CONTINUOUS COMPRESSOR ENVELOPE PROTECTION

Abstract

A transport refrigeration system is provided with a control apparatus including an inverter and a microprocessor, with the microprocessor receiving signals representative of sensed values of the compressor discharge temperature and pressure, as well as the suction pressure, and controlling the inverter to responsively provide a selective level of electrical voltage and frequency to the compressor in order to maintain a desired compressor envelope.

Description

TECHNICAL FIELD

This invention relates generally to transport refrigeration systems and, more particularly, to control of a compressor motor drive in a vapor compression system therefor.

BACKGROUND OF THE INVENTION

Transport refrigeration systems are commonly used in refrigerated trucks, truck trailers and containers in order to preserve perishable cargo during transport from one location to another. Such a system includes the necessary components for a vapor compression cycle, including a compressor which necessarily includes some type of drive means. For containers and truck trailers, this function has generally been provided by a dedicated internal combustion engine with its speed being selectively varied in order to maintain a desired compressor envelope. In refrigerated trucks and vans, however, the compressor has generally been located within the main engine drive compartment, with the compressor then being driven by direct connection to the main engine drive. Conduits then serve to provide the closed circuit refrigerant flow to the condenser and evaporator units of the system.

With such a so-called direct drive arrangement, the speed of the compressor is dependent on the speed of the main engine drive. Thus, when the truck is proceeding at higher speeds down the highway, the compressor is driven at a high speed so as to obtain a high discharge pressure. However, when the engine is idling, for example, then the compressor will be driven at a relatively slow speed, and the discharge pressure and temperature will be relatively low. In order to protect the compressor envelope, it is therefore necessary to employ the selective use of switches in order to vary the speed of the variable speed condenser and evaporator fans or to modulate valves or stop the unit.

DISCLOSURE OF THE INVENTION

In accordance with one aspect of the invention, electrical power is provided from the generator to an inverter which is, in turn, controlled by a microprocessor receiving pressure and temperature sensed conditions from the compressor in order to regulate the power being provided to the compressor so as to maintain a desired compressor envelope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic illustration of the present invention as incorporated into a transport refrigeration system.

FIG. 2 is a more detailed schematic illustration thereof.

FIG. 3 is a graphic illustration of a compressor envelope.

FIG. 4 is a control logic flow diagram of the manner in which the compressor envelope is controlled in accordance with the present invention.

FIGS. 5A-5C show graphic illustrations of the critical parameters during pull-down conditions with the present invention incorporated in the system.

DETAILED DESCRIPTION OF THE INVENTION

The invention is shown generally at 10 in FIG. 1 to include a generator 11, which is driven by the power of the vehicle engine, an inverter 12, which receives unregulated voltage from the generator 11, a vapor compression system 13, which receives regulated power regulated (i.e voltage and frequency) from the inverter 12, a box 14 which receives cooled air from the vapor compression system 13, and a controller 16, which receives box temperature measurements (i.e. return air temperatures, RAT) from the box 14 along line 17, and pressure and temperature measurements from the vapor compression system 13 along lines 18 in order to control the inverter 12 by way of line 19. The controller 16 also sends cooling demand signals to the vapor compression system 13 by way of line 21. A more detailed illustration of the system is shown in FIG. 2.

Considering first the vehicle itself and the environment surrounding that vehicle, there is included a drive engine 22, a battery 23, a stand-by power source 24 and a box with a door 26 that is opened from time to time. Both air and heat are transferred from and to the box to ambient 27, primarily when the door is open. This heat transfer, of course, will greatly affect the operation of the vapor compression system 13 and therefore the control thereof. The control of the door openings and the speed of the engine 21 is determined by the drive cycle 28, which is controlled by the operator.

As mentioned hereinabove, the engine 22 drives a generator 11 which provides unregulated voltage and current to the inverter 12 for powering the vapor compression system 13. The inverter 12 also provides power to a heater 29 that may be required under certain ambient conditions.

The vapor compression system 13 includes, in serial flow relationship, a compressor 32, a condenser 33, a thermal expansion valve 34 and an evaporator 36. An oil separator 37 may be provided downstream of the compressor 32, and a receiver 38 may be provided downstream of the condenser 33. Also, a control valve 39 may be provided between the receiver 38 and the TXV 34. The condenser 33 includes a condenser fan 41, and the evaporator 36 includes an evaporator 42, with each of these fans being independently driven at selectively variable speeds by a dc motor.

Control of the system is by way of a microcontroller 43 which receives the various inputs as indicated and then which, responsively, sends signals to the inverter 12 in order to modulate the power (i.e. voltage, frequency and/or current) being provided to the compressor 32 along line 40. In particular, the inputs to the microcontroller 43 include the discharge temperature t_d and pressure P_d and the suction pressure P_s of the compressor 32 as indicated schematically at line 18. Also passing through microcontroller 43 is the return air temperature, RAT along line 17. The various conditions of the system as maintained by the microcontroller 43 are shown in a display 44 for the convenience of the operator.

Shown in FIG. 3 is a graphic illustration of the saturated discharge temperature as a function of the saturated suction temperature. The saturated suction temperature is equivalent to the suction pressure, and the saturated discharge temperature is equivalent to the discharge pressure, with the two parameters being the critical parameters that define the envelope of a variable speed compressor. That is, in order to protect the compressor and the system operation, it is desirable to maintain the saturated suction temperature between −40° C. and 2° C. Similarly, it is desirable to maintain the saturated discharge temperature between 10° C. and 66° C. This is accomplished by varying the power being provided by the inverter 12 to the compressor 32 in response to four sensed variables, return air temperature (RAT), discharge pressure P_d, discharge temperature t_d and suction pressure P_s. This is accomplished as shown in FIG. 4.

As shown, there are four different control modules: 1) the RAT control 48, the T_d control 49, the P_s control 51 and the P_d control 52. Each of the controllers 48-52 controls its own designated variable, with only one of the four controllers is acting at one time, maintaining its variable of interest at a desired set point value. The microcontroller 43 monitors the four sensed conditions and switches control from one controller to the other as specified by the switching logic as indicated. The purpose, of course, is to maintain the desired compressor envelope during all operating conditions.

The condition under which a vapor compression system is under the greatest demand is a condition known as pull-down. This is the process of restoring the operating temperature of a refrigerated space after the introduction of an extraordinary heat load. This would occur, for example, when a new load of unrefrigerated cargo is placed in a truck such that the temperature in the box is caused to increase to a level well above the desired set point. Under these conditions it is desirable to reduce the temperature in the box to the set point temperature as quickly as is reasonably possible.

In FIGS. 5A-5C, it will be seen that where continuous control of the compressor envelope is provided as described hereinabove, the variability of these various parameters is substantially lessened. In FIG. 5A, it will be seen that there is up to a 3.5° C. better control during about 1.5 hours periods of operation as compared with the non-controlled system. In FIG. 5B, it will be seen that the envelope routing control reduces cycling substantially in the later periods of operation, thereby having a direct impact on reliability. The same is true with respect to compressor speed as shown in FIG. 5C.

While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.

Claims

1. A control apparatus for a transport refrigeration system of the type having a generator driven by an engine with the generated electrical power being supplied to a compressor of a closed loop vapor compression system, comprising:

a temperature sensor for sensing the discharge temperature of the compressor and generating a temperature signal representative thereof;

a first pressure sensor for sensing the suction pressure of the compressor and generating a suction pressure signal representative thereof;

a second pressure sensor for sensing the discharge pressure of the compressor and generating a discharge pressure signal representative thereof;

a microprocessor for receiving said temperature signal, said suction pressure signal and said discharge pressure signal and generating an inverter control signal in response thereto; and

an inverter for receiving said inverter control signal from said microprocessor and for receiving electrical power from the generator and for providing a level of electrical power to the compressor in response to said inverter control signal.

2. A control apparatus as set forth in claim 1 wherein said transport refrigeration system includes a box being refrigerated by said vapor compression system.

3. A control apparatus as set forth in claim 2 wherein said control apparatus includes a temperature sensor for sensing the temperature of air being returned from said box and for responsively sending a return air temperature signal to said microprocessor, with said inverter control signal then being responsive thereto.

4. A control apparatus as set forth in claim 1 wherein the electric power from said generator to said inverter is unregulated ac voltage.

5. A control apparatus as set forth in claim 1 wherein said electrical power from said inverter to said compressor is regulated ac current.

6. A control apparatus as set forth in claim 5 wherein said power is regulated by selectively varying the voltage, frequency and/or current.

7. A control apparatus as set forth in claim 1 wherein the level of electrical power to the compressor is controlled in order to maintain the saturated suction temperature of the compressor within a predetermined range.

8. A control apparatus as set forth in claim 7 wherein said saturated suction temperature is maintained within a range of −40° C. to 2° C.

9. A control apparatus as set forth in claim 1 wherein the level of electrical power to the compressor is controlled so as maintain a saturated discharge temperature within a predetermined range.

10. A control apparatus as set forth in claim 9 wherein said range is between 10° C. and 66° C.

11. A control apparatus as set forth in claim 1 wherein said microprocessor includes at least three different control modules, with each module being controlled in response to sensed values of, and operating to control, parameters of discharge pressure, discharge temperature and/or suction pressure.

12. A control apparatus as set forth in claim 11 and including a fourth control module which is responsive to sensed values of, and operates to control, a return air temperature.

13. A control apparatus as set forth in claim 11 wherein only one control module is operated at a time.

14. A control apparatus as set forth in claim 11 wherein the control module which controls the discharge temperature has priority over the other control modules.

15. A method of controlling a transport refrigeration system of the type having a generator driven by an engine with the generated electrical power being supplied to a compressor of a closed loop compression system, comprising the steps of:

sensing the discharge temperature of the compressor and generating a temperature signal representative thereof;

sensing the suction pressure of the compressor and creating a suction pressure signal representative thereof;

sensing the discharge pressure of the compressor and generating a discharge pressure signal representative thereof;

sending said temperature signal, said suction pressure signal, and said discharge pressure signal to a microprocessor and generating an inverter control signal in response thereto; and

sending said inverter control signal to an inverter for responsively providing the desired electrical voltage and frequency to the compressor.

16. A method as set forth in claim 15 wherein said transport refrigeration system includes a box being refrigerated by said vapor compression system.

17. A method as set forth in claim 16 and including the steps of sensing the temperature of air being returned from said box and for responsively sending a return air temperature signal to said microprocessor, with said inverter control signal then being responsive thereto.

18. A method as set forth in claim 15 wherein the electric power from said generator to said inverter is unregulated ac current.

19. A method as set forth in claim 15 wherein said electrical power from said inverter to said compressor is regulated ac current.

20. A method as set forth in claim 19 wherein said power is regulated by selectively varying the voltage, frequency and/or current.

21. A method as set forth in claim 15 wherein the level of electrical power voltage and frequency to the compressor is controlled in order to maintain the saturated suction temperature of the compressor within a predetermined range.

22. A method as set forth in claim 21 wherein said saturated suction temperature is maintained within a range of −40° C. to 2° C.

23. A method as set forth in claim 15 wherein the level of electrical power to the compressor is controlled so as maintain a saturated discharge temperature within a predetermined range.

24. A method as set forth in claim 23 wherein said range is between 10° C. and 66° C.

25. A method as set forth in claim 15 wherein said microprocessor includes at least three different control modules, with each module being controlled in response to sensed values of, and operating to control limited parameters of, discharge pressure, discharge temperature or suction pressure.

26. A method as set forth in claim 25 and including a fourth control module which is responsive to sensed values of, and operates to control, a return air temperature.

27. A method as set forth in claim 25 wherein only one control module operates at a time.

28. A method as set forth in claim 25 wherein the control module which controls the discharge temperature has priority over the other control modules.