CA1135368A - Control for refrigerator compressors - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A device for controlling a single variable capacity compressor, or multiple variable capacity compressors in either parallel flow or series flow chilling systems, senses temperature and load conditions and uses feedback in conjuc-tion with synchronously and asynchronously timed logic circuitry to automatically control loading, unloading and sequencing of the compressor or compressors in the system in selected increments, thereby yielding a significant reduction in energy consumption as well as increased precision in maintaining a desired chill temperature. Additional circuitry provides for remote manual control as well as safety shut-off and failure indication. Alternative embodiments using solid state logic, including a microprocessor embodiment, are also disclosed.

Description

11353~
CONTROL FOR REFRIGER~TOR CoMPRESSVRS

B~CKGROUND OF THE INVENTION
Refrigeration systems, as for air conditioning in large buildings, typically employ multiple compressors in either parallel or series flow for chilling of a controlled medium such as process water, cold plenum, chill water or return water. In many such systems the chilling capacity of the compressor may be varied. Some systems do not require the chilling capacity of a multiple compressor system, and use only a single, variable capacity compressor. Each such compressor, for example the York , Model HT 250 with electrical control, typically has in-ternal controls and safety devices which provide for failure shutdown and overload protections. However, such internal con-trols are effective only in controlling the individual compressor and thus have limited value in a multiple compressor system.
Further, the internal controls do not react automatically to changes in the demand for chilling capacity, and are thus of limited value in even single compressor systems.
Because of the substantial power demands of such refrigeration systems, especially in view of the rising cost of energy, and the inconvenience resulting from system failures, it is important not only that a compressor system be kept operat-ing, but that the system operate efficiently. Toward this end, various forms of control devices have been employed to turn on or turn off and increase or decrease chilling capacities of the individual variable capacity compressors within the system. For screw-type compressors, capacity is varied by the relative posi-tion of the coolant inlet nozzle. For centrifugal compressors, capacity is varied by varying vane position. However, capacity need not in all cases vary directly with vane position. Never-theless, for purposes of simplicity capacity and vane position are used herein as being substantially interchangeable.
Certain of these prior art control devices have --1-- ~ ;
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suffered from overloading problems on initial startup, raising the possiblity of nuisance failures - i.e., a shutdown of the system when actually no system failure exists. For example, on initial startup many control devices permit the system compres-sors to increase to maximum capacity over a short time period, thereby causing such nuisance failures as low refrigerant tempera-ture shutdown, or overshoot on water temperature control, when actually no system failure exists. Such a rapid increase in capacity also results in a large short term increase in power consumption. A similar problem occurs when a first compressor is at maximum capacity and demand requires the additional chill-ing capacity of another system compressor, resulting in increased power consumption for an extended period of time on startup of the second compressor. Other prior art control devices have at-tempted to avoid these sorts of nuisance failures by a circuit commonly referred to as a "load limited", which essentially senses the load current of the compressor and compares that cur-rent to a known maximum value. The control device then allows only a percentage increase in load current during a given period.
The difficulty with the load limiter type of control device is especially apparent in variable capacity centrifugal compressors, in either multiple or single compressors systems.
In centrifugal compressors load current increases are not neces-sarily indicative of variations in compressor capacity. For example, when a small amount of material of a viscosity different from the state of the normal refrigerant is accidentally drawn into the compressor, load current will increase without varying the capacity. This can cause a current "spike" in the load limiter circuit, which in turn causes the load limiter control to improperly reduce capacity.
Another problem found in conventional control devices has been a difficulty in maintaining a balance in capacities of the system compressors. Most multiple compressor systems employ compressors of equal capacity. To maximize the operating efficien-cy of such a system, thereby minimizing power demancl, i-t is desir-able to equalize, or balance, the capacity of each machine at any given moment during operation. For systems employing compress-ors of different capacities, a weighted balance is desirable.
This balancing requires two steps: first, that the capacities of the operating compressors be initially balanced;
and, second, that increases or decreases in capacity be distributed between the operating compressors so as to subs-tantially maintain a balance in chilling capacities. The distribution of increases or decreases may be accomplished either ~y signalling both compressors to load or unload a given amount, or by alternating the loading (or unloading) of the compressors. Typically, the latter option is chosen to minimize energy dernand: however, accurate sensing of cornpressor capacity, coupled with an efficient alternating control has proven difficult.
In single compressor systems, accurate control has also proven difficult. Most prior art control systems can provide only a few incremental changes in chilling capacity, such that gross changes in demand are required before the control device will respond with a variation in capacity.
SUMMARY OF THE INVENTION
In one broad aspect, the invention comprehends a refrigeration system control device for incrementally varying the chilling capacity of at leas-t one varia~le capacity compressor used for controlling the temperature of a medium. The control device is connectahle to temperature sensiny means for measuring the temperature of the medium, the means providing an output signa1 and including means for generating a prescribed cyclic ON and OFF
signal. Timing means provide a signal having a period rnatched to the time required for the capacity of the variable capacity compressor to vary, by a prescribed increment, the perLod being less than the ON time of the cyclic signal. ~ Eirs-t load and 3~

unload means is adapted for connec-tion to said tim:ing means and to the temperature sensing means, thereby being responsive to temperature variation and causes the capacity of the at least one compressor to be varied by the increment defined by the timing means in response to a signal from the temperature sensiny means only during the ON portion of the signal from the generating means.
More particularly in accordance with the i.nventi.~n disclosed, a control device senses the temperature of the control-led medium by means of any -temperature sensing device which prov:id-es a floating con-tro]. signal and, during an appropriate timing cycle, delivers commands to a lead compressor or a lag compressor, typically variable capaci-ty centrifugal or screw compressors, to adjust chilling capacity in accordance with the demand indicated by the temperature of the controlled medium. of course, in a single compressor system, signals will always be delivered to that comp-ressor. Because centrifugal compressors are generally more difficult than screw compressors to accurately control, the present disclosure is directed to centrifugal compressors. However, con-version of the present system for use on screw-type compressors will be apparent tc those skilled in the art.
During a typi.cal cycle :Eor either a single or multiple compressor system, initial application of power is follo~ed by an indication that additional chilling capacity i.s required. In typical prior artl as noted previously, such a demand is met by increasing compressor capacity to some maximum load, t-or example 100~ capacity. This rapid increase results in an unnecessary surge in capacity andr correspondingly, power consumption. In accordance with the present invention, however, a cycle ti.mer permits only an incremental increase in capacity (the magnitude of the increase being manually preset) durin~ a portion of each cycle. The remaind-er of the cycle permits the controlled medium to react to the change in chilling capacity. If, at the beginnillg of the next cycle, additional chilling capacity is required, another incremental increase in compressor capacity is permitted followed by the `` ` 1~3S361~
reaction time mentioned above. The incremental increases contin~iuntil the controlled medium stabilizes at the desired -temperature.
By permitting only incremental increases, power surges are avoided, thus eliminating a large source of nuisance failures and signific-antly reducing power consumption.
For multiple compressor systems, in some situations asingle (lead) compressor will be unable to satisfy demand for increased chilling. In such cases, the present invention automat-ically directs, independently of cycle timing, that the capacity of the lead compressor be decreased to some arbitrary value and that a second, or lag, compressor be started and brought to an arbitrary chilling capacity. Typical]y, the sum of capaclties of both (or multiple) compressors is less than the maximum capacity of a single compressor, due to the heightened efficiency associated with increased surface area. By automatically reduciny the capa-city of the lead compressor prior to starting the lag compressor, increased power consumption is reduced.
Once the lead and lag compressors have reached their respective capacities, cycle timing is permitted to direct the incremental loading or unloading of the system. Thereafter, should a demand for increased chilling be received during the "on" portion of the timer cycle, the incremental increases in capacity will be alternately directed to the lead and lag compressors. Because o~
the new configuration for alternating the signals varying compress-or capacity, the present invention more efficiently and accura-tely maintains an operatillg balance among system compressors.
If chilling capacity exceeds demand, capacity is incre-mentally decreased in a manner similar to that described above.
If demand drops below that necessary to efficiently utilize more than one compressor, the lag compressor is stopped, leaving only the lead compressor operating. The multiple compressor system then operates in a manner substantially similar to a single compressor, with all incremental increase or decrease signals being directed to a single comp~essor. The capacity of the lead B - 5 _ ~

comp~essor is then i~crementally increased or desreased to satisfy demand. If demand decreases sufficiently the lead compres sor is also stopped automatically.
Other aspects of the present invention provide for detection an.1 indication of compressor failures, with automatic load shifting to the remaining compressors, as well as local or remote manual control to override automatic sequencing.
Other and further aspects of the invention disclosed herein will become apparent in the course of the following detail-ed description.
IN THE DRAWINGS
Figure 1 illustrates the control device in a refriger-ation system utilizing two compressors.
Figure 2 diagramatically illustrates in block diagram form the control device of Figure 1, and the interconnections therebetween.
Figures 3a-3d show a schematic diagram of the control device of Figures 1 and 2 using relays.
Figure 3 shows the interrelationship between Figures 3a-3d.
Figures 4a-4e illustrate a logic gate equivalent of the control device of Figures 3a and 3d.
Figure 5 shows a functional block diagram of the system of Figure 2, suitable for implementation with a micro-processor and associated circuitry.
Figures 6a-b show a circuit diagram of a control device for use with a single compressor system.

- ~3S368 DESCRIPTION OF THE PREFERRED EMBODIMENTS
Attention is now directed to Figure 1~ which illustrates a control device 1 interconnected with a first compressor 2 and a second compressor 3. The first compressor

2 is connected to the control device 1 at terminals 5, 7, 9, 13, 15, 17 and 19, which communicate present vane inclination, system on, compressor oil failure, compressor safety failure, compressor start, increase vane inclination and decrease vane inclination signals respectively. The second compressor 3 is connected to the control device 1 at terminals 21, 23, 25, 29, 31, 33 and 35 in an analogous manner; that is, the function communicated at terminal 21 corresponds to the function communicated at terminal 5, and so on such that the functions communicated at terminals 19 and 35 correspond.
Also connected to control device 1 are temperature control switches 37, connected between terminals 36 and 38, and 39, connected between terminals 36 and 40, which indicate a required increase or decrease in chilling respectively. Switches 37 and 39 are typically controlled by a thermostat which may, for example, be placed in the chilled water return line. Further, low temperature shutdown switch 41 is connected to control device 1 at terminals 42 and 44. Switch 41 functions to shut down lead compressor when demand for chilling falls below some predetermined value; however, once chilling demand has been established in the controlled space, the temperature of the controlled medium seldom falls low enough to activate low temperature shutdown switch 41. Also a part of control device 1 for use in controlling the first compressor 2 is four vane indication switches 43, 45, 47 and 49. Normally open vane switch 43, connected to the con-trol device 1 at terminals 58 and 60, is used to indicate some minimum capacity of the compressor 2, so as to permit shut-down in a manner discussed in detail hereinafter when compressor 2 ~' ~135~

is not needed for adequate chilling Vane indication switch 45, connected at terminals 62, 64, and 66, provides both normally open and normally closed contacts and is used in a manner discussed later to indicate a return condition when a lag compressor is being started up. Normally open vane indication switch 47, connected at terminals 68 and 70, is used to indicate a maximum capacity of the compressor 2, so as to permit a second compressor to be called into operation upon increased demand for chilling. Normally closed minimum vane indication switch 49, connected at terminals 11 and 15, is used to position the vanes at some arbitrarily determined minimum capacity, for example twenty per cent, immediately after the compressor starts so as to maximize the efficiency of compressor operation.
Switches 51, 53, 55 and 57 are connected to the control device 1 for use with second compressor 3. Switch 51 is connected at terminals 72 and 74 and is analogous to switch 43; switch 53 is connected at terminals 76, 78 and 80 and is analogous to switch 45. Switch 55 is connected at terminals 82 and 84 and is analogous to switch 47; switch 57 is connected at terminals 27 and 31 and is analogous to switch 49.
For proper operation of the control device 1, it is necessary that vane switches 49 and 57 be set for a lesser vane position than switches 43 and 51; for example, 20% and 25%, respectively, of maximum vane position. Further, vane switches 45 and 53 must be set for a greater vane position than switches 43 and 51, for example 30%. Vane switches 47 and 55 are typically set on the order of 85~ of maximum vane angle, but must be set for a vane position which causes switches 47 and 55 to operate before the overload protection devices internal to the compressor are triggered.
Also connected to control device 1 are flow switches 93, 95, 97 and 99. Flow switches 93 and 95 are safety switches to insure chill water flow through first compressor 2, and are series connected to the control device 1 at terminals 7 and 96.
Flow switches 97 and 99 are analogous but intended for use with second compressor 3 and are connected to the control device 1 at terminals 23 and 100.
Attention is now directed to Figure 2, which illus-trates in block diagram form the internal circuitry of the control device 1 as shown in Figure 1. In Figure 2, a timer 200 operates synchronously and independently of the remaining circuit elements, and communicates a timing signal to an R3 transfer circuit 202, an R4 transfer circuit 204, a first compressor load and unload circuit 206 and a second compressor load and unload circuit 218. During a portion of the timing cycle generated by the timer 200, the first compressor load and unload circuit 206 is permitted to accept a signal from the temperature control switches 37 and 39, shown in Figure 1 through terminals 36, 38 and 40. If an increase in chilling capacity is needed, normally open switch 37 will be closed.
Should neither the first compressor 2 nor the second compressor

3 be in operation -- that is~ should the system require initial start-up as it would, for example, early in the morning -- load and unload circuitry 206 responds to the demand for increased chilling capacity by signaling the first compressor 2 via termi-nal 17 to increase vane inclination. The load circuit 206 at the same time signals a pump start circuit 208 to complete a circuit between terminals 365 and 367 of the control device 1, thereby starting the system pump (not shown). In some systems, more than one pump is used, as one pump per compressor. In such systems the pump may be started from a compressor start signal. When the pump start circuit 208 responds to the load circuit 206, it in turn also signals the lead compressor to start through compressor start circuit 210 or 212. Whether g ~J .

~11353~8 first compressor 2 or second compressor 3 is assigned as the lead compressor is determined automatically by sequence circuit 216 in a manner described hereinafter. For purposes of example, let it be assumed that first compressor 2 is the lead compressor, whereupon first compressor start circuit 210 signals, via terminal 15, the first compressor 2 to begin operation, which will thereafter be permitted to occur in a manner described in greater detail in connection with Figure 3a.
At this point the system pump and a lead compressor have been started, and chilling has begun. Assuming that the initial chilling capacity of the single lead compressor is in-sufficient to satisfy demand, as manifested by normally open switch 37 being closed, the next "on" cycle of the timer 200 will permit the first load and unload circuitry 206 to respond to the demand for increased chilling capacity by signalling, for a predetermined time period, an increase in vane inclination angle of first (lead) compressor 2 through terminal 17. secause the signal to increase capacity lasts for only a predetermined limited time, each "on" cycle of timer 200 permits only an incremental increase, typically five per cent, in chilling capacity of the system. Further, while the "on" portion of the cycle generated by timer 200 is of limited duration, typically twenty seconds, the "off" portion of the cycle is of much greater duration, perhaps as much as ten minutes. sy setting the "off" portion of the cycle of time 200 much longer than the "on" portion, the controlled medium is given an opportunity to react to the increased chilling capacity of the refrigeration system, thereby automatically limiting the capacity of the system to approximately the minimum necessary to satisfy demand.
This in turn minimizes power consumption. In addition, by allowing only incremental increases in capacity and requiring a reaction time between increases, power consumption is reduced ' ~, ' ~135~6~

and problems such as overshoot and low refrigerant temperature shutdown are substantially avoided.
Should the system require additional chilling, the preceding sequence is repeated until the lead compressor, in this example first compressor 2, has reached a maximum capacity, typically 85% of rated capacity. Maximum capacity is indicated by the closing of normally open switch 47, which prevents load circuitry 206 from further increasing the capacity of compressor 2 because an unload signal overrides a load signal in a manner described hereinafter. When the maximum capacity of first compressor 2 is reached, but additional chilling is required, a lag compressor start circuit 214 reacts by initiating loading of the second compressor, in this example second compressor 3, through second load and unload circuit 218. The lag compressor start circuit 214 also signals second compressor start circuit 212 to start second compressor 3. The start of compressor 3 begins as soon as compressor 2 hits the maximum capacity, or 85%; this usually occurs during the "on" cycle of timer 200.
However, the internal controls on the compressor will prevent the machine from actually starting for up to several minutes.
At the same time that the lag compressor start circuit 214 signals second compressor 3 to start, it also signals compressor 2 to unload to a predetermined value, for example 30% of maximum capacity, through first compressor load and unload circuit 206, because the normally open contacts of vane switch 45 complete a circuit through the unload portion of circuitry 206. While it is not necessary that first compressor 2 be unloaded for any circuit problem, maximum efficiency of the circuit requires that power demand be minimized. This is accomplished by unloading the first compressor 2 to a predetermined value and allowing the lag compressor, second compressor 3 in this example, to begin loading to some minimum value via a signal supplied to the ~13S368 second compressor start circuit 212 ~rom the lag compressor start circuit 214. Once loading of second compressor 3 begins, the normally closed contacts of vane switch 53 will require the compressor 3 to continue to load until the predetermined minimum capacity is reached, whereupon the normally closed contacts of switch 53 open. Compressors typically used in refrigeration systems have an internal delay for startup, and therefore first compressor 2 will unload to the predetermined value before second compressor 3 will begin loading to the minimum value. For match-ed compressors, the unload value on the compressor 2 will be ty-pically equal to the load value on the compressor 3 for maximiz-ing the energy savings available with the current invention;
however, such equalization is not required and a weighted balance will be optimal for systems not using matched compressors. At this point, the lag compressor start circuit 214 has responded to a signal from the vane switch 47 and signaled the start circuit 212 to begin operation of the compressor 3. Both compressors 2 and 3 therefore are now both operating, although at substantially less than maximum capacity --e.g., 30% capacity. It should be noted that the sum of the chilling capacities of the compressors is preferably less than the maximum capacity of the single com-pressor since an increase in chilling efficiency is observed with the increase in surface area available for chilling from two compressors.
Should a demand for increased chilling capacity continue to exist, as manifested by normally open switch 37 being closed, during the next "on" cycle of the timer 200 the lead compressor, the first compressor 2 in this example, will load as described hereinbefore. It should be noted that although the compressors 2 and 3 are both operating at this point, a demand for increased chilling capacity does not result in a simultaneous increase in capacity for both compressor 2 and compressor 3. Rather, during the first "on" cycle of the timer 200 in which a demand for C

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increased chilling capacity is indicated, the lead compressor,in this case compressor 2, increases capacity (or loads). In the next subsequent "on" cycle of timer 200 during which a demand for increased chilling capacity exists compressor 3 loads.
This is accomplished in the following manner.
During the first "on" cycle of the timer 200 in which a demand for increased capacity exists after both compressors have become operational at a minimum capacity, an R3 transfer circuit 202 will require that the lead compressor (compressor 2) incrementally increase capacity as previously described. Once the first compressor 2 has incrementally increased in capacity, the R3 transfer circuit 202 will cause the capacity of the second compressor 3 to increase during the next "on" cycle of timer 200 during which the temperature switch 37 indicates a demand for additional chilling. Once the second compressor 3 has loaded, the R3 transfer circuit 202 causes the first com-pressor 2 to load on next demand. In this manner demands for increased capacity are met by alternately increasing the capa-city of each compressor until demand is satisfied. Because only incremental increases (or decreases) in capacity are permitted due to circuit timing, system stability can be maintained with temperature switches sensitive to a one degree variation. Thus the temperature of the controlled medium may be maintained within a one degree range. This is in contrast to "proportional sensing" control devices which are made stable only be permitting wide variations, perhaps as much as ten degrees, in the temper-ature of the controlled medium.
Eventually system capacity will increase sufficiently to satisfy demand, and normally open switch 37 will remain open during the "on" cycle of timer 200. In such event no load or unload signals are generated by circuits 206 and 218, and the i - 13 -1135~6~8 R3 transfer circuit 202 will remain dormant until the next demand for increased capacity.
In the event of a demand for decreased chilling capacity of the compressors 2 and 3, normally open switch 39 will close and normally open switch 37 will remain open, thereby signaling the load and unload circuitry 206 and the R4 transfer circuit 204 during the appropriate "on" cycle of timer 200 that a decreased capacity is necessary. At that point, the unload circuitry 206 signals the compressor vane inclination motor via terminal 17 on control device 1, as shown in Figure 1, to decrease the inclination angle of the vanes on the centrifugal compressor 2. The R4 transfer circuit 204 performs a function analogous to that of R3 transfer circuit 202 in that it causes an alternating sequence in incremental unloading, while R3 circuit 202 caused alternation during loading.
It should be noted that for the particular embodiment of the invention shown in Figure 3, the capacity of the lead compressor will be decreased first; however, it is not necessary that the lead compressor unload first in all circumstances. As will be apparent to those skilled in the art, either the lead or the lag compressor may be unloaded first since unloading is similar to loading in that the compressors alternate to maintain a relatively balanced capacity. Thus, should a continuing demand for decreased capacity manifest by the closing of normally open switch 39, during the next "on" cycle the lag compressor, the second compressor 3 in this example, will unload via a signal applied to the second compressor load and unload circuitry 218.
The compressor load and unload circuitry 218 is virtually identical to the circuitry 206 except that the circuitry 206, as described hereinbefore, controls the first compressor 2 whereas load and unload circuitry 218 controls second compressor 3. Further, and similar to unloading for first compressor 2, a 11353i8 signal from the temperature control switch 39 during the appropriate "on" cycle of the timer 200 signals the load and unload circuitry 218 and the R4 transfer circuit 204 to decrease the capacity of the compressor 3 by decreasing the vane inclina-tion angle of the compressor 3 via terminal 33 of the controldevice 1.
Should a demand for decreased chilling continue to exist, both compressors will alternately back down by incremental amounts as described for loading until one of the compressors 2 or 3 reaches a predetermined minimum capacity, for example 25~, below which it is inefficient to operate both compressors. This minimum capacity is determined by vane switches 43 and 51 for compressors 2 and 3, respectively. At this point the invention requires that the second compressor be brought down incrementally to 25% during the subsequent "on" cycles of timer 200 during which a demand for decreased chilling exists. Should it occur that the first compressor to reduce to 25% is the lead compressor, the system will require that the capacity of the lag compressor be reduced by the previously explained increments until both compressors are at 25~ of maximum capacity, or some other pre-determined minimum value. In the event that an intermittent demand for decreased chilling results in the lead compressor being directed to reduce capacity below the predetermined minimum, such as 25~ herein, load and unload circuitry 206 will internally sense the condition via vane switch 49, whereby the circuit 206 will be prevented from reducing the capacity of the compressor 2 below 20~. Should the lag compressor be signaled to reduce below the minimum capacity, vane switch 57 and circuitry 218 will prevent further reduction in capacity. Thus, only the compressor which is not at minimum capacity may be reduced in capacity, still incrementally, such that eventually both compressors reach the minimum capacity detected by switches 1~3S36~

49 and 51. When both compressors reach the predetermined minimum capacity, lag compressor start circuitry 214 receives a signal from vane switches 43 and 51 and signals the lag compres-sor, in this case compressor 3, to shut down. In the event that the demand for chilling capacity is greater than that which 25%
or the predetermined minimum of the lead compressor 2 can satisfy, loading begins on the first compressor 2 as previously described. It will be noted that only compressor 2 is operating at this point. In the event that the minimum capacity of compressor 2 is more than sufficient to meet demand for chilling the system, the control device 1 transfers control of compressor 2 to the low temperature control 41. In response to diminished demand for chilling, vane switch 41 signals pump start circuit 208 and shuts down both the first compressor 2 and the compres-sor pump via the compressor start circuit 210 and the pumpstart circuit 208.
The foregoing description has assumed normal operation of all system components including the compressors 2 and 3. In some circumstances this may not be the case, and a system failure will occur. In this event, the failure shut off circuitry 220 will recognize from the compressors 2 and 3 or the flow switches 93, 95, 97 and 99 that a compressor failure has occurred or that the control device 1 has failed. The failure circuitry 220 then prevents the operation of one or both compressors, depending upon the type of failure. For example, should a compressor fail to start when ordered to do so by the control device 1, the failure circuitry 220 will signal the remaining compressor to start automatically but will prevent the system from being automatically restarted following the next system shutdown. However, manual override means are provided for operating the lone compressor while the remaining failed compressor is being repaired.

11353t~8 Attention is now directed to Figures 3a, 3b, 3c and 3d which, taken together, illustrate a preferred embodiment of the invention using relays by which it is possible to understand the detailed operation of the control device 1. The basic transfer and flow of the system was described in connection with Figure 2; the following description merely repeats in greater detail that description.
On initial start-up, power is applied to terminals 94 and 98. This causes, as shown in Figure 3a, power to be applied to variable timer relay 301 through normally closed contacts 303 of normally closed bimetal of signal timer 305.
When the timer relay 301 energizes, the normally open contacts 307 thereof close and, after the timing period of the on signal timer 305 is to create an asymmetric squarewave at the timer signal relay 309. For the embodiment herein described, the off cycle at the timer signal relay 309 is typically adjustable up to ten minutes due to adjustable timer relay 301, and the"on"cycle at timer signal relay 309 is typically fixed at approximately 20 seconds. However, these figures are substantially arbitrary and those s}~illed in the art will recognize many possible variations.
During the "on" portion of the cycle at timer signal relay 309, normally open contacts 311 (shown in Figure 3b) of timer signal relay 309 close and permit power to pass between terminal 36 and either, depending upon conditions described hereinafter, terminal 38 or 40. As discussed in connection with Figure 1, the temperature control switch 37 is connected between terminals 36 and 38, and temperature control switch 39 is connected between terminals 36 and 40. As further described in connection with Figure 1, normally open switch 37 is closed and switch 39 is open if a demand for increased chilling is required by the thermostat in the controlled medium, or wherever load conditions may be conveniently sensed ~135;~

to provide a floating control signal. Thus, it can be seen that when switch 37 is closed, power passes between terminals 36 and 38 of ~igure 3b. Let it be assumed that, for purposes of example only, switch 37 is closed. Let it further be assumed that an automatic lead-lag switch 313 is in the lead position and that two sequence relays 315 and 317 are de-energized.
It should be noted that relays 315 and 317 are merely parallel relays and, depending upon the number of contacts on the particular re]ay used, may be a single relay. Let it further be assumed that first safety relay 319 and second safety relay 321 are energized, and that first oll failure relay 323, second oil failure relay 325, first flow switch 331 and second flow switch 333 are all energized, while the first safety indication relay 327 and the second safety indication relay 329 are de-energized.
Given these initial conditions, a demand for increased chilling, indicated by switch 37 being closed, results in the following sequence:
When the time signal relay 309 energizes, an adjust-able solid state control timer 335 (Figure 3b) also energizes, and in turn, closes normally open contacts 337, which energizes a second control timer relay 339. This results in a continuous circuit between terminal 94 and 98 through contacts 311 of the timer signal relay 309, terminal 36, switch 37, terminal 38, normally closed contacts 341 of R3 relay 343, normally closed contacts 345 of second control timer relay 339, normally closed contacts 347 of first unload relay 349 and first load relay 351. It should be noted that second control timer relay 339 does not energize until control timer 335 times out, so con-tacts 345 open to terminate the load cycle. In addition, the signal from terminal 13 through contacts 341 of R3 relay 343 ~L~3~3~

also cause a second bimetal time delay timer 353 to start timing as will be discussed in more detail hereinafter. Control timer 335 is typically an adjustable timer with a time period which may vary up to the time period "on" cycle of timer signal relay 309. Because timer 335 controls relay 339, it can be seen that first load relay 351 will be energized only for as long as timer 335 is adjusted.
When first load relay 351 energizes, normally open contacts 355 thereof, located in the pump start circuit 208 (Figure 3a) close and make a complete circuit through normally closed contacts 357 of a first unload relay 349 and normally closed contact 359 of a second unload relay 361, thereby energiz-ing a pump start relay 361. When the pump start relay 361 energizes, normally open contacts 363 thereof close and make a complete circuit between terminals 365 and 367, which cause the compressor pump (not shown) to start. Further, when the first load relay 351 energizes, normally open contacts 369 (Figure 3c) thereof close, and permit a complete circuit between terminals 5 and 17. A voltage is supplies to terminal 5 whenever the controls internal to the compressor indicate a demand for chilling. By setting the compressor's internal controls to a value less than the anticipated chill water temperature, the compressor controls can be used as a signal source for loading or unloading the compressors through control device 1. In so doing, the control device 1 is operable without overriding any controls internal to the compressor. Terminal 21 is controlled by the second compressor 3 analogously. The closing of this circuit signals the control panel of compressor 2 to increase the vane inclination angle on that compressor to a predetermined minimum value determined by the position of the vane switch 49, typically set at 20% of maximum capacity. When power is first applied to terminals 94 and 98, first program motor relay 370 ~5 - 19 -1, J
. ~..

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and second program motor relay 372 energize, causing (during normal operation) normally open contacts 374 of relay 370 and normally open contacts 376 of relay 372 to close. First and second minimum vane relays 469 and 483 also energize through normally closed vane switches 49 and 57, respectively. This causes a completed circuit between terminals 5 and 17 and 21 and 33 until a minimum vane angle is reached as indicated by the opening of switches 49 and 57 (Figure l), regardless of the con-dition of first load relay 351 or second load relay 413.
The first compressor 2 is started from a signal through terminal 15 in compressor start circuitry 210 (Figure 3d).
Terminal 15 is connected to the control panel of the compressor 2 and is always at a supply voltage sufficient to trigger the com-pressor's internal start circuitry. Thus, a signal applied to terminal 7 completes a circuit to terminal 15 through the normally closed contacts 371 of fifth bimetal delay timer 373, the normally open contacts 375 of energized first safety relay 319, normally closed contacts 377 of de-energized sequence relay 315, normally open contacts 379 of energized pump start relay 361, and normally closed contacts 381 of first sequence relay 315. Thus, a signal is supplied to terminal 15, which starts compressor 2;
however, a signal is also supplied to terminal 383, adjacent to terminal 15, and should a failure exist in the internal controls of the compressor 2, the failure circuit 220 will cause the com-pressor to shut down in the following manner:
As set up in the initial conditions, the relays 323 and 331 are all energized while relay 327 is energized. In the event of a failure, normally open contacts 385 of the first safety indication relay 327 can cause ninth bimetal delay timer 387 and fifth bimetal timer 373 (as a backup) to signal first safety relay 319 which shuts out a signal to terminal 15.

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The failure shutdown circuit 220 will be discussed in greater detail in connection with Figure 3a. In a similar manner, normally closed contacts 389 of first oil failure relay 323 and normally closed contacts 391 of first flow switch relay 331 can cause a start signal to terminal 15 to be interrupted.
Assuming that no failure occurs, the condition of the circuit at this point is that the compressor pump start circuit 208 has activated the pump start circuit via terminals 365 and 367 and the compressor 2 has been started.
Should a demand for increased chilling continue to exist, as through switch 37 being closed, the next "on" cycle of timer signal relay 309 triggers control timer relay 335 in the manner hereinbefore described. Control timer relay 335 then signals second control timer relay 339, which in turn permits first load relay 351 to energize for the time period of the con-trol timer 335, thus permitting incremental loading of the com-pressor 2 as described before. Control timer relay 335 is typically an adjustable solid state timer which permits the increase in capacity to be varied over a predetermined range.
Typically, such an increase might be 5% during each "on" cycle;
for many of the compressors used with such a controller, vane angle increases approximately at 1% per one-tenth second.
However, the time re~uired varies with each type of compressor.
If the "off" cycle of timer 200 is five minutes and the "on"
cycle is 20 seconds, during which the control timer pulse occurs, a 15% increase in capacity is available over roughly a 15 minute span.
Loading of the lead compressor, first compressor 2 in this example, will continue as described above with an incremental increase occuring during each "on" cycle of the timer 200. Should ! ~ 21 ,~

1~353~3 the demand for increased chilling capacity continue to exist for a long enough period of time, compressor 2 will load to a maximum capacity beyond which operation of the compressor becomes inefficient relative to energy consumption. The capacity at which a compressor becomes inefficient is normally determined by the design of the compressor itself; typically, a maximum efficient capacity might be 85%. Should the demand for increased chilling capacity cause the capacity of the compressor 2 to reach the predetermined maximum value, the capacity of compressor 2 will be reduced to a predetermined value, and the compressor 3 will be automatically started by the following sequence. When compressor 2 reaches 85% of capacity, normally open vane switch 47 will close, completing the circuit between terminals 70 and 68 (Figure 3c). This, in turn, completes a circuit between terminals 68 and 78 through normally closed contacts 393 of a first sequence control relay 395. It should be noted that the sequence control relay 397 and the first sequence control relay 395 are merely parallel relays and could be a single relay.
Once a circuit is completed to terminal 78 (and also terminal 64) a circuit is completed through the normally closed contacts on vane switch 53 causing power to reach terminal 80. When power is applied to terminal 80 (Fig. 3c), a circuit is com-pleted through normally closed contacts 399 of sequence relay 317, thereby energizing recycle relay 401. It should be kept in mind that sequence relay 317 is de-energized, in compliance with the assumed initial conditions. When recycle relay 401 energizes, it is locked intothe energized state via the normally closed contacts of vane switch 53, and normally open contacts 403 thereof complete a circuit between terminal 68 and sequence control relays 395 and 397, until the vane position ~ - 22 -1~35~6~8 of second compressor 3 has reached the position which opens the normally closed contacts of vane switch 53. At the same time, normally open contacts 405 of recycle relay 401 close to main-tain a continuous circuit between terminals 68 and 78. In addition, normally open contacts 407 of a Recycle Relay 401 close and through normally closed contacts 409 of the sequence relay 317 and normally closed contacts 411 (Fig. 3b) of the second unload relay 361. The second load relay 413 is energized.
When vane switch 47 indicates that first compressor 2 has reached the maximum efficient capacity, thereby signaling the start of the lag compressor, the capacity of lead compressor, compressor 2 here, is automatically reduced to some substantially smaller capacity, in the following manner. When a voltage is applied to terminal 68 from vane switch 47, this voltage reaches terminal 64 as described above, which supplies power to vane switch 45 as shown in Fig. 1. Since the normally open con-tacts of vane switch 45 have closed by the time first compressor 2 reaches the capacity for which vane switch 47 is set, appli-cation of a voltage to terminal 64 causes the voltage to reach terminal 62 (Fig. 3c). This causes first unload relay 349 (Fig. 3b) to energize through normally closed contacts 509 (Fig. 3c) of se~uence relay 317, normally open contacts 510 of energized recycle relay 401, and normally closed contacts 505 of sequence relay 317. When first unload relay 349 energizes, normally closed contacts 347 thereof cause first load relay 351 to de-energize and also cause a circuit to be completed be-tween terminals 5 and 19 through normally open contacts 374 of first program motor relay 370, normally closed contacts 467 of first minimum vane relay 469 and normally open contacts 465 of energized first unload relay 349. This causes first (lead) ~ - 23 -11353~1~
compressor 2 to unload until the closed, but normally open, contacts of vane switch 45 reopen. As previously noted, switch 45 is typically set for 30% of maximum vane position, so the vane position, and the related chilling capacity of compressor is reduced to 30% by this signal originating from lag compressor start circuit 214, at the same time that second compressor 3 is being loaded and started.
The second compressor 3, the lag compressor in this example, is started by a signal applied to terminal 31 from terminal 23 in the following manner. The voltage at terminal 23, which exists continuously, is applied to terminal 31 via normally closed contacts 417 of seventh delay timer 419, and then through manual reset switch 421 or normally open contacts 423 of second safety relay 321. The voltage then passes through normally open contacts 425 of eleventh delay timer 427, whereupon a second safety relay 321 is energized. The voltage at terminal 23 passes through normally closed contacts 429 of sequence relay 315. When lag compressor start circuit 214 receives a signal from vane switch 53, which causes the second compressor start relay 395 to energize as described above, normally open contacts 431 of energized second compressor start relay 395 close and the voltage at terminal 23 is applied to terminal 31 normally closed contacts 433 of the first se~uence relay 315. A voltage signal at terminal 31 starts the lag compressor or second compressor 3. As with lead compressor 2, a failure internal to compressor 3 may manifest through normally closed contacts 435 of the second flow switch relay 333, or through the normally closed contacts 437 of the second oil failure relay 325, or through the normally open contacts 439 of the second safety failure relay 329. Any of these failures ~13S36;8 cause seventh bimetal delay timer 419 and ninth bimetal delay timer 427 to activate, opening the circuit between terminals 23 and 31, thereby shutting down lag compressor 3. Timers 419 and 427, as with timers 373 and 387, are normally closed timers with a time duration of from 10 seconds or less to more than 60 seconds, depending upon the compressor used.
Assuming no failure is detected, the system is now in the condition that the lead compressor 2 is operating at a pre-determined low capacity, for example thirty percent (30%), and lag compressor 3 has been started and has been brought up to some pre-determined capacity, typically also thirty percent (30%).

Should the control medium still require additional chilling capacity, again manifested through temperature control switch 37, the control device 1 will require an increase in capacity on the lead compressor 2 as follows. In connection ~ith Figure 2, it was pointed out that either lead compressor 2 or lag compressor 3 could be the initial compressor to load during operation of both lead and lag compressors. However, it was further pointed out that, for the particular embodiment shown in Figure 3, the lead compressor, assumed to be compressor 2, will load first because normally closed contacts 444 of second compressor start relay 397 faster than normally open contacts 445 thereof close, which causes the R3 relay 343 to be de-energized at the beginning of the next "on" cycle of timer 200. Thus, with the R3 relay 343 de-energized, loading occurs on the lead compressor 2 in the same manner as the initial loading pre-viously described. However, in addition to the relays activated during loading as described in connection with initial start up, a second bimetal delay timer 353, a normally open timer, closes and starts timing. This, in turn, closes normally open contacts . . :

~13S368 441 thereof found in R3 transfer circuit 202 (Fig. 3b).
It should be noted that the time period of the delay timer 353 must be greater than the time period of the "on" cycle of the timer signal relay 309. This causes a completed circuit to occur between terminal 94 and 98, or power, after the "on"
cycle of timer:200 has terminated; that is, a completed clrcuit exists through normally closed contacts 443 of timer signal relay 309, normally open contacts 441 and normally open contacts 445 of energized second compressor start relay 397.
This energizes the R3 transfer relay 343. Once R3 relay 343 energizes, it is locked into the energized state via normally open contacts 447 of relay 343 and normally closed contact 449 of first bimetal delay timer 451 and normally open contacts 445 of second compressor start relay 397. As will be discussed hereinafter, a failure on second safety relay 321 will cause normally open contacts 453 to open and will also de-energize the R3 relay 343. However, during normal operation once R3 relay 343 energizes, normally closed contacts 341 thereof `
(adjacent to terminal 38) open and at the same time normally open contacts 455 thereof close. This causes, during the next "on"cycle of timer 200 during which a demand for increased chilling exists, the resulting load signal to be channeled to second load relay 413, rather than first load relay 351. It can be seen that the path is similar to that for the first load relay 351: that is, the current flows through contacts 455 of the R3 relay 343, then through normally open contacts 457 of the second control timer relay 339, through normally closed con-tacts 411 of unload relay 361 and thence through second load relay 413. This causes the second compressor 3 the lag compressor, to increase in capacity through a signal applied between terminals . - 26 -~L~3S3~

21 and 33 as described previously. At the same time, normally closed first bimetal delay timer 451 is started timing. As with the second delay timer 353, the time period for the first bimetal delay timer 451 is slightly longer than the "on" cycle of timer signal relay 309. Therefore, normally open contacts 459 of the timer signal relay 309 will have opened and at the same time normally closed contacts 449 of the first delay timer 451 will also be open, causing the R3 relay 343 to de-energize.
This, in turn, causes the lead compressor 2 to increase vane position on the next cycle in which a demand for increased capacity exists.
It can be seen from the above that because of R3 transfer circuit 202, increases in capacity are alternately channeled between the lead compressor 2 and the lag compressor 3, during the appropriate "on" cycle of the timer 200. Thus, an approximate balance in vane positions of compressors 2 and 3 is maintained, and the efficiency of the system maximized, thereby minimizing power consumption.
Should additional chilling capacity be required, the lead compressor 2 and the lag compressor 3 will alternately load as previously described. Since good system design will prevent both the first compressor 2 and the second compressor 3 from being loaded to maximum capacity, the system is permitted to increase to the necessary capacity at some value less than that maximum. Thus, the system should settle into a chilling capacity where switch 37 is no longer closed--that is, no demand for increased chilling--and normally open switch 39 remains open.
This of course requires that a "dead band" exist between switches 37 and 39, but this band may be reduced to as little as one degree while maintaining system stability, depending upon the compressor ~13536~

used. In such a condition, R3 relay neither energizes nor de-energizes, depending upon the last state during which a demand was required, during subsequent "on" cycles of timer 200 and therefore R3 transfer circuit 202 remains dormant while no demand for increased chilling exists.
At some point, the system will require less chilling capacity and a demand for decreased chilling will manifest by the closing of normally open switch 39, also controlled by the thermostat. Switch 39 is also typically provided with a set of normally closed contacts for application in pneumatic compressor controls system to insure that a pneumatic compressor failure causes control device 1 to lock into an unload mode. As with the process of increasing capacity, a command to decrease capacity is only received by the controller during the "on"
portion of the timer signal relay 309 cycle. That is, during the "on" signal of the timer signal relay 309, normally open contacts 311 thereof close and because of the condition of switch 39 a circuit i5 completed between terminals 36 and 40. This in turn causes first unload relay 349 to energize by completing a circuit between terminals 94 and 98 through normally closed contacts 461 of the R4 transfer relay 463 and normally closed contacts 465 of the control timer relay 339. When first unload relay 349 ener-gizes, a circuit is completed between terminal 5 and terminal 19 through normally open contacts 374 of a first control relay 370, normally open contacts 465 of first unload relay 349 and through normally closed contacts 467 of first minimum vane relay 469. In this manner, a signal is sent to the first compressor 2 indicating that the vane angle should be decreased. At the same time that first unload relay 349 is energized, a fourth bimetal delay timer 469, a normally open bimetal timer, is energizedand starts timing.

~ - 28 -113~i3~8 slightly longer than the "on" signal of timer signal relay 309.
This in turn will cause the R4 relay 471 to de~energize, because the lock-in circuit~-comprised of normally open contacts 489 of R4 relay 471, normally closed contacts 491 of third delay timer relay 487 and contacts 475--will be broken at contacts 491 as well as normally open contacts 493 of timer signal relay 309. Thus the R4 relay 471 will de-energize, permitting first unload relay 349 to unload the first compressor 2 on the next cycle of timer 200 during which a demand for decreased chilling exists. Again, the similarity between R4 transfer circuit 204 and R3 transfer circuit 202 should be noted, keeping in mind that the R3 transfer circuit 202 applies to loading compressors 2 and 3 when a demand for increased chilling capacity exists, whereas the R4 transfer circuit 204 operates during unloading when a demand for decreased chilling capacity exists. Because the transfer circuits for loading and unloading are independent, the maximum imbalance in vane positions between compressor will be incremental increase or decrease.
As described above, lead compressor 2 and lag com-pressor 3 will continue to unload so long as a demand for de-creased chilling capacity is manifested by the condition of temperature switch 39. Should a demand for increased chilling occur, loading will occur exactly as described previously in connection with R3 transfer circuit 202 and load and unload circuits 206 and 218. However, should a demand for increased capacity not occur, and the system continue to demand decreased chilling, both compressors will alternately unload until one compressor reaches some predetermined minimum vane position fixed by vane switches 43 and 51, typically 25~ of maximum vane position, below which operation of both compressors 2 and 3 is inefficient. of course, both compressors will not reach the predetermined minimum capacity at the same time; rather, one of , 1~3~3~3 the compressors will reach the predetermined minimum and perhaps attempt to decrease below that minimum capacity during the cycle in which an unload signal exists. However, whichever compressor reaches the minimum capacity first will be prevented from reducing the vane angle o~ that compressor to less than the setting of either vane switch 49, (if compressor 2 reaches minimum capacity first) or vane switch 47 (for compressor 3), typically twenty percent of maximum vane angle. That this occurs can be seen by observing the circuit between terminals 5 and 19, normally closed contacts 467 of first minimum vane relay 469 (which is controlled by vane switch 49) will be opened when unload circuit 206 attempts to decrease the vane angle of compressor 2 below the setting of switch 49. This will break the circuit between terminals 5 and 19 causing a loss of power to terminal 19 which prevents compressor 2 from unloading further in response to a signal from control device 1. Similarly, should compressor 3 reach minimum capacity and attempt to con-tinue to decrease in capacity, normally closed contacts 481 of second minimum vane relay 483 will open due to the opening of vane switch 57, thereby opening the circuit to terminal 35. It should be noted that this occurs because terminals 48 and 72 are connected to terminal 98 through normally open contacts 495 of the energized second compressor start relay 397. If a demand for decreased chilling capacity continues to exist, the remain-ing compressor will decrease in capacity until it reaches the predetermined minimum. At this point, second compressor start relay 397 will de-energize since vane switch 51 or 43 will open and remove power to terminals 48 and 72. This in turn causes the lag compressor 3 to shut down since normally open terminals 431 of second compressor start relay 395 open, thereby shutting ~ - 31 -11353~i~

off power to terminal 31 found in compressor start circuit 212 (Figure 3d). Thus, when compressors 2 and 3 reach 25~ of capacity, the lag compressor 3 automatically ceases operation, leaving the lead compressor 2 operating at 25% (or possibly 20%) of maximum vane position. Should additional capacity for chilling be required, during the next "on" cycle of timer 200, switch 37 will be closed and loading will begin as described previously solely for the first compressor 2. However, should even less chilling capacity be sufficient to satisfy the system, command of the compressor 2 will be transferred to low limit shut down switch 41 via terminals 42 and 44. It should be noted that switch 43 no longer has an effect since relay 397 is no longer energized. Should demand for chilling be so limited as to permit low temperature shut down switch 41 to control, the opening of switch 41 will disconnect the circuit to pump start relay 361 (Figure 3a), thereby de-energizing that relay and opening normally open contacts 497 thereof in pump start circuit Z08 as well as disconnecting power to terminal 15 via normally open contacts 379 of pump start relay 361. This both turns off the compressor pump and turns off compressor 2. The system is thus entirely shut down.
When the system shuts down, automatically lag switch 313 alternates its position in response to an alternator signal applied to terminal 6, such that what had previously been the lead compressor will now become the lag compressor and vice versa. The alternator signal is generated externally to the control device 1. In addition, the lag switch 313 is typically provided with manually selectable positions for setting either the first compressor 2 or the second compressor 3 as the lead compressor. Therefore, since in this example compressor 2 had ~ - 32 -11353~8 been the lead compressor, the compressor 3 will upon next start-up be the lead compressor. This occurs because the alternation in position of lead lag switch 313 permits sequence relays 315 and 317 to energize immediately upon application of power to terminals 94 and 98 at initial start-up. In such a circumstance, all normally closed contacts of sequence relays 315 and 317 will open and remain open until the next system shut-down; in contrast, all normally open contacts of sequence relays 315 and 317 will close until the next alternation of the switch 313, barring a failure. While this will affect the order in which compressors 2 and 3 are called into operation, and subsequently the loading and unloading thereof, operation is in all other respects the same as previously described. This is apparent from the parallelism of normally open and normally closed contacts for the sequence relays 315 and 317. For example, normally open contacts 499 (Figure 3c) are analogous to normally closed contacts 399, normally open contacts 501 are analogous to normally closed contacts 409, normally open contacts 503 are analogous to normally closed contacts 505 and normally open contacts 507 are analogous to normally closed contacts 509, all found in lag compressor start circuit 214 and load and unload circuits 206 and 218 (Fig. 3b). Further, in the compressor start circuits 210 and 212 (Fig. 3d), normally open contacts 511 are analogous to normally closed contacts 377, while normally open contacts 513 are analogous to normally closed contacts 433 and normally open contacts 515 are analogous ~L13S36~3 As with second bimetal delay timer 353, fourth bimetal delay timer 469 has a time duration slightly longer than the "on"
portion of timer signal relay 309. Because of this, the R4 transfer relay 463 will energize during the time period in which bimetal timer 469 is closed and the timer signal relay 309 is de-energized. This circuit is completed through normally closed contacts 443 of the timer signal relay 309, then normally open contacts 473 of the timer signal relay 309, then normally open contacts 473 of the fourth bimetal timer 469, and normally open contacts 475 of the second compressor start relay 397. It should be noted here that this circuit ls quite similar to the circuit which energized R3 transfer relay 343 during the load sequence.
When R4 relay 471 energizes, normally closed contacts 461 open, thus preventing first unload relay 349 from energizing during the next on-cycle of timer 200. At the same time, normally open contacts 477 of R4 relay 471 close when relay 471 energizes.
Thus,during the next "on" cycle of the timer 200 in which a demand for decreased chilling is present, a circuit is completed which causes the second unload relay 361 to energize. This circuit is completed between power supply terminals 94 and 98 through contacts 311 of the timer signal relay 309, terminals 36 and 40, contacts 477 of the R4 relay 471, normally closed contacts 479 of the second control timer relay 339 and thence the second unload relay 361. This in turn causes an unload signal to be generated between terminals 21 and 35 through normally closed contacts 481 of second minimum vane relay 483 and normally open contacts 485 of the energized unload relay 361.
When the second unload relay 361 energizes, third bimetal delay timer 487, a normally closed timer analogous to first bimetal delay timer 451, will open and remain open for a timer period , - 29 -11353~8 to normally closed contacts 381. Thus,it can be seen that the sole effect of the lead lag switch 313 is to merely alter the sequence in which compressors 2 and 3 operate, thereby assuring substantially equal use of each compressor.

It has thus far been assumed the vane switches 43, 45, 47, 49 (first compressor 2), 51, 53, 55 and 57 (second compressor 3) are directly controlled by the vanes of the first compressor 2 and the second compressor 3. In fact, this is not correct, although the switches recited above are triggered from signals generated by program motors timed to duplicate the actual vane positions. By using program motors timed to operate at speeds substantially identical with those of the actual compressor vane motors, it is possible to make control device 1 as a unit needing no mechanical connection with the first compressor 2 or second compressor, instead using only electrical connection.

For example, when the vane position on the com-pressor 1 is being increased through a signal at terminal 5, a circuit is completed to terminal 17 through contacts 374 of eontrol relay 370, and either contacts 369 of first load relay 351 or contacts 368 of the minimum vane relay 469. This applied power to the actual compressor vanes. At the same time, power is applied to the increase control of program motor 621 through corresponding contacts 623 of the control relay 370 and contacts 625 of the first load relay 351 or contacts 627 of the minimum vane relay 469. This causes the specially selected program motor to track the variation in position of the actual vanes of the first compressor 2.

~13536~

During unloading the operation is only slightly different. Again contacts 629 of minimum vane relay 469 correspond to contacts 467, and contacts 631 of first unload relay 349 correspond to contacts 465. However, normally closed contacts 633 of the first control relay 370 provide a by-pass rather than a series connection to control the unload line of program motor 621. This is because the internal compressor controls may require the compressor 3 to unload whereupon power would be removed (by the compressor ltself) from terminal 5 and applied to terminal 19. This requires that a pathway not dependent on the minimum vane relay 469 or the first unload relay 349 exist for unloading the program motor. Contacts 633 of the control relay 370 provide the requisite pathway.
With reference to the second compressor 3, operation of second program motor 635 is analogous. Contacts 637 of second control relay 372 correspond to contacts 376; contacts 639 of second load relay 413 correspond to contacts 415; and contacts 641 of minimum vane relay 483 correspond to contacts 366 thereof, all to control increase line 643 of program motor 635. For unloading, contacts 645 of the second minimum vane relay 483 correspond to contacts 481 and contacts 647 of second unload relay 361 correspond to contacts 485. As with the first compressor 2, contacts 649 of the second control relay 372 provide a by-pass to the decrease line 651 of second program motor 635 so that the second program motor 635 may respond to 11353tj~

vane position changes signal ed by controls internal to the compressor. An alternative to the program motors and vane switches herein described is a multipositiont relay- or solenoid -actuated switch as described in connection with Figures 6a-6b herein. In the solid state embodiments of the invention shown in Figures 4a-4e, and 5, the program motors are replaced by timers.
The foregoing description has assumed proper, normal operation of both the compressors 2 and 3 as well as the control device 1. Inevitably, however, failures will occur which pre-vent the operation of either one or both compressors. In this event, the present invention includes various means for advan-tageously employing the internal fail detection devices of the compressor and also for detecting and signalling to the user failures within the entire system including the control device 1. First and second oil failure relays 323 and 325, first and second safety indication relays 327 and 329 and first and second flow switch relays 331 and 333, all located in failure shutoff circuit 220 (Fig. 3a), have previously been discussed.
These relays indicated failures internal to the compressor itself. As previously discussed, a failure causing any of these relays to depart from its normal state will cause first safety relay 319 or second safety relay 321 to de-energize and will shut down the failed compressor 2 or compressor 3, through the opening of normally closed contacts 371 of fifth bimetal timer 373 and normally closed contacts 322 of ninth , ~.................................................................... .

~L1353~1~

bimetal timer 387 (with respect to compressor 2), or normally closed contacts 417 of seventh bimetal timer 419 and normally closed contacts 425 of eleventh bimetal timer 427 (with respect respect to compressor 3). Each of these timers typically has a time duration adjustable up to 60 seconds. At the same time, normally open contacts 517 of safety relay 319 or normally open contacts 519 of safety relay 321 will disconnect the common side of vane switches 43, 51, 55 and 47 through terminals 60, 74, 84, and 70, respectively. This will prevent the operation of more than one compressor during the "on" cycle in which the failure continues to exist, since vane switches 47 and 55 control the start of the lag compressor. Further, should a failure occur after the second compressor has started, the failed compressor will be directed to completely unload - i.e., retwrn to minimum vane position - through normally closed contacts 531 (Fig. 3a) of first safety relay 319 or normally closed contacts 533 of second safety relay 321, which directly feed to first unload relay 349 (Fig. 3c) and second unload relay 361, respectively, through respective lines 530 and 532.
2~ In addition to causing the failed compressor to unload and shut down, the failure shut down circuitry 220 will also cause the control device 1 to recognize whichever com-pressor has not failed as the lead compressor by a signal direct-ed to sequence circuitry 216 (Fig. 3b). For example, should first compressor 2, previously assumed to be the lead compressor, fail, first safety relay 319 would de-energize and normally closed contacts 535 (Fig. 3b) of relay 319 would return to the closed position. This causes sequence relays 315 and 317 to energize, which places second compressor 3 in the lead position in the manner previously described. In contrast, should second compressor 3 fail, second safety relay 321 is de-energized, ~iL353~

causing the close of normally closed contacts 453 thereof. This results in the de-energization of sequence relays 315 and 317, or switch 313. Thus, if second compressor 3 fails, first compressor 2 is designated the lead compressor. Also, the de-energization of relay 319 or 321 is indicated to the user through failure indication lamp 537 controlled by normally closed contacts 539 of relay 319 or lamp 541 controlled by normally closed contacts 543 of relay 321.
In addition to forcing the non-failed compressor into the lead position, failure shut off circuit 220 prevents the entire system from automatically restarting after the next system shutdown. When shutdown occurs and a failure has been indicated by the de-energizing of either relay 319 or 321, the de-energizing of pump start relay 361 causes normally closed contacts 543 thereof to close. This causes both first unload relay 349 and second unload relay 361 to energize, thereby pre-venting automatic energizing of either first load relay 351 (because of normally closed contacts 347 of now-energized relay 349~ or second load relay 413 (through normally closed contacts 411 of relay 361). Thus it is seen that an unload signal has priority over a load signal.
System start switch 545 (Figure 3) typically a momentary contact switch, is provided to permit manual operation of pump start circuit 208. However, because of normally open contacts 547 of first safety relay 319, normally open contacts of second safety relay 321, normally closed contacts 357 of energized first unload relay 349 and normally closed contacts 359 of energized second unload relay 361, if both compressors 2 and 3 have failed, pump start relay 361 cannot be locked in until safety relays 319 and 321 have been manually reset by ,~

~3536i~3 switch 421 (second safety relay 321) and swItch 551 (~lrst safety relay 319).
Another typical failure in refrigeration systems, also detected and compensated for by the present invention, is the false start of the lag compressor--that is, the lag compressor received a start signal from the control device, but fails to start. Certain prior art control devices assume the lag compressor has started when in fact it has not. By means of normally closed bimetal false start timer 553 (Figure 3c), false starts of the lag compressor are detected. During normal operation, recycle relay 401 is energized until the vanes on the lag compressor have increased in position sufficiently to open the normally closed contacts of either vane switch 45 or 53. In the event of a false start, however, recycle relay 401 will remain energized indefinitely since the vane position on the lag compressor cannot change until the compressor has started due to the internal compressor controls. By setting the time duration of the false start timer 553 for a period longer than the period during which recycle relay 401 should be energized, a false start will permit timer 553 time out, thereby opening normally closed contacts 555 thereof and disconnecting power to vane switches 43, 47, 51 and 53. As previously described, this causes a loss of power to the lag compressor start circuit, leaving only the lead compressor operating.
Because of start-up delays internal to compressors used in such refrigeration systems, the lead compressor will have unloaded to the setting of either vane switch 45 or 53 (typically 30%
of maximum vane position) prior to the opening of contacts 555.

-39~

~353~

Thus the lead compressor will begin loading again at the next "on" cycle of timer 200. If demand for increased chilling again requires that the lag compressor be started, after incremental increases in lead compressor capacity, vane switch 47 or 55 will again energize the lag compressor start circuit 214.
Another failure which occasionally occurs and which can cause system overload damage is the failure of the tempera-ture controller. Should the temperature controller (switches 37 and 39) fail to a state indicating continuous demand for increased chilling capacity, when in fact the system has cooled the controlled medium to a point sufficient to permit control device 1 to transfer system control to low temperature shut-down circuit 41, the system will be automatically shut down in the following sequence. The lead compressor will load to maximum capacity from the signal received from failed tempera-ture control switch 37. When the lead compressor reaches maxi-mum capacity, load and unload circuit 206 or 218 will cause the lead compressor to unload, by energizing the appropriate unload relay 349 or 361. This opens contacts 357 or 359 (Figure 3a); since low temperature switch 41 has already opened, the pump start relay 361 de-energizes. This causes both unload relays 349 and 361 to energize through normally closed contacts 557 of the timer signal relay 309, and normally closed contacts 559 and 543 of the pump start relay 361, there-by reducing vane position on both compressors to a minimum position. The lead compressor is also stopped when the pump start relay 361 de-energizes through normally open contacts 379 thereof. Because of the delays inherent in compressors typically used in such refrigeration systems, the lag compressor will not start even though it receives a temporary start signal since vane position on both compressors will be such ,:

,353t~E3 that vane switches 43 and 51 pre~ent second co~pressor start relays 395 and 397 from locking in~ Thus the entire system will shut down, thereby avoiding problems of icing or other overload damage. Further, because contacts 557 are controlled by timer signal relay 309, the system is not permitted to attempt to restart until the next "on" cycle of timer 200, which prevents cycling. It should also be noted that the contacts 559 and 543 of the pump start relay 361, together with contacts 557 of timer signal relay 309, place the load and unload circuits 206 and 218 in an unload mode on any system shutdown where the safety relays 319 and 321 are energized, thereby complementing the un-load circuitry internal to the compressor.
Also provided are manually set switches 559 and 561 (Figure 3b) for manually controlling the loading and unloading, respectively, of the first compressor 2 and the second compressor 3. By placing switches 313, 421, 551, 545, 559 and 561 at a location remote from control device 1, remote manual control of the system is possible.
Attention is now directed to Figures 4a-4e, which are logic gate schematic diagrams suitable for performing functions substantially identical with those performed by the circuit shown in Figures 3a-d. Not shown in Figures 4a-e are the conventional analog-to-digital and digital-to-analog con-verters and buffers necessary to convert signals to the appropri-ate voltage levels suitable for use within the control device 1or the first and second compressors 2 or 3. Correspondence be--tween the circuit elements of Figures 3a-3d and Figures 4a-e is shown whenever possible by assigning to the appropriate logic gate the same numerical designator as found on the corresponding circuit elements shown in the relay embodiment of Figures 3a-d.

C

~13S3~8 For example, a first safety latch 370 found in Fi~ure 4d cor-- responds to first safety relay 370 found in Figure 3d. To further aid in establishing the correspondence between the embodiment of Figures 4a-d and Figures 3a-d, the logic gate embodiment of Figures 4a-d is shown in functional blocks as in Figure 2, with the signal sources for each block designated by the numeral associated with the gate generating the respective signal.
As to the particular elements shown in Figures 4a-e, particular attention should be paid to first delay timer 451, second delay timer 353, third delay timer 487 and fourth delay timer 469. All four timers are conventional one-shot timers with time durations as for their bimetal counterparts, and triggered on a positive transition. For timers 353 and 469, the output is taken from the true (Q) output; whereas for timers 451 and 487, the output is taken from the complement (Q) output.
Variable time duration is provided by using a variable resistor in the RC time control circuit of the one-shot. Control timer 339 is also a variable time duration one-shot triggered by a positive transition with a true (Q) output.
In Figure 4d, particular attention should be given to first safety latch 319 and second ~afety latch 321. Both are conventional bistable latching circuits with outputs taken from the complement (Q) output lines. In such a configuration the reset line of latch 319 is that fed by reset switch 551 and associated circuitry; the set line is fed by an "and" gate 701 with input signals from gates 319, 373 and 387. Similarly, for latch 321, the reset line is fed by reset switch 421 and associ-ates logic as shown and the set line is fed by an "and" gate 703 with input signals from gates 321, 419 and 427. Thus, the func-tion observed at the output of gates 319 and 321 is analogous to its relay counterparts.

C

. , .

1~L3531~8 Attention is also directed to solid state triggers 705 and 707 found in Figure ~d. These triggers are conventional solid state control devices, triggered by typical logic levels, for permitting the AC signal at terminals 7 and 23, respectively, to reach terminals 15 and 31, respectively.
Fifth delay timer 373, seventh delay timer 387, ninth delay timer 419 and eleventh delay timer 427, all shown in Figure 4d, are variable time duration one-shot timers trig-gered by a positive transition with complement (Q) outputs.
The time duration corresponds to that used in the corresponding bimetal delay timers.
Attention is especially directed to Figure 4e, which illustrates a solid state counterpart of program motors 621 and 635. Since program motors 621 and 635 operate by matching times with the vane inclination motors on the compressors 2 and 3, a conventional up-down counter can serve the same func,ion as the program motor. Program counter 709 with associated combinational and sequential logic 711 is intended to replace the first pro-gram motor 621 and vane switches 43, 45, 47 and 49 while second program counter 713 and associated combinational and sequential logic 715 replace the second program motor 635 and vane switches 51, 53, 55 and 57. The program counters 709 and 713 are typi-cally eight bit up-down counters with gate 717 feeding the count up enable line while gate feeding the count down enable 25 line of the counter 713 and gate 723 feeds the count down enable thereof. The clock of the counter 709 is controlled by the "or"signal of suitably buffered terminals 17 and 19. The clock signal of counter 713 is fed by the "or" signal of suit-ably buffered terminals 33 and 35. The reset line of counter 30 709 i5 controlled by the combination of signals from gates 360, 395 and 315, as shown. The reset line of counter 713 is controlled by the combination of signals from gates 360, 395 ~13S3~8 and 315r as shown~
Combinational and sequential logic circuits 711 and 715 provide outputs at various counts of the counters 709 and 713, respectively, plus latching circuitry so that the conjunctive use of the counters 709 and 713 with associated respective circuits 711 and 715 provide functionally the same "open" and "closed" signals provided by the program motors 621 and 635 and their respective vane switches. In addition, the three multi-position switches 733, 735 and 737 permit the user to select three settings for the counts at which signals are supplied to terminals 11, 58, 62, and 66, respectively. Simi-larly, the three multi-position switches 739, 741 and 743 pro-vide three settings for the signals supplied to terminals 27, 72, 76 and 80, respectively. While the particular settings will vary for the speed at which the compressor changes capacity, typical settings for a machine which can reach maximum capacity in 30 seconds might be a range of 4, 5, and 6 seconds on terminal 11; 5, 6 and 7 seconds on terminal 58, and 6, 7, and 8 seconds on terminals 62 and 66, with a constant setting of perhaps 27 seconds on terminal 68. Similar settings would be used on terminals 27, 72, 76 and 80, and 82.
Attention is further directed to a first control latch 370 and a second control latch 372, which are conven*ional latches with the set line of the first latch 370 being control~
led by a suitably buffered and converted signal from terminal 5, and the clear line thereof is controlled by a suitably buffered and converted signal from terminal 19. Similarly buffered and converted signals from terminals 21 and 35 control the set and clear lines, respectively, of second control latch 372. Also, solid state triggers 725, 727, 729, 731 and 745 (Figure 4a) ,.

~353fi8 are as previously clescribed for tl-e triggers 705 and 707 (~igure ~d). Also sllown in Figure~4cl are first delay line 7g7, second delay line 7~9, tllird delay line 751 and fourth delay line 753. Eacll of these delay lines typically ~rovides a time delay on the order of seventy seeonds, to per~it temporary coml~ressor ~ai]ures ot occur, as on startup, without signaling a system failure. The remaining loyie gates are sllown in eonventional symbology and may be implemented with any positive loyie technology.
~igure 5 illustrates another alternative embodiment of the multiple compressor eon-trol device shown in FicJures 1-4, employing a microprocessor such as tlle Fairchild F8 ~or sequencing and decision~ akincJ. Tlle systern inputs are as descrihed for the embodiMent sl~owll in Figures 3a-3d; ho~ever, the llO v signals must be converte~d to logic signa]s compatible witll tlle micro-processor and associated circuitry. More specifically, a signal on line 754 indicates that the first compressor, Cl, (not shown) is ready to load. A signal on line 755 indicates that the seeond com~ressor, C2 (not shown), is ready to load. A si~nal on line 75G indieates, from tlle room thermostat, time eloe~, switeh, or otller suitable means, that increased cooling is required.
A signal on line 757 indieates tl~at decreased cooliny is re-quired. SicJIlaLs Oll lines 758 and 759 indicate water flow (or other coolant) in compressors Cl and C2, respeetively.
A siyllal on line 760a determines the "lead" and "lag"
compressor for the next operational eyele. As before, a manually operated switcll 760b ean override the incominc3 signal.
External load col~nands for the eompressors Cl and C2 are indi-eated Oll lines 761a and 762a, respeetively, with the commands 30 typical]y being provided by manually actuated switches 761b and 762b, respectively, or the like. Similarly, external -- ~5 -." 1135368 commallds to unloa(~ the compressors C;l alld C2 are indicated on lirles 7G3a and 764a, respectively, Erom switches 763b ancl 7~41~, respectively, or tlle like.
~acll of these eleven input signals is then operated on by level sllifters 765a-k to be made compatible with the logic ]cvels of t:l~e microprocessol^ 766, to whicll thcy cacll supply a sigrlal. ~ach level shifter al.so supplies a si~nal to a convelltiollal failure detector circuit 767a, whicll activates an indicator 767b of any suitable type upon detection of a failure.
~ lso supplying inputs to the microprocessor 766 is a chiller failure reset swi~ch 768, and switches 769a-b for manually sta~-ting compressors Cl and C2, respectively. ~ system reset switch 770 also provides a signal to the micro-rocessor, as does a mode selector (automatic or manual) switc~l 771.
lag compressor lockout switch 772 and an anti-recycle switch 773 also provide control signals to the microprocessor.
Other inputs to the microprocessor 766 are provided by a plurality of switches 774a-f. The switch 774a sets into the microprocessor a percentage capacity to which the compressor will be increased on start-up as a minimum operating capacity, for example 32%~ 5%. The switcl~ 774}~ selects the stabilization time ("off" cycle of the timer 200 describcd in Fig. 2) from zero-to nine minutes. The switches 774c and 774d are used to set into the microprocessor the exact time required for the compressors Cl and C2, respectively, to go from zero percent capacity to 100% capacity. That is, the time for the vane angles to increase, in a frce-runnillc~ mode, from zcro to 100~ is - ~6 -~13S3~8 . . .
measul-ed ancl tlle time is set .illtO the maciline at switches 7i4c all(l 774d. 'rypically, less than'olle ll~llldred seconds is re~ lrerl Lor maYimum vane ch~nge.
011CC~ the vane change times are set, programming the microprocessor for the incremental chancJes hereinbefore clescribecl for loading and ~nloadinc3 is straightEorwarcl. ~rhe internal clock of the microprocessor is used as a time base, and the vane challge time can l~e divided into any suitable inerem~nts.
Incremental increases and decreases in capacity are provided by counting the pulses of the mieroprocessor timebase until the desired incremental change lS achieved.
The remainillc3 switeh inputs are similar. Tlle anti-recycle switch 774e sets into the microprocessor a period of time during wllicll the compressor will not restart if shutdown has occurred d-le to low demand for chilling. The water flow failure switch 774f sets into the microprocessor a selected time for the compressor to start, after a start signal has been received, before a failure condition will be signaled.
Referring again to the use of the microprocessor's internal clock as a timebase, it should be recocJnizecl that the counting registering capacities of the microprocessor permit internal setting of the capacitles a-t which lag compressor start, lag compressor shutdown, maximum operating capacity, and system shutdown occur. If adjustment of an~ of these capacities is desired, additional input switches, similar to the minimum vane setting switch 774a, may readily be provided.
The last remaining input to the microprocessor 766 is - ~7 -provide(l l~y an e~terllal memory 77S, whicll stores infornlation in ~he event- o~ a systcm or compressor failure, as indicated by t~le microprocessor. Thus, the memory 775 ~ill sigllal the micro~)rocessor 7fiG not to attempt to start a failed compressor - 5 in accorclance with the c3uidelines cliscussed in connection with Figures 1-4, above. Since the memory is generally required to retain informatioll W}len the system shuts down, and to provide that information on next attempted start-up, and independent power suppiy (not s}lown) should be provided if a volatile memory is used.
Once the mircoprocessor 756 has interpreted the inputs, it si~nals, through buffer relays or the like, each compressor, or.causes a failure signal to be detected at the failure detect circuit 767a. Corresponding indicators are also activated, as well as c3eneral system indicators. Thus, a command to load the compressor Cl activates a buffer relay and associated indicator 776. A general system indicator 777a, indicating that the system in general is operational, and a pair of indi-cators 778a-b, indicatincJ that the comporessor Cl is operationa]
and under automatic control, may also be provided. ~lso a general system indicator 777b, indicating that an increase in cooling capacity is required, may also he provided. Each of the above indicators is preferably controlled by the microprocessor, although different combillatiolls of input signasl may be required to activate a givell indicator.
Once the buffer relay 776 has been signaled, either a failure signal is provicled to the failure detection circuit 767a, or a ]oad sic~nal is provicled to the compressor Cl.

~35368 Li~cwise, the micrc)processor may si~nal a buEfer re]ay and indicator 779 to require the cornpressor Cl to be unloaded or that a failure si~nal bc ~enerated. Similarly, the second compressor C2 may be loaded or unloaded via si~nals to buffer - relays and indicators 780 and 781, respectively, and the auto-matic operation of the compressor C2 may be indicated on indicators 782a-b. Of course, Cl start relay 783 will have been activated at the initial loading of the first compressor, while t11e C2 start relay 784 will have beell activated at the initial loadinc3 of the second compressor. ~ failure of either compressor on startup will actlva-te an indicator 785a or 785b, resl)ectively, and will also si~nal bufEer relays 786a or 785b, respectively, and will also signal ~uffer relays 786a and 786h, resl~ectively, to provide a remote failure indication.
If either or both compressors Cl and C2 are operating, their percenta~e loads may be indicated by means of load displays 787 ancl 788. Other system indicators which may be provided include a system balance indicator 790, a system load decrease inclicator 791, and a recycle indicator 792, each activated by appropriate inputs to the mieroprocessor. ~he microprocessor 766 also controls an au~iliary start relay 793, and can control power to the thermostat through a buffer relay 794. It should be noted that the output buffer relays described herein perform a leval conversion, since the inputs thereto from the microprocessor are digital loc7ic si~nals, wllile the sic~nals provi(led to the compressors themselves are conventional line current.

_ ,~9 _ 1~353~3 It can thus be seen that the inVelltiorl described l-lerei.ll ma~ be embodi.ed in an electromecllallical syst:em usin~
relays and the li~e, or a "dedicated" dic3ital system, or a microprocessor system. For the systems thus far described, 5 the present invent.ion has been discussed in connection with a plurality of compressors. Ilowever, the inventioll is readily adaptable to si.nyle compressor systems.
Referring now to Figures 6a and 6b, there is sho~-7n therei.n circui~ry adapted ~or implementing the present invention ~or use with a sinyle variable capacity refrigeration compressor. Si.milar to tlle multi.ple compressor embodiments described above, power is applied to terminal 800 from an external control source such as an air thermostat, time cloc~, swltch or the like; this in turn applies power to a switch 802.
The switc}l 8n2 is preferably a three position switch, having an "off" position, a momentary "manual start" position, and an "autom.atic start" position. For most applications, t?le "automatic start" position will h~ve been pre-selected.
~ssuming the switch 802 (and there~ore also switches 802, Fiy. 6b) is in the automatic position, power is then applied to a thermos-tat (not shown) of the coolinc3 system at terminal 8n~ thl-ouc3ll nornlally closed contacts 803 associated Witil a panel failure timer 8~2 and normally closed contacts 805 associated witll an anti-recycle timer 936. ~t the same time, power is applied to a stabilization timer 806 throuc~h a normally closed pair of contacts 8n8 associated with "on cycle" timer 8]0. Both tlle stabilization timer 806 and the "on cycle" ti.mer 810 are adjustable timers of the sort described 35~

previously, wi.t:ll tl~e tirner 806 havillcJ a 5-9 minute cycle, and the ti.mer 810 having an "on" cycle of approximately twenty seconds.
~lso, a safety relay 812 ener~izes and closes a normally open pair of contacts 814 associated therewitll and connected between the terminal 800 and the relay 812.
I~hen the stabilization timer times out, normally open contacts 824 associated therewith close, and start the "on cycle" timer 810 to start timing. While the "on cycle" timer 810 is timing, the system i.s responsive to demands ~or increased or decreased chill:illcJ ~rom tlle tllermostat. If the tllermostat indicates that coo].incJ is required, power is applied to a terminal 8l.6. Tllis energizes a load timer 818 and also activates an increase capacity inclicator 820 such as a light or otller suitable means displayed on a user-visible panel (not shown).
When the load timer 818 energizes, a circuit is completed to a load relay 822 througll normally open contacts 824 associated with the timer 806, normally closed contacts 825 associated with a panel failurc relay 972, normally open contacts 826 associated Wit]l the load timer 818, normally closed contacts 828 associated with a vane incremellt relay 830, and finally through normally closed contacts 832 associated with a maximum vane relay 834 (Fisure 6b). Whell the load relay 822 energizes, a circuit is completed to a compressor start relay 836 through normally open (but now closed) contacts 838 associated with the load relay 822, and normally closed contacts 844 associa-ted with a low load recycle relay 84h.

``- ` 1135~
~h~ he compressol~ start relay 836 enerc~izes, cont~cts 848 associated therewitl; close, ancl a circuit is complel:cd between terminals 850 and 852. TerMinal 850 is connected to the internal controls of the compressor, and a sigllal Erom the compressor ~not 5l10WI1) is provided thereat when the start circuit internal to the compressor is ready ~or use. Termillal ~52 is connectecl to the compressor start eircuit;
thus a completed circuit l~etween terminals 850 and 852 causes the compressor to start. This causes an indicator 845 and an elapsecl time indicator 847 to be activated. ~lso, normally open contacts 849 associated Wit]l compressor start relay 836 close and co~nplete a circuit betweell terminals 851 alld 853, causinc3 the compressor pump to start.
011CC the compressor starts, the internal controls of the comvressor signal (if no failure occurs) that the machine is capable of loacling, or increasing capacity. This signal is received at a terminal 854, wllicll causes a control relay 894 to energize, there beinc3 a neutral at terminal 855. This causes a sigrlal to }~e applied to a terminal 856, througll normally open contacts ~57 associated witll -the relay 855, normally closed contacts 858 of a deenergized minimum vane relay 860 and normally open contacts 859 oE the compressor start relay 836. ~lso, when the control relay 894 energizes, contacts 894a assocaited therewit}l extinguish an indicator 894b, which had indicated that the chiller was self-operatinc3; i.e. - not controlled l~y the panel if operating at all. ~n inclicator 841a is also caused to activate when a sicJnal is applied to the ternlinal 854, and anotller indicator 8411~ is activated when 113S;~

a signal is applied to terlninal 8$6. The terminal 856 is conllectecl to the vane angle control,of the compressor, and the signal from the terminal 85~ therefore causes tl-e valle angle, and capacity, of tlle compressor to increase a set amount from a starting position of zero capacity.
'I'Jle amount of increase in capacity is controlled by a load programmer 900 anc1 associated circuitry, whicll is actuated whell the vane angle control of the compressor is actuated. T}le load l?rocJrammer 900 comprises a load programmer coil 90L, an unload programmer coil 902, alld a plurality of contacts, exelnplary ones of whicll are showll in FicJure 6b. The load progralluller 900 is prefera~ly a multiposition relay or solenoid-actuated switch. For example, the programmer may have twenty-six contact points designed to represent the entire range of compressor capacity. Thus, each contact of the programmer represents a four percent incremental lncrease in the capaclty of the compressor, with the first contacts representing a zero percent capacity and the last contacts representing 100%
capacity. Obviously, the nun~er of contacts chosen is to a large extent arbitrary, although greater numbers of contaets will generally result in improved accuracy. The present choice of twenty-six contacts, or twenty-five increments, has been chosen to prov:ide good accuracy wi.-tllout undue cycling in an electro-mecJlallical emhocliment. Ilowever, as has already been described, microprocessor embodiments of the invention may use two percent increments. By energi~ing the load progranuner coil 901, the programmer 900 is eaused to select contacts represental:ive of an increase in compressor capacity. Similarly, `-` . 1135~

a decrease in capacity is represented by energizing tlle unload progralluner coil 902.
To be useful, the time for the proc3rammer to move ~rom one contact point to the next must closely approximate S the time for the actual vane angle to chan(]e ~y the increment representecl by tlle programmer. This is accomplished in the follo~Jing manller. I~hell the increase load sigllal (such as that applied to the terminal ~56) is generated, a circuit is completed to enerc~ize a programmer tlmer 903. The programmer lo timer is adjustable for a range of up to 15 seconcls. I~llen .the timer 903 times out, normally open contacts 90~ associated therewith close. This enerc~lzes a programmer release timer 905, of tlle same type as -the timer 903, and permits either the load program~ner 900. l3y adjusting the programmer timer 903, the programmer 900 can be timed to increment at the same rate at which the vane angle of the compressor chanyes.
~ hen the compressor is initially started-, for example each morning, the vane angle i.s typically set for the minimum capacity, for example ~ero percent. Upon receipt of the first increase capacity signal, the capacity is increased to an arbitrary minimum start level, for example 32%. Since a 32%
capacity is substantia].ly greater thall a one step increase (four percent for this example) and, as will be explained in greater detai.l here:imaftcr, the programll)er 900 ls generally permitted to increase (or decrease) by only a single step for each cycle, special circuitry must be provided to permit the programmer, as well as the compressor itself, to increase capacity to the mi.ni.mum starting level within a single cycle.

- 5~ -.

'l`he increase to the minimum startillg capacity is accomplishecl in the followiny manner. When tl~e initial signal to increase capacity is generated, the control relay 894 ener~izes and closes the contacts 906 associated therewitllr The closing of the contacts 906 permits the programmer timer 903 to energize, or start timin~, at the same time that the com~ressor vane ang]e begins to increase by means of a current path througll normally closed contacts 907 associatecl Wit}l the programmer release timer 905, the contacts 906, ancl normally close~ contc-lcts 908 associated witll a supplemelltal minimu.rn vane relay 865. Thus, while the progran~er timcr 903 is timing, the vane angle on the compressor is being commensurately increased by a signal from terminal 854 to termillal ~56.
I~hell the proc~rammer timer 903 times out, contacts 904 associated therewith close, thereby energiziny the programmer release timer 905 and also energizing the programmer load coil 901 through normally closed contacts 869 associated wi-th an unload relay 874, and contacts 909 associated with the supple-mental minimum vane relay 8h5. Only a momelltdry pulse is required to shift tlle programmer by one step; thus the pro-grammer release timer 905 is set to time out quickly, for example, after l/10th sccond. I~hen the programmer release timer 905 times out, the contacts 907 associated therewith open and reset the programmer timer 903. This in turn opens the contacts 904 associated with the timer 903, WlliCll breaks the current path to the programmer load coll 901 (resulting in an increase of only one step) and also resets the release timer 905.
Ilowever, since the compressor vane angle has not yet ~.~35368 reached the minimum startina pOsitiOIl, the current patl~
to the progralllmer timer 903 still exists due to the contacts 908 associated ~1ith the minimum vane relay 860; tllis causes the timer 903 to begin timing as before. Likewise, the contacts 858 contillue to permit tlle vane angle on the compressor to increase by the signal at terminal 856. 'I`he programmer 900 thus contillues to indicate incremental increases in capacity, and the compressor vane angle continues to increase until the minimum vane relay 8G0 enerc3izes. This occurs whell the pro--qranuner 900 completes a circult between terlninals "1\" and "B" ( shown on bot]l Flgures 6a and 6b) . ~1hen the programmer 900 reaches the step 910 which has been pre-selected as the minimum vane posltion for start up (32% capacity, or the eight) step, for tile present eY~ample), the signa] at terminal "~"
is transmitted through the contacts 910 to energize the latch relay 862, t~hich throucJh its contacts 861 causes tlle minimum vane relay 860 to enerclize. In turn, normally open contacts 863 associated therewith are caused to close, thereby energizin~
a supplemental minimum vane relay 8fi5. 'l`his causes the minimum vane contacts 858 ancl the supplemental minimum vane contacts 908 to open, thereby hreaking the current paths to both the E)rogrammer timer 903 and the terminal 856. Thus, botll the programmer 900 and the vanes of the compressor stop increasing once the minilllum vane position is reached.
l~lso, once the minimum vane relay 860 energizes, oth i t and the supplemental minimum vane relay 865 are locked into the energized state by means of minimum vane contacts 864 and contacts 866 associated with tlle compressor start relay : ' .

. 1~353t:;8 836, ~s wi]l be apparent hereinafter, the mi.nimum vane relay 8~0 wi.ll therefore remai.n energized ulltil the com-pressor start relay 836 deenergizes. This occurs only upon a system failure, or Oll normal shut clown, ~oth of ~ ich are more comple-tely described hereillafter.
If, after the vane angle has been increasecl to the minimum position as described above, the thermostat still indicates a need .~or increased cooling, a sigllal is provided at the ter~inal 816, and the above process is in rnajor portion repeateci. That is, po~ler is applied to the terminal 816 to energi.ze the load timer 818 whell "on cycle" timer 810 is timing. ~gain, the load relay 822 enerc3izes. Since the compressor start rel.ay ~36 has remained energi%ed from the initial loading cycle, the compressor and pump have both started.
sy completing the circuit between terminals 854 and 856, whic~
is accomplished throuc3h normally open contacts 868 associated with the now-cnerc3ized load relay 822, an increase signal is applied to the valle angle control of the compressor.- llowever, the vane anyle is permi.tted to increase by only one increment of the programmer, or fo~r percent, because of the programmer timer 902.
. I~hen the load relay 822 energizc~s, contacts 910 associated therewith cause the programmer timer 903 to start timing. When the programmer timer 903 times out, the contacts 904 associated therewitll caused the load programmer coil 901 to energize through normally open contacts 911 associated with the load re].ay 822. The closing of the contacts 904 also starts the progranllner release timer 905 timing. Ilowever, wllen the 1~353~B

.
programmeT^ timer 903 times out, another normally open contact associclted therewitll closes, and eneryi7.es th~ vane increment relay 830.
EnercJizing of the relay 830 causes the contacts 828 associated therewith to open, thereby deenergizing the load relay 822. ~^1hc~n the load relay 822 deenergizes, contacts 910 open and reset the programmer timer 903. This in turn opens the contacts 904, which resets the programmer release timer ~06. Because of the time setting on the release timer 90S, and the inherent speed at whicll such contacts open or deenergize, the programme~ release timer will be reset well before it times out. Thus the programmer will increment only one step. Likewise, when the load relay 822 deenergizes, its contacts 868 open and remove the signal from the terminal 856.
Thus both the prOc3ralMler 900 and the compressor vanes undergo oen incremental increase. It should be noted that the vane increment re]ay 830, once energi2ed, is locked into the energized state via its own contacts 913. This prevents cycling when the programmer timer 903 is reset. If additional increases in capacity are required, the above process is re-peated until the thermostat connected to terminal 816 indicates that no increase is req~ired.
Presuming that the system has been designed with sufficient capacity to adequately chill the allotted space, ~5 the thermostat will eve~tually indicate that no further increase in capacity is required. Likewise, no decrease in capacity will be required. In such a case, the system will be in balance, ancl an lndicator 914 will be activatecl, by a - , .

~L1353~

eurrent path tl~rougll normally elosed contacts 915 assoeiated with the ]oaci timer 818, and normally closed contacts 916 associated wi th Ull load timer 872.
~t some pOillt, as toward evening, the thermostat S will indicate cluring the llonll cyele, at terminal 870, that a decrease in caE~aeity is required. This causes the un]oad timer 872 to energize, and also lic~hts an unload indicator 874.
When the unload t;.mer 872 energizes, a circuit is completed to unload relay 874 through normally open contaets 876 associated wi.th the unload timer 872, normally cl.osed contaets 878 assoeiated with tlle relay 830, normal].y open eontaets 873 assoeiated with the supplementcll minimum vane relay 865, normally open eontaets 880 assoeiated with the eompressor start relay 836, and normally open eontaets 882 assoeiated with the zero percent capaeity positi.on of thc? programmer 900, shown in Fig. 6b. Once the unload relay 874 energizes, normally open contacts 89() associated tllerewith elose and, through normally open eontacts 892 assoeiated with the control relay 894 a cireuit is eompleted between terminals 854 ancl 896, thereby signaling the eompressor to cleerease capaeity by deereasing vane angle. ~n indieator 897 is also aetivatc?d. ~gain, the programmc?r timer 903 and vane inerement relay 83n eom~ine to cause only an incrc)mental decrease.
More specifieally, when the unload timer energizes, contacts 917 associated therewith elose and eause the programmer timer 903 to start timing as an unload signal is applied to terminal 896. Whell the programmer timer 903 times out, eontaets 904 elose and a si.gllal is applied to the unload procJra~ner 1: L353~

coil 902 througll normally open conta;cts 918 associated with the urlload relay 902. l'llis signal causes the programmer 900 to decrement; however, the clecl^emellt is limited to one step by a sequence similar to loading. When the programmer timer 903 times out, the contacts 912 close and energize the vane increment relay 830, whicll then locks itself in via contacts 913.
Tlle ellerc)i~.ing of tlle relay 830 opens the con~acts 878, wllicll deenercJizes the unloac-l relay 874. This in turn opens contacts 917, therehy resetting the programmer timer 903.
This causes the procJrammer release timer 905 to be reset before it times out, resultillcJ in only a single, incremental decrease both in the compressor vane angle, and in the programmer 900.
The cal>acity of the eompressor will increase or decrease as clemanded by the thermostat in aecordance with the above sequences. ~s clemand for chilling decreases, capacity will be correspondinyly reduced. ~t some point, the capacity of the compressor wi]l be reduced to an arbitrarily low value, preferably less than the minimum vane opening; for example, a 20% vane opening may be used. When capacity has been 50 reduced. a latch relay 930 (Figure 6b) enerCJizes and causes norma]ly open contacts 932 (fig. 6a) associated therewith to close. (It should be noted that the latch relay 862 was released when the programmer decremented below the minimum starting capacity, at step 931.) This completes a current path from the terminal 800 to a low load recycle relay 845, throuc;h norJnal]y closecl contacts 803 associated with the failure timer 842 and normally closed contacts 80S associated Wit]l an anti-. ` ~13S3~

recycle timer 936, as well as normal.ly open contacts 938 associated with the minimum vane relay 860, ancl a normally closed lock-out switch 939. This completed path causes the low load recycle relay 84G to energize, lights a low load indicator 940, and starts the timin~ of the anti-recycle timer 936. The recycle relay 846 locks itself in by means of norma]ly open contacts 942 wllicll shunt tlle contacts 938 and 932.
Whell the recycle relay 846 energizes, normally closed contact~ 844 associ.atcd therewitll open, causing the compressor start relay 83G to deenerc~ize. This shuts off both the compressor and the compressor pump, through loss of a siynal at termi.nals 852 and 83. Thus the system shuts down chilling in response to lost demand. The compressor.is then caused to unload to %ero capacity by means of normally closed contacts 919 associated witll the relay 836, which keeps an unload signal on both the programmer 900 and the terminal 896 (from the terminal 920 and contacts 921 associated with the control relay 849) until the zero capacity contacts 886 on the programmer 900 break the circuit to the unload relay 874.
The system i.s permitted to again sense demand, via the thermostat, once the anti-recycle timer 93G llas timed out.
The anti-recycle timer 936 preferably has a timiny cycle wllich may be varied between 0 and 60 minutes, to permit stabiliza-tion of the temperature in the space being chilled. When theanti-recycle timer times out, contacts 805 associate~ therewith open, and break the current path to recycle~relay U~6. When ~- l ) U/ ~. J 'I
~L~3536~

the recycle relay 846 deener~izes, the system is restored to tlle "start" status described above, and is permitted to sense tlle therMostat in the manner tllere described. Recycling in this manner can be prevented merely by opening the recycle lockout switcll 939.
~ s with the multiple compressor control device described previously, various fault detection circuits and safety devices have been built into the single compressor control device shown in Figures 6a-b. One such circuit is the panel failure timer 842, which is preferably an adjustable timer with up to a 60 minute cycle. Whenever either the load relay 822 or the unload relay 874 energize, the panel failure timer 842 is started timing through either the load timer 818 or tlle unload timer 872. ~s previously notecl, the load timer 818 energizes in response to the thermostat's demand for chilling a-t the terminal 816, which causes normally open contacts 940 associated therewith to close.
IE the compressor start relay 836 is not yet ener-gized, the panel Lailure timer 842 may be started timing throug}l normal]y closed contacts 954 associated with com-pressor start relay 836 and normally closed contacts 952, associated with the compressor start relay 836. If the start relay 836 is energizecl, the panel failure timer 842 is started timing -througll the contacts 950, normally closed contacts 956 associated witll the load relay 822, and normally closed contacts 958 associated with tlle unload relay 874. If an unload signal is received at the terminal 870 from the thermostat, the unload timer 872 is energized. In turn, normally open contacts 960 113S3~1~
. .
associated tllerewith alld corlllectc~d in parallel wi.th the contacts 950, close to start the panel failure timer 842 timin~. Nor-mally closed contacts 955 associated wi.th the minimum vane relay 860 ensure that the compréssor start relay 83G also enercJizes properly.
Once the pallel ~ai].ure timer 842 starts timing, either load relay 822 or unload relay 874 must energi.ze before the panel failure timer 842 times out or the compressor will be prevented Erom intreasing in load capacity; this oE course olllv occurs in the event of a panel failure. On initial start-up, the compl^essor start relay 836 mllst also energize to open contacts 952. In normal operation, the timer 842 is reset either Wllen the l.oad relay 822 enerc~ize.s and opens contacts 956 or whell the unload relay 874 energizes and opens contacts 958.
In the evellt neither load relay 822 nor unload relay 872 resets ~allel failure timer 842 before it times out, normally open contacts 970 associated with the timer 842 close and enercJi7e a panel failure relay 972 and failure indlcators 974. Likewi.se normally eloseci contacts 976 associated with the timer 842 open, ancl shut off a "system operational" lndicator 978, as well as an lndlcator 979 wilich is extinc~ui.shecl by the opening of panel. Eailure relay contacts 922. Whell tl-e panel failure relay ener~izes, eontacts 9~0 associated therewith close and lock in the panel failure timer 842. This causes the contacts 840 to open ancl deenergi.ze the compressor start relay 836, whicll ~revents increasil-~g the vane angle o~ the compressor despite demands for increased capacity from the thermostat because ol~ control relay contacts 857.

.

135;~
, . . .
~lthough thc compressol- is prevcl-ted from increasillcJ
capacity Ol-CC' tlle pallC 1 failure relay l~as enerylze(1 tlle compressor is permitted to continue runlling at the same capacity by means of panel failure relay contacts 982 (connected between terminals 850 and 352) and 934 (connected between terminals 851 and 853). The compressor is permi-ttecl to decrease capacity t~lrough the internal controls of the compressor, W]liC]I override the control device when the panel failure relay 972 is energi~ed.
Momentary contact swithces 986 and s8a are provided to manually ener3ize the load re]ay 822 and the unload relay 874, respectively, tllereby overridlncJ the automatic contro]s of the system whell the start switch 802 is used in the manual mode. It should be noted that in tlle event of a system failurc the system shuts down without unloading the proyrammer 900.
Thus the load capacity at time of failure is available as a diagnostic aid. ~lso as an operational aid indicators 988 may be provided to inclicate operatinc3 load capacity.
~lthough the above detailed clescription of a single compressor control device embodyinc3 the present invention has dealt substantially Witll an electromecllanical system it is to be understood tllat tllis system may readily be embodied in soli~ state systems such as shown for the double compressor systems in Figs. 4a-43 and Fig. 5.
Ilavinc3 fuLly described the invention it is to be understood that it is not to be limited to the details herein set forth, but that tlle invention is of the full scope of the appended clai.ms.

. ~4

Claims (26)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A refrigeration system control device for incrementally varying the chilling capacity of at least one variable capacity compressor used for controlling the temperature of a medium, said control device being connectable to temperature sensing means for measuring the temperature of the medium, said means providing an output signal, comprising means for generating a prescribed cyclic ON and OFF
signal, timing means providing a signal having a period matched to the time required for the capacity of said variable capacity compressor to vary by a prescribed increment, said period being less than the ON time of the cyclic signal, and a first load and unload means, adapted for connection to said timing means and to the temperature sensing means thereby being responsive to temperature variation, for causing the capacity of said at least one compressor to be varied by the increment defined by said timing means in response to a signal from said temperature sensing means only during the ON portion of the signal from said generating means.

2. A device according to Claim 1 wherein the range of capacities of said variable capacity compressor is divided into at least twenty-five said increments.

3. A device according to Claim 1 further including means for causing the capacity of said variable capacity compressor to be increased to a minimum starting capacity when a first signal demanding an increase in capacity is received from said temperature sensing means during the ON portion of the cyclic signal.

4. A device according to Claim 1, 2 or 3 including means for preventing said first load and unload means from responding to signals from said temperature sensing means indicating a need for increased chilling capacity for a pre-determined period of time after said chilling capacity of said variable capacity compressor has been reduced to a predetermined low level, even though said signals from said temperature sensing means are present during the ON
portion of the cyclic signal.

5. A device according to Claim 1, 2 or 3 including failure detection means for detecting failures in said control device or said compressor and for preventing said load and unload means from attempting to increase the capacity of said compressor if a failure is detected.

6. A device according to Claim 1 wherein said timing means has a total timing sequence substantially identical with the time for said compressor to change capacity successively from a minimum setting to a maximum setting but is mechanically independent therefrom and generates a predetermined number of timing signals representative of the actual capacity of said compressor.

7. A device as claimed in Claim 6 wherein said timing means is a program motor and vane switches.

8. A devise as claimed in Claim 6 wherein said timing means is a counter.

9. A refrigeration system control device according to Claim 1 for controlling two variable capacity compressors comprising second load and unload means adapted for connection to said temperature sensing means for causing the capacity of said second compressor to vary by a second increment defined by said timing means in response to a signal from said temperature sensing means during the ON portion of the signal from said generating means, and transfer means for automatically activating either said first load and unload means or said second load and unload means during the ON portion of the signal from said generating means.

10. A device according to Claim 9 wherein said transfer means causes said variations in capacity to be alternated between said first compressor and said second compressor.

11. A refrigeration system control device for use with two variable capacity compressors comprising a first timing signal means for generating a cyclic ON and OFF signal, a first load and unload means adapted for connection to temperature sensing means for automatically generating a signal for varying the capacity of the first of said compressors in response to a signal from said temperature sensing means during the ON signal of said first timing signal means, second timing signal means for generating a prescribed period in response to said signal from said first load and unload means, said second timing signal means having first and second states, a second load and unload means adapted for connection to said temperature sensing means for automatically generating a signal for varying the capacity of the second of said compressors in response to a signal from said temperature sensing means during the ON signal of said first timing signal means, third timing signal means for generating a second prescribed period in response to said signal from said second load and unload means, said third timing signal means having first and second states, and a first transfer means for automatically activating either said first load and unload means or said second load and unload means during the ON signal of said first timing signal means in response to the signals from said second and third timing signal means, wherein said first load and unload means is activated only when said third timing signal means is in a first predetermined state and said second load and unload means is activated only when said second timing signal means is in a first predetermined state, and wherein said prescribed periods match the times required to change the respective capacities of the compressors by predetermined increments.

12. A device according to Claim 11 further comprising a fourth timing signal means activated by said first load and unload means, said fourth timing signal means having first and second states, a fifth timing signal means activated by said second load and unload means, said fifth timing signal means having first and second states, and a second transfer means for automatically activating either said first load and unload means or said second load and unload means during the ON signal of said first timing

Claim 12 - cont'd means in response to signals generated by said fourth and fifth timing signal means, wherein said first load and unload means is activated when said fifth timing means is in a first predetermined state and said second load and unload means is activated when said fourth timing means is in a first predetermined state, said second timing signal means being activated only when said first load and unload means generates a signal for increasing the capacity of said first compressor, said third timing signal means being activated only when said second load and unload means generates a signal for increasing the capacity of said second compressor, said fourth timing signal means being activated only when said first load and unload means generates a signal for decreasing the capacity of said first compressor, and said fifth timing signal means being activated only when said second load and unload means generates a signal for decreasing the capacity of said second compressor.

13. A device as claimed in Claim 12, further including a first means for generating a system pump start signal in response to said signal generated by said first load and unload means for increasing the capacity of said first compressor, and a second means for generating a compressor pump start signal in response to said signal generated by said second load and unload means for increasing the capacity of said second compressor.

14. A device as claimed in Claim 13 further comprising means for generating a first compressor start signal in response to said signal generated by said compressor pump start means.

15. A device as claimed in Claim 14 further comprising means for sensing that said first compressor is operating at a predetermined maximum load and generating a signal therefrom, and means for generating a signal for starting said second compressor in response to said signal from said maximum load signal means.

16. A device as claimed in Claim 15 further comprising means for generating a first signal in response to said first compressor maximum load signal which causes said first load and unload means to generate a signal. for decreasing the capacity of said first compressor, and means for generating a second signal in response to said first compressor maximum load signal which causes said second load and unload means to generate a signal for decreasing the capacity of said second compressor.

17. A device as claimed in Claim 16 further comprising first means for causing said first load and unload means to terminate said capacity-decreasing signal when the capacity of said first compressor has reached a predetermined capacity, and second means for causing said second load and unload means to terminate said capacity-increasing signal when said second compressor has reached a predetermined capacity.

18. A device according to Claim 11 further comprising sequencing means having first and second states for selecting said first compressor as a lead compressor and said second compressor as a lag compressor when said sequencing means is in said first state, and for selecting said second compressor as a lead compressor and said first compressor as a lag compressor when said sequencing means is in said second state, said lead compressor being started and loaded to maximum capacity before said lag compressor is started.

19. A device according to Claim 17 further comprising means for sensing the capacity of the first of said compressors, said sensing means comprising a program motor responsive to the signals generated by said first load and unload means for varying the capacity of the first of said compressors, and means for sensing the capacity of the second of said compressors, said sensing means comprising a program motor responsive to said signals generated by said second load and unload means for varying the capacity of the second of said compressors.

20. A device as claimed in Claim 19 where said capacity sensing means are adapted to be additionally responsive to unload signals generated by said first and second compressors.

21. A device according to Claim 11, 17 or 18 having first sensing means for detecting a failure of one of said compressors and generating a signal thereupon, first lockout means for disconnecting power from said failed compressor in response to a signal from said first sensing means, and second lockout means for preventing said remaining compressors from restarting following a system shutdown in response to a signal from said first lockout means.

22. A device according to Claim 11 further including means for causing the capacity of each of said variable capacity compressors to be increased to a minimum starting capacity when the respective load and unload means receives a first signal for an increase in capacity from said temperature sensing means during the ON portion of the cyclic signal.

23. A control device according to Claim 22 wherein said transfer means causes said first compressor to receive all load and unload signals until the capacity of said first compressor is increased to a predetermined maximum.

24. A device according to Claim 23 wherein said transfer means responds to said first compressor reaching said predetermined maximum capacity by causing the capacity of said first compressor to be reduced to a predetermined level and the capacity of said second compressor to be increased to a minimum starting level.

25. A device according to Claim 11, further including means for preventing said first and second load and unload means from responding to the signal from said temperature sensing means indicating a need for increased chilling capacity for a predetermined period of time after the capacities of both of said compressors have been reduced to a predetermined low level, even though said signal from said temperature sensing means is present during the ON portion of the cyclic signal.

26. A device according to Claim 25 wherein said second compressor is caused to cease chilling if the chilling capacity thereof is reduced below a minimum operating level.