U.S. patent application number 12/375255 was filed with the patent office on 2009-12-17 for refrigerant system with multi-speed pulse width modulated compressor.
Invention is credited to Alexander Lifson, Mark A. Lifson, Michael F. Taras.
Application Number | 20090308086 12/375255 |
Document ID | / |
Family ID | 39268734 |
Filed Date | 2009-12-17 |
United States Patent
Application |
20090308086 |
Kind Code |
A1 |
Lifson; Alexander ; et
al. |
December 17, 2009 |
REFRIGERANT SYSTEM WITH MULTI-SPEED PULSE WIDTH MODULATED
COMPRESSOR
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.
Inventors: |
Lifson; Alexander; (Manlius,
NY) ; Taras; Michael F.; (Fayetteville, NY) ;
Lifson; Mark A.; (Fairport, NY) |
Correspondence
Address: |
CARLSON, GASKEY & OLDS, P.C.
400 WEST MAPLE ROAD, SUITE 350
BIRMINGHAM
MI
48009
US
|
Family ID: |
39268734 |
Appl. No.: |
12/375255 |
Filed: |
October 6, 2006 |
PCT Filed: |
October 6, 2006 |
PCT NO: |
PCT/US06/39583 |
371 Date: |
January 27, 2009 |
Current U.S.
Class: |
62/115 ;
418/55.1; 62/498 |
Current CPC
Class: |
F04C 18/0215 20130101;
F25B 2600/0252 20130101; F04C 23/008 20130101; F25B 2400/13
20130101; F25B 41/22 20210101; F04C 29/042 20130101; F25B 49/025
20130101; F04C 28/08 20130101; F25B 2600/0261 20130101 |
Class at
Publication: |
62/115 ; 62/498;
418/55.1 |
International
Class: |
F25B 1/00 20060101
F25B001/00; F01C 1/04 20060101 F01C001/04 |
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)
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.sub.DESIRED. The desired
capacity is matched to the Q.sub.TIME-AVERAGED capacity. The
Q.sub.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.
* * * * *