A CAPACITY CONTROL DEVICE FOR A COMPRESSOR IN A REFRIGERATING SYSTEM

Abstract: A capacity control device (10) for a variable displace­ ment compressor (1) in a refrigerating or air conditioning system is disclosed. The capacity control device (10) controls the operation of a variable displacement device (9) in the compressor (1) to change the capacity of the compressor (1) from a large to small volume and vice versa. This capacity control device (10) is responsive to the required refrigerating capacity of the compressor (1) operating at small volume. The required small volume rotation frequency is compared to the actual rotation frequency to determine whether the compressor (1) should operate at large or small volume. If the required small volume rotation frequency is smaller than the actual rotation frequency, the capacity control device (10) actuates the variable displacement device (9) to enable the compressor (1) to operate at small volume. If the required small volume rotation frequency is larger than the actual rotation frequency, the compressor (1) is operated at large volume or capacity.

Description: A CAPACITY CONTROL DEVICE FOR A COhlPRERSSVR IN A REFRIGERATiNG SYST > I BACKGROUND OF THE INVENTION This invention relates to 2 control system for controlling the operation of an air conditioning system, and more particularly, to o control circuit for a variable displacement compressor in an automobile air conditioning system. In conventional air conditioners, thermal control in the room generally is accomplished by the intermittent -operation of the air conditioning compressor through a magnetic clutch or motor. The magnetic clutch or motor is connected to the compressor and activated by a signal from a thermostat disposed in the room. Once the temperature in the room has been lowered to a desired temperature, the refrigerating or cooling capacity of the air conditioner for supplemental cooling because of further temperature changes in the room, or for keeping the room at the desired temperature, need not be as large. Therefore, after the room has been cooled to the desired temperature, conventional air conditioning compressors ate Intermittently operated in response to the thermostat signal. Compressors having a large cooling capacity are operated even more intermittently so that the large bad required to drive such compressors is intermittently applied to the compressor drive mechanism. h air conditioning systems for automobiles, the compressor usually is driven by the engine of the automobile. Since the r.p.m. of the automobile engine changes continuously; the rotation frequency of the compressor also changes which in turn rapidly changes the cooling capacity of the air conditioncr. Automobile air conditioners generally are designed so that when the compressor is drivcn by the engine at normal driving speed, the air conditioner operates at optimum capacity. Therefore, when the compressor is driven by the engine at lower operating speed, or during idling, the cooling capacity of the air conditioner Is insufficient in compnrison with the refrigerating or cooling load. Also, when the compressor is driven by the engine at high operating speed, the cooling capacity Is higher than necessary. One known mechanism for controlling the excessive cooling capacity of an air conditioner operates by heating part of the cool air generated by the air conditioner. Part of the cool air from the evaporator of the air con ditioner passes a heating unit which beats the cool air and this heated air 15 mixed with the remaining cool air to control the temperature of the air which is blown into the room or compartment. In this mechanism, the ratio of cooling capacity to beating capacity is controlled in response to driving frequency This mechanism for controlling air temperature is complicated and considerable energy is spent to reheat the cool air by using engine powder. In another known embodimcnt, the magnetic clutch which is disposed on the compressor and connected between the engine and the compressor to transmit rotating motion is intermittently operated to control the operation of the compressor. However, since the magnetic clutch may be operated at high rotation frequency, a sudden force is generated at the momcnt the clutch is engaged and this sudden force is transmitted to the engine and the compressor. Also, the temperature of the air blown Into the compart mcnt drastically changes upon operation of the compressor. SUMMARY OF THE INVENTION It is a primary object of this invention tb provide a capacity control device for an air conditioning system, particularly an automobile air conditioner, which controls the rcfrigenting or cooling capacity of the air conditioner compressor in response to operating conditions to maintain the correct balance between the referigerating capacity and the refrigeration or cooling toad at all timcs. It is another object of this invention to provide a capacity control device for an air conditioning system, particularly an automobile air conditioner, which reduces sudden forces acting on the engine and the compressor. ft is a furthcr object of this invention to provide a capacity control device for an air conditioning system, particularly an automobile air conditioner, which improves performance and efficiency. The present invention is directed to a capacity control device for a compressor in a refrigerating or air conditioning system. The compressor includes a variable displacement device for changing the capacity of the a.mpres50r The capacity control device controls the operation of the variable displacernent device to change the capacity of the compressor from a large capacity to a small capacity, in response to the required refrigerating capacity and the actual rotation frequency of the compressor. The capacity control device includes a frequency evaluation circuit for evaluating the required rotation frequency of the compressor, whcn the compressor is operating at small volume or capacity, in order to satisfy the required refrigerating capacity. The required refrigerating capacity is determined by the frequency evaluation circuit as a function of the temperature of the incoming air. A voltage representing the required rotation frequency of the compressor operating at small volume, i.e., thc required small volume rotation frequency, is compared in a voltage comparator with a voltage representing the actual rotation frequency of the compressor. This latter voltage is generated by a frequency to voltage converter which converts an incoming pulse signal representing the rotation of the compressor to a DC voltage signal. If the required small volume rotation frequency is smaller than the actual rotation frequency, the voltage comparator generates a signal which causes the compressor to operate at small volume and vice verso if the small volume rotation frequency is larger than the actual rotation frequency. Further objects, features and aspects of this invention will be understood from the following detailed description of the preferred e-bodIments of this invention rcferring to the annexed drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure l is a schematic diagram of an air conditioning or refrigerating system including a refrigerant circuit and a capacity control device in accordance with this Invention. Figure 2 is a characteristic diagram of the refrigeraring or cooling capacity of a variable displacemcnt compressor which is used In the refrigerant circuit of Figure 1. Figure 3 is a circuit diagram of an embodiment of a capacity control device for the compressor of Figure I. Figure 4 is a diagram illustrating the relationship between the refrigerating capacity and air temperature. Figure 5 is a flow diagram illustrating the operation of the capacity control device of Figure 3. Figure 6 is a circuit diagram of another embodiment of the capacity control device. DETAILED DENCRIYTION OF THE PREFERRED EMBODIMENTS Referring to Figure I, the basic construction of an air conditioning or refrigerating system, particularly, an automobile air conditioner, is shown. This 2ir conditioning system comprises compressor 1, condenscr 2, recciver and dryer 3, a thermostatic expansion device or valve 4 and evaporator 5. These five basic comtxan^nts are coupled in series to form a conventional closed vapor cycle refrigeration circuit. Refrigerant gas is compressed in compressor I and supplied to condenser 2 where it is condensed to liquid refrigerant and cooled by forced ventilation of fan 6. The liquid refrigerant is accumulated in receiver and dryer 3, which rcmoves the moisture and mctal chips in the liquid ref rigerant. The liquid refrigerant within receiver and dryer 3 is delivered to thermostatic expansion valve 4. The refrigerant expands as it flows through valve 4 and emerges as a twophase mixture of liquid and gas, but primarily liquid. As the two-phase mixture then flows through evaporator 5, which acts as a heat exchanger in conjunction with blower 7 which supplies air to the vehicle compartment, heat is transferred from the incoming air to the refrigerant and the entirety of thc refrigerant vaporizes and assumes its gaseous state. The refrigerant gas at the evaporator outlet then is passed to the suction inlet of compressor I. Compressor I, which is shown in block diagram form in Figure @, is a variable displacement compressor. In the preferred embodiment, compressor I is a scroll type compressor including a variable displacement device as shown in copending European patent application No. 82301205.9 filed on March 9, 1982 and copending European patent application No. 82@@@102.0 filed on December 15, 1982. Compressor I is controlled by operation of magnetic clutch 8 and variable displacement device 9 which changes the refrigerating capacity thereof. M;gnctic clutch 8 is controlled by operation of 1 conventional thermostat 8 and variable displacement device 9 is controlled by operation of capacity control circuit 10, which is described in furthcr detail below in conncction with Figure 3. Capacity control circuit lo is connected to voltage source E by on/off switch S. As mentioned above, compressor X includes a variable displacement device which is capable of changing the refrigerating or cooling capacity of the compressor. The capacity of the compressor shown in Figure I can be changed to one of two conditions, C1 or C2, as shown in Figure 2. In Figure 2, curve C is the normal capacity of prior art compressors. The variable displacement device of compressor X includes at least one valve as shown in patent applications Nos. 82301205.9 and SQ 366701.(3 and this valve is controlled by solenoid 9 shown in Figure I. Solenoid 9 is connected to voltage source E through capacity control circuit 10 and main air conditioning, on/off switch S. Referring to Figure 3, a circuit diagram of capacity control circuit lo for controlling the variable displacement device of compressor 1 is shown. Mne capacity control circuit includes relay fly1 to control operation of solenoid valve SV which actuates the variable displacement device to change the capacity of compressor I. Capacity control circuit lo further includes small volume frequency evaluation circuit A which detects air temperature and computes the required rotation frequency 2s a function of temperature of compressor I operating at small volume C2 (i.e., the required small volume rotation freqjency), frequency to voltage converter circuit B to detect the actual rotation frequency of the compressor and comparison circuit C to compare the voltage outputs from small volume frequency evaluation circuit A and converter circuit B to control operation of relay Ry1. Small volume frequency evaluation circuit A includes temperature sensing thcrmistor Th and comparator 11. Thermistor Th is mounted on the air inlet of evaporator S in order to detect the temperature of incoming air. A voltage divider formed by thermistor Th and resistor RI supplics voltage Va to inverting input terminal (-) of comparator II via resistor R4. Voltage Va, which is inversely rclated to the temperature of the incoming air, is reference voltage Yb, which is generated by a voltage divider formed by resistors R2 and K3. Voltage vb is applied to a norl invcrting input terminal (+) of comparator 1I. A feedback resistor ft5 is connected across comparator 31 from the output terminal of comparator I I to the inverting input tcrminal (-). The relationship between the needed or required refrigerating capacity Q for maintaining a suitable temperature in the compartment and the incoming air temperature t is shown by Figure 4 and can bc cxpresscd as Q=T(t). As shown, the required refrigerating capacity is directly rclated to the incoming air temperature. Also, as shown by Figure 2, the refrig crating capacity Q is directly related to the rotation frequency of the compressor. Accordmgly, as apparent from Figure 2, the rotation frequency Nc2 at which compressor I can continue to operate at small volume C2 can be easily calculated as a function of the required refrigerating capacity Q. Since refrigerating capacity also varies as a function of temperature as discussed above in connection with Figure +, the required rotation frequency of the compressor can be easily dctermined as a function of incoming air tcmpcrature. Frequency evaluation circuit A takes Into account the above relationships between the refrigerating, capacity Q and both the incoming air temperature (t) and the rotation frequency Nc2 of the compressor operating at small volume C2. Frequency evaluation circuit A generates art Output voltage which corresponds to the required rotation frequency Nc2 for the compressor to continue operating at small volume C2 as a function of refrigerating capacity. The refrigerating capacity is determined in frequency evaluation circuit A as a function of incoming air temperature based on thc relationship shown in Figure 4 and the rotation frequency Nc2 is determined as a function of refrigerating capacity based on the relationship shown In Figure 1. The output voltage Vo of frequency evaluation circuit A is given by Vo=Vb+R5 (Vb-Va), where Va is the voltage across thermistor Th, Vb Is R4 a reference voltage, Rs is the resistance of resistance R and R4 Is the resistance of resistor R4. The values of resistors RX and R5, as well as the values of the other elements in frequency cvaluation circuit A, are set in accordance with the relationships shown in Figures 2 and 4. Frequency to voltage converter circuit B receives a pulse signal from a conventional rotation sensor (not shown) mounted on the engine or compressor. For example, such a sensor could be mounted adjacent the rotating shaft of the compressor or engine to sense the rotation frcquency of the shaft. The pulse signal, which corresponds to the rotation frequency of the engine or compressor, is supplied to terminal 13 of converter circuit B. Converter circuit B is a conventional frequency to voltagr converter which includes a rectifier circuit basically formed by diodes D1 and D2 and a voltage comparator circuit basically formed by voltage comparator 14. The output of voltage comparator 14, which is also the output Vd of converter circuit B, is a voltage signal which varies in accordance with variations in the rotation frequency of compressor l, The voltage output Vo of small volume frequency evaluatiori circuit A is compared at comparator 12 of comparison circuit C with the output voltage Vd of frequency to voltage converter circuit B. The output of comparator 12 is couplcd to the base circuit of switching transistor Tr. Relay RyI is connected to the collector of transistor Tr and its operating contact is connected in series with solenoid valve SY. When transistor Tr is conductive, relay Ryl is energized so that solenoid valve SV actuates the variable displacement device to change the displacement of compressor from large volume Cl to small volume C2. In operation, If the output voltage Vo of small volume frcquency evaluation circuit A is smaller than the output voltage Vd of converter circuit 8, i.e., the required rotation frequency of compressor t operation±3 at small volume C2 in order to produce adequate refrigerating capacity (the required small volume rotation frequency) is smaller than the actual rotation frequency, then the output of comparator 12 has a low. voltage level. When comparator I2 generates a low voltage level, transistor Tr Is rendered conductive and rclay Ryl is energized. The energization of relay Ryl actuates solenoid valve SV which in turn actuates the variable displacement device of compressor 1 tu operate compressor I at small volume C2. On the other hand, if the output Vo of evaluation circuit A is larger than the output Vd of converter circuit B, the output of comparator 12 has a high voitage level. Transistor Tr then is rendered nonconductive and relay Ryl is not cnergized. Thus, the displacement of compressor I is changed to large volume CI. The operation of capacity control circuit 10 now will be described with reference to the flow diagram shown in Figure 5. When main switch S is actuated to connect the voltage source at start step IOI, the capacity control circuit is ready for operation. At step 102, the air tcniperature is detected and used by small volume frequency valuator circuit A to compute the required refrigerating capacity Q of compressor I. After computing the required capacity at Step I03, the required rotation frequency Nc2 to obtain the required refrigerating capacity from step 102 is determined. At step IO4, the actual rotation frequency NE of compressor I 13 compared to the calculated or required rotation frequency Nc2 from step 103. If the actual rotation frequency NE of the compressor is higher than the calculated rotation frequency Nc2, the required refrigerating capacity Q is already obtained. In this case, the compressor Is driven at small volume C2 as indicated by step 105. On the other hand, if the actual rotation frequency NE is smaller than the required rotation frequency Nc2, the required refrigerating capacity has not yet been obtained so that the compressor is driven at large volume C1 as indicatcd by step 106. Referring to Figure 6, another embodiment of the present invention is shown In this embodiment, the evaluation circuit A is replaced by a voltage divider A' which defines the predetermined rotation frequency Nc2 of the compressor. The voltage divider A' comprises resistors R6 and R7 which provide a refercnce voltage Vc to the non-inverted input tcrminal (+) of comparator 12. The refercnce voltage Vc' gcncrated by voltage divider AT, which corresponds to the predetermined rotation frequency Nc2, Is compared at comparator 12 with output voltage Vd of converter circuit B. In other words, the actual rotation frequency of the compressor is compared to the predetermined rotation frequency NC2 by comparator circuit C to determine whether the compressor should be operated at small volume C2 or lae volume C2 If the actual rotation frequency is smaller than the prc- determined rotation frequency Nc2, i.e., voltage Vd is smaller than voltage Vc, then the output of comparator C is low which turns on transistor Tr and energizes rely Ry1. Relay Ryl In turn actuates solenoid SV which causes the ccmpressor to Operate at small volume C2. If the actual rotation frequency is larger than the predetermined rotation frequency NC2S then the compressor operates at large volume C. This invention has been described in detail in connection with the preferrcd embodiments, but these embodiments are examples only and this invention is not restricted thcreto. It will be easily understood by those skilled in the art that other variations and modifications can be easily made within the scope of this invention.

Claims: CLAIMS: I. A capacity control device for a compressor in a refrigerating system, said compressor including a variable displacement device to change the capacity of said compressor between a small and large volume, said capacity control device comprising: a voltagc source; actuating means for actuating said variable displacement device; and control circuit means coupled to said voltage source and said actuating means for energizing said actuating means to actuate said variable displacement device to change the capacity of said compressor, said control circuit means including evaluating means for evaluating the required refrigerating capacity of said refrigerating system and generating an output signal corresponding to the required rotation frequenGbRf said compressor at tlie small volume capacity as a function of the required refrigerating capacity, rotation measuring means coupled to said compressor for generating an output signal corresponding to the actual rotation frequency of said compressor and comparing means connected to said evaluating means and said rotation measuring means for comparing the output signals of said evaluating means and said rotation measuring means and generating a control signal to control the operation of said actuating means to change the capacity of said compressor. 2. The capacity control device of claim I whercin the control signal gencrated by said comparing means energizes said actuating means to change the capacity of said compressor to a small volume when the required rotation frequency is smaller than the actual rotation frequency. 3. The capacity control device of claim 2 wherein the control 5i1al generated by said comparing means de-cnergizes said actuating means to change the capacity of said compressor to; large volume when the required rotation frequency is larger than the actual rotation frequency, 4. The capacity control device of claim 1 wherein said evaluating means comprises temperature detecting means for detecting the temperature of the air to be cooled by said refrigerating system arid wherein said evaluating means is responsive to the temperature detected by said temperature detecting means to evaluate the required refrigerating capacity. 5. The capacity control device according to claim 1 wherein said rotation measuring means comprises a frequency to voltage converter circuit. 6. A capacity control device for a compressor in a refrigerating system, said compressor including a variable dispiacement device to change the capacity of said compressor between a small and large volume, said capacity control device comprising: a voltage source; actuating means for actuating said variable displacement device; and control circuit means coupled to said voltage source and said actuating means for energizing said actuating means to actuate said variable displace ment device to change the capacity of said comprcssor, said control circuit means including voltage divider means for providing an output signal corrcsponding to a predetermined rotation frequency of said compressor at the small volume capacity, rotation measuring means coupled to said compressor for generating an output signal corrcsponding to the actual rotation frequency of said compressor and comparing means connected to said voltage divider means and said rotation measuring means for comparing the output signals of said evaluating means and said rotation measuring means and generating a control signal to control the operation of said actuating means to change the capacity of said compressor. 7. The capacity control device of claim 6 wherein the control signal generated by said comparing means energizes said actuating means to change the capacity of said compressor to a small volume when the predctermlned rotation frequency is smaller than the actual rotation frequency. 8. The capacity control device of claim 7 wherein the control signal generated by said comparing means dcencrgizet said actuating means to change the capacity of said compressor to a large volume when the prcdetermined rotation frequency is larger than the actual rotation frequency. g. In a refrigerating system where refrigerant flows through a closcd refrigeration circuit including an evaporator, a compressor, a condenser and a thermostatic expansion device, said compressor having a variable displaccment device to change the capacity thereof, a capacity control dcvice to control the operation of said variable displacement device to change the capacity of said compressor compriing; a voltig source; actuating means for controlling said variable displacement device; ; anci control circuit means coupled to said voltage source and said actuating means for energizing said actuating means to actuate said variable displace ment device to change the capacity of said compressor, said control circuit means including evaluatirg means for evaluating the required refrigerating capacity of said refrigerating system and generating an output signal corresporiding to the required rotation frequency of said compressor at the small volume capacity as a function of the required refrigerating capacity, rotation measuring means coupled to said compressor for generating an output signal corresponding to the actual rotation frequency of said compressor and comparing means connected to said evaluating means and said rotation measuring means for comparing the output signals of said cvaluating means and said rotation measuring means and generating a control signal to control the operation of said actuating means to change the capacity of said compressor. 10. The capacity control device of claim g wherein the control signal generated by said comparing means cncrgizes said actuating means to change the capacity of said compressor to a small volume when the required rotation frequency is smaller than the actual rotation frequency. II. The capacity control device of claim 10 wherein the control signal g(*ncrated by said comparing means dcbcnergizes said actuating meatus to change the capacity of said compressor to a large volume when the required rotation frequency is larger than the actual rotation frequency.

Inventors: UBUKATA, TADAYOSHI
Applicants: SANDEN CORPORATION
Application year: 1982
Main classification: F04C29
IPC codes: F04C28, B60H1, G05D23, F04D27