electrodes 46 and 41 and upon a decrease in suction pressure the mercury is caused to separate from the electrodes 46 and 41. `By suitably adjustlng the tension of the spring 44 the pressure setting of the controller 40 may be varied at will. For purposes of illustration it may be assumed -that mercury switch 45 closes when the suction pressure rises to some predetermined high value such as 40 lbs. which is indicative of defrosting of the evaporator II. The switch may open at some lesser value such as 38 lbs. Hence the only time switch 45 closes is when the suction pressure rises to a high value corresponding to the defrosting temperature `of the evaporator II.

The relay or starter 22 is also controlled by a high lpressure cut-out generally designated at 58 and this cut-out may comprise a bellows 5I connected by a pipe 52 to the high pressure side of the compressor I3. The bellows 5I operates a lever 53 against the action of a tension spring 54. Thelever 53 operates a mercury switch 55 having electrodes 56 and 51. By suitably adjust*- ing the tension of the spring 54 the pressure 'setting of the high pressure cut-out 56 may be adjusted at will. For purposes of.illustratlon it is assumed that the switch 55 is opened when the high pressure rises to afpredetermined high value such as 180 lbs. and is closed when the high pressure decreases to a predetermined lower value such as 160 lbs.

'Ihe relay or starter 22 may also be controlled by an overload cut-out generally designated at 68 and this cut-out may` include a resistance heater element 6I located in the compressor circuit for heating the bimetallic element 62. Upon the occurrence of an overload condition in the compressor circuit the bimetallic element 62 is flexed toward the left allow contacts 63 and 64 to separate. Suitab e resetting means, not shown, may be provided for reclosing the contacts 63 and 64. Since such a construction is well known in the. art, a further description is not considered necessary. i

Power is supplied `to the compressor motor I4 and to the control system by means of line wires 66 and 61 leading from some source of power not shown. With the parts thus far described in the positions shown in the drawing the relay or starter 22 is deenergized and the compressor I3 is shut down. Assume now that the suction pressure has risen to 40 lbs. indicating that defrosting of the evaporator II has occurred and that the space or box temperaturehas increased to 40, the

switches

45 and 36 will then be closed to complete a starting circuit from theline wire 66 through contacts 63 and 64 of the overload cut-out 60, electrodes 56 and 51v of the high pressure cut-out 50, electrodes 31 and 38 of the temperature responsive controller 32, electrodes 46 and," of the suction pressure controller 48, and relay coil 23 back to the line wire 61. Completion of this starting circuit energizes the relay coil 23 to move the switch arms 24 and 25 into engagement with the contacts 21, 28, respectively. Movement of the switch arm 25 into engagement with the contact 28 completes a load circuit from the line wire 66 through contact 28, switch arm 25, compressor motor I4 and heater element 6I back to the other line wire 61. Completion of this circuit causes operation of the compressor, I3.

Movement of the switch arm 24 into engagement with the contact 21 completes a maintaining circuit for the relay coil 23 which is independent of the suction pressure controller 48 and this maintaining circuit may be traced from the line wire 66 through contacts 63 and 64 of the i -overload cut-out 68, electrodes 56 and .51 of the high pressure cut-out 5I),v electrodes 31 and 38 of the space or box thermostat 32, contact 2-1, switch arm 24, and relay coil 23 back to the line wire 61. Completion of this circuit maintains lthe relay or starter 22 energized until the box temperature decreases to 38. VAccordingly the compressor is placed in operation upon a call for cooling by the boxA thermostat 32 when the suction pressure controller 40 indicates that defrosting of the evaporator II has taken. place and wires 66 and 61.

after vthe compressor has oncebeen placed in operation it is'maintained in operation under the control of the box vthermostat 32 until the box temperature is decreased to 38. 'I'he overload cut-out 60 and the high pressure cut-out 5I are located in both the starting and maintaining circuits for the starter 22 and therefore the compressor I3 is shut down in case of overload conditions in the compressor circuit and high pressure conditions on the high pressure side of the refrigerating apparatus.4

A switch formedby a switch arm and a contact 1I is located in parallel with the electrodes 46 and 41 of the suctionpressure controller 48 and when this switch is closed, as is shown in Figure 1, the suction pressure controller 48 is shunted out or rendered inoperative whereby the relay or starter 22 may be placed in operation in'dependently of the suction pressure so that the compressor I3 may be started even though defrosting of the evaporator II has not occurred. The switch arm 10 is maintained in engagement with Ithe contact 1I for a predetermined number of cycles of operation of compressor I3 and then the switch arm 10 is moved out of engagement with the contact 1I so that the compressor cannot be started on,its next cycle until the. defrosting of the evaporator II has been accomplished.

The opening and closing of the switch formed `by the switch arm 10 and the contact 1I is controlled by the momentary contact arrangement formed by the switch 26 and the contact 28,01' the relay or starter 22 and a plurality of sequentially operated relays generally designated at 14, 15, 16, and 11. The relay 14 may comprise a relay coil 18 for

operating switch arms

18, 80, and

` 8| with respect to

contacts

82, 83, and 84. When the relay coil-18 is energized switch arms 18 and 8| are movedinto engagement with contacts 82 `and 84, respectively, and switch arm 80 is moved out of engagement with contact 83. The switch arms 19 and 80 are so arranged that they overlap in their action, that is, switch arm 19 engages contact 82 before switch arm 80 disengages contact 83. When the relay coil 18 is deenergized the switch arms 19 and 8| are moved out of engagement with their

respective contacts

82 and 84 and the switch arm 80 is moved into engagement with contact 83 by means of springs gravity or other means, not shown." Therelays and 16 are exactly the same as the relay 14 and therefore like reference characters for the parts there--l of have been utilized. However, in addition relay 16 operates switch arm 10 with respect to contact 1I. Relay 11 may comprise a relay 4coil 86 for operating a switchv arm 81 with respect to a contact 88. When the relay coil 86 is energized the switch arm 81 is moved out of engagement with the contact 88 and when the relay coil 86 is deenergized the switch arm 81 is moved into engagement with the contact 88 by'means of springs gravity or other means, not shown. Power is supplied to the relays 14 to 11, inclusive,- by means of a transformer'SS having a secondary 9| and a primary!!! connected across the line Assume now that the relays 14 to 11 arel in the positions shown inl the drawing and that the relay or starter 22 is deenergized. Upon a Ycall for cooling by the box thermostat 32 jthe starter 22 is energized to place the compressor I3 in operation in the manner pointed out above.

Since the switch arm 10 is engaging the contact1 1I this can occur even though the evaporator Il has not defrosted. Energization of the starter or relay 22 causes theswitch arm 26 to momentarily engage the contact 29 which completes a momentary circuit from the secondary 9| through contact 88 and switch arm 81 of relay 11, contact 29, switch arm 26 and contact 88, switch arm 88 and relay coil 18 of the relay 14 back to the secondary 9|. Completion of this circuit energizes the relay coil 18 which moves the switchv arm v19 4into engagement with the contact 82 to complete a maintaining circuit for the relay coil 18 which may be traced from the secondary 9| through contact 88, switch arm 81 and contact 82, switch arm 19 and relay coil 18 of the relay 14 back to the secondary 9|. This maintaining circuit for the relay coil 18 is completed before the momentary pull-in circuit thereof is broken by reason of lthe overlapping contact arrangement. The parts are so arranged that the switch arm 26 of the starter 22 disengages the contact 29 beforethe switch arm 8| of the relay 14 engages the contact 84. It follows thenthat when the starter 22 is energized to operate the compressor the first relay 14 is pulled in and is maintained in. When the call for cooling is satisfied the relay or starter 22 drops out to stop the compressor I3 but the switch arm 26 willnot engage the conf tact 29 by reason of the piece of insulating material 38 carried by the contact 29. Hence no circuits are completed by the dropping out of the relay or starter 22.

Upon the next call for cooling by the box thermostat 82 the relay 22 is again energized to start the compressor I8 and to complete a momentary contact between the switch arm 26 and contact 29 of the relay 22. This momentary contact completes a momentary circuit from the secondary 9| through contact 88 and switch arm 81 of the relay 11, contact 29 and switch arm 26 of the starter`22, switch arm 8| and contact 84 of the relay 14, and contact 88, switch arm 88 and relay coil 18 .of the relay 15 back to the secondary 9|. Completion of this *circuit energizes the relay coil 18 of the second relay '15 to move the switch arms 19 and 8| thereof into engagement with'their

respective contacts

82 and 84. Movement of the switch arm 19 into engagement with the contact 82 completes a maintaining circuit for the relay'coil 18 of the relay in the same manner as pointed out above for relay 14 and therefore the relay 15 is maintained in. Hence when the starter 22 is energized the second time to place the compressor I3 in operation the second relay 15is pulled in and maintainedy in. The .compressor continues to operate until the box temperature is satisfied whereupon the relay 22 is deenergized to stop the compressor I3.

The third time that the box thermostat calls for cooling to pull in the starter 22 the switch arm 26 again momentarily engages the contact 29 to complete a momentary circuit from the secondary 9| through contact 88 and switch arm 81 of relay coil 11, contact 29 and switch arm 2 6 of starter 22, switch arm 8| and contact 84 of relay 14, switch arm 8| and contact 84 o! relay 15 and contact 83, switch arm'88 and re- .lay coil 18 of the relay 16 back to the secondary completes a maintaining circuit for the relay coil 18 of the relay 16 to maintain this relay in. Ac-l cordingly the third time that the compressor I8 is placed in operation the switch arm 18 is caused to disengage the contact 1|. Since this breaks the shunt circuit around the electrodes 46 land 41 ,of the suction pressure controller 48', the relay .or starter 22 then cannot be energized until the suction pressure has risen to 40-lbs. which is indicative of the defrosting'of the evaporator II and until the box thermostat 82 callsfor cooling.

When-these two conditions exist, that is, high suction pressure and high box temperature, the

tact 84 of relay 15, switch arm 8| and contactl 84 of relay 16 and relay coil 86 of relay 1'I- back to the 'secondary 9|. Completion o f this circuit causes .the 'switch arm 81 to Idisengage the contact 88 whereupon all of the circuits through the

relays

14, 15, 16, and 11 are vbroken to drop these relays out. The parts are then returned to the positions shown in Figure 1, and hence requires a complete recycling of operation before defrosting ofv the evaporator II will again occur.

By reason of this arrangement the compressor I8 is placed in operation three times irrespective of the value of the suction pressure and the fourth time that.. the thermostat 32 requires operation of the compressor I8 the compressor cannot be operated until the evaporator coil II has defrosted.A In other words, the compressor I8 will be cycled normally for three times and then will defrost on the fourth time. It is obvious that more relays'may be interposed or less re# lays maybe utilized so that the compressor could operate 'any number of times, say two times or eight times, before deirosting of the evaporator is brought about.

Referring -now to Figure 2 there is provided another, arrangement for obtaining substantially of the invention contemplatesthe use of .a unitary control arrangement generally designated 'at |88 and which may be of the form shown and described in application Serial No. 196,447, filed by Albert L. Judson and Carl G. Kronmiller on March 1'1, 1938. This unitary control arrangement |88 is shown diagrammatically in Figure 2 to comprise a base or housing |8| in which is mounted a bellows |82 connected by a pipe |88 to the high pressure side of the refrigerating w apparatus. The bellows |82 operates a lever |84 pivoted on a fulcrum member |85 against the action of a tension spring |86.- One end of the tensionspring |86 is connected to the lever |84 and the-other end is connectedto a nut |81 screw threadedly mounted von a screw 88. By.

rotating the screw |88'the pressure setting of the bellows |82 may be adjusted at will. ,The

" contact adapted to engage a contact post ||1.

Upon an increase in high pressure the abutment ||4 ilrst engages the contact member ||5 to separate its associated contacts and then the abutment surface I engages the contact member I to separate its associated contacts. For purposes "ofillustration it is assumed that the contacts between contact member and the contact post ||1 make and break at 80 lbs.,and that the contacts between the contact member I and the c terminal contact ||3 make and break at 180 lbs. Y Also mounted in the casing |0| is a bellows |28 connected by a pipe |2| to the suction or pressure side of 'the refrigerating apparatus. The bellows operates a lever |22 pivoted on a fulcrum member |23 against the action of a tension spring |24. One end of the tension spring |24 is connected to the lever |22 and theother end is connected to a nut |25 screw threadedly mounted' on a screw |28. By suitably rotating the screw |28 the pressure setting of the bellows |20 may be adjusted. The leyer |22 carries by means of an insulating pad |21 contacts |28 and 3| which are electrically coupled together. Contact |28 is adapted to engage a relatively stationary contact carried by a contact member |29 suitably anchored on a terminal |30. Contact |3| is adapted to engage a contact carried by a contact member |32 also suitably mounted on the` terminal ||8. Cam's |33 and |34 are utilized for independently adjusting the positions of the contact members |29 and |30 with respect to the contacts |28 and 3|, respectively. For purposes of illustration it is assumed that the cams |33 and |34 are so positioned the contact |28 engages and disengages the contact of the contact member A.|29 at substantially 15 lbs; and that the contact |3| engages and disengages the contact of the contact member |32 at substantially 40 lbs. As pointed out above a 40 lb. suction pressure occurs onlyafter the evaporator has defrosted and accordingly contact |3| will not engage the contact member 32 until defrosting of the evapo-` rator has been accomplished.

shown.

'-Ehejunitary control arrangement |00 also includesf anoverload cut-out generally designated at |485which may comprise a casing |41 supporting aheater |48 which operates a bi'.netallic latch reengaged by suitable manipulation of the lever 'I'he unitary control arrangement is also provided `with a control terminal |53 which is electrically connected to the contact |43 and a power terminal |54. Since the above construction oi' the unitary control arrangement is fully shown in the above referred to Judson and Kronmiller application a further description. of the construction thereof is not considered necessary.

Power is supplied to the compressor motor |4 and to the unitary control arrangement |88 by means of line wires and |8| extending from a source of power, not shown, to terminal |48 of the overload cut-out |48 and load terminal |84, respectively. Wires |82 and |83 lead from the load terminal |54 and the contact terminal |4| to the compressor motor |4. Assume that the parts thus far described are in the position shown in Figure 2, the relay |31 is deenergized and hence the compressor motor |4 is not operating. sult the suction pressure increases and the box temperature increases. When the suction pressure increases to a defrosting value of say 40 lbs., contact |3| engages contact member |312 and contact |28 engages contact member |29 and when the space temperature rises to 40 electrodes 31 and 3 8 are bridged to complete a starting circuit from the line wire |80 through contact terminal 3, contact member terminal -||2, electrodes 31 and 38 of the box temperature responsive controller 32, terminal |30, contact member |29, contacts |28 and |3|, contact member |32I terminal 8, contact member H5, contact post |1, contact post |44, contacts |88 and 15| of the overload cut-out |48, relay coil |38 and terminal 54 back to the other line wirev III.

Movement of the bridge member |42-into ena. gagement with the contacts |43 and |44 complete's a maintaining circuit for the relay coil I 88 independently of the contact members ||8 and 32 which may be traced from the line wire |88 through contact posts H3, contact member terminal ||2, electrodes 31 and 38 of the box temperature responsive controller |32, terminal |30, contact member |29, contact |28, contact post |43, bridge member |42, contact post |44, contacts |50 and |5| vof the overload cut-out |48, relay coil |38 and terminal |54 back to the other line wire |8I. Completion of this circuit main- -tains the relay coil 38 energized until thebox thermostat 32 is satisfied or until the suction pressure is pumped down to 15 lbs. or until the high pressure rises to lbs. Likewise an overvalue of 80 lbs.

. 2,166,602 'load condition 'win cause shutting down of the compressor.

Summing upbriefly the relay |31 is energized to cause operation of the compressor only when the box temperature rises to 40, the suction pressure rises to adefrosting value of 46 lbs. and the high pressure decreases to a predetermined low After these conditions have been established and the compressor is placed in operationit is maintained in operation even though the suction pressure is decreased and the high pressure is increased until the space or box temperature decreases to 38 or unless the suction pressure should decrease to a predetermined low value of 15 lbs. or the high pressure should increase to a predetermined high value of 180 lbs. By reason of this arrangement the, compressor cannot be placed in operation until defrosting has occurred `and until the high pressure decreases to a. predetermined low value which providesa minimum starting torque for the com pressor motor.

clockwise rotation of the gear |14. Hence every time that the compressor motor I4 is operated the slider |16 is inched along in a clockwise direction. The conducting track |11 is connected to the terminal |53 of the unitary control arrangement |66 and the slider |16 is connected to the terminal |6 which supports the contact members ||5 and |32.

When the slider |16 is engaging the insulating button 16 as shown in Figure'2 or is engaging the insulating button |19 the mechanism |64 has no effect upon the operation of the unitary control arrangement |66 and when the parts are in this position the unitary control arrangement |66 acts in conjunction with the box thermostat 32 to give a defrosting control in the manner However, when the slider |16 engages the conducting tract |11 contact 3| and contact member |32 are shunted out of the starting circuit for ,the relay |31 whereupon the 'compressor can be placed in operation even though the suction pressure has not risen to the defrosting value of 40 lbs. This auxiliary starting chant for the relay |11 which shunts ont contact |31 and contact member |32 to start the compressor motor |4 may be traced from the line wire |66 through contact terminal III, contact member terminal ||2, electrodes 31 and 36, terminal |36, contact member |29, contact |26, contact postl |43, terminal |53, conducting track |11, slider |16, terminal H6, contact Y' member II5, contact post contact post |44, contacts |56 and |5| of the overload cut-out |46. relay coil |31, and terminal |54 back .to the other line wire |6|.v Completion of this circuit energizes the-relay |31 to cause operationlof the compressor motor |4 an'd to complete-the above referred to maintaining circuit for the relaycoilv |38. A

Accordinglywhen the slider |16 is engaging the conducting track |11 the low pressure cut-in switch formed bythe contact |3| and the contact member |32 is shunted out or rendered inoperative to allow the compressor -motor |4 to be placed in operation even though the suction pressure ha not risen to 40 lbs. Since the slider |16 is `inche in a clockwise direction every time the compressor motor 4 is placed in 'operation the slider |16 will eventually engage one of the insulating-buttons |18 or |19 at which time this shunt circuit is broken whereupon defrosting oi' the evaporator must be brought about before the compressor motor I4 may again be started. The mechanism |64 may be adjusted either by changing the gear ratio between the gears |12 and |14 or by changing the number oi' ratchet teeth picked up by the pawl |16 to vary the number of cycles of operation of the compressor that may be obtained before the evaporator is caused to be defrosted. d

From the above it is seen that I have provided a control arrangement for a refrigerating apparatus wherein the refrigerating apparatus is started and stopped to maintain desired condi# tions such as 'temperature conditions in a box or space, wherein means responsive to a condition which is a measure of defrosting of the levaporator are provided for preventing starting of the reirigerating apparatus until defrosting of the evaporator. has occurred and wherein means are provided for rendering this last mentioned means operative and inoperative fordefrosting the evaporator following `a, predetermined number oi cycles of operation of the refrigerating apparatus.

Although for purposes of illustration two forms of this invention have been disclosed other forms thereof may become apparent to those skilled in the art upon reference to this. disclosure and therefore this invention is to be limited only by the scope of the appended claims and prior art.-

I claim as my invention: l

1. In amechanical' refrigerating system havi ing a compressor and an evaporator for maintaining desired conditions, the combinationof.

means responsive to changes in the condition by the operation of the compressor to renderV said second lresponsive means inoperative for a predetermined number of compressor' operations and then rendering said second responsive means operative whereby the evaporator is after the compressor has operated a mined number of times.

2. In a mechanical refrigerating system having a compressor and anl evaporator for maintaining desired conditions, the combination of, means responsive to changes in the condition being maintained to start and stop the compressor for maintaining desired conditions, means responsive to suction pressure for preventing the first responsive means from starting the compressor until the suction pressure has risensufiiciently to defrost the evaporator, and means controlled by the operation of the compressor to render said second responsive means inoperative for a predetermined number of compressor operations and then rendering said second responsive means operative whereby the evaporator is defrosted after the compressor has operated a predetermined number of times.

3. In a mechanical refrigerating system having a compressor and an evaporator for maintaining desired conditions, the combination of, means responsive to changes in the condition being maintained to' start and stop the compressor for maintaining desired conditions, means responsive to a condition which is a measure of defrosting of the evaporator to prevent the iirst responsive means from starting the compressor until defrosting of the evaporator has been established, means to render normally said second responsive means inoperative whereby the compressor is controlled by the iirst responsive means without defrosting of the evaporator, and means operative as an incident'to a predetermined number of cycles of operation of the compressor for rendering said last mentioned means inoperative whereby the evaporator is defrosted.

4. In a mechanical refrigerating system having a compressor and an evaporator" for maintaining desired conditions, the combination of, means responsive to changes in the condition being maintained to start and stop the compressor for Amaintaining desired conditions. means responsive to suction pressure for preventing the first responsive means from starting the compressor until the suction pressure has risen sufciently to defrost the evaporator, means to render normally said second responsive means inoperative` whereby the compressor is controlled by the rst responsive means without defrost- 'ing of the evaporator, and means operative as an incident to a predetermined number of cycles of operation of Athe compressor for rendering said last'mentioned meansinoperative whereby the evoporator is defrcted'. Y

5. In a mechanical refrigerating system having an electrically operated compressor and an evaporator for maintaining desired conditions, the combination of, a .starter for the compressor including an operating coil, a lad switch for the compressor and a maintaining switch, nrst switching means, means responsive to variations in the condition being'maintained to close the grst switching means when operation of thev Y; compressor is required to maintain the desired condition, second switching means, means responsive to a condition-which is a measure of defrosting of the evaporator for closing the second switching means when the evaporator is defroste'd, a starting circuit for the starter including the `ilrst and second switching means and the operating'c'oil for `operating-the compressor when' both switching means are closed, a maintaining circuit for the starter including the rst switching means, the maintaining switch and the predeteroperating coil for maintaining the compressor in operation after it has started, third switchingV means, an auxiliary starting circuit for the starter including the first and third switching means and the operating coil for starting the compressor when both switching means are closed, and

means operated as an incident to operation of the compressor for maintaining the third switching means closed for-a predetermined number of cycles of the compressor and then openingv the Same.

6. In a mechanical refrigerating system having an electrically operated compressor and an evaporator for maintaining desired conditions, the combination of, a starter tor the compressor including an operating coil, a load switch for the compressor and a maintaining switch, ilrst switching means, means responsive to variations in the condition being maintained to close ythe first switching means when operation of thecompressor is required to maintain the desired condition, second switching means, means responsive to a condition which is a measure of defrosting of the evaporator for closing the second switching means when the evaporator is defrosted, a starting circuit for the starter including the rst and second switching means and the operating coil for operating the compressor when both switching means are closed, a maintaining circuit for the starter including the iirst switch- 1 ing means, the maintaining switch and the operating coil for maintaining the compressor in operation after it has started, third switching means, an auxiliary starting circuit for the starter including the rst and third switching means and theoperating coil for starting the compressor when both switching means are closed, means i'or normally maintaining the thir'd switching means closed, and means for opening the third switching means every time that the compressor has operated a predetermined number of times. Y

7. In a mechanical refrigerating system having an electrically operated'compressor and an evaporator for maintaining desired conditions, the

combination of, a starter for the compressor including an operating coil, a load switch for the compressor and a maintaining switch, rst switching means, means responsive to variations in the condition being maintained to close the rst switching means when operation of the compressor is required to maintain the desired condition, second switching means, meansv responsive to suction pressure for closing'the second 'switching means when the suction pressure has risen su'iiiciently to defrost the evaporator,

a starting circuit for the starter including the first and second switching means and the operating coil for operating the compressor when both switching. means' are closed, a maintaining circuit for the starter including the rst switching means, the maintaining switch and -t-he operating coil for maintaining the compressor in operation after it has started, third switching means, an auxiliary starting circuit for the 'starter including the rst and third switching means and the operating coil for starting the compressor when both switching means are closed, and means operated as an incident to operation of the compressor for maintaining the third switching means closedfor apredetermined number of cycles of the compressor and then opening the same.

8. In a mechanical ref rigerating system having' an electrically operated compressor and an evaporator for maintaining desired conditions, the combination of, first switching means, means responsive to variationslin the condition being maintained to close the first switching means when operation of the compressor is required to maintain the desired condition, second switching means, means responsive to a condition which is a measure of defrosting of the evaporator for closing the second switching means when the evaporator is defrosted, a control circuit for the compressor including both of said switching means to operate the compressor when both switches are closed whereby defrosting of the evaporator is required before the compressor is operated by this circuit, third switching means, a second control circuit for the compressor including the first and third switching means to operate the compressor when both switches are closed whereby the compressor is operated by this cir- -cuit without defrosting, and means operated as an incident to operation of the compressor for opening and closing the third switching means for defrosting the evaporator at intervals.

9. In a mechanical refrigerating system having an electrically operated compressor and an evaporator for maintaining desired conditions, the combinationof, first switching means, means responsive-to variations in 'the condition being maintained to close the first switching means when operation of the compressor is required to maintain the desired condition, second switching means., means responsive to variations in suction pressure for closing the second switching means when the suction pressure has risen sufficiently to cause defrosting of the evaporator, a"control circuit for the compressor including both of said switching means to operate the compressor when both switches are closed whereby. defrosting of the evaporator is required before the compressor is operated by this circuit, third switching means, a second control circuit for the compressor including the iirst and third switching means to operate the compressor when both switches are closed whereby the compressor is operated by this circuit without defrosting, and meansoperated as an incident to operation of the compressor for opening and closing the third switching means for defrosting the evaporator at intervals.

10. In a mechanical refrigerating system hav- 'ing an electrically operated compressor 'and an evaporator for maintaining desired conditions, the combination of, first switching means, means responsive tov variations in the condition being maintained to close the first switching means when operation ,of the compressor isrequired to y maintain the desired condition, second switching means, means responsive to a condition which is a measure of defrosting of the evaporator for closing the second switching means when the evaporator is defrosted, a control circuit for the compressor including both of said switching means to operate the compressor when both switches are closed whereby defrosting of the evaporator is required before the compressor is operated by this circuit, third switching means, a second control circuit for the compressor including the flrst and switching means to operate the compressor when both switches are closed whereby the Vcom- -pressoris operated by this `circuit without defrosting. means operated as an incident to operation.

of the compressorifor opening and closing thev third switching'means for defrosting the evaporator at intervals, said last named means including, a switch closed momentarily uponv starting oi the compressor, a plurality of relays, one of said relays operating said third switching means, and means controlled by said momentarily closed switch for operating said relays in sequence.

11. In a mechanical refrigerating system having an electrically. operated compressor and an evaporator for maintaining desired conditions, the combination of, iirst switching means, means responsivey to variations in the condition being maintained to close the first switching means when operation of the compressor is required to maintain the desired condition, second switching means, means responsive to variations in suction pressure for closing the second switching means when the suction pressure has risen sufficiently to cause defrosting of the evaporator, a control circuit for the compressor including both ofsaid switching means to operate the compressor when both switches are closed whereby defrostingof-the without defrosting, means operated as an incident to operation of the compressor for opening and closing the third switching means for defrosting' the evaporator at intervals, said last named means including, a switch closed momentarily upon starting of the compressor, a plurality of relays, one of said relays operating said third switching means, and means controlled by said momentarily closed switch for operating said relays in sequence.

l2. In a mechanical refrigerating system having an electrically operated compressor and an evaporator for maintaining desired conditions, the combination of, first switching means, means responsive to .variations ln the condition being maintained to close the first switching means when operation of the compressor is required to maintain the desired condition, second switching means, means responsive to a condition which is a measure of defrosting of the evaporatorfor closing the second switching means when the evaporator is defrosted, a control circuit for the compressor including both of said switching means to operate the compressor when both switches are closed whereby defrosting of the evaporator is required before the compressor is operated by this circuit, third switching means, a second control circuit for the compressor including the rst and third switching means to operate the compressor when both switches are closed whereby the compressor is operated by this circuit without defrosting, means operated as an incident to operation of the compressor for opening and closing the third switching means for defrosting the evaporator at intervals, said last named means including, an electromagnet energized when the compressor is operated and a pawl and ratchet arrangement operated by said electromagnet for operating said third switching means.

13. In a mechanical refrigerating system having Aan electrically operated compressor andan evaporator for maintaining desired conditions, the combination of, first switching means, means responsive to variations in the condition being maintained to close the first switching -'means when operation of the compressor is required to maintain the desired condition, second switching means, means responsive to variations in suction pressure for closing the second switching means when the suction pressure has risen suiiiciently to cause defrosting of the evaporator, a control cirv cuit for thescompresson-lnciudingboth of said switching means to'operate the compressor when both switches are closed whereby'defrosting of the evaporator isl required before the compressor is operated by this circuit, third switching means, a second control circuit for the compressor including the first and third switching means to operate the -compressor -whenboth' switches are ,closed whereby the compressor is operated by circuit without defrosting, means operated as'an incident to operation of the compressor for opening and closing the third switching means for defrosting the evaporator at intervals, said last named `means including, an electromagnet energized when the compressor is pperated and a 4pa'wl and ratchet arrangement operated-"by said lectro? magnet for operating said thirdy switching means.

14. In combination, a plurality of relays having operating coils, maintaining switches; andl ,transfer switches, a momentary contact switch, means forvcompletin-'g a starting circuit for theoperating coil vof a firstrelay to pull in the same including the momentary contact switch, means for com pleting a maintaining circuit for the operating coil of the first relay to maintain the same pulled in including the maintaining switch thereof, means for completing a vstarting circuit forthe operating coil of another relay to pull in' the saine awcmj including themomeutary-contact switch and the transfer switch of the first relay, and means for completing a taining circuit for the operating cou or. th otherlelay't'o maintain the same pulled 'in `the maintaining switch thereof.

. l5. In combination, a plurality of relays having operating coils, maintaining switches and transfer switches, a momentary contact switch, means for completing a starting circuit for the operating coil 1 ofa'flrst relay to'puli in the same including the momentary contact switch, means for completing I out all of the relays when pulled in including an operatingcoil, and means for ycompleting a circuit to the operating coil of said last mentioned relay,

including the momentary contact switch and the transfer switches of said plurality of relays.

L. MCGRATH.