REFRIGERANT SYSTEM WITH MULTI-SPEED PULSE WIDTH MODULATED COMPRESSOR
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.
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.
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.
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 INVENTIONIn 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.
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.
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.
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.
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.
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.
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.
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.
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
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.
Claims
1. A refrigerant system comprising:
- a compressor for compressing a refrigerant and delivering this refrigerant to a downstream condenser, refrigerant passing through said condenser, through an expansion device, through an evaporator, and returning from said evaporator to said compressor;
- a motor for driving said compressor, said motor being operable at at least two speeds; and
- a pulse width modulation control for said motor, said pulse width modulation control operating said motor between a higher speed and a lower speed at a certain cycling rate to vary the refrigerant system capacity.
2. The refrigerant system as set forth in claim 1, wherein said motor has only two speeds, with said pulse width modulation control cycling said motor between a higher speed and a lower speed.
3. The refrigerant system as set forth in claim 1, wherein said refrigerant system is also provided with an economizer function, and said economizer function being selectively actuated to assist in achieving a desired system capacity.
4. The refrigerant system as set forth in claim 1, wherein said refrigerant system is also provided with an unloader function, and said unloader function being selectively actuated to assist in achieving a desired system capacity.
5. The refrigerant system as set forth in claim 1, wherein said compressor is a scroll compressors.
6. The refrigerant system as set forth in claim 5, wherein a flow control device is provided for controlling the amount of refrigerant passing to a back pressure chamber for holding orbiting and non-orbiting scroll members of said scroll compressor in contact with each other, and said flow control device being selectively and periodically closed to block flow of compressed fluid to the back pressure chamber to assist in achieving a desired system capacity.
7. The refrigerant system as set forth in claim 1, wherein a suction modulation valve is placed on a suction line leading to said compressor, and is controlled to limit the amount of refrigerant reaching said compressor to assist in achieving a desired system capacity.
8. The refrigerant system as set forth in claim 1 wherein a cycling rate is adjustable.
9. The refrigerant system as set forth in claim 1 wherein the cycle time can vary from 1 second to 30 seconds.
10. The refrigerant system as set forth in claim 1 wherein the cycling rate is defined by at least one of reliability, temperature control, and efficiency considerations.
11. The refrigerant system as set forth in claim 1 wherein a time period at the lower speed is defined by lubrication consideration of the compressor elements.
12. The refrigerant system as set forth in claim 1, wherein said compressor is selected from a set consisting of a screw compressor, a reciprocating compressor and a rotary compressor.
13. The refrigerant system as set forth in claim 1, wherein said refrigerant system is one of an air-conditioning system, a heat pump system, a container refrigeration system, a tractor-trailer refrigeration system and a supermarket refrigeration system.
14. The refrigerant system as set forth in claim 1, wherein said motor is positioned outside a compressor shell.
15. The refrigerant system as set forth in claim 1, wherein said motor is positioned inside a compressor shell.
16. The refrigerant system as set forth in claim 1, wherein said motor operation between a high and low speed is accomplished by solid state contactors.
17. A method of operating a refrigerant system comprising the steps of:
- a) providing a compressor for compressing a refrigerant and delivering refrigerant to a downstream condenser, refrigerant passing through said condenser, through an expansion device, through an evaporator, and returning from said evaporator to said compressor;
- providing a motor for driving said compressor, said motor being operable at least at two speeds; and
- cycling said motor between a higher speed and a lower speed at a certain rate in a pulse width modulation manner to vary the refrigerant system capacity.
18. The method as set forth in claim 17, wherein said motor has only two speeds, with said pulse width modulation control cycling said motor between a higher speed and a lower speed.
19. The method as set forth in claim 17, wherein said refrigerant system is also provided with an economizer function, and said economizer function being selectively actuated to assist in achieving a desired system capacity.
20. The method as set forth in claim 17, wherein said refrigerant system is also provided with an unloader function, and said unloader function being selectively actuated to assist in achieving a desired system capacity.
21.-32. (canceled)
Type: Application
Filed: Oct 6, 2006
Publication Date: Dec 17, 2009
Inventors: Alexander Lifson (Manlius, NY), Michael F. Taras (Fayetteville, NY), Mark A. Lifson (Fairport, NY)
Application Number: 12/375,255
International Classification: F25B 1/00 (20060101); F01C 1/04 (20060101);