Patent Description:
A chiller system is a refrigerating machine or apparatus that removes heat from a medium. Commonly a liquid such as water is used as the medium and the chiller system operates in a vapor-compression refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required. As a necessary byproduct, refrigeration creates waste heat that must be exhausted to ambient or, for greater efficiency, recovered for heating purposes. A conventional chiller system often utilizes a centrifugal compressor, which is often referred to as a turbo compressor. Thus, such chiller systems can be referred to as turbo chillers. Alternatively, other types of compressors, e.g. a screw compressor, can be utilized.

In a conventional (turbo) chiller, refrigerant is compressed in the centrifugal compressor and sent to a heat exchanger in which heat exchange occurs between the refrigerant and a heat exchange medium (liquid). This heat exchanger is referred to as a condenser because the refrigerant condenses in this heat exchanger. As a result, heat is transferred to the medium (liquid) so that the medium is heated. Refrigerant exiting the condenser is expanded by an expansion valve and sent to another heat exchanger in which heat exchange occurs between the refrigerant and a heat exchange medium (liquid). This heat exchanger is referred to as an evaporator because refrigerant is heated (evaporated) in this heat exchanger. As a result, heat is transferred from the medium (liquid) to the refrigerant, and the liquid is chilled. The refrigerant from the evaporator is then returned to the centrifugal compressor and the cycle is repeated. The liquid utilized is often water.

A conventional centrifugal compressor basically includes a casing, an inlet guide vane, an impeller, a diffuser, a motor, various sensors and a controller. Refrigerant flows in order through the inlet guide vane, the impeller and the diffuser. Thus, the inlet guide vane is coupled to a gas intake port of the centrifugal compressor while the diffuser is coupled to a gas outlet port of the impeller. The inlet guide vane controls the flow rate of refrigerant gas into the impeller. The impeller increases the velocity of refrigerant gas, generally without changing pressure. The diffuser increases the refrigerant pressure without changing the velocity. The motor rotates the impeller. The controller controls the motor, the inlet guide vane and the expansion valve. In this manner, the refrigerant is compressed in a conventional centrifugal compressor. The inlet guide vane is typically adjustable and the motor speed is typically adjustable to adjust the capacity of the system. In addition, the diffuser may be adjustable to further adjust the capacity of the system. The controller controls the motor, the inlet guide vane and the expansion valve. The controller can further control any additional controllable elements such as the diffuser.

When the pressure behind the compressor is higher than the compressor outlet pressure, the fluid tends to reverse or even flow back in the compressor. As a consequence, the pressure will decrease, inlet pressure will increase and the flow reverses again. This phenomenon, called surge, repeats and occurs in cycles. The compressor loses the ability to maintain the peak head when surge occurs and the entire system becomes unstable. A collection of surge points during varying compressor speed or varying inlet guide vane angle is called a surge line. In normal conditions, the compressor operates in the right side of the surge line. However, during startup/emergency shutdown, the operating point will move towards the surge line because flow is reduced. If conditions are such that the operating point approaches the surge line, flow recirculation occurs in the impeller and diffuser. The flow recirculation, which causes flow separation, will eventually cause a decrease in the discharge pressure, and flow from suction to discharge will resume. Surging can cause the compressor to overheat to the point at which the maximum allowable temperature of the unit is exceeded. Also, surging can cause damage to the thrust bearing due to the rotor shifting back and forth from the active to the inactive side. This is defined as the surge cycle of the compressor.

Therefore, techniques have been developed to predict surge. See for example <CIT>.

<CIT>discloses a compressor having a housing assembly with a suction port and a discharge port. A shaft is mounted for rotation about an axis and an impeller is mounted to the shaft to be driven in at least a first condition so as to draw fluid in through the suction port and discharge the fluid from the discharge port. A magnetic bearing system supports the shaft. A controller is coupled to a sensor and configured to detect at least one of surge and pre-surge rotating stall and, responsive to said detection, take action to prevent or counter surge.

<CIT> discloses a control system and a method for controlling a centrifugal chiller through which a fluid to be chilled passes includes a chilling apparatus having at least one of each of the following components; an evaporator, a compressor, preferably a magnetic bearing compressor, a condenser and an expansion device. A plurality of sensors measure and generate signals representing operating conditions within the chilling apparatus. A chiller control unit including a signal processor receives the signals generated by the plurality of sensors. The chiller control unit further includes a memory device that stores information relating to thermodynamic properties of specific fluids and a comparison device programmed with a comparison algorithm that compares the received signals generated by the plurality of sensors to thermodynamic properties of the specific fluid contained in the memory device. Based on the comparison, the control unit generates at least one control signal to vary operation of one or more of the evaporator, compressor, condenser and expansion device to ensure the chiller system is operating at maximum efficiency.

<CIT> discloses a method for avoiding an unstable working condition in centrifugal compressors, which unstable working condition is better known under the term "surge" or "surging" condition, characterised in that it consists in measuring and/or calculating the forces on the bearings of the rotor; in detecting timely an exceptional imbalance of radial forces on the bearings which occurs before the centrifugal compressor ends up in an unstable condition; and, when the above-mentioned exceptional radial imbalance has been detected, in changing the operational parameters of the centrifugal compressor so as to avoid "surge". <CIT> discloses a centrifugal compressor comprising a casing, a guide vane, an impeller, a radial magnetic bearing, and an axial thrust bearing, as well as sensors to detect shaft positions.

In a conventional centrifugal compressor, differential pressure between a hub side pressure and a shroud side pressure is detected. The differential pressure is then compared to set values to predict surge. While this technique works relatively well, it is desirable to predict surge more quickly and accurately.

Therefore, one object of the present invention is to provide a centrifugal compressor that predicts surge more quickly and/or accurately.

Another object of the present invention is to provide a centrifugal compressor that predicts surge without overly complicated construction and/or additional parts.

A centrifugal compressor according to the present invention is defined by claim <NUM>. Dependent claims relate to preferred embodiments.

These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments.

Referring now to the attached drawings which form a part of this original disclosure:.

Selected embodiments will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments are provided for illustration only and not for the purpose of limiting the invention, which is defined by the appended claims.

Referring initially to <FIG>, a chiller system <NUM> is illustrated which includes a centrifugal compressor in accordance with an embodiment of the present invention. The chiller system <NUM> is preferably a water cooled chiller that utilizes cooling water and chiller water in a conventional manner. The chiller system <NUM> illustrated herein is a single stage chiller system. However, it will be apparent to those skilled in the art from this disclosure that the chiller system <NUM> could be a multiple stage chiller system. The chiller system <NUM> basically includes a controller <NUM>, a compressor <NUM>, a condenser <NUM>, an expansion valve <NUM>, and an evaporator <NUM> connected together in series to form a loop refrigeration cycle. In addition, various sensors S and T are disposed throughout the circuit as shown in <FIG>. The chiller system <NUM> is conventional except that the chiller system predicts surge in accordance with the present invention.

Referring to <FIG>, in the illustrated embodiment, the compressor <NUM> is a centrifugal compressor. The centrifugal compressor <NUM> of the illustrated embodiment basically includes a casing, <NUM>, an inlet guide vane <NUM>, an impeller <NUM>, a diffuser <NUM>, a motor <NUM> and a magnetic bearing assembly <NUM> as well as various conventional sensors. The controller <NUM> receives signals from the various sensors and controls the inlet guide vane <NUM>, the motor <NUM> and the magnetic bearing assembly <NUM> in a conventional manner, as explained in more detail below. Refrigerant flows in order through the inlet guide vane <NUM>, the impeller <NUM> and the diffuser <NUM>. The inlet guide vane <NUM> controls the flow rate of refrigerant gas into the impeller <NUM> in a conventional manner. The impeller <NUM> increases the velocity of refrigerant gas, generally without changing pressure. The motor speed determines the amount of increase of the velocity of refrigerant gas. The diffuser <NUM> increases the refrigerant pressure without changing the velocity. The motor <NUM> rotates the impeller <NUM> via a shaft <NUM>. The magnetic bearing assembly <NUM> magnetically supports the shaft <NUM>. In this manner, the refrigerant is compressed in the centrifugal compressor <NUM>.

The centrifugal compressor <NUM> is conventional, except that the centrifugal compressor <NUM> predicts surge in accordance with the present invention. In particular, controller <NUM> uses data received from the magnetic bearing assembly <NUM> of the centrifugal compressor <NUM> in order to predict surge. More specifically, the controller <NUM> in the illustrated embodiment uses a shaft position signal, a magnetic bearing current signal through a magnetic bearing controller in order to predict surge, as explained in more detail below.

Referring to <FIG>, the magnetic bearing assembly <NUM> is conventional, and thus, will not be discussed and/or illustrated in detail herein, except as related to predicting surge in accordance with the illustrated embodiment. Rather, it will be apparent to those skilled in the art that any suitable magnetic bearing can be used without departing from the present invention. As seen in <FIG>, the magnetic bearing assembly <NUM> preferably includes a first radial magnetic bearing <NUM>, a second radial magnetic bearing <NUM> and an axial (thrust) magnetic bearing <NUM>. The first and second radial magnetic bearings <NUM> and <NUM> may be disposed on opposite axial ends of the motor <NUM>, or can be disposed on the same axial end with respect to the motor <NUM> (not illustrated). Various sensors, discussed in more detail below, sense radial and axial positions of the shaft <NUM> relative to the magnetic bearings <NUM>, <NUM> and <NUM>, and send signals to the magnetic bearing control section <NUM> in a conventional manner. The magnetic bearing control section <NUM> then controls the electrical current sent to the magnetic bearings <NUM>, <NUM> and <NUM> in a conventional manner to maintain the shaft <NUM> in the correct position. Since the operation of magnetic bearings and magnetic bearing assemblies such as magnetic bearings <NUM>, <NUM> and <NUM> of magnetic bearing assembly <NUM> are well known in the art, the magnetic bearing assembly <NUM> will not be explained and/or illustrated in detail herein, except as related to predicting surge in accordance with the present invention. Specifically, in the illustrated embodiment, vibrations of the magnetic bearing are sensed and used to predict surge, as discussed in more detail below.

The magnetic bearing assembly <NUM> is preferably a combination of active magnetic bearings <NUM>, <NUM>, and <NUM>, which utilizes non-contact position sensors <NUM>, <NUM> and <NUM> to monitor shaft position and send signals indicative of shaft position to the magnetic bearing control section <NUM>. Thus, each of the magnetic bearings <NUM>, <NUM> and <NUM> are preferably active magnetic bearings. Each active magnetic bearings typically include a proportional-integral-derivative controller (PID controller, or PID). A PID uses information from position sensors <NUM>, <NUM> and <NUM> to adjust the required current to the magnetic bearings <NUM>, <NUM>, and <NUM> of the bearing assembly <NUM> to maintain proper rotor position both radially and axially, as would be apparent in light of the disclosure. Active magnetic bearings are well known in the art, and thus, will not be explained and/or illustrated in detail herein, except as related to predicting surge in accordance with the present invention.

Referring to <FIG>, <FIG>, and <FIG>, in the illustrated embodiment the controller <NUM> includes a magnetic bearing control section <NUM>, a surge prediction section <NUM>, a surge control section <NUM>, a variable frequency drive <NUM>, a motor control section <NUM>, an inlet guide vane control section <NUM>, and an expansion valve control section <NUM>. The controller <NUM> may also include any PIDs as processes of a magnetic bearing control section <NUM> as illustrated in <FIG>. The magnetic bearing control section <NUM>, the surge prediction section <NUM>, the surge control section <NUM>, the variable frequency drive <NUM>, the motor control section <NUM> and the inlet guide vane control section <NUM> form parts of a centrifugal compressor control portion of the controller <NUM> that is electrically coupled to an I/O interface <NUM> of the compressor <NUM>. The magnetic bearing control section <NUM> may be connected to current sensors <NUM>, <NUM>, and <NUM> to monitor current supplied to the magnetic bearings <NUM>, <NUM>, and <NUM> of the bearing assembly <NUM>.

Because the magnetic bearing control section <NUM> is connected to several portions of the magnetic bearing assembly <NUM> and communicates with various sections of the controller <NUM>, the various sections of the controller <NUM> can receive signals from the sensors <NUM> to <NUM> of the compressor <NUM>, perform calculations and transmit control signals to parts of the compressor <NUM> such as the magnetic bearing assembly <NUM>. Similarly, the various sections of the controller <NUM> can receive signals from the sensors S and T, perform calculations and transmit control signals to the compressor <NUM> (e.g., the motor) and the expansion valve <NUM>. The control sections and the variable frequency drive <NUM> can be separate controllers or can be mere sections of the chiller controller programmed to execute the control of the parts described herein. In other words, it will be apparent to those skilled in the art from this disclosure that the precise number, location and/or structure of the control sections, control portion and/or controller <NUM> can be changed without departing from the present invention so long as the one or more controllers are programed to execute control of the parts of the chiller system <NUM> as explained herein.

The controller <NUM> is conventional, and thus, includes at least one microprocessor or CPU, an Input/output (I/O) interface, Random Access Memory (RAM), Read Only Memory (ROM), a storage device (either temporary or permanent) forming a computer readable medium programmed to execute one or more control programs to control the chiller system <NUM>. The controller <NUM> may optionally include an input interface such as a keypad to receive inputs from a user and a display device used to display various parameters to a user. The parts and programming are conventional, except as related to predicting surge, and thus, will not be discussed in detail herein, except as needed to understand the embodiment(s).

The magnetic bearing control section <NUM>, either directly or indirectly from one or more PID, receives signals from the sensors <NUM>, <NUM> and <NUM> of the magnetic bearing assembly <NUM>, and transmits electrical signals to the bearings <NUM>, <NUM> and <NUM> to maintain the shaft <NUM> in the desired position in a conventional manner during normal operation when no surge is predicted. At least one of a PID and the magnetic bearing control section <NUM> is programmed to execute a magnetic bearing control program to maintain the shaft <NUM> in the desired position in a conventional manner. In the illustrated embodiment, the magnetic bearing control section <NUM> may control (e.g. executes a magnetic bearing control program) the magnetic bearing assembly <NUM> using the hardware and/or software of controller <NUM>. However, it will be apparent to those skilled in the art from this disclosure that the magnetic bearing control section <NUM> as well as the other controls sections of the controller <NUM> may be independently implemented by one or more additional separate controllers including the same components of controller <NUM>, but that are connected to the controller <NUM> even though not illustrated.

Referring to <FIG>, the magnetic bearing control section <NUM> is illustrated to be a portion of controller <NUM> as a single independent controller directly integrated into the magnetic bearings, connected to a plurality of PID controllers corresponding to the magnetic bearings, or connected to a single PID controller connected to to each magnetic bearing. These are merely three examples of possible structures for the magnetic bearing control section <NUM> and are not intended to limit the invention as defined by the appended claims. The magnetic bearing control section <NUM> is electrically directly connected, or indirectly connected through one or more PID, to any of the sensors <NUM> to <NUM>, and an amplifier <NUM>, <NUM> or <NUM> of each respective magnetic bearing of the magnetic bearing assembly <NUM>. Each magnetic bearing <NUM> includes a plurality of position sensors <NUM>, a plurality of actuators <NUM> and at least one amp <NUM>. Similarly, each the magnetic bearing <NUM> includes a plurality of position sensors <NUM>, a plurality of actuators <NUM> and at least one amp <NUM>. Likewise, each magnetic bearing <NUM> includes a plurality of position sensors <NUM>, a plurality of actuators <NUM> and at least one amp <NUM>. The amplifiers <NUM>, <NUM> and <NUM> of each magnetic bearing <NUM>, <NUM>, and <NUM> may be a multi-channel amp to control the position sensors thereof, or can include separate amplifiers for each position sensor <NUM>, <NUM> and <NUM>. In either case, the amplifiers <NUM>, <NUM> and <NUM> are electrically connected to the actuators <NUM>, <NUM> and <NUM> of each respective magnetic bearing <NUM>, <NUM>, and <NUM>.

The magnetic bearing control section <NUM> is connected to current sensors <NUM>, <NUM>, and <NUM> in the case that the magnetic bearing control section <NUM> is to monitor current delivered to each actuator <NUM>, <NUM> and <NUM> of the magnetic bearing assembly <NUM> (see <FIG>); connected to position sensors <NUM>, <NUM>, and <NUM> in the case that the magnetic bearing control section <NUM> is to monitor the position of shaft <NUM> (see <FIG>).

The magnetic bearing control section <NUM> is programmed to execute control of each respective actuator <NUM>, <NUM> and <NUM> of the magnetic bearings <NUM>, <NUM>, and <NUM> to maintain a desired position of shaft <NUM>. The magnetic bearing control section <NUM> controls the magnetic bearing assembly <NUM> by either generating or adjusting the control signal sent to each amplifier <NUM>, <NUM> and <NUM> of the magnetic bearing assembly <NUM>. The control signal indicates the current which each amp must output to a respective actuator <NUM>, <NUM> and <NUM> of the magnetic bearing assembly <NUM>. Each amplifier <NUM>, <NUM> and <NUM> may have several channels to independently control each actuator <NUM>, <NUM> and <NUM> of magnetic bearing assembly <NUM> respectively, each actuator <NUM>, <NUM> and <NUM> of magnetic bearing assembly <NUM> may have a unique corresponding amplifier, or a combination as would be understood in light of the disclosure.

The magnetic bearings <NUM>, <NUM>, and <NUM> include current sensors <NUM>, <NUM>, and <NUM> disposed between the amplifier <NUM>, <NUM> and <NUM> and the actuator <NUM>, <NUM> and <NUM> of each magnetic bearing, respectively. The current sensors <NUM>, <NUM>, and <NUM> sense the current being provided to each actuator <NUM>, <NUM> and <NUM> of the magnetic bearing assembly <NUM> by either monitoring the current output by each amplifier <NUM>, <NUM> and <NUM> of the magnetic bearing assembly <NUM>, or by monitoring the current provided to each amplifier <NUM>, <NUM> and <NUM> of the magnetic bearing assembly <NUM> (not illustrated). The current sensors <NUM>, <NUM>, and <NUM> are connected to the surge prediction section <NUM>, and generate current signals that indicate the current being provided to each actuator <NUM>, <NUM> and <NUM> of the magnetic bearing assembly <NUM>. In this manner, the surge prediction section <NUM> can be configured to monitor the current being supplied to the actuators <NUM>, <NUM> and <NUM> of each of magnetic bearings <NUM>, <NUM>, and <NUM>. Alternatively, the surge prediction section <NUM> may be configured to individually monitor the current supplied to any combination of the magnetic bearings <NUM>, <NUM>, and <NUM>. The current sensors <NUM>, <NUM>, and <NUM> are used in the techniques illustrated in <FIG>, but the current sensors <NUM>, <NUM>, and <NUM> may be omitted in the technique illustrated in <FIG>, unless used for some other purpose than surge prediction.

The variable frequency drive <NUM> and motor control section <NUM> receive signals from at least one motor sensor (not shown) and control the rotation speed of the motor <NUM> to control the capacity of the compressor <NUM> in a conventional manner. More specifically, the variable frequency drive <NUM> and motor control section <NUM> are programmed to execute one or more motor control programs to control the rotation speed of the motor <NUM> to control the capacity of the compressor <NUM> in a conventional manner. The inlet guide vane control section <NUM> receives signals from at least one inlet guide vane sensor (not shown) and controls the position of the inlet guide vane <NUM> to control the capacity of the compressor <NUM> in a conventional manner. More specifically, the inlet guide vane control section <NUM> is programmed to execute an inlet guide vane control program to control the position of the inlet guide vane <NUM> to control the capacity of the compressor <NUM> in a conventional manner. The expansion valve control section <NUM> controls the opening degree of the expansion valve <NUM> to control the capacity of the chiller system <NUM> in a conventional manner. More specifically, the expansion valve control section <NUM> is programmed to execute an expansion valve control program to control the opening degree of the expansion valve <NUM> to control the capacity of the chiller system <NUM> in a conventional manner. The motor control section <NUM> and the inlet guide vane control section <NUM> work together and with the expansion valve control section <NUM> to control the overall capacity of the chiller system <NUM> in a conventional manner. The controller <NUM> receives signals from the sensors S and optionally T to control the overall capacity in a conventional manner. The optional sensors T are temperature sensors. The sensors S are preferably conventional pressure sensors and/or temperature sensors used in a conventional manner to perform the control.

Referring now to <FIG>, structure and operation of the centrifugal compressor <NUM> will now be explained in more detail. As mentioned above, the centrifugal compressor <NUM> is adapted to be used in the chiller <NUM>. The casing <NUM> has an inlet portion 31a and an outlet portion 31b. An outlet port <NUM> of the outlet portion 31b is disposed between the impeller <NUM> and the diffuser <NUM>. The inlet guide vane <NUM> is disposed in the inlet portion 31a. The impeller <NUM> is disposed downstream of the inlet guide vane <NUM>. The impeller <NUM> is attached to the shaft <NUM>, which is rotatable about a rotation axis X. The radial magnetic bearings <NUM> and <NUM> rotatably support the shaft <NUM>. Thus, in the illustrated embodiment, there are a pair of radial magnetic bearings <NUM> and <NUM> disposed on opposite axial sides of the motor <NUM>. In any case, at least one radial magnetic bearing <NUM> or <NUM> rotatably supports the shaft <NUM>. The thrust magnetic bearing <NUM> supports the shaft <NUM> along the rotational axis X by acting on a thrust disk <NUM>. The thrust magnetic bearing <NUM> includes the thrust disk <NUM> which is attached to the shaft <NUM>. The thrust disk <NUM> extends radially from the shaft <NUM> in a direction perpendicular to the rotational axis X. The motor <NUM> is arranged and configured to rotate the shaft <NUM> in order to rotate the impeller <NUM>. The diffuser <NUM> is disposed in the outlet portion 31b downstream from the impeller <NUM> with an outlet port of the outlet portion 31b disposed between the impeller <NUM> and the diffuser <NUM>.

Referring to <FIG>, <FIG>, the position sensors <NUM>, <NUM>, and <NUM> sense the location of the shaft <NUM>. The position sensors <NUM> are illustrated as being axially offset from the actuators <NUM> for the sake of illustration, but may be disposed on the same plane as the actuators <NUM> of magnetic bearing <NUM>. Unnumbered backup (mechanical) bearings are located axially adjacent the position sensors <NUM> and <NUM> in a conventional manner. Likewise, the position sensors <NUM> are illustrated as being axially offset from the actuators <NUM> for the sake of illustration, but may be disposed on the same plane as the actuators <NUM> of magnetic bearing <NUM>. The position sensors <NUM> and <NUM> detect a radial position of the shaft <NUM>. Preferably, the magnetic bearing <NUM> includes four position sensors <NUM> radially arranged about shaft <NUM> as illustrated in <FIG>, and the magnetic bearing <NUM> has a configuration identical to the magnetic bearing <NUM>, except the location of the magnetic bearing <NUM>. Thus, the magnetic bearing <NUM> also includes four position sensors <NUM> radially arranged about shaft <NUM> (not all illustrated). The position sensors <NUM> detect the axial position of shaft <NUM> along rotational axis X, and are disposed axially offset from the thrust disk <NUM>. Preferably, magnetic bearing <NUM> includes two position sensors <NUM>, each of the position sensors <NUM> being disposed on opposite sides of the thrust disk <NUM> as illustrated in <FIG>.

All of the position sensors <NUM>, <NUM>, and <NUM> output a positional signal which indicates the position of the shaft <NUM>. The position sensors <NUM> output positional signals indicating the position of the shaft <NUM> at magnetic bearing <NUM>. The position sensors <NUM> output positional signals indicating the position of the shaft <NUM> at magnetic bearing <NUM>. The position sensors <NUM> output positional signals indicating the axial position of the thrust disk <NUM> of the shaft <NUM>. Because only certain movements of the impeller <NUM> may be relevant to predicting surge, a position signal may be any combination of the positional signals indicating the position of shaft <NUM> from the position sensors <NUM>, <NUM>, and <NUM>. By non-limiting example, surge may be predicted by monitoring for any changes to the rotational axis X at one of the rotational magnetic bearings <NUM> and <NUM>; a change in the axial position of shaft <NUM> at magnetic bearing <NUM>; or changes in position of shaft <NUM> indicated at positions monitored by any combination of magnetic bearings <NUM>, <NUM>, and <NUM>.

According to the invention, surge is predicted based on a radial position signal and an axial position signal, as specified by claim <NUM>. The position sensors <NUM>, <NUM>, and <NUM> may send the position signal to the magnetic bearing control section <NUM> directly, or indirectly through one or more PID. The surge prediction section <NUM> may receive the position signal directly from the position sensors <NUM>, <NUM>, and <NUM>; indirectly through one or more PID; or from the bearing control section <NUM>.

During operation, the magnetic bearing control section <NUM>, or one or more PID, receive the positional signals and generates control signals. A control signal is sent to each amplifier <NUM>, <NUM> and <NUM> of the magnetic bearing assembly <NUM>. Each control signal indicates an amount of current to be output by a corresponding amplifier <NUM>, <NUM> and <NUM> of the magnetic bearing assembly <NUM>. The magnetic bearing control section <NUM>, or one or more PID, is programmed to calculate the control signal based on the positional signals. The magnetic bearing control section <NUM> preferably shares at least one of the positional information and the control signals of with the various components of controller <NUM> such as surge prediction section <NUM>. The control signals are generated based on the positional signals.

The surge prediction section <NUM> of controller <NUM> is programmed to predict surge. In accordance with the present invention, the surge prediction section <NUM> predicts surge based on a position signal (<FIG>). Alternatively, but not according to the invention, it may predict surge based on the current signal (<FIG>), and the force or control signal (<FIG>), as described in further detail below. The surge prediction section <NUM> may be executed by the hardware and/or software of controller <NUM>, or may be independently implemented one or more outside controllers as mentioned above.

According to a first method that is compatible with the invention, as illustrated in <FIG>, the surge prediction section <NUM> is programmed to predict surge based on the position signal. In S100, the surge prediction section <NUM> receives the position signal, and determines a shaft position value as indicated by the position signal in S102. In S104, the surge prediction section <NUM> then compares the shaft position value as indicated by the position signal with a predetermined position value. The predetermined position value is usually an ideal shaft position, and by comparing the shaft position value with the predetermined position value, the surge prediction section <NUM> determines an amount the shaft <NUM> has shifted. The predetermined position value is set based on the components and size of the chiller system <NUM> based on experiments conducted by the manufacturer. Alternatively, testing can occur onsite to determine such values. If the shaft position value indicated by the position signal differs from the predetermined position value by an amount equal to or greater than a threshold in S104, the surge prediction section <NUM> proceeds to S108 in which the surge prediction section <NUM> predicts that surge will occur. In S110, upon predicting that surge will occur, the surge prediction section <NUM> outputs a signal to the surge control section <NUM> indicating that surge will occur. Since vibration occurs during surge, displacement amount is indicative of vibration amount. Therefore, displacement amount can be used to determine vibration amount, which indicates surge can be predicted.

After outputting the signal in S <NUM>, the surge prediction section <NUM> returns to SI <NUM>, i.e. receiving the position signal. If the shaft position value indicated by the position signal differs from the predetermined position value by an amount less than the threshold in SI04, the surge prediction section <NUM> proceeds to SI06 in which the surge prediction section <NUM> predicts that no surge will occur. Upon predicting that no surge will occur in SI06, the surge prediction section <NUM> returns to receiving the position signal in SI00. It will be apparent to one of ordinary skill in the art in light of the disclosure, that the method to predict surge based on the position value of the shaft <NUM> may be determined in alternative manners.

According to a second method that is not compatible with the invention, as illustrated in <FIG>, the surge prediction section <NUM> can be programmed to predict surge based on force output by each actuator <NUM>, <NUM> and <NUM>, which can be calculated based on the current signal(s) sensed by the current sensors S3, <NUM> and <NUM> as well as other information. By way of non-limiting example, in S200, the surge prediction section <NUM> receives the current signals from any combination of current sensors <NUM>, <NUM>, and <NUM>. In S202, the surge prediction section <NUM> determines the current value that is being supplied to individual magnetic bearings <NUM>,<NUM>, and <NUM> based on the current signals from the current sensors <NUM>, <NUM>, and <NUM>. The value for force output by each actuator <NUM>, <NUM> and <NUM> may then be determined using the following equation: <MAT> Wherein F is the force output, fx is the magnetic permeability of the magnet of the actuator, N is the number of coil turns of the actuator, i is the current supplied to the actuator, A is the pole face area of the actuator, and g is the air gap thickness between the actuator and the shaft <NUM>, or thrust disk <NUM>, respectively. In S204, the surge prediction section <NUM> then calculates a force output by aggregating the force output at each actuator <NUM>, <NUM> and <NUM> of each respective magnetic bearing <NUM>, <NUM>, and <NUM>, respectively. In S206, the surge prediction section <NUM> then compares the force value output by each actuator <NUM>, <NUM> and <NUM> to a predetermined set of force values for each of magnetic bearings <NUM>,<NUM>, and <NUM>, respectively.

The predetermined set of force values are set based on the components and size of the chiller system <NUM> based on experiments conducted by the manufacturer. Alternatively, testing can occur onsite to determine such values. If any of the force values calculated from the current signal differs from the predetermined set of force values for each actuator <NUM>, <NUM> and <NUM> by an amount greater than a threshold value, the surge prediction section <NUM> continues to S210 and predicts that surge will occur. Upon predicting that surge will occur, the surge prediction section <NUM> proceeds to S212 and outputs a signal to the surge control section <NUM> indicating the prediction that surge will occur, and returns to receiving the current signal in S200. If any of the force values calculated from the current signal differs from the predetermined position value by an amount less than a threshold in S206, the surge prediction section <NUM> predicts that no surge will occur in S208. Upon predicting that no surge will occur, the surge prediction section <NUM> returns to receiving the current signal in S200. It will be apparent to one of ordinary skill in the art in light of the disclosure, that the method to predict surge based on the force values for each actuator <NUM>, <NUM> and <NUM> and/or the exact method of calculating force for each actuator <NUM>, <NUM> and <NUM> may be determined in alternative manners.

According to a third method that is not compatible with the invention, as illustrated in <FIG>, the surge prediction section <NUM> may be programmed to predict surge based on the current signal(s) output to the actuators <NUM>, <NUM> and <NUM>. The current signal(s) can be sensed by the current sensors <NUM>, <NUM> and <NUM> or may be based on a control signal indicative of this information. By way of non-limiting example, in S300, the surge prediction section <NUM> receives the current signals from any combination of current sensors <NUM>, <NUM>, and <NUM>. The surge prediction section <NUM> proceeds to S302, in which the surge prediction section <NUM> determines a current value for each of the magnetic bearings <NUM>, <NUM>, and <NUM>. In S304, the surge prediction section <NUM> then compares the current value that is to be supplied to each actuator <NUM>, <NUM> and <NUM> of the magnetic bearing section <NUM> to a predetermined set of current values for each actuator <NUM>, <NUM> and <NUM> of the magnetic bearing section <NUM>. If any of the current values indicated by the control signals differ from the predetermined set of control values for each of the magnetic bearings <NUM>, <NUM>, and <NUM> by an amount greater than a threshold value in S304, the surge prediction section <NUM> predicts that surge will occur in S308. Alternatively, the current signal(s) can be sensed by the current sensors <NUM>, <NUM> and <NUM> and compared directly to threshold values. In either case, the threshold values are set based on the components and size of the chiller system <NUM> based on experiments conducted by the manufacturer. Alternatively, testing can occur onsite to determine such values. Upon predicting that surge will occur in S308, the surge prediction section <NUM> outputs a signal to the surge control section <NUM> indicating the prediction that surge will occur and returns to receiving the control signal in S310. If any of the control values calculated from the control signals differ from the predetermined control values by an amount less than a threshold in S304, the surge prediction section <NUM> predicts that no surge will occur in S306. Upon predicting that no surge will occur in S306, the surge prediction section <NUM> returns to receiving the control signal in S300. It will be apparent to one of ordinary skill in the art in light of the disclosure, that the method to predict surge based on the control signal may be determined in alternative manners if needed and/or desired.

The surge control section <NUM> is programmed to prevent surge. The surge control section <NUM> is electrically connected to the surge prediction section <NUM>. The surge control section <NUM> is also electrically connected to at least one of the variable frequency drive <NUM>, the motor control section <NUM>, the inlet guide vane control section <NUM>, and the expansion valve control section <NUM>. The surge control section <NUM> is programmed to prevent surge, upon receiving the signal predicting surge will occur, by adjusting an operation of the chiller system <NUM>. By non-limiting example, the surge control section <NUM> may be programmed to increase an operation range of the compressor <NUM> in response to a signal indicating surge from the surge prediction section <NUM>.

More specifically, by non-limiting example, the surge control section <NUM> may increase the operation range of the compressor <NUM> by adjusting the control of at least one of the motor control section <NUM> and the inlet guide vane control section <NUM>. The surge control section <NUM> may adjust the control of the motor speed via the motor control section <NUM> in a manner that increases the operation range of the compressor <NUM>. Similarly, the surge control section <NUM> may adjust the inlet guide vane position via the inlet guide vane control section <NUM> in a manner that increases the operation range of the compressor <NUM>. It should be apparent to one of ordinary skill in the art, in light of this disclosure, that conventional methods of preventing surge may also be implemented by the surge control section <NUM>.

Referring to <FIG>, surge is the complete breakdown of steady flow in the compressor, which typically occurs at a low flow rate. <FIG> illustrates a surge line SL, which connects the surge points S1, S2, and S3 at rpm1, rpm2, and rpm3, respectively. These points are the peak points in which pressure generated by the compressor is less than the pipe pressure downstream of the compressor. These points illustrate initiation of the surge cycle. Broken line PA illustrates a surge control line. The distance between line PA and SL show the inefficiency of surge control methods. By reducing the difference between a surge control line PA and surge line SL, the compressor <NUM> can be controlled to be more efficient. One advantage of the aforementioned surge detection methods is that it is more accurate than previously known methods of detecting surge; thus the surge control line PA may be closer to surge line SL when compared to previous methods.

In understanding the scope of the present invention, the term "comprising" and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, "including", "having" and their derivatives. Also, the terms "part," "section," "portion," "member" or "element" when used in the singular can have the dual meaning of a single part or a plurality of parts.

The term "detect" as used herein to describe an operation or function carried out by a component, a section, a device or the like includes a component, a section, a device or the like that does not require physical detection, but rather includes determining, measuring, modeling, predicting or computing or the like to carry out the operation or function.

The term "configured" as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function.

The terms of degree such as "substantially", "about" and "approximately" as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed.

Claim 1:
A centrifugal compressor (<NUM>) comprising:
a casing (<NUM>) having an inlet portion and an outlet portion;
an inlet guide vane (<NUM>) disposed in the inlet portion;
an impeller (<NUM>) disposed downstream of the inlet guide vane (<NUM>), the impeller (<NUM>) being attached to a shaft (<NUM>) rotatable about a rotation axis,
at least one radial magnetic bearing (<NUM>, <NUM>) rotatably supporting the shaft (<NUM>);
an axial thrust bearing (<NUM>), wherein the axial thrust bearing (<NUM>) is a magnetic thrust bearing, the axial thrust bearing (<NUM>) including a thrust disk (<NUM>) which is attached to the shaft (<NUM>) and the axial thrust bearing (<NUM>) supporting the shaft (<NUM>) along the rotation axis by acting on the thrust disk (<NUM>);
four position sensors (<NUM>, <NUM>) arranged to detect a radial position signal indicative of the shaft's radial position relative to the at least one radial magnetic bearing (<NUM>, <NUM>);
at least two second sensors (<NUM>) arranged to detect an axial position signal indicative of the shaft's axial position at the at least one axial thrust bearing (<NUM>);
a motor (<NUM>) arranged and configured to rotate the shaft (<NUM>) in order to rotate the impeller (<NUM>);
a diffuser (<NUM>) disposed in the outlet portion downstream from the impeller (<NUM>) with a port of the outlet portion being disposed between the impeller and the diffuser;
the centrifugal compressor being characterized in that
it is adapted to be used in a chiller and in that it comprises
a controller (<NUM>) programmed to predict surge based on:
- the radial position signal and the axial position signal of the at least two second sensors.