Abstract:
A refrigerant system is provided with a compressor having a motor that is operable at least at two distinct speeds. The pulse width modulation control is provided to cycle a compressor motor operation between its at least two speeds at a specific rate to exactly match thermal load demands in a conditioned space. The present invention reduces cycling and other efficiency losses as have been experienced in the prior art, as well as minimizes cost and may improve reliability. Also, the present invention can be utilized in conjunction with other known unloading techniques.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates to a refrigerant system wherein a compressor motor may operate at least at two speeds, and wherein a pulse width modulation control is provided to allow cycling the motor between the distinctive speeds at a specified adjustable rate to vary the refrigerant system capacity. 
         [0002]    Refrigerant systems are utilized to condition a secondary fluid, such as air. Compressors and refrigerant systems are typically sized to meet a maximum capacity demand. However, for the most part, a cooling or heating capacity demand is relatively low, and therefore, the refrigerant system needs to be unloaded by some means. In a single-circuit refrigerant system, a motor for a compressor is typically cycled between on and off operational stages. Alternatively, a suction modulation valve, or a variable speed compressor may be utilized. All of these methods of unloading have various drawbacks. When the unit is cycled on and off, the temperature and humidity of the environment to be conditioned cannot be precisely controlled. In the air conditioning case, this insufficient temperature control creates discomfort to the occupant of the indoor environment. In the refrigeration case, the inadequate temperature control can lead to spoilage of goods that need to be refrigerated and kept within a specified temperature range. Also, cycling losses occurring during compressor start-stop operation are detrimental to the efficiency of the refrigerant system. Similarly, the use of a suction modulation valve has undesirable consequences, since an increased pressure ratio across the compressor reduces the refrigerant system efficiency and increases compressor discharge temperature. Also, a suction modulation valve adds cost to the unit and becomes an additional reliability risk. In the case of a variable speed compressor, a variable speed drive is also a significant cost adder. Furthermore, a compressor speed, while controlled by a variable speed drive, often cannot be reduced below a certain value to meet tight temperature control requirements. The variable speed drive systems introduce extra losses due to inefficiencies of the variable speed drives themselves. These extra losses are normally on the order of 5-6%. Also, additional expensive means to cool the variable speed drive are often required. Lastly, the use of a variable speed compressor and associated components introduce extra complexity into the system design, potentially leading to reliability problems. 
         [0003]    One of the methods proposed in the past to vary the system capacity, was to rapidly cycle the refrigerant system components. For example, it is known to rapidly cycle a suction valve between open and closed positions (a so-called pulse width modulation control), to control the amount of refrigerant delivered to a compressor. In this manner, the capacity provided by the overall refrigerant system is reduced. Though this method is highly advantageous, it is often not as efficient as a variable speed option, and in some cases may lead to excessively high discharge temperatures. Therefore, there is a need to further develop and advance the use of the pulse width modulation technique. Rapid cycling or pulse width modulation techniques have not yet been utilized to control the speed of a multi-speed compressor motor. 
       SUMMARY OF THE INVENTION 
       [0004]    In a disclosed embodiment of this invention, a compressor in a refrigerant system is provided with a multi-speed motor. A motor can be designed to operate at two or more distinct speeds by, for instance, being wound with pole changing windings. A particular motor speed can be selected by changing external connections. Switching between the motor speeds can be accomplished by so-called solid state contactors. (Though more expensive than normal switching controls, the solid state contactors offer higher reliability when fast switching between the speeds might be required.) A control for selecting the motor operating speed is provided with pulse width modulation capability. When it is determined that a reduced capacity should be provided, the pulse width modulation control cycles the compressor motor between a higher and lower speed at a required rate to satisfy thermal load demands in a conditioned space. In this manner, the capacity of the refrigerant system is reduced and precisely tailored to the required capacity. Further, pulse width modulation of the compressor motor can be used in conjunction with other unloading techniques such as switching on and off an economizer circuit, employment of a compressor bypass valve and utilization of a suction modulation valve. In this invention, the compressor motor is cycled sufficiently fast between its operating speeds that the cycling rate would normally be faster than the system thermal inertia. In other words, the cycling rate is selected to be fast enough not to significantly affect the temperature of the air supplied to the environment to be conditioned when the compressor is switched from one operating speed to another. 
         [0005]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0006]      FIG. 1A  is a schematic view of a refrigerant system incorporating the present invention. 
           [0007]      FIG. 1B  shows another schematic incorporating the present invention. 
           [0008]      FIG. 1C  shows another schematic related to the present invention. 
           [0009]      FIG. 2  is a speed versus time graph for one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0010]      FIG. 1A  shows a refrigerant system  20  incorporating a compressor  21  with a multi-speed motor  22  driving a shaft  24 . The compressor  21  is illustrated as a scroll compressor having an orbiting scroll  26  interfitting with a non-orbiting scroll  28 . It has to be noted, that although the description is primarily related to a scroll compressor type, any other compressor (screw, reciprocating, rotary, etc.) capable of running at multiple speeds is within the scope of the invention. The present invention would apply to different types of refrigerant systems. For example, these systems may include air conditioning units, heat pump units, chiller systems, and different types of refrigeration units including container units, truck and trailer units and supermarket cabinets and display cases. The present invention would also apply to different compressor-motor configurations, where the motor can be a part of a hermetic or semi-hermetic compressor shell that also includes compressor pumping elements (in this case, the compression elements can be referred to as the compressor). Alternatively, the motor  200  can be located outside the shell  202  containing the compression elements (a so-called open-drive compressor design), see  FIG. 1C . 
         [0011]    The motor  22  is a motor that can operate at least at two speeds, although the invention would extend to motors operable at multiple discrete speeds (or nearly discrete speeds, as it is the case with the induction motors, where the motor speed can vary slightly at each distinct speed due to a motor slip). A control  23  can control the motor to operate at a desired speed from a set of multiple discrete speeds mentioned above for a certain period of time. A switching device is included into the control  23  to switch from one operating speed to another operating speed. The switching rate can also be controlled, if desired. 
         [0012]    Refrigerant, having been compressed by the compressor  21 , passes through a discharge line  30 , a condenser (or a gas cooler in transcritical operation)  32 , and flows toward a main expansion device  33 . As shown, the refrigerant system  20  can incorporate, as an option, an economizer cycle, including an econonizer heat exchanger  34 , where a tapped portion of refrigerant passes through an economizer expansion device  37 , and then through the economizer heat exchanger  34 . As known, the expanded (to a lower pressure and temperature) refrigerant in the tap line  36  cools a refrigerant in a main refrigerant circuit, also flowing through the economizer heat exchanger  34  toward the main expansion device  33 , to provide higher cooling thermal potential in an evaporator  40 . While the tapped refrigerant is shown passing in the same direction through the economizer heat exchanger  34 , in practice, the two refrigerant flows are typically arranged in a counterflow configuration. In case the economizer expansion device is not equipped with a shutoff capability, an extra flow control device such as a valve  54  may be added to enable an economizer function when additional capacity is desired and to disengage it when extra capacity is not required. Although the economizer flow is tapped upstream of the economizer heat exchanger  34 , as known in the art, downstream tap point locations are feasible and are within the scope of the invention. 
         [0013]    Downstream of the main expansion device  33 , the refrigerant passes through the evaporator  40 , and then to a line  41 . A suction modulation valve  42  (also an optional component for the purposes of this invention) is shown for controlling the amount of refrigerant passing to a suction line  44  and back to the compressor  21 . 
         [0014]    Other optional feature in this refrigerant system includes an unloader bypass line  48  incorporating an unloader valve  50  and selectively communicating at least a portion of partially compressed refrigerant from the compressor  21  to a line  46  and then to the suction line  44  to reduce the capacity of the refrigerant system  20  when desired. 
         [0015]    A return line  52  returns the tapped refrigerant, typically in a vapor state, downstream of the economizer heat exchanger  34  through the valve  54  and line  46  to an intermediate point in the compression process. In this embodiment, the same ports are selectively utilized to inject the economized refrigerant when the valve  54  is open and to unload the compressor when the valve  50  is open. In other possible schematics, the economizer and unloader functions do not have to be mutually exclusive and may be engaged simultaneously. 
         [0016]    The economizer function, the unloader function, and the suction modulation valve are all known techniques of varying the capacity provided by the refrigerant system  20  to match thermal load demands in an environment to be conditioned. 
         [0017]    The present invention provides additional control over this capacity by utilizing a pulse width modulation technique from control  23  to rapidly switch the motor  22  between its higher and lower speeds. Thus, as shown in  FIG. 2 , the motor may be cycled between the high speed and the lower speed at a specific rate to provide an average desired capacity Q DESIRED . The desired capacity is matched to the Q TIME-AVERAGED  capacity. The Q TIME-AVERAGED  capacity is calculated as an integrated average of capacities delivered at high and low speeds of operation. In the illustrated example, the high speed is approximately twice as high as the lower speed (for example, the motor speed can be switched between 3500 RPM operation and 1750 RPM operation), and the time of operation at each speed may be controlled to achieve an exact time-averaged speed, in order to provide the desired capacity to precisely match thermal load demands in a conditioned space. It should be pointed out that this invention overcomes one of the limitations of the variable speed compressor, where the variable speed drive has to operate the compressor at a reduced speed for prolonged periods of time. The prolonged operation at a reduced speed can lead to compressor damage, since the amount of lubricating oil delivered to compressor components that needed to be lubricated can be reduced to an unacceptably low level. In this invention, such situations would not occur, as the amount of time the compressor spends at a low operating speed is very short (normally, in the range from 1 to 30 seconds). In this case, the oil supply at a low-speed operation is not interrupted for prolonged period of time and is replenished as soon as the compressor is being brought back to a higher speed. Therefore, no additional provisions to enhance oil lubrication of the compressor components (bearings, seals, etc.) are required. This would not be the case if the compressor had operated at a lower speed for significant amount of time. Thus, in the pulse width modulation mode of operation, the compressor operating envelope can be extended to a much lower speed than in the case of a continuous variable speed operation. The required heating or cooling system capacity defines the ratio of how much time the compressor should operate at a high speed vs. operational time at a low speed. The cycling rate (how fast the compressor is cycled between the high and low speed) is normally determined by reliability and efficiency considerations. A too low cycling rate may present lubrication problems at lower speeds as well as cause unacceptable variations in the temperature of the air delivered to the conditioned environment within the time interval between the high and low speed of operation, as discussed above. On the other hand, an excessively high cycling rate may introduce reliability problems associated with the switching device or potential thermodynamic efficiency degradation. 
         [0018]      FIG. 1B  is included to show an embodiment, wherein one of the various capacity control techniques mentioned in the  FIG. 1A  embodiment also includes an unloader function that controls the amount of refrigerant passing to a back pressure chamber  306  behind one of the scroll members  302  and  304  (the scroll member  304  in this case), and thus the refrigerant pressure in the back chamber  306 , to allow the scroll members to engage and disengage with each other to compress and circulate a required amount of refrigerant throughout the refrigerant system. This technique will reduce overall system capacity by reducing the time-averaged amount of compressed refrigerant vapor. For instance, as shown, a valve  310  controls the amount of a higher pressure fluid from a source  308  reaching the back pressure chamber  306  such that an orbiting scroll  302  and non-orbiting scroll  304  can move in and out of contact with each other to control system capacity. If the valve  310  is controlled by a control  312  in a pulse width modulation manner at a specific rate, the scroll elements  302  and  304  will engage and disengage accordingly, providing required refrigerant flow for the system capacity to match thermal load requirements in a conditioned space. The valve  310  may be positioned internally or externally to the compressor  21 , as well as the controls  23  and  312  may be separate stand-alone controls or combined with the control for the refrigerant system  20 . This technique is known, and is mentioned here as another feature that can be utilized in combination with the inventive pulse width modulation control of the drive speed of the compressor motor to precisely tailor provided system capacity to desired capacity. 
         [0019]    Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.