US20060130504A1 - Method and apparatus for control of a variable speed compressor - Google Patents

Abstract

A vapor compression system includes a fluid circuit circulating a refrigerant in a closed loop. The fluid circuit has operably disposed therein, in serial order, a variable speed compressor, a first heat exchanger, an expansion device and a second heat exchanger. A first blower device is associated with the first heat exchanger. A speed of the first blower device is dependent upon a speed of the compressor. A second blower device is associated with the second heat exchanger. A speed of the second blower device is dependent upon the speed of the compressor.

Description

BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• The present invention relates to a vapor compression system and, more particularly, to a method and apparatus for controlling a variable speed compression system.
• 2. Description of the Related Art
• Vapor compression systems are used in a variety of applications including heat pump, air conditioning, and refrigeration systems. Conventional air conditioners include fixed speed compressors. A thermostat or another type of temperature sensor may be used in controlling the operation of the compressor. The temperature sensor may be located at the inlet side of the evaporator and may be used to sense the room temperature. When the sensed temperature becomes lower than a set temperature selected by the user, either manually or by remote control, the compressor is switched off. When the measured temperature of the room rises again to a level above the set temperature, the compressor is switched on and the cycle continues. This type of intermittent operation not only results in more power consumption but also results in variation of room temperature leading to conditions uncomfortable to the user. Fixed speed air conditioners are found to have low capacity when most needed, say for instance, when ambient temperatures are high, and have higher capacity when ambient conditions are low.
• A variable speed compressor can be used in order to avoid some of the drawbacks of a fixed speed compressor. A variable speed compressor may have a variable capacity that is based on the demands of the room being cooled.FIG. 1 plot the capacities of fixed and variable speed compressors as a function of ambient load conditions. The capacity of a variable speed compressor may increase with increasing ambient load, while the capacity of a fixed speed compressor may decrease with increasing ambient load. Variable speed of the compressor may be enabled by an inverter and other related electronic controls.
• A variable speed compressor has the advantage of being able to operate for extended periods at near 100% duty cycle at a reduced frequency that is just enough to maintain a required level of cooling. The reduced frequency of the variable speed compressor provides a level of power consumption that is much lower than that of a fixed speed compressor.
• The majority of the industrial loads driven by motors comes under two major broad categories, i.e., constant power and constant torque application.FIG. 2 illustrates the characteristics of a constant torque/constant power application in which constant torque is achieved up to a base frequency, i.e., compressor speed, of approximately 50 Hz. Above the base frequency, the torque decreases due to the fact that the output voltage of an inverter that drives the compressor cannot be increased due to limitations of the input voltage. In this case, the voltage increases up to a base frequency above which the voltage is constant. Thus, the voltage to frequency ratio V/f is constant up to the base frequency and decreases above the base frequency.
• In the case of a given load, such as a compressor, the torque value remains constant throughout the operating frequency.FIG. 3 illustrates the characteristics of such a constant torque application. The motor is designed for a maximum voltage and maximum frequency zone considering the available input voltage so that V/f is constant throughout the operating range. The power increases linearly with frequency.
• In both types of systems associated withFIGS. 2 and 3, there is a considerable torque drop at lower frequencies due to a significant stator resistive drop. This results in the voltage applied across the stator being less than the voltage applied across the terminal. The stator thus creates lesser flux, thereby creating lesser torque at lower frequencies. To overcome this problem, additional voltage, generally known as “boost voltage”, may be applied to the stator terminal. The user may select a predetermined boost voltage and a frequency up to which the boost voltage is to be applied.
• One drawback of a constant torque application is that the user must select the boost voltage, which may not result in optimum efficiency from the motor. Another drawback of a constant torque application is that the performance characteristics of the motor, specifically the efficiency, eventually degrade even though V/f may remain constant. The degradation of motor performance characteristics may be due to the fact that different losses of the motor, e.g., stator copper loss, rotor conductor loss, hysteresis loss, eddy current loss, and stray loss, behave differently with respect to the change in frequency. For example, the hysteresis loss and eddy current loss may be a greater part of the total loss at higher frequency than at lower frequency.
• Inverter driven compressor motors are typically protected for locked rotor, over-current and over and under voltage. However, single phasing protection is not included because it involves addition of current sensing circuits at the output of the inverter and means for sensing protection of the same. Normally, inverters used to drive compressor motors are only suited to carry out that particular function. The other accessories are not controlled, or are controlled by another control system in window air conditioners.
• Normally, inverters used to drive compressor motors are only suited to carry out that particular function. The other accessories are not controlled, or are controlled by another control system in window air conditioners.
• What is needed in the art is a method of operating a compressor such that a wide range of cooling demands may be met in an efficient manner.
SUMMARY OF THE INVENTION
• The present invention provides a vapor compression system including a variable speed compressor that is capable of providing different capacities on the basis of the demands of a room or rooms being cooled. A method of efficiently operating a compressor in a plurality of modes is provided, with each mode corresponding to a different level of cooling demand.
• The compressor may be an inverter driven compressor with an electronic controller. The controller may operate the compressor in one of three different modes (quick chill, standard or energy saver mode) when it functions as an air conditioner and also may operate the compressor in a heating cycle mode when the system is employed as a heat pump.
• The controller may seek to operate the compressor motor at a constant torque as the rotational speed or frequency (f) of the compressor is varied. The three different modes of operation may be defined by three different algorithms for controlling the speed of the compressor motor and the speed of the condenser and evaporator fans. The condenser and evaporator fans may each have high/medium/low speeds and the speed at which the fans are operated may be determined by the speed of the compressor motor, with higher compressor motor speeds corresponding to higher fan speeds. Fan motor speeds may be determined for specified compressor speeds without regard to whether the system is operating as a heat pump or air conditioner.
• It may be desirable for the system to quickly cool the room to the desired temperature but do so in a manner that gradually reduces the speed of the compressor and then places it in energy saving mode. The system may operate in accordance with the operating mode that has been selected out of the three different operating modes.
• If the quick chill mode is selected, the compressor may initially operate at its highest operating frequency with the fan speeds at high. When the difference between the desired temperature and the actual room temperature is at 1° C., the speed of the compressor may be reduced and the fans speeds may be reduced to medium. If this does not result in an increase in the room temperature, the speed of the compressor may be further reduced. This process may be repeated with the compressor and fan speeds being repeatedly reduced if the temperature of the room does not increase, until eventually, the desired temperature is achieved and the system is switched off.
• If the standard operating mode is selected, the system may operate generally similar to the quick chill mode but the temperature differential at which the compressor and fan speeds are stepped downward may differ. In the standard operating mode, the compressor may be stepped down from its highest operating frequency when the temperature of the room is 10° C. warmer than the desired temperature instead of 1° C.
• If the energy saving mode is selected, the frequency of the compressor motor may be varied as the temperature differential between the actual room temperature and the desired room temperature varies. The compressor motor speed may be chosen to maximize the efficiency of the compressor.
• A digital signal processor (DSP) may be used to provide both power factor correction (PFC) and compressor-motor control. The DSP may also receive inputs from a voltage sensor measuring the DC current. When the variation of the DC current extends beyond a defined window of variation, it may be interpreted to be single phasing on the output side of the inverter. The inverter may then be switched off in order to protect the compressor motor from high currents.
• The invention comprises, in one form thereof, a vapor compression system including a fluid circuit circulating a refrigerant in a closed loop. The fluid circuit has operably disposed therein, in serial order, a variable speed compressor, a first heat exchanger, an expansion device and a second heat exchanger. A first blower device is associated with the first heat exchanger. A speed of the first blower device is dependent upon a speed of the compressor. A second blower device is associated with the second heat exchanger. A speed of the second blower device is dependent upon the speed of the compressor.
• The present invention comprises, in another form thereof, a method of controlling a room temperature, including determining a first temperature differential between a first actual temperature and a desired temperature. A subsequent temperature differential between a subsequent actual temperature and the desired temperature is ascertained after a compressor is operated. A speed of the compressor is decreased if the subsequent temperature differential is less than an immediately preceding temperature differential. The operating, ascertaining, and decreasing steps are repeated so long as the speed of the compressor is greater than a threshold speed and the subsequent temperature differential is greater than a threshold temperature differential.
• The present invention comprises, in yet another form thereof, a vapor compression system including a variable speed compressor. A control device repetitively determines a temperature differential between an actual room temperature and a desired room temperature. The compressor is selectively operated in a first mode and a second mode. The first mode includes decreasing a speed of the compressor after each temperature differential determination so long as a most recently determined temperature differential is less than a second most recently determined temperature differential. The second mode includes substantially continuously varying the speed of the compressor based upon the determined temperature differentials.
• An advantage of the present invention is that the compressor may be operated in three different cooling modes having different cooling performance characteristics and different efficiencies. Thus, one of the three modes may be selected for each particular set of cooling conditions such that cooling demands may be met in an efficient manner.
• Another advantage is that simple algorithms are employed which require a low level of electronic computations. Thus, processing time and power are reduced, and implementation is simplified.
• Yet another advantage is that different fan motors speeds such as high, medium and low are defined for compressor speeds within a fixed range to thereby provide better performance in terms of cooling and energy efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
• The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:
• FIG. 1 is a representative plot of capacity versus ambient load for a fixed speed compressor and a variable speed compressor;
• FIG. 2 is a representative plot of various aspects of general purpose motor performance versus speed for constant torque/constant power applications;
• FIG. 3 is a representative plot of various aspects of compressor motor performance versus speed for constant torque applications;
• FIG. 4 is a schematic diagram of one embodiment of a vapor compression system of the present invention;
• FIG. 5 is a perspective view of one embodiment of a room air conditioner of the present invention including the vapor compression system ofFIG. 4;
• FIG. 6 is a flow chart illustrating one embodiment of a method of the present invention for controlling a room temperature;
• FIG. 7 is a representative plot of compressor frequency versus a temperature differential in the energy saving cooling mode;
• FIG. 8 is a schematic diagram of another embodiment of a control circuit of the present invention for powering and controlling a compressor motor;
• FIG. 9 is a representative plot of inverter output voltage versus AC input voltage for the control circuit ofFIG. 8;
• FIG. 10 is a flow chart of one embodiment of a constant output voltage follower algorithm of the present invention;
• FIG. 11 is a flow chart of one embodiment of an algorithm of the present invention for making a DC side current measurement and frequency output correlation for deciding on single phase protection; and
• FIG. 12 is a block diagram illustrating an embodiment of a control circuit of the present invention for powering and controlling a three phase compressor motor.
• Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplification set out herein illustrates an embodiment of the invention, the embodiment disclosed below is not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise form disclosed.
DESCRIPTION OF THE PRESENT INVENTION
• FIG. 4 illustrates one embodiment of avapor compression system20 of the present invention including a fluid circuit circulating refrigerant in a closed loop. Acompressor mechanism22 compresses the refrigerant from a suction pressure to a discharge pressure.Compressor mechanism22 may be a variable speed, reciprocating type compressor mechanism, for example. The refrigerant is then cooled in a heat exchanger in the form of a gas cooler orcondenser24. The pressure of the refrigerant is then reduced by anexpansion valve26. The refrigerant then enters another heat exchanger in the form of anevaporator28 where it is boiled and cools a secondary medium, such as air, that may be used, for example, to cool a room or a refrigerated cabinet. The refrigerant discharged fromevaporator28 enterscompressor mechanism22 to repeat the cycle. Afan motor30 is operatively coupled to and drivesblower devices31a,31bto thereby blow air acrosscondenser24 andevaporator28, respectively. The heat-exchanging properties ofcondenser24 andevaporator28 may be enhanced by the moving air. Schematically represented fluid lines orconduits23,25,27, and29 provide fluid communication betweencompressor mechanism22,gas cooler24,expansion valve26,evaporator28 andcompressor mechanism22 in serial order.
• In one embodiment,compressor mechanism22 is a digital reciprocating compressor controlled and powered by anelectronic control circuit32 which provides a heating cycle mode and three cooling cycle modes, i.e., quick chill, standard, and energy saving modes.Control circuit32 may select one of the modes based upon the external ambient temperature and a set temperature selected by the user.
• In the embodiment shown inFIG. 4,control circuit32 includes apower inverter34 for drivingcompressor22 in a controlled manner.Inverter34 receives inputs from a power factor correction module (PFC)36 and from a voltage/frequency control module (V/f control)38.Control circuit32 also includes adigital signal processor40, amicrocontroller42, and arectifier44.
• FIG. 5 illustrates one embodiment of aroom air conditioner46 which may includevapor compression system20.Room air conditioner46 may includecompressor22,condenser24,expansion device26 in the form of a capillary tube,evaporator28,fan motor30,blowers31a,31b,anoutdoor inverter box34a,andindoor inverter box34b,a conduit34cfor carrying power and communication betweenboxes34aand34b,aremote controller48 for transmitting aset temperature signal50, an indoorroom temperature sensor52, an externalambient temperature sensor54, a hermetically coveredcompressor terminal56, acompressor discharge tube58 and acompressor suction tube60.
• According to the present invention, in order to achieve improved energy efficiency, the speed, i.e., the revolutions per minute (rpm) or frequency, ofcompressor22 is varied according to the cooling demand of the room or rooms. Both speed and frequency may be referred to interchangeably herein, and it is to be understood that 60 rpm is equivalent to 1 Hz. The cooling demand may be in the form of a temperature differential between the room temperature and the set temperature selected by the user. For different compressor speeds or frequencies, the voltage applied tocompressor22 is varied or selected based on an algorithm to thereby produce a superior level of motor efficiency.
• The control circuit may combine the power factor correction (PFC) and motor control into a single digital signal processor (DSP), thus allowing the PFC to be operated in constant output mode until a certain voltage point, and then switch into a voltage follower mode, thereby increasing the input window for voltage.
• DSP40 may correlate the measured current in the DC link of the compressor motor with compressor speed to protect against single phasing.Control circuit32, apart from controlling and protectingcompressor22, also may control other window air conditioner accessories using split architecture.
• Initially, based upon the external ambient temperature, the control cycle for either heating or cooling is selected. If the cooling control cycle is selected, three different ambient temperature bands may be defined to represent the possible load conditions. A given ambient temperature represented by a load allows the user to select a set temperature within a predetermined range so as to get energy and cooling benefits. In one embodiment, quick chill, standard and energy saver modes may be selected based upon the measured temperature differential between the set temperature and the room temperature. Based on the respective algorithms for these modes, the compressor speed may be varied based on the demand of the room as represented by the temperature differential in order to achieve a room temperature that is close to the set temperature. Compressor speed may be reduced or increased depending on the drop or rise of the room temperature for a specified predetermined time period.
• In the energy saving mode, condenser speed may be varied on the basis of the differential between the room and set temperatures in order to improve the energy efficiency. The quick chill mode may provide faster cooling, while the standard mode may provide a performance comparable with regular air conditioners but with better energy efficiency. The selection of a specified mode may decide the compressor speed or frequency, f. Based on this frequency, a desired voltage to be applied tocompressor22 may be calculated or otherwise determined from the V/f algorithm corresponding to the mode. The optimum V/f curve is not constant, but rather may vary with respect to the particular motor and the load system designed for the specific purpose.Compressor22 may draw much less power than known compressors operating with constant V/f by virtue ofcompressor22 being operated at a high efficiency for a given speed. More particularly, a desired level of voltage may be applied tocompressor22 in order to achieve improved performance.
• Three modes of operation namely, quick chill, standard and energy saver modes may each correspond to a respective algorithm to control the speed of the compressor motor and the fan motors of the evaporator and condenser based on the room demand of capacity that is represented by the differential between the room temperature and the set temperature. The present invention may be applicable to both split air conditioners and room air conditioners.
• Microcontroller42 may process the cooling or heating logic requirements, as well ascontrol fan motor30 and the user interface which may include displays, buttons and/or remote controls, such asremote controller48.Microcontroller42 may estimate the amount of cooling required and transmit the data toDSP40, which viaPFC36 and V/f control38, may controlinverter34 and thus the motor ofcompressor22. The data transmission may be performed serially.
• DC Link ripple cancellation and power factor correction (using constant power output) may be used to obtain wide incoming voltage functionality. Power Factor correction may be applied in either constant voltage or voltage follower modes.
• PFC36 and theincoming rectifier44 may be designed for a maximum current rating. This current rating may define the maximum power that can be drawn at various voltages (assuming thatPFC36 keeps the incoming power factor constant). Thus, the hardware may be designed by keeping the lowest operating input voltage in mind for the maximum power to be drawn. Once this hardware is in place, the minimum input voltage operation may be fixed.PFC36 may switch from constant output to voltage follower mode when the minimum input voltage is reached, thereby allowingcompressor22 to still operate beyond this point.
• Single phasing sensing is provided by the present invention by sensing the current on the DC side of the inverter. The compressor motor may be protected for high winding temperature by providing three thermistors, one on each winding, and then measuring the resistance at a 2-pin terminal connected to the thermistors. This winding temperature may be verified. Based on this verification, only the compressor may be turned off to protect the motor from high winding temperature.
• The ambient temperature may be sensed by externalambient temperature sensor54, which may be provided on the condenser side of the room air conditioner. Based upon the ambient temperature,control circuit32 may determine whether to operate in the heating mode or in one of the three cooling modes. In one specific embodiment, the heat pump mode is selected if the ambient temperature T_ais less than a set point of 17° C. The user may select any desired set point. If T_ais greater than or equal to 17° C., then one of the cooling modes is selected.
• Although the selection of the mode may be made bycontrol circuit32, a manual override may be provided wherein the user may select the operating mode. For example,remote controller48 may be used to select heating or cooling, and further may be used to select one of the three cooling modes.
• Based upon the sensed external ambient temperature, limits on the set temperature may be established. In one embodiment, the set temperature limits are defined as shown below:
• 1. 17° C.<T_a≦37° C. then T_x≧16° C.
• 2. 37° C.≦T_a≦43° C. then T_x≧20° C.
• 3. 43° C.<T_athen T_s≧25° C.
  where
• T_a=Ambient temperature or outside temperature
• T_r=actual room side temperature
• T_s=set temperature
• T_r−T_s=ΔT (for cooling mode)
• T_s−T_r=ΔT₁(for heating mode)
• ΔT, ΔT₁are the temperature differential measures of room condition
• P₁, P₂, P₃are predefined time periods for which the compressor operates at a desired speed
• Remote controller48 may allow the user to select the desired set temperature within the above limits based upon the band in which the ambient temperature falls.
• Condenser andevaporator fan motor30 may operate at different speeds, such as high, medium and low speeds. Based on the speed ofcompressor22, the speed offan motor30 may be set as given below.

  	Compressor speed (Hz)	Fan motor speed
  	Above 50	High
  	40-50	Medium
  	Below 40	Low

• Hence,fan motor30 may be operated at a speed that is dependent upon the speed ofcompressor22. Further, based on the compressor speed or frequency, the speed offan motor30 may be predefined. The discrete operating speeds of blower motor31, i.e., High, Medium, Low, each may correspond to a respective range of compressor speeds. Further, the speeds ofblowers31a,31bmay be dependent upon a speed ofcompressor22. Althoughcondenser blower31aandevaporator blower31bare shown inFIG. 4 as having acommon motor30, it is possible for the condenser blower and the evaporator blower to have separate motors. Further, these separate motors may be operated at the same speeds or at different speeds.
• When the quick chill mode is selected,compressor22 starts out at its highest frequency and continues to do so until ΔT (room temperature minus set temperature) becomes equal to 1° C. When ΔT is 1° C., the speed ofcompressor22 switches to 50 Hz and continues as per the following algorithm given below. This quick chill mode results in faster cooling. In the quick chill mode, the pull down time required to lower the room temperature to a desired level is about60% of that of a conventional air conditioner with a fixed speed compressor.

  Logic Table for speed change as a function

  of Temperature in the Quick Chill Mode

  		Fan
  Temperature	Compressor	motor	Time
  (° C.)	speed (rpm)	speed	Duration	Remarks
  Tr> Ts+ 1	Highest	High	NA
  Tr= Ts+ 1	3000	Medium	P1	If there is no rise in
  				temp., then go to
  				next lower
  				compressor speed;
  				otherwise go to next
  				higher speed
  Tr≦ Ts+ 1	2400	Medium	P2	If there is no rise in
  				temp., then go to
  				next lower
  				compressor speed;
  				otherwise go to next
  				higher speed
  Tr≦ Ts+ 1	2100	Low	P3	If there is no rise in
  				temp., then go to
  				next lower
  				compressor speed;
  				otherwise go to next
  				higher speed
  Tr≦ Ts+ 1	1800	Low	NA	If there is no rise in
  				temp., then continue
  				at present
  				compressor speed;
  				otherwise go to next
  				higher speed
  Tr= Ts	Switch off	Low	NA	Switch off regardless
  				of present
  				compressor speed

• After switching off,compressor22 may be restarted at a lower frequency, e.g., at 30 Hz (1800 rpm) when T_r=T_s+0.5° C.
• As shown in the above logic table and in the flow chart ofFIG. 6, amethod600 of controlling a room temperature includes determining a temperature differential ΔT between an actual temperature T_rand a desired temperature T_s(S602). In a next step S604, a compressor is operated in order to reduce the temperature differential. After the compressor is operated, a subsequent temperature differential between a subsequent actual temperature and the desired temperature is ascertained (S606). In S608, it is determined whether the subsequent temperature differential is above a threshold temperature differential. If not, the compressor may be shut off (S610). In the embodiment shown in the above logic table, the threshold temperature differential is zero, i.e., the compressor is switched off if T_r=T_s. If the subsequent temperature differential is above the threshold temperature differential, then it is determined whether the subsequent temperature differential is less than an immediately preceding temperature differential (S612). If not, the compressor speed may be increased (S614), and operation of the compressor may be continued (S604). If the subsequent temperature differential is less than the immediately preceding temperature differential, then it is determined whether the compressor speed is above a threshold speed (S616). In the embodiment shown in the above logic table, the threshold speed is 1800 rpm, which is the minimum speed in the logic table. If the compressor speed is not above the threshold speed, e.g., is equal to the threshold speed of 1800 rpm, then operation of the compressor may be continued at the present compressor speed of 1800 rpm (S604). If the compressor speed is above the threshold speed, then the compressor speed may be decreased (S618), and operation of the compressor may be continued (S604).
• When the standard mode is selected,compressor22 may be operated similarly to a conventional cooling cycle of an air conditioner with a fixed speed compressor. The speed change may be as per the following algorithm:

  Logic Table for speed change as a function

  of Temperature in the Standard Mode

  		Fan
  Temperature	Compressor	motor	Time
  (° C.)	speed (rpm)	speed	Duration	Remarks
  Tr> Ts+ 10	Highest	High	NA
  Tr≦ Ts+ 10	3000	Medium	NA
  Tr≦ Ts+ 1	2400	Medium	P1	If there is no rise in
  				temp., then go to
  				next lower
  				compressor speed;
  				otherwise go to next
  				higher speed
  Tr≦ Ts+ 1	2100	Low	P2	If there is no rise in
  				temp., then go to
  				next lower
  				compressor speed;
  				otherwise go to next
  				higher speed
  Tr≦ Ts+ 1	1800	Low	P3	If there is no rise in
  				temp., then continue
  				at present
  				compressor speed;
  				otherwise go to next
  				higher speed
  Tr= Ts	Switch off	Low	NA	Switch off regardless
  				of present
  				compressor speed

• After switching off,compressor22 may be restarted at a lower frequency, e.g., at 30 Hz (1800 rpm) when T_r=T_s+0.5° C.
• When the quick chill mode is selected, the compressor may be operated at its highest speed if the T_r−T_stemperature differential exceeds 1° C. In contrast, when the standard mode is selected, the compressor may be operated at its highest speed if the T_r−T_stemperature differential exceeds a higher value of 10° C.
• In both the quick chill mode and the standard mode, the time periods P₁, P₂, P₃between the individual ascertainments of the T_r−T_stemperature differential are dependent upon the speed of the compressor.
• When the energy saving mode is selected, compressor frequency is varied based on the temperature differential between the room temperature and the set temperature (ΔT) as illustrated inFIG. 6. As the temperature difference ΔT increases, the frequency ofcompressor22 is also increased. The frequency may increase substantially linearly as shown inFIG. 7. This energy saving mode may result in more energy savings as compared to conventional fixed speed compressor energy consumption and also as compared to the quick chill and standard modes of the present invention.
• Whenroom air conditioner46 is operated in the heating cycle mode, a valving system (not shown) may be activated such that the direction of flow of refrigerant is reversed. Thus,condenser24 may function as an evaporator, andevaporator28 may function as a condenser.
• In all three of the cooling modes and in the heating mode,control circuit32 may repetitively determine the T_r−T_stemperature differential between the actual room temperature and the desired room temperature.
• According to the present invention, the set temperature limits may be defined based on the external ambient temperature measured bytemperature sensor54. While defining these set temperature limits, due care is taken to consider the comfort level of the occupants and reduce the energy consumption.
• The algorithms of the present invention may be employed with both a reduced level of computation ability and a reduced level of processing time. The quick chill mode results in faster cooling and the standard mode provides a level of air conditioner performance that is comparable to that of known air conditioners. However, in both the quick chill mode and the standard mode the speed changeover is based on a predefined time period, which reduces the delay in the speed changeover as compared to known inverter compressor control algorithms. In these known algorithms, the speed change is based on the capacity required for the room, the calculation of which may require substantial processing power and time.
• The algorithms employed in the energy saving mode may also be computationally simple. The speed change may be based purely on the delta T, i.e., the temperature differential ofroom temperature sensor52 and the set temperature. Compressor speed may change only when delta T changes.
• All of the three algorithms for the quick change, standard and energy saving modes, respectively, are aimed at quickly getting room temperature to the set temperature and then gradually reducing the compressor speed. Thus, the algorithms may reduce the power consumption of the air conditioner and achieve a room temperature that is close to the set temperature with minimal temperature fluctuations.
• The present invention may avoid the problems associated with the prior art by making use of a desired V/f curve. No user interface is required to select the boost voltage or to select a different predetermined V/f curve. The desired V/f curve is not constant, but rather varies with respect to the particular motor and the load system designed for the specific purpose. The power drawn is much less as compared to the conventional constant V/f curve because the compressor may be operated at or near the maximum possible efficiency for the given speed by applying a desired voltage for improved performance. The efficiency is better as compared to that of the conventional constant V/f algorithm, thereby offering energy saving.
• An embodiment of acontrol circuit132 of the present invention for powering and controlling acompressor motor121 is shown inFIG. 8. Arectifier section144 may convert an incoming AC voltage into a DC voltage. A powerfactor correction circuit136 may operate in continuous conduction mode, thereby functioning as a boost stage. After the powerfactor correction circuit136 are one or moreDC Link capacitors162, which may smoothen the DC voltage. Aninverter stage164 includingIGBTs166 may convert the DC voltage on the DC link162 to AC voltage for themotor121.
• Power factor stage136 may assist in the correction of the power factor and thereby may remove the discontinuity in the current flow. This may reduce harmonics and improve compliance with standards IEC61000-3-2 and IEC61000-3-4.
• The present invention may implement the motor control as well as the power factor correction in a singledigital signal processor140. The digital outputs are controlled from theDSP140 to trigger the devices for theinverter164 and thePFC136.
• As illustrated in the plot ofFIG. 9,PFC136 may operate in a constant output voltage mode as long as the incoming voltage is equal to or above the constant to voltage follower point. In this case the DC Link voltage may be held constant irrespective of the incoming voltage as long as the incoming voltage is above the constant to voltage follower point. This may ensure that the output V/f ratio is maintained constant. Until the incoming voltage is within this band, the output frequency and voltage corresponds to the maximum demand possible. In this mode the compressor can run up to its maximum designed speed to provide for enhanced cooling or run at slower speeds to provide for energy savings.
• The constant to voltage follower point may be determined on the basis of the hardware design of the incoming rectifier current capability and the PFC component current rating selections. According to the present invention, the PFC switches from constant output to a voltage follower mode below the voltage constant to voltage follower point, as illustrated inFIG. 9.
• Below the constant to voltage follower point, the output voltage onto theDC Link162 from thePFC stage136 operates in a voltage follower mode. That is, the DC Link voltage is not constant, but rather may be maintained at a percentage of the incoming voltage. Thus, the DC Link voltage may vary in proportion to the incoming voltage.
• As the DC Link voltage decreases, the motor control algorithm may start to limit the output voltage to the motor, possibly allowing a maximum voltage out depending on the incoming voltage and the DC bus utilization. The V/f control, such as V/f control38, may then limit the frequency due to the voltage limitation to ensure that the V/f is maintained as required. Thus, the speed of the compressor motor may be limited due to reduction in the voltage applied to the compressor motor. With reduction in compressor speed, the power consumed by the motor decreases. As the power consumed decreases, the required input current decreases as the power factor on the input side is maintained consistently high by the PFC. Thus, the input side current may be kept within the current rating selection of the rectifier and the PFC sections.
• The present invention using the algorithm illustrated inFIG. 10 allows the compressor to run at lower speeds, but still provides cooling when the input voltage goes below the specified value for that particular inverter when it operates in constant output mode for the PFC section. If the controls (power factor correction and motor control) were split into two different processors, then a high volume of data exchange would be needed to carry out both the switchover of PFC (from constant output to the voltage follower) and the limitation of speed in the motor control section on basis of the incoming voltage. This would consume a lot of the peripherals and the processing bandwidth of both the control elements. This problem is avoided in the present invention by combining both types of the control software into a common digital signal processor.
• In the embodiment of the present invention illustrated inFIG. 8, the current is sensed only on the DC side of the inverter. This current on DC side may be used to protectmotor121 for single phasing in addition to locked rotor and over-current protection. The DC Link current may be measured and compared with the expected current based on the motor speed as is fed to this logic from the motor speed control algorithm, as illustrated inFIG. 11.
• The DC Link current may be steady as long as the inverter load is symmetrical. That is, the current flow may be consistent with little variations. However, in reality, this current may vary slightly irrespective of the consistency of the load. However when single phasing occurs, that is, when one of the phases stops conducting, then the current out frominverter164 may be dependent only on two phases from the inverter, and the total power may be drawn from only these two phases. Now, the current on the DC Link may vary. When theIGBT166 of the non-conducting phase comes on, there may be no conduction, but when the IGBT of the conducting phase switches on, there may be high current conduction. Thus, there may be significant variation in the DC side current in case of single phasing. Hence, if the variation in DC side current is high (beyond a certain defined window of variation based on the speed and external ambient), the fault is understood to be single phasing on the output ofinverter164.Inverter164 is then switched off and thuscompressor motor121 is protected.
• According to the present invention, a warning may be issued when the compressor is in need of service, or when there is an impending failure of the compressor or the air conditioner. The variable speed compressor may be provided with an additional two pin terminal (not shown) and three thermistors (not shown), i.e., one thermistor on each respective phase of a three-phase motor of the compressor. These thermistors may be connected to a two pin terminal where the temperature of the winding can be measured in the form of resistance of these thermistors. Beyond a certain winding temperature, such as approximately 130° C., resistance increases dramatically. This temperature can be tapped from an electronic controller, such asDSP40.
• Based on the measured winding temperatures, it can be estimated to determine whether to issue a warning that maintenance service is needed or that the appliance or the compressor is failing.
• It is possible to estimate the motor current based upon the ambient temperature. The electronic controller also measures the current as a protection measure. Power may be removed from the motor based on ambient temperature measurements or current measurements in order to protect the motor from high current.
• In the present invention, the control functions may be classified as appliance control (indoor control) and inverter control (outdoor control). The two controls work together to form a complete control system. The appliance control may function as the master controller and determine requirements on the basis of the user settings and the various measured parameters. The appliance control may determine the amount of cooling needed to satisfy the user requirement. A signal indicative of this required cooling amount may be transmitted to the inverter controller. On receipt of this data, the inverter controller may control the speed of the compressor to provide the desired cooling. The inverter controller may be capable of handling the outdoor side accessories, such as the fan, temperature measurement, etc. In this manner there may be a hardware/software combination for the indoor section and another hardware/software combination for the outdoor section. As most of the controls needing large amounts of processing are performed in the outdoor section, the indoor section may be a smaller portion of the control system.
• The inverter controls and the appliance controls may be separated into two printed circuit boards (PCBs) each.FIG. 12 illustrates the split architecture of separate indoor and outdoor processing units. In both the inverter controls and the appliance controls there is a control PCB whose hardware may be the same for each type and rating of air-conditioner. The other PCB in the inverter control may be the power PCB. The power PCB may be connected to the control PCB, and may have different designs corresponding to different ratings. The software on the control PCB may be the same for each application.
• In the appliance section, the user and appliance interface section of the PCB may vary with the application, while the hardware of the control PCB may be the same for each application. The software may be modified to suit the particular appliance. In this manner, the inventory control and hardware-software library maintenance is simplified. The window air-conditioner system can easily be made into a split system without any change in hardware or software, further simplifying the version control issues.
• The present invention has been described herein as being used to cool or heat a room of a building. However, it is to be understood that the present invention may also be used to cool or heat any space or compartment, such as a refrigerated compartment in a drink dispenser or refrigerator.
• While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles.

Claims (20)

1. A vapor compression system comprising:

a fluid circuit circulating a refrigerant in a closed loop, said fluid circuit having operably disposed therein, in serial order, a variable speed compressor, a first heat exchanger, an expansion device and a second heat exchanger;

a first blower device associated with said first heat exchanger, a speed of said first blower device being dependent upon a speed of said compressor; and

a second blower device associated with said second heat exchanger, a speed of said second blower device being dependent upon the speed of said compressor.

2. The system ofclaim 1 wherein said first heat exchanger comprises a condenser, said second heat exchanger comprising an evaporator.

3. The system ofclaim 1 wherein the speed of said first blower device is approximately equal to the speed of said second blower device.

4. The system ofclaim 3 further comprising a blower motor operatively coupled to each of said first blower device and said second blower device.

5. The system ofclaim 4 wherein the blower motor has a plurality of discrete operating speeds each corresponding to a respective range of compressor speeds.

6. A method of controlling a room temperature, said method comprising the steps of:

determining a first temperature differential between a first actual temperature and a desired temperature;

operating a compressor;

ascertaining a subsequent temperature differential between a subsequent actual temperature and the desired temperature after said operating step;

decreasing a speed of said compressor if the subsequent temperature differential is less than an immediately preceding temperature differential; and

repeating said operating, ascertaining, and decreasing steps so long as the speed of said compressor is greater than a threshold speed and the subsequent temperature differential is greater than a threshold temperature differential.

7. The method ofclaim 6 comprising the further step of selecting one of a first mode and a second mode of operating said compressor, said compressor being operated at a highest speed in the first mode if the first temperature differential exceeds a first value, said compressor being operated at the highest speed in the second mode if the first temperature differential exceeds a second value greater than the first value.

8. The method ofclaim 7 wherein said selecting step includes selecting one of the first mode, the second mode, and a third mode in which the speed of said compressor is substantially continuously varied based upon determined temperature differentials.

9. The method ofclaim 6 comprising the further step of increasing the speed of said compressor if the subsequent temperature differential is greater than an immediately preceding temperature differential.

10. The method ofclaim 6 wherein the threshold speed comprises a minimum speed.

11. The method ofclaim 6 wherein the threshold temperature differential is approximately zero.

12. The method ofclaim 6 comprising the further step of operating a fan motor at a speed dependent upon the speed of the compressor.

13. The method ofclaim 6 wherein a time period between said ascertaining steps is dependent upon the speed of the compressor.

14. A vapor compression system comprising:

a variable speed compressor; and

a control device configured to:

repetitively determine a temperature differential between an actual room temperature and a desired room temperature; and

selectively operate said compressor in a first mode and a second mode, the first mode including decreasing a speed of said compressor after each said temperature differential determination so long as a most recently determined temperature differential is less than a second most recently determined temperature differential, the second mode including substantially continuously varying the speed of said compressor based upon the determined temperature differentials.

15. The system ofclaim 14 wherein the first mode includes increasing the speed of said compressor if the most recently determined temperature differential is greater than the second most recently determined temperature differential.

16. The system ofclaim 14 wherein the speed of said compressor is decreased after each said temperature differential determination so long as the speed of said compressor is greater than a threshold speed.

17. The system ofclaim 16 wherein the threshold speed comprises a minimum speed.

18. The system ofclaim 16 wherein said control device is configured to operate said compressor so long as the actual room temperature is greater than the desired room temperature.

19. The system ofclaim 16 further comprising a fan motor configured to blow air across a heat exchanger that is in fluid communication with said compressor, said control device being configured to operate said fan motor at a speed dependent upon the speed of the compressor.

20. The system ofclaim 16 wherein said control device is configured to repetitively determine the temperature differential at time intervals that are dependent upon the speed of the compressor.

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