Start-stop control system and method for gas foil bearing machines

Abstract

An HVAC system includes an unloader, a centrifugal compressor, a gas foil bearing, a VFD, and a controller. The controller is programmed to start the centrifugal compressor from a stopped state by operating the unloader to remove a load from the centrifugal compressor, accelerating a motor to a first speed that is higher than a takeoff speed of the gas foil bearing and lower than an operating speed of the centrifugal compressor, operating the motor at the first speed for a period of time, operating the unloader to apply a load to the centrifugal compressor, and accelerating the motor to the operating speed.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. patent application No. 17 / 009,535, filed on September 1, 2020, which is incorporated herein by reference in its entirety. Technical Field

[0003] The field of the present disclosure relates generally to control systems and, more particularly, to control systems for machines including gas foil bearing assemblies. Background Art

[0004] Gas foil bearing (GFB) machines are particularly useful in HVAC applications, such as two-stage refrigerant centrifugal compressors. HVAC compressors have a drive shaft operatively connected to a motor supported by gas foil bearings between the impeller stages. The drive shaft can be positioned between the impeller stages, rotating the impellers at a speed to compress the refrigerant to a selected pressure in the HVAC system. The compressor bearings are typically equipped with one or more features to reduce friction between the compressor bearings and the drive shaft. Once the shaft rotates quickly enough, the gas pushes the foil away from the shaft, preventing contact. The shaft and gas foil bearing are separated by the high pressure of the gas, which is generated by the rotation of the gas into the bearings through viscosity effects. A high speed of the shaft relative to the gas foil bearing is required to induce a gas gap, and once this is achieved, wear does not occur. These bearings offer several advantages over conventional bearings, including reduced weight due to the elimination of an oil system, stable operation at higher speeds and temperatures, low power losses at high speeds, and a longer life with minimal maintenance.

[0005] Current gas foil bearings deform in response to the pressures generated within the compressor. Gas foil bearings are subject to wear and tear during startup and shutdown operations. More specifically, operating GFB machines below takeoff speed and experiencing compressor surge events lead to accelerated wear of the bearings and bearing coatings. Surge is a characteristic behavior of centrifugal compressors that can occur when the inlet flow rate decreases to the point where the pressure head generated by the compressor is insufficient to overcome the pressure at the compressor discharge. Once surge occurs, the compressor's output pressure decreases dramatically, leading to a backflow within the compressor. When a centrifugal compressor surges, there is actual gas backflow through the compressor impeller. Surge typically begins in one stage of a multi-stage compressor and can occur very rapidly. Compressors are particularly susceptible to surge events during startup and shutdown due to their lower operating speeds. The severity of surge events and the resulting damage increase with compressor speed. Minimizing the time that a GFB machine operates below its takeoff speed and minimizing the number and severity of surge events experienced by the compressor increases bearing life.

[0006] This background section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present disclosure described below and / or claimed. This discussion is believed to be helpful in providing the reader with background information to better understand the various aspects of the present disclosure. Therefore, it should be understood that these statements are to be understood from this perspective and not as admissions of prior art. Summary of the Invention

[0007] In one aspect, an HVAC system is described that includes an unloading device, a centrifugal compressor, a gas foil bearing, and a controller. The centrifugal compressor includes a compressor housing, a motor having a drive shaft rotatably supported within the compressor housing, and an impeller connected to the drive shaft and operable to compress refrigerant gas as the drive shaft rotates. The gas foil bearing is supported by the compressor housing and supports the drive shaft. A controller is connected to the motor and the unloading device. The controller is programmed to start the centrifugal compressor from a stopped state by operating the unloading device to remove a load from the centrifugal compressor, accelerating the motor to a first speed that is greater than a takeoff speed of the gas foil bearing and less than an operating speed of the centrifugal compressor, operating the motor at the first speed for a period of time, operating the unloading device to apply a load to the centrifugal compressor, and accelerating the motor to the operating speed. The controller is also programmed to stop the centrifugal compressor from an operating state by operating the unloading device to remove the load from the centrifugal compressor, decelerating the motor toward a minimum speed greater than zero, and, when the motor speed reaches the minimum speed, removing power from the motor and allowing it to coast to a stop.

[0008] In another aspect, a controller for controlling a centrifugal compressor having a gas foil bearing that supports a shaft of an impeller driven by a motor is described. The controller includes a motor interface for connecting to the motor, an unloading interface for connecting to an unloading device, a processor, and a memory. The memory includes instructions that, when executed by the processor, cause the controller to start the centrifugal compressor from a stopped state by operating the unloading device to remove a load from the centrifugal compressor, accelerating the motor to a first speed that is greater than a takeoff speed of the gas foil bearing and less than an operating speed of the centrifugal compressor, operating the motor at the first speed for a period of time, operating the unloading device to apply a load to the centrifugal compressor, and accelerating the motor to the operating speed. The memory also includes instructions that, when executed by the processor, cause the controller to stop the centrifugal compressor from an operating state by operating the unloading device to remove the load from the centrifugal compressor, decelerating the motor toward a minimum speed greater than zero, and, when the speed of the motor reaches the minimum speed, removing power from the motor and allowing the motor to coast to a stop.

[0009] In yet another aspect, a method of controlling a centrifugal compressor having a gas foil bearing that supports a shaft of an impeller driven by a motor is described. The method includes starting the centrifugal compressor from a stopped state by operating an unloading device to remove a load from the centrifugal compressor, accelerating the motor to a first speed that is greater than a takeoff speed of the gas foil bearing and less than an operating speed of the centrifugal compressor, operating the motor at the first speed for a period of time, operating the unloading device to apply a load to the centrifugal compressor, and accelerating the motor to the operating speed. The method also includes stopping the centrifugal compressor from an operating state by operating the unloading device to remove the load from the centrifugal compressor, decelerating the motor toward a minimum speed greater than zero, and removing power from the motor when the speed of the motor reaches the minimum speed and allowing the motor to coast to a stop.

[0010] Various refinements of the features presented in connection with the above-described aspects exist. Other features may also be incorporated into the above-described aspects. These refinements and additional features may exist individually or in any combination. For example, the various features discussed below with respect to any of the illustrated embodiments may be incorporated into any of the above-described aspects individually or in any combination. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The following figures illustrate various aspects of the disclosure.

[0012] Figure 1 is a perspective view of the assembled compressor.

[0013] Figure 2 It is taken along line 2-2 Figure 1 A cross-sectional view of a compressor with the external ducting removed.

[0014] Figure 3 It passes through Figure 2 , which illustrates a drive shaft supported within a foil bearing assembly that is retained within the sleeve of the bearing housing using a pair of retaining clips.

[0015] Figure 4 Is suitable for Figure 1 A cross-sectional view of another embodiment of a bearing housing for use in a compressor of FIG. 1 illustrates a drive shaft supported within a foil bearing assembly retained within the bearing housing between a retaining lip formed within the bearing housing at one end and a retaining clip at an opposite end.

[0016] Figure 5 is an exploded view of the elements of the foil bearing assembly arranged relative to the bearing housing and drive shaft.

[0017] Figure 6 is a block diagram of a start-stop control system for a gas foil bearing (GFB) machine.

[0018] Figure 7 The present invention is a flow chart of a method for starting a centrifugal compressor from a stopped state.

[0019] Figure 8 It is a surge current characteristic curve diagram for centrifugal compressors.

[0020] Figure 9 is a flow chart of a method for stopping a centrifugal compressor from an operating state.

[0021] Figure 10 is a deceleration graph showing estimated surge speed and minimum speed as actual speed decreases during a centrifugal compressor stop.

[0022] Figure 11 is a flow chart of an example startup routine for a centrifugal compressor.

[0023] Figure 12 is a flow chart of an example shutdown routine for a centrifugal compressor.

[0024] Figure 13 is a flow chart of an example emergency stop routine for a centrifugal compressor.

[0025] Corresponding reference numerals indicate corresponding parts throughout the drawings. DETAILED DESCRIPTION

[0026] For the sake of brevity, an example will be described with respect to an HVAC compressor. However, the methods and systems described herein can be applied to any suitable gas foil bearing (GFB) machine. In a start-stop control system for a GFB machine, the following startup routine will prevent additional deformation of the bearings: disconnect the load from the compressor, then quickly accelerate the compressor to an idle speed higher than the takeoff speed of the bearing (~10k RPM), and maintain this idle speed until any initial surge stops. Additionally, the following stop routine will also prevent deformation of the bearings: disconnect the load and then slowly decelerate the compressor to the estimated surge speed plus a margin, and then allow the compressor to coast to a stop.

[0027] Reference Figure 1 The compressor shown in the form of a two-stage refrigerant compressor is generally indicated by 100. The compressor 100 generally includes a compressor housing 102, which is formed with at least one sealed cavity, and each stage of refrigerant compression is completed in the sealed cavity. The compressor 100 includes: a first refrigerant inlet 110, which introduces refrigerant vapor into the first compression stage (at Figure 1a first refrigerant outlet 114; a refrigerant transfer conduit 112 for transferring the compressed refrigerant from the first compression stage to the second compression stage; a second refrigerant inlet 118 for introducing refrigerant vapor into the second compression stage (in the Figure 1 (not shown); and a second refrigerant outlet 120. Refrigerant transfer conduit 112 is operatively connected at opposite ends to first refrigerant outlet 114 and second refrigerant inlet 118, respectively. Second refrigerant outlet 120 delivers compressed refrigerant from the second compression stage to the cooling system in which compressor 100 is incorporated. Refrigerant transfer conduit 112 may also include a refrigerant discharge port 122 for adding (or removing) refrigerant from compressor 100 as needed.

[0028] Reference Figure 2 The compressor housing 102 encloses a first compression stage 124 and a second compression stage 126 at opposite ends of the compressor 100. The first compression stage 124 includes a first impeller 106 configured to add kinetic energy to the refrigerant entering through the first refrigerant inlet 110. The kinetic energy imparted to the refrigerant by the first impeller 106 is converted into increased refrigerant pressure (i.e., compression) as the refrigerant velocity slows as the refrigerant passes into a sealed cavity (e.g., a diffuser) formed between the first bearing housing 200 and a portion of the outer compressor housing 102. Similarly, the second compression stage 126 includes a second impeller 116 configured to add kinetic energy to the refrigerant entering through the second refrigerant inlet 118, passed from the first compression stage 124. The kinetic energy imparted to the refrigerant by the second impeller 116 is converted into increased refrigerant pressure (i.e., compression) as the refrigerant velocity slows as the refrigerant passes into a sealed cavity (e.g., a diffuser) formed between the bearing housing 200a and the second portion of the outer compressor housing 102. The compressed refrigerant is discharged through the second refrigerant outlet 120 (not in the Figure 2 ) exits the second compression stage 126.

[0029] Reference Figure 2 , the first stage impeller 106 and the second stage impeller 116 are connected at opposite ends of the drive shaft 104. The drive shaft 104 is operatively connected to a motor 108 positioned between the first stage impeller 106 and the second stage impeller 116 so that the first stage impeller 106 and the second stage impeller 116 rotate at a selected rotational speed to compress the refrigerant to a preselected pressure exiting the second refrigerant outlet 120. Any suitable motor may be incorporated into the compressor 100, including but not limited to an electric motor. The drive shaft 104 is supported by a gas foil bearing assembly 300 positioned within the sleeve 202 of each bearing housing 200 / 200a, as described in further detail below. Figure 2 As shown in FIG, each bearing housing 200 / 200 a includes a mounting structure (not shown) for connecting the corresponding bearing housing 200 / 200 a to the compressor housing 102 .

[0030] Reference Figure 2 Each bearing housing 200 / 200a supports the drive shaft 104, and the drive shaft 104 protrudes through the bearing housing 200 / 200a opposite to the sleeve 202, and the impeller 106 is connected to the protruding end of the drive shaft 104. Figure 3 and Figure 5 , a gas foil bearing assembly 300 is positioned within the cylindrical bore 206 within the bearing housing 200. The drive shaft 104 fits tightly within the gas foil bearing assembly 300, which includes an outer compliant foil or outer compliant foil layer 302 positioned adjacent the inner wall of the sleeve 202, an inner compliant foil or inner compliant foil layer 306 (also referred to as a "top foil") positioned adjacent the drive shaft 104, and a bump foil or bump foil layer 310 positioned between the inner foil layer 306 and the outer foil layer 302. The foils or layers 302 / 306 / 310 of the gas foil bearing assembly form a substantially cylindrical tube sized to receive the drive shaft 104 with a relatively small or no clearance design as determined by existing foil bearing design methods. The components of the foil bearing assembly 300, such as the outer foil layer 302, the inner foil layer 306, and the bump foil layer 310, can be constructed of any suitable material that enables the foil bearing assembly 300 to function as described herein. Suitable materials include, for example, but are not limited to, metal alloys. In some embodiments, for example, each of the outer foil layer 302, the inner foil layer 306, and the bump foil layer 310 is constructed of stainless steel (e.g., 17-4 stainless steel).

[0031] Refer again Figure 3 The foil bearing assembly 300 in the illustrated embodiment further includes a pair of foil retainers 312a / 312b positioned adjacent to opposite ends of the layers 302 / 306 / 310 to inhibit the layers 302 / 306 / 310 from sliding in the axial direction within the cylindrical bore 206 of the sleeve 202. A pair of foil retaining clips 314a / 314b positioned adjacent to the foil retainers 312a / 312b, respectively, secure the layers 302 / 306 / 310 in place. Figure 3 The foil retaining clips 314a / 314b may be removably connected to the bearing housing 200.

[0032] In other embodiments, Figure 4As shown in FIG, each bearing housing 200 includes a foil retention lip 214 that is integrally formed (e.g., cast) with the bearing housing 200 and projects radially inwardly from the radially inner surface 204 defining the cylindrical bore 206. In the illustrated embodiment, the foil retention lip 214 is positioned at the cylindrical bore 206 proximate the impeller 116 (at Figure 2 204). The foil retention lip 214 is sized and dimensioned to protrude a radial distance from the radial inner surface 204 that overlaps at least a portion of the layers 302 / 306 / 310 of the foil bearing assembly 300. The foil retention lip 214 may extend completely around the circumference of the radial inner surface 204, or the foil retention lip may include two or more segments that extend over a portion of the circumference of the radial inner surface 204 and are separated by a space that is flush with adjacent radial inner surfaces 204. The bearing housing 200a (at Figure 4 ) are formed in a similar manner.

[0033] Figure 4 The foil bearing assembly 300 of the embodiment illustrated in FIG also includes a single foil retention clip 314 positioned adjacent the ends of the layers 302 / 306 / 310 opposite the foil retention lip 214 to inhibit axial movement of the layers 302 / 306 / 310 within the cylindrical bore 206 of the sleeve 202. In this embodiment, the foil retention clip 314 snaps into a circumferential groove 212 formed in the radial inner surface 204 of the cylindrical bore 206 near the motor end 218 of the cylindrical bore 206.

[0034] The foil retention lip 214 may be positioned in any area of the cylindrical bore 206 proximate the impeller end 216, including but not limited to immediately adjacent the opening of the cylindrical bore 206 at the impeller end 216. Alternatively, the foil retention lip 214 may be positioned in any area of the cylindrical bore 206 proximate the motor end 218, including but not limited to immediately adjacent the opening of the cylindrical bore 206 at the motor end 218. In such an embodiment, the foil retention clip 314 may be positioned in any area of the cylindrical bore 206 proximate the motor end 218. Figure 4 2. The impeller 216 engages in a circumferential groove 212 formed in the radially inner surface 204 of the cylindrical bore 206 near the impeller end 216 in an arrangement that is substantially the opposite of the arrangement shown in FIG.

[0035] Refer again Figure 4The foil bearing assembly 300 is installed within the bearing housing 200 by inserting the foil bearing assembly 300 into the cylindrical bore 206 of the bearing housing 200 at the motor end 218. The foil bearing assembly 300 is then advanced axially into the cylindrical bore 206 toward the impeller end 216 until the layers 302 / 306 / 310 contact the foil retention lip 214. The foil retention clip 314 then snaps into the circumferential groove 212 of the cylindrical bore 206 near the motor end 218 to lock the foil bearing assembly 300 in place.

[0036] In other embodiments, any suitable method for attaching the foil bearing assembly 300 within the sleeve 202 may be used. Non-limiting examples of suitable methods include retainers and retaining clips, adhesives, set screws, and any other suitable securing method.

[0037] The bearing housing 200 / 200a may also serve as a mounting structure for various components including, but not limited to, radial bearings such as the foil bearing assembly 300 described above, thrust bearings, and sensing devices (not shown) such as proximity probes, pressure transducers, thermocouples, key phasers, etc. used as feedback for passive or active control schemes.

[0038] The foil bearing assembly 300 can be configured in any suitable manner, without limitation. For example, the foil bearing assembly 300 can be configured with two, three, four, or additional layers, without limitation. The bump foil 310 of the foil bearing assembly 300 can be formed from a radially resilient structure to provide a resilient surface for the rotating drive shaft 104 during operation of the compressor 100. The bump foil 310 can be formed from any suitable radially resilient structure, including, but not limited to, an array of deformable waves or other features designed to deform and rebound under intermittent compressive radial loads, and any other elastically resilient material capable of compressing and rebounding under intermittent compressive radial loads. The bump foil 310 can be connected to at least one adjacent layer, including, but not limited to, at least one of the outer layer 302 and the inner layer 306. In some embodiments, the bump foil 310 can be connected to both the outer layer 302 and the inner layer 306. In other embodiments, the bump foil 310 can be free-floating and not connected to any layer of the foil bearing assembly 300.

[0039] Reference Figure 6, an example embodiment of the system 400 includes a centrifugal compressor 404. The system 400 includes the compressor 404 having a compressor housing 405, an unloading device 401, and a user interface 415, a controller 410. The compressor includes a motor 406, an impeller 407, and a gas foil bearing 409. The system 400 also includes a variable frequency drive (VFD) 416 having a current sensor 408 and a motor interface 413 in communication with the motor 406. The compressor housing 405 and the compressor 404 including the motor 406, the impeller 407, and the gas foil bearing 409 can be connected to Figures 1 to 5 The compressor 404 is similarly configured to the compressor 100 described in the system 400 or may be configured differently. The compressor 404 is not limited to the specific configuration of the system 400. The compressor 404 includes a controller 410 for controlling the startup routine, shutdown routine, and operation routine of the compressor 404. The controller 410 includes a processor 411, a memory 412, and an unloading interface 414. The memory 412 includes instructions that are executed by the processor 411 to cause the controller 410 to perform various methods.

[0040] Unloader 401 in system 400 removes and / or reduces the load on the compressor during startup and shutdown routines to handle surge events and prevent accelerated wear on gas foil bearings 409. In centrifugal compressor 404, excessively low flow or excessively high pressure increases cause the angle of attack within the impeller to vary, leading to separation and stall. This results in unstable compressor flow and, if used, shock to bearings and gears, as well as unstable system pressure. This is caused by decreased inlet density and flow due to issues such as over-throttling, hot exhaust gas, cold return gas, or clogged condenser inlet filters. When these process conditions force compressor 404 to operate at a low flow rate and ensure that compressor 404 always handles a flow rate higher than the surge value, unloader 401 opens as necessary to allow gas delivered by compressor 404 to recirculate to the suction section. By coupling unloader 401 to compressor 404, flow is maintained to prevent compressor 404 from entering a stall / surge cycle. In this example, unloader 401 is a bypass valve or discharge valve in acceptable applications. Regardless of how slowly the compressor motor 406 accelerates during startup or decelerates during shutdown, a bypass valve, such as a refrigerant bypass valve, provides an alternative path for gas, thereby stopping the pressure rise of the compressor 404 and thus limiting any potential surge. In other embodiments, the unloading device 401 is an expansion valve. An expansion valve removes pressure from the liquid refrigerant to allow expansion or a change of state from liquid to vapor in the evaporator of the compressor 404, and expansion valves are included in many HVAC systems. Other embodiments of the unloading device 401 include variable orifice or variable diameter valves, such as servo valves, and fixed orifice or fixed diameter valves, such as solenoid valves and pulse width modulation (PWM) valves, which are configured to control opening and closing according to a duty cycle. Other embodiments of the unloading device 401 may include, but are not limited to, a variable diffuser or a variable inlet guide vane (VIGV). Although many types of unloading devices are described herein, the unloading device 401 may also be any suitable device that reduces the load on the compressor 404. The strategic opening of the unloader 401 at the start-up routine and the stop-down routine of the compressor 404 is determined by the system 400 .

[0041] Unloader 401 is operatively coupled to controller 410, and controller 410 is configured to control at least one operating parameter of unloader 401, such as the opening of the bypass valve, according to one or more control schemes described in detail below. Controller 410 controls the removal or reduction of load on compressor 404 according to one or more control schemes based on measurements or other data received from current sensor 408, and is configured to monitor one or more conditions of compressor 404. Current sensor 408 senses the current of motor 406, and controller 410 determines whether surge in compressor 404 has ceased when the sensed current of motor 406 is substantially constant. Non-limiting examples of suitable sensors used in the one or more control schemes include temperature sensors, pressure sensors, flow sensors, current sensors, voltage sensors, rotational speed sensors, and any other suitable sensors. In other embodiments, controller 410 controls the removal or reduction of load on compressor 404 according to one or more schemes that do not rely on measurements or other data received from sensors, and instead operates based on pre-set timing.

[0042] In some embodiments, the removal or reduction of the load is controlled in response to a detected state of the compressor 404. In these embodiments, the compressor system 100 includes at least one unloader 401 controlled by a controller 410. In some embodiments, the reduction or disconnection of the load on the compressor 404 is controlled by operating the at least one unloader 401 according to one or more feedback control schemes based on the detected state of the compressor 404. The feedback or closed-loop control schemes used to achieve the reduction or disconnection of the load on the compressor 404 may include, but are not limited to, a PID controller, a PI controller, a fuzzy logic controller, and any other suitable control scheme that can be used to reduce or disconnect the load on the compressor 404.

[0043] The control system 400 includes a motor interface 413 for connecting the VFD 416 to the motor 406, an interface for connecting the controller to the driver, and an unloading interface 414 for connecting the controller 410 to the unloading device 401, so that the processor 411 can execute the following instructions: These instructions are stored in the memory 412 to reduce or disconnect the load from the compressor 400 during the starting method and the stopping method.

[0044] The control system 400 includes a user interface 415 configured to output (e.g., display) and / or receive (e.g., from a user) information associated with the system 400. In some embodiments, the user interface 415 is configured to receive an activation input and / or a deactivation input from a user to activate and deactivate (i.e., turn on and off) or otherwise enable operation of the system 400. Additionally, in some embodiments, the user interface 415 is configured to output information associated with one or more operating features of the system 400, including, for example, but not limited to, warning indications, the status of the gas foil bearing 409, and any other suitable information.

[0045] The user interface 415 may include any suitable input and output devices that enable the user interface 415 to function as described herein. For example, the user interface 415 may include input devices including, but not limited to, a keyboard, a mouse, a touch screen, a joystick, a throttle, buttons, switches, and / or other input devices. Furthermore, the user interface 415 may include output devices including, for example, but not limited to, a display (e.g., a liquid crystal display (LCD) or an organic light emitting diode (OLED) display), a speaker, indicator lights, instruments, and / or other output devices. Furthermore, the user interface 415 may be part of a different component, such as a system controller (not shown). Other embodiments do not include the user interface 415.

[0046] In some embodiments, the system 400 can be controlled by a remote control interface. For example, the system 400 can include a communication interface (not shown) configured to connect to a wireless control interface that enables remote control and activation of the system 400. The wireless control interface can be implemented on a portable computing device, such as a tablet or smartphone.

[0047] Controller 410 is generally configured to control the operation of compressor 404. Controller 410 controls operation through programming and instructions from another device or controller, or is integrated with control system 400 through a system controller. In some embodiments, for example, controller 410 receives user input from user interface 415, and controller 410 controls one or more components of system 400 in response to such user input. For example, controller 410 can control the power supplied to motor 406 based on the user input received from user interface 415. Additionally, in some embodiments, controller 410 can regulate or control the power supplied to system 400, such as from an energy storage device.

[0048] Controller 410 may generally include any suitable computer and / or other processing unit, including any suitable combination of computers, processing units, and / or the like that may be communicatively coupled to one another and may operate independently or in conjunction with one another (e.g., controller 410 may form all or part of a controller network). Controller 410 may include one or more modules or devices, one or more of which may be enclosed within system 400 or may be located remotely from system 400. Controller 410 may be part of compressor 404 or separate, and may be part of a system controller in an HVAC system. Controller 410 and / or components of controller 410 may be integrated or incorporated into other components of system 400. In some embodiments, for example, controller 410 may be incorporated into motor 406 or unloading device 401. Controller 410 may include one or more processors 411 and associated memory devices 412 configured to perform various computer-implemented functions (e.g., performing calculations, determinations, and the functions disclosed herein). As used herein, the term "processor" refers not only to integrated circuits, but also to controllers, microcontrollers, microcomputers, programmable logic controllers (PLCs), application-specific integrated circuits, and other programmable circuits. Additionally, the memory device 412 of the controller 410 may generally be or include a memory element, including, but not limited to, computer-readable media (e.g., random access memory (RAM)), computer-readable non-volatile media (e.g., flash memory), a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disk (DVD), and / or other suitable memory elements. Such a memory device 412 may generally be configured to store suitable computer-readable instructions that, when implemented by the processor, configure or cause the controller 410 to perform the various functions described herein, including, but not limited to, controlling the system 400, controlling the operation of the motor 406, receiving input from the user interface 415, providing output to an operator via the user interface 415, controlling the unloader 401, and / or various other suitable computer-implemented functions.

[0049] Reference Figure 7, describes a method 500 for starting a centrifugal compressor from a stopped state. Method 500 may also be referred to as a startup routine or startup method. This startup method 500, as well as other methods described herein, can be implemented using the aforementioned system 400. Processor 411 executes instructions for method 500, and memory 412 stores these instructions. Startup method 500 begins by operating 501 an unloading device 401 to remove load from centrifugal compressor 404. In an exemplary embodiment, the unloading device is a bypass valve between the discharge and suction sections of compressor 404. Opening the valve reduces the pressure ratio of compressor 404 and increases the mass flow rate of compressor 404. Next, motor 406 is accelerated 502 to a first speed that is higher than the takeoff speed of gas foil bearing 409 and lower than the operating speed of centrifugal compressor 404. By rapidly accelerating to the first speed while compressor 404 is unloaded, compressor 404 operates for a reduced amount of time below the takeoff speed of gas foil bearing 409, helping to prevent wear on gas foil bearing 409 and address potential surge events during startup. The motor 406 then runs 503 at the first speed for a certain period of time. In an example embodiment, the period of time that the motor 406 runs is set or predetermined by the manufacturer or by a user via the user interface 415. In other embodiments, the period of time is a variable period of time that begins when the motor reaches the first speed and ends when the surge of the centrifugal compressor stops, and the period of time can be estimated, calculated, or measured. Regardless of whether the compressor 404 experiences a surge event, the system 400 accelerates the compressor 404 to a first speed that is higher than the takeoff speed of the gas foil bearing 409 to minimize wear on the gas foil bearing 409. In some cases, there may not be a surge event during the startup routine when the motor 406 runs at the first speed for a certain period of time. If a surge event has already occurred during the startup method 500, the current sensor 408 will sense the current of the motor 406, and the controller 410 determines that the surge of the compressor 404 has stopped when the sensed current of the motor 406 is approximately constant current. In some embodiments, the system 400 can incorporate various learning algorithms to monitor and store system measurements over multiple startups and optimize the time period during which the motor is operated at the first speed based on historical trends over time and the significance of surge events stored in the memory 412. In some embodiments, the system 400 monitors and stores measurements associated with all surge events experienced over the life of the compressor 404 and sets the time period to the longest surge event currently stored in the memory 412. In this case, each time a new surge event occurs that is longer than the time period currently set by the system 400, the time period is reset to be longer than the most recent surge event for future iterations of the startup method 500.In some embodiments, a machine learning algorithm or neural network may be employed by system 400 to predict the frequency and duration of surge events for compressor 404 based on different simulated environments in which compressor 404 will reside and set the time period based on these simulations.

[0050] Reference Figure 8 , a surge current characteristic curve 600 during the starting method 500 is shown including a speed curve 601 and a motor current curve 602 . Figure 8 The motor speed is shown being accelerated 502 to a first speed and the motor 406 is operated 503 at the first speed for a period of time 605. During the operation 503 of the motor 406 at the first speed for a period of time 605, an area of possible surge 603 has been identified by oscillations in the motor current curve 602. The compressor 404 remains at the no-load speed until the current oscillation pattern of surge has ceased 604 and the compressor 404 is indicated as fully started.

[0051] Return to reference Figure 7 In the startup method 500, after the motor 406 runs 503 at the first speed for a certain period of time 605, the unloading device 401 is operated 504 to apply a load to the compressor 404. In some embodiments, applying the load to the compressor 404 includes closing the bypass valve. Figure 8 In all cases of surge, operation 504 of the unloading device to apply a load to the compressor 404 will only begin after the current oscillation pattern of the surge has ceased 604. If there is no surge during startup, the motor 406 is operated at a first speed for a certain period of time 605, which is generally predetermined by the manufacturer or user via the user interface 415. Finally, the motor 406 is then accelerated 505 to the operating speed and the compressor 404 completes the startup routine and begins the operating routine of the compressor 404. In some embodiments, the controller 410 accelerates the motor 406 to the first speed at a first acceleration and accelerates the motor to the operating speed at a second acceleration that is less than the first acceleration. In other embodiments, the first acceleration and the second acceleration are the same. In other embodiments, the second acceleration is less than the first acceleration.

[0052] Reference Figure 9, describes a method 700 for stopping a centrifugal compressor 404 from an operating state. The method 700 may also be referred to as a stop routine or a stop method. First, the unloading device 401 is operated 701 to remove the load from the compressor 404, similar to the starting method 500. It should be noted that the controller 410 can implement the starting method 500, operate and stop the method 700, or implement any combination of these three stages of the compressor 404. Next, the motor 406 is decelerated 702 toward a minimum speed greater than zero. Finally, power is removed 703 from the motor 406 when the speed of the motor 406 reaches the minimum speed, and the motor 406 is then allowed to coast to a stop. In some embodiments, the minimum speed is an estimated surge speed plus a margin, and surge of the compressor 404 may occur below the estimated surge speed.

[0053] Reference Figure 10 , shows a deceleration curve 800 for an estimated surge speed 801 and a minimum speed 802 as the actual speed 803 decreases during the stopping method 700. The actual speed deceleration curve 803 shows the stopping method 700 as the motor 406 decelerates toward a minimum speed 802 greater than zero. At the point where the actual speed is at the minimum speed 804, power is removed and the motor 406 is allowed to coast to a stop. The estimated surge speed curve 801 is an estimated compressor speed below which there is a risk of a surge event. To help avoid a surge event, a margin 805 is added to the estimated surge speed curve 801 for additional protection of the compressor 404 from falling below the estimated surge speed curve 801 during the deceleration 702. The estimated surge speed plus the margin curve 802 is the standard that the controller 410 will prevent the compressor 404 speed from falling below during the deceleration 702 toward the minimum speed. In some embodiments, the estimated surge speed is retrieved by the controller 410 from a lookup table and varies with the pressure ratio of the HVAC system in which the compressor 404 resides. In other embodiments, the minimum speed is retrieved by the controller 410 from a lookup table and varies with the pressure ratio of the HVAC system in which the compressor 404 resides. As described above with respect to a surge event in the startup method 500, a learning algorithm can be used to calculate a deceleration curve that includes the estimated surge speed plus the margin curve 802, the estimated surge speed curve 801, and the minimum speed in the shutdown method 700.

[0054] Reference Figure 11, describes an example embodiment of a start-up method 900. The start-up method 900 is one embodiment of the broader start-up method 500 and is considered non-limiting and may be implemented using the system 400 described above. In this embodiment, the motor 406 and the gas foil bearing 409 are given a start-up condition where their temperatures are less than 100°F. Other start-up conditions may include waiting at least 5 minutes since the compressor 404 has stopped, the compressor 404 having a requested demand (kW) greater than the minimum power, and the VFD 416 having no faults. The start-up routine begins by requesting a start and checking whether the above start-up conditions have been met, and the start-up routine does not proceed until the conditions are met. After these conditions are met, the bypass valve is set to 100% and opened to unload the compressor 404. The VFD 416 is activated and the acceleration of the motor 406 is set to A 最大 RPM / second, the speed command is set to N 空载 RPM, and the start timer is then activated. In an example embodiment, A 最大 is 4500RPM / sec and N 空载 is 10000RPM. The speed increases and experiences T 启动 seconds to reach N 启动 RPM, and method 900 will not proceed further until this criterion is met. 启动 After N seconds 启动 RPM, then the shutdown method 1000 is implemented as follows (in Figure 12 In the example embodiment, T 启动 is 0.5 seconds and N 启动 is 2000RPM. Once at T 启动 Reach N in seconds 启动 RPM, then the speed increases to N 空载 RPM. The speed must be within T 启动 Reach N in seconds 空载 RPM and will not proceed until this criterion is met. If 空载 After N seconds 空载 RPM, then execute the shutdown method 1000. In an embodiment, T 空载 5 seconds. 空载 After RPM, the end timer is activated for surge events. Method 900 checks if surge is detected, and if surge is detected, the end timer is set to T 重置 In an example embodiment, T 重置 is 0 seconds. In this case, if the start timer indicates that more than T 喘振minutes, then execute the shutdown method 1000. If the start timer indicates that less than T 喘振 minutes, the method is repeated, checking for surge until no surge is detected. When no surge is detected, the end timer is checked. If the end timer indicates that less than T 结束 seconds, then check the start timer again to see if T 喘振 minutes. In addition, if more than T 喘振 minutes, the shutdown method 1000 is executed, and if less than T 喘振 minutes, the method repeats the surge detection step. In an exemplary embodiment, T 喘振 is 2 minutes and T 结束 If the end timer indicates that more than T 结束 seconds, the bypass valve is R 打开 % per second is set to B 打开 %, the acceleration step is reduced to A 最小 RPM / second, and the speed command is set to the minimum speed N 最小 Then the actual speed is compared with N 最小 If the start timer indication is greater than T 最小 minutes and the actual speed does not reach N 最小 , then the shutdown method 1000 is executed. If the actual speed is less than T as indicated by the start timer 最小 If the minimum speed is reached within 1 minute, automatic control is enabled for compressor 404. In the example embodiment, B 打开 is 50%, R 打开 0.5% per second, A 最小 is 150RPM / sec, and T 最小 In the range of 3 minutes to 4 minutes. In other embodiments, A 最大 、N 空载 、T 启动 、N 启动 、N 空载 、T 空载 、T 重置 、T 喘振 、T 结束 、B 打开 、R 打开 、A 最小 、N 最小 and T 最小 Any other suitable value may be used. The automatic control indicated here is a single set of control algorithms to operate the compressor until shut down.

[0055] Reference Figure 12, describes an example embodiment of a shutdown method 1000. The shutdown method 1000 is one embodiment of the broader shutdown method 700 and can be implemented using the system 400 described above. In this embodiment, when shutdown is requested, such as in the startup method 900 described above, the bypass valve is set to 100% to unload the compressor. The shutdown timer is activated and the speed command is set to the minimum speed N 最小 If the actual speed is greater than N multiplied by 1.01 最小 , then check the stop timer to see if T 停止 In this case, if the stop timer indicates that less than T 停止 minutes, the speed command is set to N again 最小 , and check whether the actual speed is greater than the multiplied coefficient N 最小 If the stop timer is greater than or equal to T 停止 If the actual speed is less than the minimum speed multiplied by the coefficient, the VFD command is also disabled and the speed command is set to 0 RPM. In an example embodiment, T 停止 is 3 minutes and the coefficient is 1.01. Then wait for T 等待 minutes, and then the bypass valve is set to 0%. In the example embodiment, T 等待 The shutdown method 1000 then ends as the motor 406 coasts to a stop.

[0056] Reference Figure 13 In addition to the conventional shutdown method 1000, an emergency stop (e-stop) method 1100 may be implemented for the user to override the HVAC system via the user interface 415 or in the event of an emergency stop. When an emergency stop is requested, the VFD command is set to disabled and the speed command is set to 0 RPM. The bypass valve is set to 100% and the T 等待 After 1 minute, the bypass valve is then set to 0%. In the example embodiment, T 等待 The emergency stop method 1100 then ends as the motor 406 coasts to a stop.

[0057] The technical benefits of the methods and systems described herein are as follows: (1) minimizing the time the compressor is below takeoff speed for the gas foil bearings to prevent wear on the bearings during startup and shutdown procedures in an HVAC system, (b) employing an unloading device to minimize the number and severity of surge events observed by the compressor in the HVAC system, and (c) maintaining the compressor at no-load speed for a period of time to handle surge events during startup and shutdown procedures, and (d) reducing speed during shutdown to limit the number and severity of surge events.

[0058] When introducing elements of the present disclosure or embodiments thereof, the articles "a," "an," "the," and "said" are intended to indicate that one or more of the elements are provided. The terms "comprising," "including," "comprising," and "having" are intended to be inclusive and mean that additional elements may be provided in addition to the listed elements. The use of terms indicating a particular orientation (e.g., "top," "bottom," "side," etc.) is for convenience of description and does not require any particular orientation of the described items.

[0059] As various changes could be made in the above constructions and methods without departing from the scope of the present disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

1. An HVAC system comprising: Unloading device; as well as A centrifugal compressor, comprising: compressor housing; a motor having a drive shaft rotatably supported within the compressor housing; an impeller connected to the drive shaft and operable to compress refrigerant gas upon rotation of the drive shaft; a gas foil bearing supported by the compressor housing and supporting the drive shaft; and a controller connected to the motor and the unloading device, the controller being programmed to: The centrifugal compressor is started from a stopped state by: operating the unloading device to remove a load from the centrifugal compressor, accelerating the motor to a first speed that is greater than a takeoff speed of the gas foil bearing and less than an operating speed of the centrifugal compressor, operating the motor at the first speed for a certain period of time, operating the unloading device to apply the load to the centrifugal compressor, and The motor is accelerated to the operating speed.

2. The HVAC system of claim 1, wherein: The controller is further programmed to stop the centrifugal compressor from operating by: operating the unloading device to remove a load from the centrifugal compressor, decelerating the motor towards a minimum speed greater than 0, and When the speed of the motor reaches a minimum speed, power is removed from the motor and the motor is allowed to coast to a stop.

3. The HVAC system of claim 1, wherein: The time period is a predetermined time period.

4. The HVAC system of claim 1, wherein: The time period is a variable time period that begins when the motor reaches the first speed and ends when surge of the centrifugal compressor stops.

5. The HVAC system according to claim 4, further comprising a current sensor for sensing a current of the motor, wherein The controller determines that surge of the centrifugal compressor has ceased when the sensed current of the motor is a substantially constant current.

6. The HVAC system of claim 1, wherein: The controller accelerates the motor to the first speed at a first acceleration and accelerates the motor to the operating speed at a second acceleration that is less than the first acceleration.

7. The HVAC system of claim 2, wherein: The minimum speed includes an estimated surge speed below which surge of the centrifugal compressor may occur plus a margin.

8. The HVAC system of claim 7, wherein: The estimated surge speed is retrieved by the controller from a lookup table and varies with the pressure ratio of the HVAC system.

9. The HVAC system of claim 7, wherein: The minimum speed is retrieved by the controller from a lookup table and varies with the pressure ratio of the HVAC system.

10. The HVAC system of claim 1, wherein: The unloading device includes a refrigerant bypass valve.

11. A controller for controlling a centrifugal compressor having a gas foil bearing that supports a shaft of an impeller driven by a motor, the controller comprising: a VFD comprising a motor interface for connecting to the motor; an unloading interface, the unloading interface being used to connect to an unloading device; processor; as well as a memory comprising instructions that, when executed by the processor, cause the controller to: The centrifugal compressor is started from a stopped state by: operating the unloading device to remove a load from the centrifugal compressor, accelerating the motor to a first speed that is higher than a takeoff speed of the gas foil bearing and lower than an operating speed of the centrifugal compressor, operating the motor at the first speed for a certain period of time, operating the unloading device to apply the load to the centrifugal compressor, and The motor is accelerated to the operating speed.

12. The controller according to claim 11, wherein: The memory includes instructions that, when executed by the processor, further cause the controller to stop the centrifugal compressor from an operating state by: operating the unloading device to remove a load from the centrifugal compressor, decelerating the motor towards a minimum speed greater than 0, and Power is removed from the motor when the speed of the motor reaches the minimum speed and the motor is allowed to coast to a stop.

13. The controller according to claim 10, wherein: The time period is a variable time period that begins when the motor reaches the first speed and ends when surge of the centrifugal compressor stops.

14. The controller of claim 13, further comprising a current sensor interface that receives a signal from a current sensor representing the current of the motor, wherein The controller determines that surge of the centrifugal compressor has ceased when the sensed current of the motor is a substantially constant current.

15. The controller according to claim 11, wherein: The controller accelerates the motor to the first speed at a first acceleration and accelerates the motor to the operating speed at a second acceleration that is less than the first acceleration.

16. The controller according to claim 12, wherein: The minimum speed includes an estimated surge speed below which surge of the centrifugal compressor may occur plus a margin.

17. The controller according to claim 16, wherein: The estimated surge speed or the minimum speed is retrieved by the controller from a lookup table and varies with the pressure ratio of the HVAC system.

18. A method for controlling a centrifugal compressor having a gas foil bearing supporting a shaft of an impeller driven by a motor, the method comprising: The centrifugal compressor is started from a stopped state by: operating an unloading device to remove a load from the centrifugal compressor, accelerating the motor to a first speed that is higher than a takeoff speed of the gas foil bearing and lower than an operating speed of the centrifugal compressor, The motor runs at the first speed for a certain period of time, operating the unloading device to apply the load to the centrifugal compressor, and The motor is accelerated to the operating speed.

19. The method of claim 18, further comprising stopping the centrifugal compressor from operating by: operating the unloading device to remove a load from the centrifugal compressor, decelerating the motor towards a minimum speed greater than 0, and Power is removed from the motor when the speed of the motor reaches the minimum speed and the motor is allowed to coast to a stop.

20. The method according to claim 18, wherein The time period is a variable time period that begins when the motor reaches the first speed and ends when surge of the centrifugal compressor stops.

21. The method according to claim 20, further comprising: receiving a signal from a current sensor representing a current of the motor; as well as It is determined that surge of the centrifugal compressor has ceased when the sensed current of the motor is a substantially constant current.

22. The method according to claim 18, wherein Accelerating the motor to a first speed includes accelerating the motor to the first speed at a first acceleration, and accelerating the motor to the operating speed includes accelerating the motor to the operating speed at a second acceleration that is less than the first acceleration.

23. The method according to claim 19, wherein The minimum speed includes an estimated surge speed below which surge of the centrifugal compressor may occur plus a margin, and the method further includes retrieving the estimated surge speed or retrieving the minimum speed from a lookup table.

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