Cryogenic refrigerator and method for monitoring the same

Abstract

A cryogenic refrigerator and a monitoring method thereof are provided that are useful for predicting or preventing abnormal operation or failure of an expander motor due to long-term operation. [Solution] A cryogenic refrigerator (10) is capable of performing steady operation and a cool-down operation prior to the steady operation. The cryogenic refrigerator (10) includes an expander motor (42) that operates an expander (14) of the cryogenic refrigerator (10), an inverter (90) configured to control the operating frequency of the expander motor (42) and operable to drive the expander motor (42) at a lower operating frequency during steady operation than during cool-down operation, a current sensor (50) that measures the current supplied from the inverter (90) to the expander motor (42) and outputs a motor current signal indicative of the current, and a processing unit (100) that monitors the expander motor (42) based on at least the motor current signal during steady operation.

Description

[Technical Field]

[0001] The present invention relates to a cryogenic refrigerator and a method for monitoring a cryogenic refrigerator. [Background technology]

[0002] It is known that the rotation speed of the drive motor of the expander is increased by an inverter to shorten the cooling time when starting up a cryogenic refrigerator. During this startup operation, if an abnormal fluctuation in the motor current is detected, the motor rotation speed is reduced to protect the drive motor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 3-152353 Summary of the Invention [Problem to be solved by the invention]

[0004] During long-term operation of a cryogenic refrigerator, the load on the expander's drive motor tends to gradually increase. A load that exceeds the motor's specified limits can cause the motor to malfunction or malfunction, such as losing synchronization, which can cause the cryogenic refrigerator to make strange noises or interfere with normal operation.

[0005] An exemplary object of an embodiment of the present invention is to provide a cryogenic refrigerator and a monitoring method thereof that are useful for predicting or preventing abnormal operation or failure of an expander motor due to long-term operation. [Means for solving the problem]

[0006] According to one aspect of the present invention, there is provided a cryogenic refrigerator capable of performing a steady operation and a cool-down operation prior to the steady operation. The cryogenic refrigerator includes an expander motor for operating an expander of the cryogenic refrigerator, an inverter configured to control the operating frequency of the expander motor and operable to drive the expander motor at a lower operating frequency during steady operation than during cool-down operation, a current sensor that measures a current supplied from the inverter to the expander motor and outputs a motor current signal indicative of the current, and a processing unit that monitors the expander motor based on at least the motor current signal during steady operation.

[0007] According to one aspect of the present invention, there is provided a method for monitoring a cryogenic refrigerator. The cryogenic refrigerator is capable of performing a steady-state operation and a cool-down operation prior to the steady-state operation, and includes an expander motor for operating an expander of the cryogenic refrigerator, and an inverter configured to control an operating frequency of the expander motor and operable to drive the expander motor at a lower operating frequency during the steady-state operation than during the cool-down operation. The method includes measuring a current supplied from the inverter to the expander motor, and monitoring the expander motor based on the current of the expander motor during at least the steady-state operation.

[0008] According to one aspect of the present invention, a cryogenic refrigerator includes an expander motor that operates an expander of the cryogenic refrigerator, an inverter configured to control the operating frequency of the expander motor, and a processing unit that monitors the expander motor based on a power consumption signal that indicates the power consumption of the inverter or the expander motor.

[0009] According to one aspect of the present invention, there is provided a method for monitoring a cryogenic refrigerator, the cryogenic refrigerator including an expander motor for operating an expander of the cryogenic refrigerator, and an inverter configured to control an operating frequency of the expander motor, the method including acquiring power consumption of the inverter or the expander motor, and monitoring the expander motor based on the acquired power consumption.

[0010] Any combination of the above components or mutual substitution of the components or expressions of the present invention between methods, devices, systems, etc. are also valid aspects of the present invention. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a cryogenic refrigerator and a monitoring method thereof that are useful for predicting or preventing abnormal operation or failure of an expander motor due to long-term operation. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a diagram schematically illustrating a cryogenic refrigerator according to an embodiment. [Figure 2] 1 is a diagram schematically illustrating a cryogenic refrigerator according to an embodiment. [Figure 3] 10 is a graph showing the results of measuring the relationship between the effective value of the current flowing through the expander motor and the load duty for a plurality of operating frequency values. [Figure 4] 4 shows an example of the relationship between the operating frequency and the current threshold value of the expander motor according to the embodiment. [Figure 5] 1 is a block diagram of a motor monitoring device according to an embodiment; [Figure 6] 3 is a flowchart illustrating a method for monitoring a cryogenic refrigerator according to an embodiment. [Figure 7] 1 is a block diagram of a motor monitoring device according to an embodiment; [Figure 8] 10 is a graph showing the relationship between the power consumption and the load duty of the inverter for a plurality of operating frequency values. [Figure 9] FIG. 10 is a block diagram of a motor monitoring device according to another embodiment. [Figure 10] 10 is a flowchart illustrating a method for monitoring a cryogenic refrigerator according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the description and drawings, identical or equivalent components, parts, and processes are designated by the same reference numerals, and redundant explanations will be omitted as appropriate. The scale and shape of each part shown in the drawings are set for convenience to facilitate explanation, and should not be interpreted as limiting unless otherwise specified. The embodiments are merely examples and do not limit the scope of the present invention in any way. All features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0014] 1 and 2 are diagrams that schematically show a cryogenic refrigerator 10 according to an embodiment. Fig. 1 shows the external appearance of the cryogenic refrigerator 10, and Fig. 2 shows the internal structure of the cryogenic refrigerator 10. As an example, the cryogenic refrigerator 10 is a two-stage Gifford-McMahon (GM) refrigerator.

[0015] The cryogenic refrigerator 10 includes a compressor 12 and an expander 14. As will be described in detail later, the cryogenic refrigerator 10 includes a monitoring device that monitors the expander motor 42 that operates the expander 14, and this monitoring device includes a current sensor 50 and a processing unit 100.

[0016] The compressor 12 is configured to recover the working gas of the cryogenic refrigerator 10 from the expander 14, increase the pressure of the recovered working gas, and supply the working gas again to the expander 14. The working gas, also called a refrigerant gas, is typically helium gas, but other suitable gases may also be used.

[0017] Generally, the pressure of the working gas supplied from the compressor 12 to the expander 14 and the pressure of the working gas recovered from the expander 14 to the compressor 12 are both significantly higher than atmospheric pressure and can be referred to as the first high pressure and the second high pressure, respectively. For ease of explanation, the first high pressure and the second high pressure are also simply referred to as the high pressure and the low pressure, respectively. Typically, the high pressure is, for example, 2 to 3 MPa. The low pressure is, for example, 0.5 to 1.5 MPa, e.g., approximately 0.8 MPa. For ease of understanding, the flow direction of the working gas is indicated by an arrow.

[0018] The compressor 12 includes a compressor body 22 and a compressor housing 23 that houses the compressor body 22. The compressor 12 is also referred to as a compressor unit.

[0019] The compressor body 22 is configured to compress the working gas drawn in through its intake port and discharge it from its discharge port. The compressor body 22 may be, for example, a scroll type, a rotary type, or any other pump that pressurizes the working gas. The compressor body 22 may be configured to discharge a fixed, constant flow rate of the working gas. Alternatively, the compressor body 22 may be configured to vary the flow rate of the working gas it discharges. The compressor body 22 is sometimes referred to as a compression capsule.

[0020] The compressor 12 may include a compressor controller 24 that controls the compressor 12. The compressor controller 24 may not only control the compressor 12, but may also control the cryogenic refrigerator 10 as a whole, and may also control, for example, the expander 14 (e.g., the expander motor 42). The compressor controller 24 may be attached to the compressor 12, or may be installed on the outer surface of the compressor housing 23 and housed in the compressor housing 23, for example. Alternatively, the compressor controller 24 may be located remotely from the compressor 12 and connected to the compressor 12, for example, by a control signal line.

[0021] The expander 14 includes a refrigerator cylinder 16 and a displacer assembly 18. The refrigerator cylinder 16 guides the linear reciprocating motion of the displacer assembly 18, and forms expansion chambers (32, 34) for the working gas between the refrigerator cylinder 16 and the displacer assembly 18. The expander 14 also includes a pressure switching valve 40 that determines the timing at which the working gas starts to be drawn into the expansion chamber and the timing at which the working gas starts to be exhausted from the expansion chamber.

[0022] In this document, for convenience in explaining the positional relationships between the components of the cryocooler 10, the side closer to the top dead center of the displacer's axial reciprocating motion will be referred to as "top" and the side closer to the bottom dead center as "bottom." The top dead center is the position of the displacer where the volume of the expansion space is maximum, and the bottom dead center is the position of the displacer where the volume of the expansion space is minimum. During operation of the cryocooler 10, a temperature gradient occurs in which the temperature decreases from top to bottom in the axial direction, so the top side can also be referred to as the high-temperature side and the bottom side as the low-temperature side.

[0023] The refrigerator cylinder 16 has a first cylinder 16a and a second cylinder 16b. The first cylinder 16a and the second cylinder 16b are, for example, cylindrical members, and the second cylinder 16b has a smaller diameter than the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are arranged coaxially, and the lower end of the first cylinder 16a is rigidly connected to the upper end of the second cylinder 16b.

[0024] The displacer assembly 18 includes a first displacer 18a and a second displacer 18b that are connected to each other and move together. The first displacer 18a and the second displacer 18b are, for example, cylindrical members, and the second displacer 18b has a smaller diameter than the first displacer 18a. The first displacer 18a and the second displacer 18b are arranged coaxially.

[0025] The first displacer 18a is accommodated in the first cylinder 16a, and the second displacer 18b is accommodated in the second cylinder 16b. The first displacer 18a is capable of reciprocating in the axial direction along the first cylinder 16a, and the second displacer 18b is capable of reciprocating in the axial direction along the second cylinder 16b.

[0026] 2, the first displacer 18a accommodates the first regenerator 26. The first regenerator 26 is formed by filling a cylindrical main body of the first displacer 18a with a wire mesh such as copper or other suitable first regenerator material. The upper and lower lids of the first displacer 18a may be provided as separate members from the main body of the first displacer 18a, and the upper and lower lids of the first displacer 18a may be fixed to the main body by suitable means such as fastening or welding, thereby accommodating the first regenerator material in the first displacer 18a.

[0027] Similarly, the second displacer 18b accommodates the second regenerator 28. The second regenerator 28 is formed by filling the cylindrical main body of the second displacer 18b with a non-magnetic regenerator material such as bismuth, a magnetic regenerator material such as HoCu2, or another suitable second regenerator material. The second regenerator material may be formed in a granular form. The upper and lower covers of the second displacer 18b may be provided as separate members from the main body of the second displacer 18b, and the upper and lower covers of the second displacer 18b may be fixed to the main body by suitable means such as fastening or welding, thereby accommodating the second regenerator material in the second displacer 18b.

[0028] The displacer assembly 18 defines a room-temperature chamber 30, a first expansion chamber 32, and a second expansion chamber 34 inside the refrigerator cylinder 16. The expander 14 includes a first cooling stage 33 and a second cooling stage 35 for heat exchange with the desired object or medium to be cooled by the cryogenic refrigerator 10. The room-temperature chamber 30 is defined between the top cover of the first displacer 18a and the top of the first cylinder 16a. The first expansion chamber 32 is defined between the bottom cover of the first displacer 18a and the first cooling stage 33. The second expansion chamber 34 is defined between the bottom cover of the second displacer 18b and the second cooling stage 35. The first cooling stage 33 is fixed to the bottom of the first cylinder 16a to surround the first expansion chamber 32, and the second cooling stage 35 is fixed to the bottom of the second cylinder 16b to surround the second expansion chamber 34.

[0029] The first regenerator 26 is connected to the room-temperature chamber 30 through a working gas passage 36a formed in the upper lid of the first displacer 18a, and is connected to the first expansion chamber 32 through a working gas passage 36b formed in the lower lid of the first displacer 18a. The second regenerator 28 is connected to the first regenerator 26 through a working gas passage 36c formed from the lower lid of the first displacer 18a to the upper lid of the second displacer 18b. The second regenerator 28 is also connected to the second expansion chamber 34 through a working gas passage 36d formed in the lower lid of the second displacer 18b.

[0030] A first seal 38a and a second seal 38b may be provided so that the flow of working gas between the first expansion chamber 32, the second expansion chamber 34 and the room temperature chamber 30 is guided to the first regenerator 26 and the second regenerator 28, rather than through the clearance between the refrigerator cylinder 16 and the displacer assembly 18. The first seal 38a may be attached to an upper cover of the first displacer 18a so as to be positioned between the first displacer 18a and the first cylinder 16a. The second seal 38b may be attached to an upper cover of the second displacer 18b so as to be positioned between the second displacer 18b and the second cylinder 16b.

[0031] 1, the expander 14 includes a refrigerator housing 20 that houses a pressure switching valve 40. The refrigerator housing 20 is coupled to the refrigerator cylinder 16, thereby forming an airtight container that houses the pressure switching valve 40 and the displacer assembly 18.

[0032] 2, the pressure switching valve 40 includes a high-pressure valve 40a and a low-pressure valve 40b, and is configured to generate periodic pressure fluctuations in the refrigerator cylinder 16. The working gas discharge port of the compressor 12 is connected to the room-temperature chamber 30 via the high-pressure valve 40a, and the working gas inlet port of the compressor 12 is connected to the room-temperature chamber 30 via the low-pressure valve 40b. The high-pressure valve 40a and the low-pressure valve 40b are configured to open and close selectively and alternately (i.e., when one is open, the other is closed).

[0033] The pressure switching valve 40 may take the form of a rotary valve. That is, the pressure switching valve 40 may be configured so that the high-pressure valve 40a and the low-pressure valve 40b are alternately opened and closed by the rotational sliding of a valve disc relative to a stationary valve body. In this case, the expander motor 42 may be connected to the pressure switching valve 40 so as to rotate the valve disc of the pressure switching valve 40. For example, the pressure switching valve 40 is arranged so that the valve rotation axis is coaxial with the rotation axis of the expander motor 42.

[0034] Alternatively, the high pressure valve 40a and the low pressure valve 40b may be valves that can be controlled individually, in which case the pressure switching valve 40 does not need to be connected to the expander motor .

[0035] The expander 14 includes an expander motor 42 and a motion conversion mechanism 43. The expander motor 42 is attached to the refrigerator housing 20. The motion conversion mechanism 43 is housed in the refrigerator housing 20, similar to the pressure switching valve 40.

[0036] The expander motor 42 is connected to a displacer drive shaft 44 via a motion conversion mechanism 43, such as a Scotch yoke mechanism. The motion conversion mechanism 43 converts the rotational motion output by the expander motor 42 into linear reciprocating motion of the displacer drive shaft 44. The displacer drive shaft 44 extends from the motion conversion mechanism 43 into the room-temperature chamber 30 and is fixed to the upper lid of the first displacer 18a. The rotation of the expander motor 42 is converted into axial reciprocating motion of the displacer drive shaft 44 by the motion conversion mechanism 43, and the displacer assembly 18 reciprocates linearly in the axial direction within the refrigerator cylinder 16.

[0037] The cryogenic refrigerator 10 is supplied with power from a power source 46 such as a commercial power source (three-phase AC power source). The power source 46 is connected to the compressor 12 and the expander motor 42 by a power supply wiring 48. Since the expander motor 42 is connected to the power source 46 via the compressor 12, the compressor 12 can also be considered as the power source for the expander motor 42. The compressor 12 and the expander motor 42 may each be connected to separate power sources.

[0038] The expander motor 42 is, for example, a permanent magnet motor driven by three-phase AC. The operating frequency of the expander motor 42 is controlled by an inverter 90. The inverter 90 is installed on the power supply wiring 48. The expander motor 42 can operate at a rotation speed corresponding to the operating frequency of the expander motor 42, which is equal to the output frequency of the inverter 90. As an example, the output frequency of the inverter 90 can vary within a range of 30 Hz to 100 Hz, or within a range of 40 Hz to 70 Hz.

[0039] The current sensor 50 is connected to the expander motor 42 so as to measure the current supplied from the inverter 90 to the expander motor 42 at least during steady-state operation of the cryogenic refrigerator 10. The current sensor 50 is installed on the power supply wiring 48 between the inverter 90 and the expander motor 42.

[0040] The current sensor 50 is configured to output a motor current signal S1 indicating the measured current to the processing unit 100. The motor current signal S1 may represent an effective value of the current supplied to the expander motor 42. The current sensor 50 is communicatively connected to the processing unit 100 via wire or wireless communication. The current sensor 50 may be a three-phase ammeter that individually and simultaneously measures the three-phase currents flowing through the expander motor 42, or may be any other type of current sensor that measures the current flowing through the expander motor 42. For example, the current sensor 50 may be configured to individually and simultaneously measure the three-phase currents output from the inverter 90 to the expander motor 42 and output, as the motor current signal S1, voltage signals indicating the magnitude of each of the measured three-phase currents to the processing unit 100. The motor current signal S1 may be current waveform data indicating a time change in the current flowing through the expander motor 42 during operation of the cryogenic refrigerator 10.

[0041] The inverter 90 is configured to output output frequency information S2 indicating the output frequency of the inverter 90 (i.e., the operating frequency of the expander motor 42) to the processing unit 100. Alternatively, instead of the processing unit 100 receiving the output frequency information S2 from the inverter 90, the processing unit 100 may calculate the output frequency information S2 from the motor current signal S1 input from the current sensor 50. For example, the processing unit 100 may calculate the operating frequency of the expander motor 42 by counting the number of current peaks per unit time from the waveform of the current flowing through the expander motor 42. Alternatively, the inverter 90 may be equipped with the current sensor 50 (the inverter 90 may have a function for measuring the output current), and the processing unit 100 may acquire the motor current signal S1 from the inverter 90. A signal representing the effective value of the current output by the inverter 90 to control the expander motor 42 may be used as the motor current signal S1.

[0042] The processing unit 100 is configured to receive the motor current signal S1 from the current sensor 50 (or the inverter 90) and monitor the expander motor 42 based on the motor current signal S1 during at least steady operation of the cryogenic refrigerator 10. The processing unit 100 will be described in detail later.

[0043] In the illustrated example, the current sensor 50, the inverter 90, and the processing unit 100 are built into the compressor controller 24 and provided in the compressor 12, but this is not limiting. The current sensor 50, the inverter 90, and the processing unit 100 may be provided in the expander 14, such as by being mounted on the expander motor 42, or may be provided in another location on the power supply wiring 48.

[0044] When the compressor 12 and the expander motor 42 are operated, the cryogenic refrigerator 10 generates periodic volume fluctuations and synchronized pressure fluctuations of the working gas in the first expansion chamber 32 and the second expansion chamber 34. Typically, during the intake stroke, the low-pressure valve 40b closes and the high-pressure valve 40a opens, causing high-pressure working gas to flow from the compressor 12 through the high-pressure valve 40a into the room-temperature chamber 30, be supplied to the first expansion chamber 32 through the first regenerator 26, and be supplied to the second expansion chamber 34 through the second regenerator 28. In this way, the pressures of the first expansion chamber 32 and the second expansion chamber 34 are increased from low to high. At this time, the displacer assembly 18 is moved upward from bottom dead center to top dead center, increasing the volumes of the first expansion chamber 32 and the second expansion chamber 34. The intake stroke ends when the high-pressure valve 40a closes.

[0045] During the exhaust stroke, the high-pressure valve 40a closes and the low-pressure valve 40b opens, opening the high-pressure first and second expansion chambers 32 and 34 to the low-pressure working gas inlet of the compressor 12. This causes the working gas to expand in the first and second expansion chambers 32 and 34, resulting in the low-pressure working gas being discharged from the first and second expansion chambers 32 and 34 through the first and second regenerators 26 and 28 into the room-temperature chamber 30. At this time, the displacer assembly 18 is moved downward from top dead center to bottom dead center, reducing the volumes of the first and second expansion chambers 32 and 34. The working gas is recovered from the expander 14 through the low-pressure valve 40b and returned to the compressor 12. The exhaust stroke ends when the low-pressure valve 40b closes.

[0046] In this way, a refrigeration cycle such as a GM cycle is configured, and the first cooling stage 33 and the second cooling stage 35 are cooled to a desired cryogenic temperature. The first cooling stage 33 can be cooled to a first cooling temperature in the range of, for example, about 20 K to about 40 K. The second cooling stage 35 can be cooled to a second cooling temperature (for example, about 1 K to about 4 K) that is lower than the first cooling temperature.

[0047] The cryocooler 10 can perform both a steady-state operation and a cool-down operation prior to the steady-state operation. The cool-down operation is an operating mode in which the cryocooler 10 is rapidly cooled from room temperature to a cryogenic temperature upon startup, while the steady-state operation is an operating mode in which the cryocooler 10 maintains a cryogenically cooled state through the cool-down operation. The cryocooler 10 is cooled to a standard cooling temperature through the cool-down operation, and during steady-state operation, the cryocooler 10 is maintained within an allowable cryogenic temperature range that includes the standard cooling temperature. The standard cooling temperature varies depending on the application and settings of the cryocooler 10, but is typically about 4.2 K or less for applications such as cooling superconducting devices. In some other cooling applications, the standard cooling temperature may be, for example, about 10 K to 20 K, or even 10 K or less.

[0048] Switching from cool-down operation to steady-state operation is controlled by a controller (e.g., compressor controller 24) that controls the cryogenic refrigerator 10. The cryogenic refrigerator 10 may include a temperature sensor 52 that measures the temperature of the second cooling stage 35 (and / or the first cooling stage 33) and outputs a measured temperature signal indicative of the measured temperature. Based on the measured temperature signal from the temperature sensor 52, the compressor controller 24 may compare the measured temperature of the second cooling stage 35 with the standard cooling temperature (or the above-mentioned allowable temperature range), and may execute cool-down operation if the measured temperature is higher than the standard cooling temperature, or may transition from cool-down operation to steady-state operation if the measured temperature is equal to or lower than the standard cooling temperature.

[0049] The inverter 90 is operable to drive the expander motor 42 at a lower operating frequency during steady operation compared to cool-down operation. For example, under control of a controller (e.g., compressor controller 24), the inverter 90 may drive the expander motor 42 at a predetermined first operating frequency during cool-down operation and at a predetermined second operating frequency during steady operation, where the second operating frequency is lower than the first operating frequency. The controller may control the expander motor 42 based on a measured temperature signal from the temperature sensor 52 and a predetermined operating frequency profile, which may be defined to provide a higher operating frequency as the measured temperature increases.

[0050] Alternatively, a controller (e.g., compressor controller 24) may control the output frequency of the inverter 90 (e.g., by feedback control such as PID control) based on the measured temperature signal from the temperature sensor 52 so as to minimize the deviation of the measured temperature from the standard cooling temperature. Therefore, when the temperature measured by the temperature sensor 52 is higher than the standard cooling temperature, the operating frequency of the expander motor 42 is increased, and when the temperature measured by the temperature sensor 52 is lower than the standard cooling temperature, the operating frequency of the expander motor 42 is decreased. Thus, during cool-down operation, the initial measured temperature is room temperature, and the operating frequency of the expander motor 42 is quite high, thereby enabling rapid cooling. As the temperature decreases toward the standard cooling temperature, the operating frequency decreases. During steady-state operation, when the measured temperature rises above the standard cooling temperature due to a disturbance such as heat intrusion from the surroundings, the operating frequency of the expander motor 42 is increased, and the temperature is cooled back to the standard cooling temperature. Conversely, when the measured temperature drops below the standard cooling temperature due to a decrease in the heat load, the operating frequency of the expander motor 42 is decreased, and the temperature is returned to the standard cooling temperature. Excessive cooling can be avoided and standard cooling temperatures can be maintained.

[0051] Incidentally, as the cryogenic refrigerator 10 is operated over a long period of time, the load on the expander motor 42 tends to gradually increase. This is due to, for example, the accumulation of various fine particles in the expander 14, which originates from the lubricating oil and regenerator material in the compressor 12, which increases the pressure loss in the expander 14, and the gradual increase in the load on the expander motor 42 over the long term due to the flow of working gas in and out of the expansion chamber. Another reason is that moisture is absorbed by the displacer in the expander 14, which causes the displacer to expand slightly inside the expander 14, reducing the clearance with the cylinder and increasing the sliding resistance of the displacer.

[0052] Such an increase in the drive load of the expander motor 42 increases the risk that a load torque exceeding the specification limits of the motor (for example, the instantaneous maximum torque or other rated torque) will be applied to the expander motor 42. A load exceeding the specification limits of the expander motor 42 may cause abnormal operation or failure, such as loss of synchronism, in the expander motor 42, which may cause abnormal noise in the expander 14 or interfere with normal operation of the expander 14.

[0053] By monitoring the current flowing through the expander motor 42 and detecting abnormal fluctuations in the current due to an abnormality in the expander motor 42, it is possible to detect the occurrence of an abnormality in the expander motor 42. However, when the operating frequency (rotation speed) of the expander motor 42 is controlled by the inverter 90, the motor current fluctuates not only when an abnormality occurs in the expander motor 42, but also when the operating frequency is changed during normal operation. Furthermore, when an external magnetic field acts on the expander motor 42, such as when the expander 14 is installed in a strong magnetic field environment, the motor current may fluctuate depending on the magnitude of the magnetic field. The motor current may also fluctuate when the input voltage to the inverter 90 fluctuates. Because the motor current can fluctuate depending on various operating conditions of the expander motor 42, it is not always easy to distinguish between abnormal fluctuations in current due to an abnormality in the expander motor 42 and current fluctuations due to changes in operating conditions during normal operation of the expander motor 42.

[0054] 3 is a graph showing the relationship between the effective value of the current flowing through the expander motor 42 and the load duty measured for a number of operating frequencies. The vertical axis shows the effective value of the current supplied from the inverter 90 to the expander motor 42 and measured by the current sensor 50. The load duty shown on the horizontal axis indicates the ratio (%) of the load torque actually acting on the expander motor 42 to the maximum load torque (e.g., the instantaneous maximum torque) allowable for the expander motor 42. Therefore, when the load duty exceeds 100%, abnormal operation such as step-out of the expander motor 42 actually occurs or is highly likely to occur.

[0055] As can be seen from Figure 3, for a given load duty (any value will do, for example 90%), the inverter controls the motor current value so that it is approximately the same when the operating frequency is 40 Hz and 50 Hz. Thus, when the operating frequency is relatively low, the motor current value shows almost no dependency on the operating frequency. On the other hand, in the example of Figure 3, when the operating frequency is in the range of 50 Hz to 70 Hz, the motor current value decreases as the operating frequency value increases. Thus, when the operating frequency value is relatively high, the motor current value shows dependency on the operating frequency, and the motor current value may differ depending on the operating frequency value.

[0056] Next, let us look at the change in motor current value when the load duty is changed for a given operating frequency. Although the motor current value varies for each operating frequency, it can be seen that the motor current value tends to increase as the load duty increases.

[0057] Therefore, for a certain operating frequency (e.g., 60 Hz), the motor current value when the load duty is smaller than 100% but sufficiently large (e.g., 90% to 98%) can be used as the current threshold. When a current exceeding this current threshold flows through the expander motor 42 (i.e., when measured by the current sensor 50), a large load corresponding to the load duty is being applied to the expander motor 42. This can be regarded as a prediction of the occurrence of abnormal operation of the expander motor 42, and measures can be taken to prevent the occurrence of abnormal operation, such as issuing a warning, lowering the operating frequency, or stopping operation of the cryogenic refrigerator 10.

[0058] However, when the expander motor 42 is driven at a higher operating frequency (e.g., 70 Hz), the motor current value is significantly smaller than when the expander motor 42 is driven at 60 Hz, and does not reach the current threshold value set for 60 Hz. Therefore, comparing the motor current value of the expander motor 42 when it is driven at 70 Hz with the threshold value for 60 Hz does not provide a useful result for predicting the occurrence of an abnormality in the expander motor 42. Conversely, when the expander motor 42 is driven at a lower operating frequency (e.g., 50 Hz), the motor current value is significantly larger than when the expander motor 42 is driven at 60 Hz, and exceeds the current threshold value set for 60 Hz even at a significantly smaller load duty. Thus, comparing the motor current value of the expander motor 42 when it is driven at 50 Hz with the threshold value for 60 Hz does not provide a useful result for predicting the occurrence of an abnormality in the expander motor 42.

[0059] Therefore, when the operating frequency of the expander motor 42 is controlled by the inverter 90, it is difficult to predict or prevent abnormal operation or failure of the expander motor 42 by monitoring the current flowing through the expander motor 42 simply by setting one current threshold value. Therefore, in this embodiment, a current threshold value is set for each of multiple values ​​of the operating frequency of the expander motor 42. Figure 4 shows an example of such multiple current threshold values.

[0060] FIG. 4 shows an example of the relationship between the operating frequency and the current threshold value of the expander motor 42 according to the embodiment. FIG. 4 shows the relationship between the motor current value and the load duty according to the operating frequency shown in FIG. 3 for specific load duty values ​​(specifically, load duties of 90% and 95%). Therefore, as described above, when the operating frequency is relatively low (in the operating frequency range of 40 Hz to 50 Hz in FIG. 4), the motor current value is almost constant. When the operating frequency is relatively high (in the operating frequency range of 50 Hz to 70 Hz in FIG. 4), the motor current value decreases as the operating frequency value increases. As can be seen from FIG. 4, as the load duty value increases, the motor current value increases.

[0061] The relationship between the operating frequency and the current threshold value of the expander motor 42 shown in Figure 4 may be used as an example of an operating frequency vs. current threshold value table 62, which will be described later. If the combination of the operating frequency value and the measured motor current value is in the region above the graph (i.e., if the expander motor 42 is driven at that operating frequency and the current value measured by the current sensor 50 at this time exceeds the current threshold value corresponding to that operating frequency), it can be assumed that the occurrence of abnormal operation of the expander motor 42 is predicted. If the combination of the operating frequency value and the measured motor current value is in the region below the graph, the measured current value does not exceed the current threshold value, and therefore it can be assumed that no abnormality will occur.

[0062] In this way, by associating multiple current thresholds with multiple values ​​of the operating frequency of the expander motor 42, it is possible to accurately determine whether abnormal operation such as step-out has actually occurred or is highly likely to occur, even when the operating frequency of the expander motor 42 is controlled by the inverter 90. Therefore, measures can be taken to prevent such abnormal operation of the expander motor 42 from occurring.

[0063] Depending on the application of the cryogenic refrigerator 10 (e.g., when used to cool a superconducting electromagnet), the expander 14 may be installed and used in a strong magnetic field environment. In such cases, the external magnetic field acting on the expander motor 42 may affect the operation of the expander motor 42 and cause the current flowing through the expander motor 42 to fluctuate. According to the inventor's investigations, the motor current value tends to increase as the external magnetic field increases under a given operating frequency and a given load duty. This tendency is particularly evident when the expander motor 42 is a permanent magnet motor. For example, under operating frequencies of 50 Hz or 60 Hz and a load duty of 90% or more, the motor current value may increase, for example, by approximately 5% to 10% when the external magnetic field is 500 Gauss compared to when the external magnetic field is 0 Gauss.

[0064] Furthermore, typically, the specifications of the cryogenic refrigerator 10 allow a certain degree of fluctuation (for example, about ±10%) in the input voltage from the power supply 46 to the inverter 90. Fluctuations in the input voltage to the inverter 90 can also cause fluctuations in the current flowing through the expander motor 42. According to the inventor's investigations, at a given operating frequency and a given load duty, the motor current value tends to increase as the input voltage increases from a voltage value lower than the specified voltage value (for example, 200 V) of the power supply 46 to this specified voltage value.

[0065] Therefore, by associating multiple current threshold values ​​with multiple values ​​of the operating conditions of the expander motor 42, not only the operating frequency of the expander motor 42 but also other operating conditions of the expander motor 42 such as the external magnetic field and input voltage, it is possible to accurately determine whether abnormal operation of the expander motor 42 may occur and take measures to prevent this from happening.

[0066] FIG. 5 is a block diagram of a motor monitoring device according to an embodiment. The monitoring device includes a processing unit 100, which includes a current threshold setting unit 60 and a comparison unit 70. The current threshold setting unit 60 includes an operating frequency vs. current threshold table 62, an external magnetic field vs. current threshold table 64, and an input voltage vs. current threshold table 66. The monitoring device may include a notification unit 80 that visually notifies the user of information indicating the monitoring results, and the notification unit 80 may include, for example, a display 82. The notification unit 80 may notify the diagnostic results audibly using a speaker or the like. The notification unit 80 may also transmit the diagnostic results to a remote device via a network such as the Internet.

[0067] The processing unit 100 is configured to monitor the expander motor 42 based on the motor current signal S1 at least during steady operation. Therefore, the processing unit 100 may acquire information indicating the current operation mode of the cryogenic refrigerator 10 from a controller (e.g., the compressor controller 24) or determine the current operation mode based on the measured temperature signal S3 from the temperature sensor 52, and may monitor the expander motor 42 based on the motor current signal S1 when the current operation mode is steady operation. Additionally or alternatively, the processing unit 100 may monitor the expander motor 42 based on the motor current signal S1 when the current operation mode is cool-down operation.

[0068] The processing unit 100 is configured to acquire a current threshold value Th based on the operating conditions of the expander motor 42, and to monitor the expander motor 42 by comparing the current (e.g., the effective value of the current) of the expander motor 42 with the current threshold value Th based on the motor current signal S1. Therefore, the current threshold value setting unit 60 acquires the current threshold value Th based on the operating conditions of the expander motor 42 (e.g., the motor current signal S1, the output frequency information S2, the magnitude of the external magnetic field, the input voltage to the inverter 90, etc.) using at least one current threshold value table out of an operating frequency vs. current threshold value table 62, an external magnetic field vs. current threshold value table 64, and an input voltage vs. current threshold value table 66.

[0069] The operating frequency / current threshold table 62 associates a plurality of current thresholds with a plurality of operating frequency values ​​of the expander motor 42, the external magnetic field / current threshold table 64 associates a plurality of current thresholds with a plurality of values ​​of the external magnetic field applied to the expander motor 42, and the input voltage / current threshold table 66 associates a plurality of current thresholds with a plurality of values ​​of the input voltage to the inverter 90. The operating frequency / current threshold table 62, the external magnetic field / current threshold table 64, and the input voltage / current threshold table 66 are set in advance and stored in the processing unit 100. These current threshold tables can be set as appropriate based on the designer's empirical knowledge or on experiments, simulations, etc.

[0070] For example, the current threshold value setting unit 60 is configured to acquire the current threshold value Th corresponding to the value of the operating frequency of the expander motor 42 based on the operating frequency / current threshold value table 62 and the value of the operating frequency of the expander motor 42 .

[0071] For example, as shown in FIG. 4, the operating frequency vs. current threshold value table 62 may associate multiple current threshold values ​​with multiple values ​​of the operating frequency of the expander motor 42 so that the current threshold value decreases as the operating frequency increases in at least a portion of the range of the operating frequency of the expander motor 42 that can be controlled by the inverter 90 (for example, the high-frequency region of that range).

[0072] Furthermore, the operating frequency / current threshold value table 62 may associate, for each of a plurality of values ​​of the operating frequency of the expander motor 42, a current value of the expander motor 42 at a value of the operating frequency when a load torque that is a predetermined amount smaller than the maximum load torque allowable for the expander motor 42 acts on the expander motor 42 (i.e., at a predetermined load duty), as a current threshold value with the value of the operating frequency. The predetermined load duty value may be selected, for example, from a range of 80% or more and less than 100%, or from a range of 90% or more and 98% or less.

[0073] A plurality of operating frequency / current threshold values ​​tables 62 may be set in advance, and each of the plurality of operating frequency / current threshold values ​​tables 62 may be associated with a plurality of different load duty values. The current threshold value setting unit 60 may acquire the current threshold value Th corresponding to the value of the operating frequency of the expander motor 42 based on an operating frequency / current threshold value table 62 selected from the plurality of operating frequency / current threshold values ​​tables 62 and the value of the operating frequency of the expander motor 42.

[0074] The current threshold value setting unit 60 may be configured to obtain the current threshold value Th corresponding to the value of the external magnetic field based on the external magnetic field / current threshold value table 64 and the value of the external magnetic field applied to the expander motor 42. The value of the external magnetic field may be measured by a magnetic field sensor 54 mounted on or installed in the vicinity of the cryogenic refrigerator 10 (e.g., the expander 14), and this measured value of the external magnetic field may be input to the processing unit 100 and used by the current threshold value setting unit 60.

[0075] Alternatively, the processing unit 100 may be configured to acquire an estimate of the external magnetic field acting on the expander motor 42 based on the motor current signal S1, and to acquire the current threshold value Th corresponding to the estimated value of the external magnetic field based on the external magnetic field / current threshold table 64 and the estimated value of the external magnetic field acting on the expander motor 42. According to the inventor's investigations, the three-phase current waveforms flowing through the expander motor 42 are symmetrical when no external magnetic field is acting or the external magnetic field is sufficiently small, whereas as the external magnetic field increases, the asymmetry of the three-phase current waveforms tends to increase. Examples of this asymmetry include the difference in peak values ​​of the current waveforms among the three phases (U phase, V phase, and W phase). In particular, when the coil wiring of the expander motor 42 is star-connected, the occurrence of unbalanced currents can significantly increase the asymmetry of the current waveforms in response to the external magnetic field. For example, an estimated value of the external magnetic field of the expander motor 42 can be obtained from the motor current signal S1 based on parameters resulting from the external magnetic field that can be calculated from the motor current signal S1, such as the maximum value, minimum value, or difference between the maximum and minimum values ​​of the peaks of the current waveforms of the U phase, V phase, and W phase.

[0076] The current threshold value setting unit 60 may be configured to obtain the current threshold value Th corresponding to the value of the input voltage to the inverter 90 based on the input voltage / current threshold value table 66 and the value of the input voltage. Information indicating the value of the input voltage to the inverter 90 may be input from the inverter 90 to the processing unit 100 and used by the current threshold value setting unit 60.

[0077] A plurality of external magnetic field / current threshold tables 64 may be preset, and each of these plurality of external magnetic field / current threshold tables 64 may be associated with a plurality of different operating frequency values. That is, for example, first and second external magnetic field / current threshold tables 64 may be preset, and the first external magnetic field / current threshold table 64 may be used when the expander motor 42 is driven at a first operating frequency value (e.g., 70 Hz), and the second external magnetic field / current threshold table 64 may be used when the expander motor 42 is driven at a second operating frequency value (e.g., 60 Hz) different from the first operating frequency value. Similarly, a plurality of input voltage / current threshold tables 66 may be preset, and each of these plurality of input voltage / current threshold tables 66 may be associated with a plurality of different operating frequency values.

[0078] The comparison unit 70 compares the current (e.g., the effective value of the current) of the expander motor 42 with the acquired current threshold value Th based on the motor current signal S1, and generates monitoring result data D1 based on the comparison result. The monitoring result data D1 is sent to notification means 80, and the monitoring result is notified to the user, for example, by displaying it on a display 82. If an abnormal operation of the expander motor 42 is predicted, the notification means 80 may notify the user by sounding an alarm. Instead of (or in addition to) notifying the user immediately in this way, the monitoring result data D1 may be stored in the processing unit 100 so that it can be presented to the user as needed.

[0079] The internal configuration of the processing unit 100 is realized as a hardware configuration by elements and circuits such as a computer CPU and memory, and as a software configuration by a computer program, etc., but in the figure it is depicted as appropriate as functional blocks realized by the cooperation of these. Those skilled in the art will understand that these functional blocks can be realized in various ways by combining hardware and software.

[0080] For example, the processing unit 100 can be implemented as a combination of a processor (hardware) such as a CPU (Central Processing Unit) or a microcomputer, and a software program executed by the processor (hardware). Such a hardware processor may be configured, for example, as a programmable logic device such as an FPGA (Field Programmable Gate Array), or may be a control circuit such as a programmable logic controller (PLC). The software program may be a computer program that causes the processing unit 100 to monitor the cryogenic refrigerator 10.

[0081] Fig. 6 is a flowchart showing a method for monitoring the cryogenic refrigerator 10 according to the embodiment. First, as shown in Fig. 6, while the cryogenic refrigerator 10 is operating (for example, during steady operation), the current supplied from the inverter 90 to the expander motor 42 is measured by the current sensor 50 (S10). Then, the expander motor 42 is monitored based on the current of the expander motor 42 during at least steady operation (S20).

[0082] In S20, a current threshold value Th is acquired based on the operating conditions of the expander motor 42 (e.g., the motor current signal S1, the output frequency information S2, the magnitude of the external magnetic field, the input voltage to the inverter 90, etc.) (S21). The measured current value of the expander motor 42 is compared with the acquired current threshold value Th (S22). If the measured current value exceeds the current threshold value Th (Y in S22), the comparison unit 70 determines that the occurrence of abnormal operation of the expander motor 42 is predicted (S23), and outputs monitoring result data D1 indicating this. If the measured current value is equal to or less than the current threshold value Th (N in S22), the comparison unit 70 determines that the occurrence of abnormal operation of the expander motor 42 is not predicted (S24), and outputs monitoring result data D1 indicating this. This monitoring method then ends.

[0083] In addition, if abnormal operation of the expander motor 42 is predicted, the controller of the cryogenic refrigerator 10 (e.g., the compressor controller 24) may take measures to prevent the occurrence of abnormal operation, such as issuing a warning, reducing the operating frequency of the expander motor 42, or stopping operation of the cryogenic refrigerator 10.

[0084] The processing unit 100 periodically and repeatedly performs such monitoring. Since the increase in the drive load of the expander motor 42 is a long-term phenomenon that progresses gradually over a long span of time, it is sufficient in practice to perform this monitoring method from time to time while the cryogenic refrigerator 10 is operating (e.g., during steady operation). Alternatively, the monitoring method may be performed repeatedly at all times while the cryogenic refrigerator 10 is operating.

[0085] When the cryogenic refrigerator 10 is operated continuously for a long period of time, the majority of the operation time is in steady operation. By monitoring during steady operation, it is possible to grasp the increase in the load torque on the expander motor 42, which gradually increases due to deterioration of the components of the cryogenic refrigerator 10 over time.

[0086] As described above, according to the embodiment, by measuring the current supplied from the inverter 90 to the expander motor 42 and monitoring the expander motor 42 based on the current (e.g., the effective value of the current) of the expander motor 42 at least during steady-state operation, it is possible to predict or prevent abnormal operation or failure of the expander motor 42 due to long-term operation.

[0087] If the load on the expander motor 42 is left unattended, the cryogenic refrigerator 10 may eventually break down. If it does break down, the operation of the cryogenic system (such as a superconducting device or an MRI system) that uses the cryogenic refrigerator 10 will have to be stopped until maintenance such as repairing the cryogenic refrigerator or replacing it with a new one is completed. In the case of a sudden breakdown, it tends to take a relatively long time to restore operation.

[0088] However, according to the embodiment, the expander motor 42 can be monitored, and the monitoring results can be notified to the user of the cryogenic refrigerator 10 or a service technician who performs maintenance on the cryogenic refrigerator 10. Based on the monitoring results, it becomes possible to take measures to minimize the impact on the operation of the cryogenic system.

[0089] In some embodiments, the output voltage from the inverter 90 (i.e., the input voltage to the expander motor 42) may differ from the input voltage to the inverter 90. For example, the inverter 90 may have a function of limiting the output voltage to a specified voltage value (e.g., 200 V) when the input voltage exceeds this specified voltage value. However, the inverter 90 may not have a voltage boost function. In this case, if the input voltage is less than the specified voltage value, the input voltage and the output voltage are equal. However, if the input voltage exceeds the specified voltage value, the input voltage and the output voltage differ (the output voltage is smaller than the input voltage). As another example, the input voltage and the output voltage may differ depending on the method of controlling the motor operating frequency by the inverter 90. For example, in typical Vf control, even if the input voltage is a specified voltage value, the output voltage may fall below this specified voltage value depending on the operating frequency.

[0090] Therefore, the output voltage may be used as an example of a parameter to be referenced to set the current threshold in addition to (or instead of) the motor operating conditions mentioned above, such as the operating frequency.

[0091] 7 is a block diagram of a motor monitoring device according to an embodiment. The monitoring device includes a processing unit 100 configured to monitor the expander motor 42 based on a motor current signal S1 from a current sensor 50. The processing unit 100 may monitor the expander motor 42 at least during steady-state operation of the cryogenic refrigerator 10. The monitoring device may include notification means 80 that notifies information indicating the monitoring results, and the notification means 80 may include, for example, a display 82.

[0092] The processing unit 100 is configured to obtain a current threshold value Th based on the operating conditions of the expander motor 42, and to monitor the expander motor 42 by comparing the current (e.g., the effective value of the current) of the expander motor 42 with the current threshold value Th based on the motor current signal S1.

[0093] The processing unit 100 includes a current threshold setting unit 60 and a comparison unit 70. The current threshold setting unit 60 includes an operating frequency / current threshold table 62 and an output voltage / current threshold table 68. The operating frequency / current threshold table 62 associates a plurality of current threshold values ​​with a plurality of operating frequency values ​​of the expander motor 42, and the output voltage / current threshold table 68 associates a plurality of current threshold values ​​with a plurality of output voltage values ​​from the inverter 90. These current threshold tables are set in advance and stored in the processing unit 100. These current threshold tables can be set as appropriate based on the empirical knowledge of the designer or on experiments, simulations, etc., performed by the designer.

[0094] For example, the current threshold value setting unit 60 is configured to acquire the current threshold value Th corresponding to the value of the operating frequency based on the operating frequency / current threshold value table 62 and the value of the operating frequency of the expander motor 42. The inverter 90 is configured to output, to the processing unit 100, output frequency information S2 indicating the value of the output frequency of the inverter 90 (i.e., the operating frequency of the expander motor 42).

[0095] The current threshold setting unit 60 may be configured to obtain a current threshold Th corresponding to the value of the output voltage from the inverter 90 based on the output voltage / current threshold table 68 and the value of the output voltage. Information indicating the value of the output voltage from the inverter 90 may be input from the inverter 90 to the processing unit 100 and used by the current threshold setting unit 60. For example, the inverter 90 may be configured to output an output voltage signal S4 indicating the value of the output voltage of the inverter 90 (i.e., the input voltage to the expander motor 42) to the processing unit 100, in addition to the output frequency information S2.

[0096] A plurality of output voltage / current threshold tables 68 may be set in advance, and each of these plurality of output voltage / current threshold tables 68 may be associated with a plurality of different operating frequency values. For example, first and second output voltage / current threshold tables 68 may be set in advance, and the first output voltage / current threshold table 68 may be used when the expander motor 42 is driven at a first operating frequency value (e.g., 70 Hz), and the second output voltage / current threshold table 68 may be used when the expander motor 42 is driven at a second operating frequency value (e.g., 60 Hz) different from the first operating frequency value.

[0097] The comparison unit 70 compares the current (e.g., the effective value of the current) of the expander motor 42 with the acquired current threshold value Th based on the motor current signal S1, and generates monitoring result data D1 based on the comparison result. As in the embodiment described with reference to Fig. 5, the monitoring result data D1 is sent to the notification means 80, and the monitoring result is notified to the user, for example, by displaying it on the display 82.

[0098] Even in this way, by monitoring the expander motor 42 based on the current (e.g., the effective value of the current) of the expander motor 42, it is possible to predict or prevent abnormal operation or failure of the expander motor 42 due to long-term operation.

[0099] In the above-described embodiment, monitoring based on the motor current is described as an example, but monitoring based on the power consumption of the inverter 90 or the expander motor 42 is also possible. Such an embodiment will be described below.

[0100] 8 is a graph showing the relationship between the power consumption and load duty of the inverter 90 for a number of operating frequency values. This graph is based on measurements made by the inventors. The vertical axis shows the power consumption value of the inverter 90. The horizontal axis shows the load duty of the expander motor 42, as in FIG. 3.

[0101] As can be seen from FIG. 8, the power consumption value tends to increase as the load duty increases. Therefore, for a certain operating frequency (e.g., 60 Hz), the power consumption value when the load duty is smaller than 100% but sufficiently large (e.g., 90% to 98%) can be used as the power consumption threshold. When the power consumption of the inverter 90 (or the expander motor 42) exceeds this power consumption threshold, it means that a large load corresponding to the load duty is being applied to the expander motor 42. This can be regarded as a prediction of the occurrence of abnormal operation of the expander motor 42, and measures can be taken to prevent the occurrence of abnormal operation, such as issuing a warning, lowering the operating frequency, or stopping operation of the cryogenic refrigerator 10.

[0102] The power consumption value depends on the operating frequency of the expander motor 42. As shown in Fig. 8, the power consumption value indicating the same load duty differs when the operating frequency is 50 Hz and when it is 60 Hz. Therefore, in this embodiment, a power consumption threshold is set for each of multiple values ​​of the operating frequency of the expander motor 42.

[0103] Monitoring the expander motor 42 based on power consumption has the advantage of reducing the possibility of erroneous predictions due to the influence of external magnetic fields compared to monitoring based on the motor current described above. As described above, external magnetic fields can increase or decrease the current flowing through the expander motor 42. External magnetic fields can also cause a similar increase or decrease in power consumption. However, measurements and studies by the inventors have shown that the change in power consumption when the load duty changes by a certain unit amount (e.g., 1%) tends to be larger than the change in motor current when the load duty changes by a unit amount. In other words, the gradient of the graph in FIG. 8 tends to be larger than the gradient of the graph in FIG. 3. Therefore, the apparent change in load duty corresponding to a change in power consumption due to an external magnetic field of a certain magnitude tends to be smaller than the apparent change in load duty corresponding to a change in motor current due to an external magnetic field of the same magnitude. Therefore, monitoring the expander motor 42 based on power consumption can relatively reduce the influence of external magnetic fields.

[0104] Furthermore, as shown in Fig. 8, the relationship between the power consumption value and the load duty is closer to linearity than the relationship between the motor current value and the load duty shown in Fig. 3. This has the advantage of making it easier to monitor the operating conditions in the region where the load duty is relatively small.

[0105] 9 is a block diagram of a motor monitoring device according to another embodiment. The monitoring device includes a processing unit 100 that monitors the expander motor 42 based on a power consumption signal S5 that indicates the power consumption of the inverter 90. The inverter 90 may be configured to output the power consumption signal S5 that indicates the power consumption of the inverter 90 to the processing unit 100, in addition to output frequency information S2 and an output voltage signal S4, and the processing unit 100 monitors the expander motor 42 based on the power consumption signal S5 output from the inverter 90. The monitoring device may include notification means 80 that notifies information indicating the monitoring results, and the notification means 80 may include, for example, a display 82.

[0106] The processing unit 100 acquires a power consumption threshold value Th2 based on the operating conditions of the expander motor 42, and monitors the expander motor 42 by comparing the power consumption value of the inverter 90 with the power consumption threshold value Th2 based on the power consumption signal S5.

[0107] The processing unit 100 includes a power consumption threshold setting unit 61 and a comparison unit 70. The power consumption threshold setting unit 61 includes an operating frequency / power consumption threshold table 63 and an output voltage / power consumption threshold table 69. The operating frequency / power consumption threshold table 63 associates a plurality of power consumption thresholds with a plurality of values ​​of the operating frequency of the expander motor 42, and the output voltage / power consumption threshold table 69 associates a plurality of power consumption thresholds with a plurality of values ​​of the output voltage from the inverter 90. These power consumption threshold tables are set in advance and stored in the processing unit 100. These power consumption threshold tables can be set as appropriate based on the empirical knowledge of the designer or on experiments, simulations, etc. conducted by the designer.

[0108] For example, the power consumption threshold value setting unit 61 is configured to acquire a power consumption threshold value Th2 corresponding to the value of the operating frequency based on the operating frequency / power consumption threshold value table 63 and the value of the operating frequency of the expander motor 42. The inverter 90 is configured to output output frequency information S2 indicating the value of the output frequency of the inverter 90 (i.e., the operating frequency of the expander motor 42) to the processing unit 100.

[0109] The operating frequency vs. power consumption threshold value table 63 may associate, for each of a plurality of values ​​of the operating frequency of the expander motor 42, the power consumption value of the inverter 90 (or the expander motor 42) at that operating frequency value when a load torque that is a predetermined amount smaller than the maximum load torque allowable for the expander motor 42 acts on the expander motor 42 (i.e., when the load duty is at a predetermined value), as the power consumption threshold value for that operating frequency value. The predetermined value of the load duty may be selected, for example, from a range of 80% or more and less than 100%, or from a range of 90% or more and 98% or less.

[0110] A plurality of operating frequency / power consumption threshold tables 63 may be set in advance, and each of the plurality of operating frequency / power consumption threshold tables 63 may be associated with a plurality of different load duty values. The power consumption threshold setting unit 61 may acquire the power consumption threshold Th2 corresponding to the value of the operating frequency of the expander motor 42 based on an operating frequency / power consumption threshold table 63 selected from the plurality of operating frequency / power consumption threshold tables 63 and the value of the operating frequency of the expander motor 42.

[0111] The power consumption threshold setting unit 61 may be configured to acquire a power consumption threshold Th2 corresponding to the value of the output voltage from the inverter 90, based on the output voltage / power consumption threshold table 69 and the value of the output voltage. The inverter 90 may be configured to output, to the processing unit 100, an output voltage signal S4 indicating the value of the output voltage of the inverter 90 (i.e., the input voltage to the expander motor 42), in addition to the output frequency information S2.

[0112] A plurality of output voltage / power consumption threshold tables 69 may be set in advance, and each of these plurality of output voltage / power consumption threshold tables 69 may be associated with a plurality of different operating frequency values. For example, first and second output voltage / power consumption threshold tables 69 may be set in advance, and the first output voltage / power consumption threshold table 69 may be used when the expander motor 42 is driven at a first operating frequency value (e.g., 70 Hz), and the second output voltage / power consumption threshold table 69 may be used when the expander motor 42 is driven at a second operating frequency value (e.g., 60 Hz) different from the first operating frequency value.

[0113] Based on the power consumption signal S5, the comparison unit 70 compares the power consumption value of the inverter 90 with the acquired power consumption threshold value Th2, and generates monitoring result data D1 based on the comparison result. As in the embodiment described with reference to Fig. 5, the monitoring result data D1 is sent to the notification means 80, and the monitoring result is notified to the user, for example, by displaying it on the display 82.

[0114] The processing unit 100 may monitor the expander motor 42 based on the power consumption in this manner at least during steady operation of the cryogenic refrigerator 10. As in the embodiment shown in FIG. 5, the processing unit 100 may acquire information indicating the current operation mode of the cryogenic refrigerator 10 from a controller (e.g., the compressor controller 24), or may determine the current operation mode based on a measured temperature signal S3 from the temperature sensor 52. The processing unit 100 may monitor the expander motor 42 based on the power consumption signal S5 when the current operation mode is steady operation. Additionally or alternatively, the processing unit 100 may monitor the expander motor 42 based on the power consumption signal S5 when the current operation mode is cool-down operation.

[0115] Instead of monitoring the power consumption of the inverter 90, the processing unit 100 may monitor the expander motor 42 based on a power consumption signal S5 indicating the power consumption of the expander motor 42. In this case, the processing unit 100 may acquire the current and voltage supplied to the expander motor 42 and calculate the power consumption of the expander motor 42 from these current and voltage. The processing unit 100 may acquire the current of the expander motor 42 from a motor current signal S1 input from a current sensor 50. The processing unit 100 may acquire the voltage of the expander motor 42 from an output voltage signal S4 input from the inverter 90. Alternatively, a voltage sensor may be provided to measure the voltage supplied to the expander motor 42, and the processing unit 100 may acquire the voltage of the expander motor 42 from a motor voltage signal input from the voltage sensor. The processing unit 100 may compare the power consumption of the expander motor 42 with the acquired power consumption threshold value Th2 based on the power consumption signal S5 and generate monitoring result data D1 based on the comparison result.

[0116] Fig. 10 is a flowchart showing a method for monitoring a cryogenic refrigerator 10 according to another embodiment. First, as shown in Fig. 10, while the cryogenic refrigerator 10 is operating (for example, during steady operation), the power consumption of the inverter 90 (or the expander motor 42) is acquired (S30). Then, the expander motor 42 is monitored based on the acquired power consumption (S40).

[0117] In S40, a power consumption threshold value Th2 is acquired based on the operating conditions of the expander motor 42 (e.g., output frequency information S2, output voltage signal S4, etc.) (S41). The acquired power consumption value of the inverter 90 is compared with the power consumption threshold value Th2 (S42). If the power consumption value exceeds the power consumption threshold value Th2 (Y in S42), the comparison unit 70 determines that the occurrence of abnormal operation of the expander motor 42 is predicted (S43) and outputs monitoring result data D1 indicating this. If the power consumption value is equal to or less than the power consumption threshold value Th2 (N in S42), the comparison unit 70 determines that the occurrence of abnormal operation of the expander motor 42 is not predicted (S44) and outputs monitoring result data D1 indicating this. This ends the monitoring method.

[0118] In addition, if abnormal operation of the expander motor 42 is predicted, the controller of the cryogenic refrigerator 10 (e.g., the compressor controller 24) may take measures to prevent the occurrence of abnormal operation, such as issuing a warning, reducing the operating frequency of the expander motor 42, or stopping operation of the cryogenic refrigerator 10.

[0119] In this way, according to the embodiment, by monitoring the expander motor 42 based on the power consumption of the inverter 90 (or the expander motor 42), it is possible to predict or prevent abnormal operation or failure of the expander motor 42 due to long-term operation.

[0120] The present invention has been described above based on examples. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiments, and that various design changes and modifications are possible, and that such modifications are also within the scope of the present invention. Various features described in relation to one embodiment can also be applied to other embodiments. A new embodiment created by combining embodiments will have the combined effects of the respective combined embodiments.

[0121] In the above-described embodiment, the processing unit 100 includes three current threshold tables, namely, an operating frequency vs. current threshold table 62, an external magnetic field vs. current threshold table 64, and an input voltage vs. current threshold table 66, in order to obtain the current threshold value based on the operating conditions of the expander motor 42. However, these three tables are not essential. For example, the processing unit 100 may include only the operating frequency vs. current threshold table 62.

[0122] Alternatively, the processing unit 100 may not include the operating frequency / current threshold value table 62. In this case, the processing unit 100 may include the external magnetic field / current threshold value table 64, or the input voltage / current threshold value table 66, or both, and the external magnetic field / current threshold value table 64 (or the input voltage / current threshold value table 66) may be preset to be used at a specific operating frequency for monitoring. In this case, the processing unit 100 may determine whether the expander motor 42 is operating at the specific operating frequency for monitoring based on the output frequency information S2, and may monitor the expander motor 42 by comparing the current of the expander motor 42 with a current threshold value based on the motor current signal S1 obtained during operation at this specific operating frequency.

[0123] Furthermore, the processing unit 100 may be configured to monitor the expander motor 42 by operating the inverter 90 to drive the expander motor 42 at a monitoring operating frequency, and comparing the current of the expander motor 42 with a current threshold based on the motor current signal S1 when the expander motor 42 is driven at the monitoring operating frequency. In this way, a fixed current threshold corresponding to the monitoring operating frequency can be used to compare the current of the expander motor 42 with the current threshold. Note that even in this case, the current threshold may be adjusted based on motor operating conditions such as an external magnetic field.

[0124] In one embodiment, the cryogenic refrigerator 10 may be a single-stage GM refrigerator or other type of cryogenic refrigerator that includes an expander motor 42 for powering the expander 14.

[0125] In one embodiment, the processing unit 100 does not constitute part of the cryocooler 10, but may instead be part of the cryogenic system (eg, superconducting equipment, or an MRI system) in which the cryocooler 10 is installed.

[0126] The present invention has been described using specific terms based on the embodiments, but the embodiments merely illustrate one aspect of the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments as long as they do not deviate from the concept of the present invention as defined in the claims. [Industrial Applicability]

[0127] The present invention can be used in the field of cryogenic refrigerators and methods for monitoring cryogenic refrigerators. [Explanation of symbols]

[0128] 10 cryogenic refrigerator, 14 expander, 42 expander motor, 50 current sensor, 62 operating frequency and current threshold value table, 64 external magnetic field and current threshold value table, 90 inverter, 100 processing unit.

Claims

1. an expander motor that operates an expander of the cryogenic refrigerator; an inverter configured to control an operating frequency of the expander motor; a processing unit that monitors the expander motor based on a power consumption signal that indicates power consumption of the inverter or the expander motor.

2. The cryogenic refrigerator according to claim 1 , wherein the processing unit monitors the expander motor based on a power consumption signal output from the inverter, the power consumption signal indicating the power consumption of the inverter.

3. 3. The cryogenic refrigerator according to claim 2, wherein the processing unit acquires a power consumption threshold value based on an operating condition of the expander motor, and monitors the expander motor by comparing the power consumption of the inverter with the power consumption threshold value based on the power consumption signal.

4. The cryogenic refrigerator according to claim 2 or 3, characterized in that the processing unit includes an operating frequency / power consumption threshold value table that associates multiple power consumption threshold values ​​with multiple values ​​of the operating frequency of the expander motor, acquires the power consumption threshold value corresponding to the operating frequency value based on the operating frequency / power consumption threshold value table and the operating frequency value of the expander motor, and monitors the expander motor by comparing the power consumption of the inverter with the corresponding power consumption threshold value based on the power consumption signal.

5. 5. The cryogenic refrigerator according to claim 2, wherein the processing unit includes an output voltage / power consumption threshold table that associates a plurality of power consumption thresholds with a plurality of values ​​of the output voltage from the inverter, and obtains the power consumption threshold corresponding to the value of the output voltage from the inverter based on the output voltage / power consumption threshold table and the value of the output voltage from the inverter, and monitors the expander motor by comparing the power consumption of the inverter with the corresponding power consumption threshold based on the power consumption signal.

6. 1. A method for monitoring a cryogenic refrigerator, the cryogenic refrigerator comprising: an expander motor for operating an expander of the cryogenic refrigerator; and an inverter configured to control an operating frequency of the expander motor, the method comprising: Obtaining power consumption of the inverter or the expander motor; and monitoring the expander motor based on the obtained power consumption.

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