COMPRESSOR MOTOR CONTROL

Abstract

A compressor, which is susceptible to protective shutdowns when certain operating conditions are sensed, includes a control feature wherein, when the compressor is restarted after a period of time, it may be caused to operate in an unloaded mode such that a reoccurrence of the shutdowns is less likely.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to refrigerant systems and, more particularly, to selectively unloading the compressor of a refrigerant system.

Air conditioning and refrigeration systems typically include protective mechanisms which sense when a compressor becomes overloaded. When such condition is sensed the compressor is typically shut down for a period of time and then typically restarted again after the shutdown. In some cases, when a catastrophic failure is detected, the compressor may not be re-started until the root cause of the failure is identified by an operator, technician or maintenance personnel. Typically, the protective mechanism is associated with the measurement of the electric current in the motor and/or the motor temperature or a direct effect of these parameters on the protection device such as an electric motor line break that is tripped in response thereto.

This condition will normally occur in high temperature ambient environments when the cooling is most needed, and the refrigerant system operates at extremely high loads to satisfy these cooling demands. That is, during the period in which if the compressor motor is shut down as a result of protection device engagement, there will be no cooling for an occupant of the conditioned space or food refrigeration in the refrigeration chamber. When the motor is re-started; it will still be subjected to high load conditions and is likely to be shut down once again in a relatively short period of time. Therefore, there may be prolonged periods of higher than desired temperature conditions when there is little or no cooling being provided, thereto causing discomfort to an occupant of the climate-controlled environment or spoilage of the food in the refrigeration container. It is desirable to obtain some cooling, even if at a reduced rate, during these periods of time when extreme environmental conditions are imposed on the refrigerant system.

DISCLOSURE OF THE INVENTION

Briefly, in accordance with one aspect of the invention, at or near the time in which the compressor motor is re-started after a line break shutdown has occurred, the compressor is caused to operate at a reduced load mode such that it is less likely to result in another line break shutdown occurrence.

In accordance with another aspect of the invention, when a compressor shutdown is anticipated and likely to occur, the compressor is switched to the next less loaded mode of operation to avoid or prevent shutdown conditions.

In accordance with yet another aspect of the invention, the unloading of the compressor may be accomplished by switching the refrigerant system from an economized to a non-economized mode, from a non-economized mode to a bypass mode, throttling the flow through a suction modulation valve, or operating a suction modulation valve in the pulse width modulation manner.

In accordance with another aspect of the invention, the unloading of the compressor may be accomplished by at least one of the following means: a variable speed drive for the compressor motor, a multi-speed drive for the compressor motor, permanent disengagement of at least some of the compression elements, engagement-disengagement of the compression elements in the pulse width modulation manner, a variable air management system and a variable number of active heat exchangers in the refrigerant system.

By still another aspect of the invention, provision may be made to count the number of line break shutdown occurrences and to cause a compressor unloading only after a predetermined number of line break shutdowns occur within a predetermined period of time.

By yet another aspect of the invention, provision may be made to return to a more loaded mode of operation after certain predetermined conditions had been met.

In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the present invention as incorporated into a refrigerant system.

FIG. 2 is a flow chart depicting an exemplary method of control in accordance with the present invention.

FIG. 3 is a graphic illustration of the results of the present invention implementation.

DETAILED DESCRIPTION OF THE INVENTION

There is schematically shown in FIG. 1, a refrigerant circuit in which a compressor 11, a condenser, 12 an expansion device 13 and an evaporator 14 are connected in typical serial refrigerant flow relationship. The condenser 12 is positioned such that ambient air is passed thereover by a condenser fan 16 for purposes of de-superheating, condensing and then subcooling the refrigerant vapor being received from the compressor 11. As known, the condenser 12 of a sub-critical cycle becomes a gas cooler for trans-critical applications, where it operates above the refrigerant critical point. The evaporator 14 is positioned to receive return air from a conditioned space, which may be mixed with a portion of fresh air, to be cooled and for delivering the cooled air to the climate-controlled environment by way of an evaporator fan 17. The refrigerant is evaporated and typically superheated in the evaporator 14. Although a basic refrigerant system arrangement is depicted in FIG. 1, various options and enhancement features are feasible and known in the art. All these system configurations are within the scope and can equally benefit from the invention.

As will be understood, the refrigerant system compressor 11 is subject to a variable load depending on various factors such as ambient temperature, thermal (sensible and latent) load demand in the conditioned space, electrical power supplied to the compressor, etc. Accordingly, provision is made for operating the compressor 11 in various modes to match cooling or heating demands in the climate-controlled space.

In the exemplary refrigerant system schematic shown in FIG. 1, in order to vary the degree of loading or unloading of the compressor 11, there are provided several unloading features such as an economizer 18, a bypass valve 19 and a suction modulating valve 21. Although a discussion below is limited only to these three unloading features, other options may be provided, that may include, but are not limited to, variable or multi-speed compressor motors, variable air management systems, a variable number of components (such as heat exchangers) operating in parallel, etc. All these unloading steps are well within the scope of the invention.

For economized operation, such as, for example, when an additional capacity is needed, as during pulldown or at steady-state operation at high ambient temperatures or deep refrigeration conditions, the liquid refrigerant in the main circuit entering the evaporator expansion device 13 is further cooled by operation of the economizer heat exchanger 18. To activate the economizer heat exchanger 18, the economizer expansion device 22 is opened so as to allow a portion of the refrigerant to flow through the economizer heat exchanger 18 to absorb heat from the liquid refrigerant flowing to the evaporator expansion device 13. Therefore, thermal cooling potential of the refrigerant entering the evaporator 14 is increased, enhancing performance (capacity and efficiency) of the refrigerant system. The resultant “medium” temperature/pressure economizer refrigerant vapor is injected into the compressor 11 at a mid-point thereof. This economizer flow is mixed with the partially compressed suction refrigerant vapor, and typically cools this suction vapor to a lower temperature. Since the economizer refrigerant is entering the compressor 11 at a higher pressure then the refrigerant entering through a suction port of the compressor 11, less energy (per a unit of mass) is required to compress this refrigerant vapor from the economizer pressure to the discharge pressure, as compared to the compression of the suction vapor from the suction pressure to the discharge pressure. Regardless, overall compressor power is increased, due to additional amount of refrigerant that is injected through the economizer port (or ports) and needed to be compressed and then circulated throughout the system. Therefore, the economized mode of operation imposes a higher load on the compressor 11, in comparison to the conventional (non-economized) mode of operation. It should be pointed out that if the economizer expansion device 22 does not have a shutdown capability, an additional flow control device should be integrated into the economizer circuit. Also, as known in the art, there are many variations of the economizer cycle schematic shown in FIG. 1, all of which are within the scope of the invention.

The bypass mode of operation is activated by opening a bypass valve 19 which allows a portion of the partially compressed refrigerant vapor to pass back to a suction port of the compressor 11 so as to thereby decrease the compressor power (since a reduced amount of refrigerant is compressed from a mid-stage pressure to a discharge pressure) and refrigerant system capacity (since a lower amount of refrigerant is circulated through the evaporator 14).

To operate the refrigerant system in the suction modulation mode, the suction modulation valve 21 is selectively moved to a partially closed position so as to decrease the flow of refrigerant to the compressor 11. This is intended to further balance the compressor capacity with the thermal load in the climate-controlled space. In this mode of operation, a liquid injection solenoid valve (not shown) may be selectively opened as required to provide a sufficient amount of expanded liquid refrigerant, flow into the suction port of the compressor 11 for cooling of the compressor motor. It should be noted that the suction modulation valve 21 may be substituted by a pulse width modulation valve or the like to control refrigerant flow into the suction port of the compressor 11.

As was mentioned before, any other unloading techniques, such as, for instance, in addition to examples disclosed above, disengaging of some pistons of a multi-piston reciprocating compressor or controlling engagement of the scroll elements of a scroll compressor in a pulse width modulation manner, are within the scope of the invention.

Implementation of the various devices for loading/unloading the compressor 11 as described hereinabove is accomplished by way of a controller 23. That is, the controller 23 is connected, to the economizer expansion device 22 by way of an electrical conduit 24, to the compressor 11 by way of an electrical conduit 26, to the bypass valve 19 by way of an electrical conduit 27 and to the suction modulation valve 21 by way of an electrical conduit 28.

Also sensors 29 and 31, which are connected to the controller 23 by electrical conduits 32 and 33, respectively, may be provided. A sensor 29 is provided to sense the current in the motor driving the compressor 11, and sensor 31 is provided to sense the temperature of the drive motor for that compressor 11. One or both of these sensed conditions may be used to determine that the compressor 11 is in an overloaded condition (or is approaching an overloaded condition) and needs to be temporarily shut down. On the other hand, this temporary shutdown may be accomplished by a line break, which could be, for instance, a bi-metal plate contactor, installed on the compressor motor and which automatically reacts to excessive current and/or temperature. Other sensors can also be installed to determine if the compressor operation is approaching the overloaded condition. Such sensors may, for example, include pressure sensors installed either on the high or low pressure side of the refrigerant system, as well as temperature sensors associated with the refrigerant system condenser and/or evaporator heat exchangers. Also, the current or power sensors can sense compressor overload conditions directly.

Referring now to FIG. 2, an exemplary flow control diagram is shown to indicate the manner in which the controller 23 may operate after a line break shutdown has occurred, so as to allow the compressor 11 to be brought back into operation at an unloaded state so that at least some, although reduced, cooling can be accomplished, while the refrigerant system is operating at a reduced capacity, at a time when the compressor 11 may otherwise be forced into another line break shutdown resulting in another prolonged period of non-operation.

In a block 34, the counter, which counts the number of shutdowns in sequence, is set to C=0. Periodically, after a certain time interval expired, in which the compressor 11 was operated in a loaded mode as shown in a block 36, the system is queried as to whether a shutdown on a line, break has occurred as shown in a block 37. If not, the system continues to operate until a shutdown has occurred in which case, the control steps to a block 38. The counter then resets to C=C+1, and then the query is made in a block 39, whether C=2 (i.e. whether there have been two line breaks in sequence). In this regard, it should be understood that in this particular example, the threshold for implementing the control method of the present invention has been set at two line break occurrences. This, of course, can be varied as desired for any particular application.

If, in a block 39, C does not equal 2 (i.e. this is not the second line break in succession), or is less than 2 and equals to 1, in this particular example, then a timer is set at 0 in a block 41, and the refrigerant system operation is initiated in the same loaded mode, after the line break is returned to its operational state. If C is found to be equal to 2, then two consecutive shutdowns have occurred, and the controller 23 steps to a block 40 wherein the system is queried as to whether the time since the last line break shutdown is greater then t₁, which is a time period which has been predetermined to be an allowable time interval between consecutive shutdowns and therefore giving reason for not implementing the unloading mode of operation, since in that case, two consecutive shutdowns are unlikely to be caused by the same factor. Therefore if t has been determined to be greater than t₁, then the controller 23 steps to a block 42 to reset the counter back to C=1. Further, as before, the timer is set at t=0 in the block 41 and the refrigerant system is made to operate in the same loaded mode as shown in the block 36.

If t does not exceed t₁, then most likely the same factor caused two consecutive shutdowns, and the controller 23 steps to a block 43 to reset the timer to t=0, and then to a block 44 where the compressor motor is restarted in next in sequence unloaded mode of operation. At this point, the particular mode of operation for the compressor 11 has changed such that it made to operate in a more unloaded state than the state which caused the previous line break shutdown. For instance, for the exemplary refrigerant system shown in FIG. 1, the sequence of operational modes from the most loaded mode to the least loaded mode would be as follows: economized, fully loaded mode; non-economized fully loaded mode; non-economized bypass mode; and non-economized bypass mode with suction modulation valve.

Thus, presuming that, when the second line break shutdown occurred, the system was operating in an economized, fully loaded mode, then, in a block 44, the compressor may be restarted as operating in a non-economized, fully loaded mode. If another line break shutdown then occurs as at a block 46, then the refrigerant system would proceed to a block 47 so that the loaded mode is reset to non-economized, fully loaded operation now.

If a line break shutdown does not occur as shown in the block 46, then the refrigerant system controller 23 proceeds to a block 48 and it is queried whether the time interval in the unloaded mode of operation is greater than t₂, which is a predetermined sufficiently safe time interval after which the refrigerant system may try to restart in the loaded mode of operation. If t is greater than t₂, then the control 23 resets to the block 34 and the cycle is repeated. lithe time t is less than t₂, then the system resets to the block 44, and operation in the unloaded mode continues.

It should be understood that the above flow diagram is only one example of the method, with particular values of certain parameters being used. Thus, the number of line break shutdowns that occurs prior to the unloading step can be selected as desired, as can the values t₁ and t₂. Further, rather than counting on predetermined time intervals t₁ and t₂, the refrigerant system may be “tested” in a more loaded mode of operation by relying on the feedback from the sensors positioned within the refrigerant system and associated environments. These sensors, such as, for instance, temperature and/or pressure sensors, and their preferred locations are known in the art.

Referring now to FIG. 3, there is shown a typical example of various temperature trends that may occur during operation in more and less loaded modes. As is common, the ambient temperature T_amb tends to increase from early morning until late afternoon. Thus, when operating without the control features as described hereinabove, the line break temperature in a loaded mode of operation T_load, may reach the cutoff temperature limit T_cutoff, as shown. However, if the system is operating with the control features as discussed hereinabove, then the line break temperature in an unloaded mode of operation, T_unload, remains below the cutoff temperature limit T_cutoff in which the compressor would be shut down. Thus, the refrigerant system continues to operate and provide conditioned air to a climate-controlled space.

It should be noted that the refrigerant system control 23 may operate proactively to potentially prevent any line break shutdowns by keeping compressor operational parameters within the predetermined allowable ranges, by sequentially unloading the compressor 11. In this case, the control 23 would rely on feedback from the sensors positioned within the refrigerant system and providing information associated with the critical operational parameters for the compressor 11. These sensors, such as, for instance, temperature sensors, pressure sensors, current sensors and power sensors, and their preferred locations, are known in the art. Such sensors may directly measure critical operational parameters for the compressor 11 or provide sufficient data to the control 23 that could be translated into the critical parameter values.

It should also be understood that in the context of the above embodiments, a compressor can be selected from a variety of compressor types, including reciprocating, screw, scroll, centrifugal or axial compressors. Each compressor can be represented by multiple compressors. For example, a compressor may consist of several sequential centrifugal compressor stages. Also, multiple compressors may operate in parallel or tandem arrangement. Further, this invention can be applied to different refrigerant system types, including residential or commercial cooling and heating applications. It can also be used for providing cooling and refrigeration in supermarkets, truck-trailer and container applications.

While certain preferred embodiments of the present invention have been disclosed in detail, it is to be understood that various modification in its structure may be adopted without departing from the spirit and scope of the invention.

Claims

1. A control for operating a refrigerant system of the type having a motor-driven compressor which is subject to variable load conditions,

An identification mechanism identifying a condition indicative of an excessive compressor load when operating the compressor in a first loaded mode; and

an unloading mechanism for responsively reducing the compressor load to prevent compressor shutdown.

2. A control as set forth in claim 1 wherein said identification mechanism comprises at least one sensor.

3. A control as set forth in claim 1 wherein said compressor load is associated with a compressor motor load.

4. A control as set forth in claim 2 wherein said at least one sensor includes a compressor motor current sensor.

5. A control as set forth in claim 2 wherein said at least one sensor includes a sensor for sensing the temperature of the compressor drive motor.

6. A control as set forth in claim 2 wherein said at least one sensor includes both a compressor motor current sensor and a compressor motor temperature sensor.

7. A control as set forth in claim 2 wherein said at least one sensor includes at least one of a temperature sensor and a pressure sensor sensing operating conditions within the refrigerant system.

8. A control as set forth in claim 1 wherein said unloading mechanism automatically reduces the load on the compressor motor by switching to one of the following modes of operation: non-economized and bypass to suction.

9. A control as set forth in claim 1 wherein said unloading mechanism automatically reduces the load on the compressor motor by at least one of the following means: a suction modulation valve, a suction pulse width modulation valve, a variable speed drive for the compressor motor, a multi-speed drive for the compressor motor, permanent disengagement of at least some of the compression elements, engagement-disengagement of compression elements in the pulse width modulation manner, variable air management system and a variable number of active heat exchangers in the refrigerant system.

10. A control as set forth in claim 1 and including a compressor drive motor overload protection which is adapted to shut down the operation of the compressor.

11. A control as set forth in claim 10 wherein said motor overload protection comprises a line break shutdown apparatus.

12. A control as set forth in claim 7 wherein said motor overload protection comprises an overload sensor.

13. A control as set forth in claim 10 and including a counter for counting the number of consecutive shutdowns by the compressor drive motor overload protection and a timer for recording the time from an initial shutdown of the compressor by the line break apparatus to the time of a subsequent shutdown of the compressor by the line break apparatus.

14. A control as set forth in claim 13 and including a tinier for recording the time from an initial shutdown of the compressor to the time of a subsequent shutdown of the compressor.

15. A control as set forth in claim 14 wherein said unloading mechanism reduces the load on the compressor drive motor only when the counter has recorded a predetermined number of shut offs within a predetermined period of time.

16. A control as set forth in claim 1 and including a loading mechanism for selectively and automatically increasing, the load on the compressor when certain predetermined conditions are met.

17. A control as set forth in claim 16 and including a timer for recording the time from the last shutdown of the compressor by the line break apparatus.

18. A control as set forth in claim 17 wherein a loading mechanism increases load on the compressor drive motor when a predetermined period of time from the last shutdown has expired.

19. A method of controlling a refrigeration system of the type having a motor driven compressor which is subject to variable load conditions comprising the steps of:

sensing a condition indicative of an excessive compressor load when operating the compressor in a first loaded mode; and

responsively reducing the compressor load to prevent compressor shutdown

20. A method as set forth in claim 19 wherein said sensed condition indicative of an excessive compressor load is one indicative of an excessive compressor motor load.

21. A method as set forth in claim 19 wherein said reducing step is initiated prior to an anticipated overload shutdown.

22. A method as set forth in claim 19 wherein said recovery step is initiated after the occurrence of an overload shutdown and prior to restarting the compressor drive motor after a period if time.

23. A method as set forth in claim 19 wherein said sensing step comprises the step of sensing the compressor motor current.

24. A method as set forth in claim 19 wherein said sensing step is that of sensing the temperature of the compressor drive motor.

25. A method as set forth in claim 19 wherein said sensing step involves the sensing of both the compressor motor current and the compressor motor temperature.

26. A method as set forth in claim 19 wherein said compressor load reducing step automatically reduces the load on the compressor motor by switching to one of the following modes of operation: a non-economized and bypass to suction.

27. A method as set forth in claim 19 wherein said load reducing step automatically reduces the load on the compressor motor by at least one of the following means: a suction modulation valve, a suction pulse width modulation valve, a variable speed drive for the compressor motor, a multi-speed drive for the compressor motor, permanent disengagement of at least some of the compression elements, engagement-disengagement of compression elements in the pulse width modulation manner, variable air management system and a variable number of active heat exchangers in the refrigerant system.

28. A method as set forth in claim 19 and including the step of counting the number of consecutive shut offs by the line break apparatus and recording the time from initial shutting off of the compressor by a line break apparatus to the time for a subsequent shutting off of the compressor a line break.

29. A method as set forth in claim 28 wherein said load reducing step involves reducing the load capacity mode of operation only when the counter has recorded a predetermined number of shut offs within a predetermined period of time.

30. A method as set forth in claim 19 and including the step of selectively increasing the load capacity mode of operation under certain predetermined conditions.