Chiller device

Abstract

This invention provides a highly efficient chiller system using an inverter equipped with SiC elements. [Solution] A chiller device comprising a refrigerant circuit including a compressor, condenser, throttle device, and evaporator with a built-in motor, a bypass circuit branched from the refrigerant circuit, an inverter for driving the motor, and a control unit, wherein the motor for driving the compressor is cooled by the refrigerant flowing through the bypass circuit, the control unit for controlling the motor operates the inverter, which includes a wide-bandgap semiconductor element, at a carrier frequency of 10 kHz or higher, and the motor has a product of the square root of its rated rotational speed [rpm] and rated power [kW] of 180,000 or higher.

Description

Technical Field

[0001] The present disclosure relates to a chiller device.

Background Art

[0002] Patent Document 1 describes a compression unit including a motor that generates a rotating magnetic field based on a current supplied from an inverter circuit and rotates a rotor based on the rotating magnetic field, and a compressor that compresses a fluid by an impeller as a shaft directly connected to the rotor rotates at the same rotational speed as the rotor, wherein a switching element of the inverter circuit is a SiC element. Conventionally, as in Patent Document 1, SiC elements have been applied to inverters, but there are no application examples of SiC elements to large chiller devices or air conditioners of 100 US refrigeration tons or more, and mainly Si-IGBT (Insulated-Gate Bipolar Transistor) has been used. The reason is considered to be that SiC elements are expensive and the cost-effectiveness as a single compressor is small.

Prior Art Documents

Patent Documents

[0003]

Patent Document No. 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present disclosure is to provide a highly efficient chiller device using an inverter equipped with a SiC element.

Means for Solving the Problems

[0005] The chiller device in the first aspect is a chiller device having a refrigerant circuit including a compressor with a built-in motor, a condenser, a throttle device, and an evaporator, a bypass circuit branched from the refrigerant circuit, an inverter that drives the motor, and a control unit, wherein the motor that drives the compressor is cooled by the refrigerant flowing through the bypass circuit, the control unit that controls the motor operates the inverter, which includes a wide-bandgap semiconductor element, at a carrier frequency of 10 kHz or higher, and the motor has a product of the square root of its rated rotational speed [rpm] and rated power [kW] of 180,000 or higher. In this case, a highly efficient chiller device can be provided by using a motor with reduced heat generation and a heat exchanger with reduced heat transfer area. The chiller device in the second aspect is the chiller device in the first aspect, wherein the motor has permanent magnets. In this case, a significant reduction in magnet iron loss can be achieved by increasing the carrier frequency. A chiller device in the third aspect is a chiller device in the first or second aspect, wherein the control unit monitors or estimates the temperature of the motor's stator, rotor, or the compressor's magnetic bearing. In this case, the carrier frequency can be controlled based on the motor temperature. A chiller device according to the fourth aspect is a chiller device according to the first to third aspects, wherein when the motor is hot, the control unit increases the flow rate of the refrigerant or cooling water, or increases the carrier frequency. In this case, efficient control can be achieved by performing feedback control to increase the flow rate of the refrigerant or cooling water, or to increase the carrier frequency, when the motor temperature increases. The chiller device according to the fifth aspect is the chiller device according to the first to fourth aspects, wherein the motor has a ratio of the air gap between the rotor and the stator to the outer diameter of the rotor of 0.035 or less. In this case, the power factor is improved and heat transfer is improved. The chiller device according to the sixth aspect is the chiller device according to the first to fifth aspects, wherein the control unit sets the carrier frequency to 1.5 times or more the frequency at the rated load when the load is light (50% or less of the rated load). In this case, the degree of improvement in motor efficiency at light load can be increased. The chiller device according to the seventh aspect is the chiller device according to the first to sixth aspects, wherein the carrier frequency is 20 kHz or higher. In this case, the iron loss of the motor can be reduced by approximately 45%. The chiller device according to the eighth aspect is the chiller device according to the first to sixth aspects, wherein the carrier frequency is 30 kHz or higher. In this case, the iron loss of the motor can be reduced by approximately 24%. The chiller device according to the ninth aspect is the chiller device according to the first to eighth aspects, wherein the wide bandgap semiconductor element is SiC. In this case, even if surges become severe due to high current and high frequency, high voltage resistance can be achieved with the SiC element. The chiller device according to the tenth aspect is the chiller device according to the first to ninth aspects, wherein the device structure of the wide-bandgap semiconductor element is a MOSFET structure. In this case, the short-circuit withstand capability can be improved. The chiller device according to the eleventh aspect is a chiller device according to the first to tenth aspects, wherein the condenser is cooled with cooling water. In this case, a chiller device with higher efficiency than an air-cooled chiller device can be provided. [Brief explanation of the drawing]

[0006] [Figure 1] This figure shows an example of the configuration of a chiller device according to this embodiment. [Figure 2] This figure shows an example of the configuration of the compressor drive system of the chiller device according to this embodiment. [Figure 3] This figure shows an example of the configuration of a motor cooling circuit according to this embodiment. [Figure 4] This diagram shows the basic structure of a BPM motor. [Figure 5] This is a cross-sectional view showing the basic structure of an SPM motor. [Figure 6] (A) is a diagram showing the relationship between inverter efficiency and carrier frequency at 25% of rated load when motor cooling is considered, and (B) is a diagram showing the relationship between inverter efficiency and carrier frequency at 25% of rated load when motor cooling efficiency is not considered. [Figure 7] (A) is a diagram showing the relationship between inverter efficiency and carrier frequency at rated load when motor cooling is considered, and (B) is a diagram showing the relationship between inverter efficiency and carrier frequency at rated load when motor cooling efficiency is not considered. [Figure 8] This figure shows the relationship between the heat transfer area of ​​a heat exchanger and the overall compression efficiency. [Figure 9] This figure shows the relationship between the gap length, the rotor outer diameter ratio, and the power factor. [Figure 10] (A) is a characteristic diagram showing the relationship between carrier frequency and the magnet iron loss of the motor, and (B) is a diagram showing the relationship between carrier frequency and temperature rise. [Figure 11] (A) is a diagram showing the change in iron loss when the carrier frequency is changed from 10 kHz to 20 kHz, and (B) is a diagram showing the change in iron loss when the carrier frequency is changed from 20 kHz to 30 kHz. [Figure 12] This diagram shows the relationship between carrier frequency and ripple current ratio. [Figure 13] This table compares various efficiencies of SiC elements with a MOSFET structure and Si elements with an IGBT structure. [Modes for carrying out the invention]

[0007] The embodiments will be described in detail below with reference to the attached drawings. <Chiller device> A chiller is a device that uses a refrigerant to circulate and control the temperature of a target space, sample, or equipment. In the case of a water-cooled chiller, a refrigerant circulating inside the chiller and water used to cool the refrigerant are employed. Figure 1 shows an example of the configuration of a chiller device 1 according to this embodiment. The chiller device 1 includes a compressor 10, a condenser 20, a cooling tower 30, a cooling water pump 32, a cooling water pump drive motor 34, a first cooling circuit 36, a throttling device 40, an evaporator 50, a load 60, a chilled water pump 62, a chilled water pump drive motor 64, a second cooling circuit 66, and the like. The throttling device 40 is sometimes called an expansion valve.

[0008] A refrigerant circuit 70, which is a circuit through which refrigerant circulates, includes a compressor 10, a path connecting between the compressor 10 and a condenser 20, the condenser 20, a path connecting between the condenser 20 and an expansion device 40, the expansion device 40, a path connecting between the expansion device 40 and an evaporator 50, the evaporator 50, and a path connecting between the evaporator 50 and the compressor 10. The first cooling circuit 36 is a circuit that cools the condenser 20 with cooling water. In the first cooling circuit 36, cooling water circulates between the condenser 20 and a cooling tower 30. The second cooling circuit 66 is a circuit that cools a load 60 with cooling water. In the second cooling circuit 66, cooling water circulates between the evaporator 50 and the load 60.

[0009] In the chiller device 1, the refrigerant changes phase and circulates through the refrigerant circuit 70. The compressor 10 compresses a gas, which is the refrigerant, by an impeller driven by a motor to form a high-temperature and high-pressure gas. The high-temperature and high-pressure refrigerant discharged from the compressor 10 flows into the condenser 20 in the direction shown by f1 through the refrigerant circuit 70. The refrigerant is condensed in the condenser 20 through heat exchange with the cooling water to become a low-temperature and high-pressure refrigerant. The cooling water from the cooling tower 30 flows through the first cooling circuit 36 in the direction of f4. The condenser 20 has a cooling water pump 32, and the refrigerant flows in when the cooling water pump driving motor 34 drives the cooling water pump. Therefore, in the condenser 20, the heated refrigerant is cooled by the cooling water to liquefy the refrigerant. The cooling water refluxes to the cooling tower 30 in the direction of f5 through the first cooling circuit 36.

[0010] Next, the liquefied refrigerant flows into the throttle device 40 in the direction shown by the flow f2 via the refrigerant circuit 70. The refrigerant becomes a low-temperature and low-pressure refrigerant by being expanded and depressurized by the throttle device 40, and flows into the evaporator 50 in the direction of the flow f2 via the refrigerant circuit 70. From the evaporator 50, the cooling water is cooled by driving the cooling water pump 62 by the cooling water pump drive motor 64, and flows into the load 60 in the direction of the flow f6 via the second cooling circuit 66. The cooling water heated by cooling the load 60 flows into the evaporator 50 in the direction of the flow f7 from the load 60. In the evaporator 50, the refrigerant evaporates by heat exchange with the cooling water and becomes a low-pressure gas. At this time, the refrigerant takes heat from the surroundings by the heat of vaporization and vaporizes. The gas of the vaporized refrigerant returns to the compressor 10 in the direction of the flow f3 via the refrigerant circuit 70.

[0011] <Configuration of Compressor Drive System> FIG. 2 is a diagram showing a configuration example of the compressor drive system 5. The compressor drive system 5 refers to a system that drives the compressor 10. The chiller device 1 includes the compressor drive system 5. The compressor drive system 5 includes a control panel 12, a power source 13, an inverter 14, an internal power source 15, and a magnetic bearing controller 16. The compressor 10 has a motor 11, an impeller 130 (see FIG. 3), and a mechanism (magnetic bearing 120) that holds the rotating shaft by magnetic force.

[0012] The magnetic bearing controller 16 controls the magnetic bearing 120. The inverter 14 is a power conversion circuit that converts power to DC, then converts it back to AC, and supplies the motor 11 with a current converted to an arbitrary frequency. The inverter 14 controls the rotational speed of the motor 11. The inverter 14 has a rectifier circuit and a three-phase output DC-AC inverter circuit using two sets of switching elements for each phase. The switching elements are SiC elements as wide-bandgap semiconductor elements. SiC elements have high resistance to surges caused by high current and high frequency. For this reason, they are suitable when operating at high carrier frequencies. Furthermore, it is preferable that the SiC elements have a MOSFET structure. Compared to the IGBT structure, the MOSFET structure has improved short-circuit withstand time (SCWT), which is the time that a power device can withstand without being destroyed during a short circuit. Furthermore, because the IGBT structure generates tail current, it requires a long dead time, making it unsuitable for high carrier frequency operation. However, the MOSFET structure does not generate tail current, allowing for a shorter dead time, making it suitable for high carrier frequency operation. The control panel 12 controls the entire compressor drive system 5.

[0013] Figure 3 shows an example of the configuration of a motor cooling circuit according to this embodiment. In this embodiment, the refrigerant in the refrigerant circuit 70 is also applied to the cooling of the motor 11. By cooling the motor 11, the temperature rise of the motor 11 is suppressed, and the motor efficiency is improved.

[0014] The motor 11 includes a stator 110, a rotor 112, and a magnetic bearing 120. A shaft 125 is coaxially fixed to the radially inward side of the rotor 112. The shaft 125 extends axially. An impeller 130 is fixed to one axial end of the shaft 125. The rotation of the impeller 130 compresses the refrigerant flowing into the impeller chamber (not shown) in which the impeller is housed. The refrigerant circuit 70 (path indicated by flows f1 to f3) which circulates sequentially through the compressor 10, condenser 20, throttle device (expansion valve) 40, evaporator 50, and compressor 10, was explained in the explanation of chiller device 1 in Figure 1. Here, we will explain the configuration in which a bypass circuit 72 branched from the refrigerant circuit 70 cools the motor 11 of the compressor 10.

[0015] The bypass circuit 72 for cooling the motor 11 is configured as a bypass path through which the refrigerant flows in the order of condenser 20, motor 11, and evaporator 50. The bypass path shares the refrigerant circuit 70 with the path through which the refrigerant flows from the evaporator 50 to the motor 11 with a flow rate of f3, and then flows back to the condenser 20 with a flow rate of f1. However, the refrigerant flowing through the bypass path does not flow through the refrigerant circuit 70, which passes through the throttling device 40 and evaporator 50, but instead branches off at the condenser 20 and flows through the bypass circuit 72. The bypass circuit 72 is a circuit that enables the flow of refrigerant from the condenser 20 to the motor 11 with a flow rate of f9 and from the motor 11 to the evaporator 50 with a flow rate of f8. By cooling the motor 11 with the refrigerant flowing through this bypass circuit 72, the motor's temperature rise can be suppressed. The bypass path from the condenser 20 to the motor 11 can be branched as needed to provide multiple paths. In this case, valves can be placed on the branched bypass paths to adjust the flow rate.

[0016] The bypass circuit 72 described above branches off from the refrigerant circuit 70 at the condenser 20 and rejoins the refrigerant circuit 70 at the evaporator 50, but the branching point and flow direction are not limited to this. For example, the bypass circuit 72 may be a path that flows from the throttling device (expansion valve) 40 to the motor 11 and from the motor 11 to the evaporator 50. Alternatively, for example, the bypass circuit 72 may be a path composed of an upstream branch of the condenser 20 and a downstream branch of the evaporator 50. In other words, the bypass circuit 72 may have a path that branches off from the refrigerant circuit 70 at an intermediate point between the motor 11 and the condenser 20, flows into the motor 11, and then flows out at an intermediate point between the evaporator 50 and the motor 11.

[0017] The control unit 200 (not shown) of the control panel 12 (see Figure 2) monitors or estimates the temperature of the motor 11 using sensors that measure the temperature of the stator 110, rotor 112, or magnetic bearing 120, impeller 130, etc. When the motor 11 is hot, the control unit 200 increases the flow rate of the cooling refrigerant or increases the carrier frequency.

[0018] Figure 4 shows an example of the basic structure of motor 11. Motor 11 is a PMSM (Permanent Magnet Synchronous Motor). Figure 4(A) shows a cross-section of motor 11, and Figure 4(B) is an enlarged view of rotor 112. Motor 11 comprises rotor 112 and stator 110. Rotor 112 is integrally constructed with shaft 125.

[0019] Figure 4(A) illustrates the cross-section of the motor 11. The rotor 112 is a BPM (Buried Permanent Magnet) type rotor. The rotor 112 is positioned radially inward of the stator 110, facing the stator 110 with an air gap 114 separating them. The rotor 112 comprises a rotor core 111 and permanent magnets 115. The rotor core 111 is a cylindrical member made of a magnetic material. A shaft hole for inserting a shaft 125 (see Figure 3) is formed in the center of the rotor core 111. Multiple permanent magnets 115 are arranged around the rotor core 111 such that their poles facing the stator 110 alternate. The permanent magnets 115 are sintered magnets containing rare earth elements, but are not limited to these.

[0020] The stator 110 comprises a stator core and a drive coil. The stator core is a cylindrical member made of a magnetic material. The drive coil is a coil through which a drive current flows to rotate the rotor 112. The drive coil generates a rotating magnetic field when current flows through it. The motor 11 is directly connected to the shaft 125 so as to rotate as a single unit without the use of gears or the like. Figure 4(B) shows a detailed explanation of the rotor 112.

[0021] The air gap 114, which is the gap between the rotor 112 and the stator 110, worsens the power factor when it increases. Widening the gap weakens the magnetic field acting on the rotor 112 (the magnet torque decreases). To achieve the same torque, the current flowing through the coil must be increased, which manifests as a decrease in the power factor. In addition, in a BPM-type rotor, which is a type of IPM (Interior Permanent Magnet), the space between the BPM permanent magnet 115 and the surface of the rotor 112 is made of laminated steel (metal), so losses occur due to eddy currents generated by changes in magnetic flux. The definition of an air gap is the gap between the outer diameter of the rotor and the inner diameter of the stator, where the magnetic resistance is as high as air in the magnetic circuit. In other words, as shown in Figure 4(B), the air gap 114 is the gap between the outer diameter of the rotor 116 and the inner diameter of the stator 110. On the other hand, in the case of an SPM (Surface Permanent Magnet) motor, as shown in Figure 5, a protective sleeve 119 is often provided on the outside of the permanent magnet 115 attached to the rotor 112. The protective sleeve 119 is usually made of resin, and since its magnetic resistance is equivalent to that of air in the magnetic circuit, it can be treated as air. For this reason, the protective sleeve 119 is not included in the gap between the outer diameter of the rotor and the inner diameter of the stator, which forms the air gap 114. In other words, in the case of an SPM motor, the air gap 114 is the gap from the outer surface of the permanent magnet 115 to the tip of the teeth 117 (inner diameter of the stator).

[0022] Generally, motor heat generation is due to copper loss, iron loss, and mechanical loss. Copper loss is power loss arising from the winding resistance of the motor coils and the current flowing through them. Iron loss is loss due to changes in the magnetic field. Iron loss includes loss due to hysteresis and loss due to eddy currents generated in magnetic materials. Mechanical loss is loss due to friction and air resistance within the motor. Of these losses, mechanical loss accounts for a small proportion. In medium to large motors, the number of turns cannot be increased, so when high-speed rotation is performed using an inverter, the effect of ripple current becomes significant, and iron loss tends to be more pronounced. In small motors, it is easier to increase the number of turns, so copper loss due to winding resistance tends to be a higher proportion.

[0023] The permanent magnet 115 of the rotor 112 generates heat due to eddy currents. This heat generation may cause the permanent magnet 115 to demagnetize. In the configuration with the bypass circuit 72 described above, the coolant may be supplied to the vicinity of the outer surface of the permanent magnet 115 of the motor 11. In this case, the permanent magnet 115 is efficiently cooled by the coolant. In other words, by directly or indirectly cooling the permanent magnet 115 with the coolant, the heat generation of the permanent magnet 115 is suppressed, and the demagnetization of the permanent magnet 115 can be prevented.

[0024] Here, the ratio of the air gap 114 between the rotor 112 and the stator 110 to the outer diameter 116 of the rotor 112 is called the gap ratio. If the gap ratio is large, the power factor decreases and iron loss decreases, but copper loss increases. This results in a motor with excessive copper loss. On the other hand, if the gap ratio is small, copper loss decreases and iron loss increases. How to set the gap ratio appropriately when considering the overall efficiency of the chiller device 1 will be discussed later.

[0025] Here, we will explain the types of elements that make up inverter 14 (see Figure 2), the inverter efficiency when the carrier frequency is changed, the motor efficiency, and the cooling efficiency. Inverter efficiency is the ratio of output power to input power. Motor efficiency is the ratio of mechanical output to input power. Cooling efficiency is the ratio of heat absorbed to input power. Motor × inverter efficiency is the efficiency obtained by multiplying the motor efficiency and the inverter efficiency. In other words, motor × inverter efficiency is the ratio of mechanical output to input power. When motor cooling is considered, motor × inverter efficiency refers to the efficiency obtained by multiplying the motor efficiency, inverter efficiency, and cooling efficiency.

[0026] Figure 6(A) shows the relationship between motor × inverter efficiency and carrier frequency at 25% of rated load, considering the cooling efficiency of motor 11 supplied by the refrigerant from the bypass circuit 72 in Figure 3. Figure 6(B) shows the relationship between motor × inverter efficiency and carrier frequency at 25% of rated load, without considering the cooling efficiency of motor 11. The horizontal axis represents carrier frequency [kHz], and the vertical axis represents inverter efficiency [%]. The rated load is the load point at 100% load, which is used to calculate the energy consumption efficiency of the chiller system. Here, the elements constituting the inverter 14 are a MOSFET-structured SiC element and an IGBT-structured Si element. In Figures 6(A) and (B), the MOSFET-structured SiC element is denoted as SiC, and the IGBT-structured Si element is denoted as IGBT. Hereafter, the MOSFET-structured SiC element will also be denoted as SiC element, and the IGBT-structured Si element will be denoted as IGBT element.

[0027] Both Figure 6(A) and (B) show that the SiC element has higher motor × inverter efficiency than the IGBT element at carrier frequencies from 2kHz to 20kHz. As shown in Figure 6(A), under a 25% load and considering the cooling of the motor 11, the SiC element achieves maximum motor × inverter efficiency at a carrier frequency of 10 kHz. The IGBT element achieves maximum motor × inverter efficiency at a carrier frequency of 5 kHz. On the other hand, as shown in Figure 6(B), under the case of a 25% load and without considering cooling of the motor 11, the SiC element achieves maximum motor × inverter efficiency at a carrier frequency of 6 kHz. The IGBT element achieves maximum motor × inverter efficiency at a carrier frequency of 4 kHz. The results in Figures 6(A) and (B) show that the SiC element has higher motor × inverter efficiency than the IGBT element. When using a SiC element and considering the cooling of the motor 11, the motor × inverter efficiency is maximized at a carrier frequency of 10 kHz.

[0028] Figure 7(A) shows the relationship between inverter efficiency and carrier frequency at rated load when motor cooling is considered. Figure 7(B) shows the relationship between inverter efficiency and carrier frequency at rated load when motor cooling is not considered. In both Figures 7(A) and (B), at all carrier frequencies, SiC elements have a higher motor × inverter efficiency than IGBT elements when applied to the inverter. Under 100% load conditions and without considering motor cooling, SiC elements achieve maximum motor-inverter efficiency at a carrier frequency of 2kHz. IGBT elements also achieve maximum motor-inverter efficiency at a carrier frequency of 2kHz. Under rated load conditions and considering motor cooling, SiC elements achieve maximum motor-inverter efficiency at a carrier frequency of 3kHz. IGBT elements achieve maximum motor-inverter efficiency at a carrier frequency of 2kHz. The results in Figures 7(A) and (B) show that SiC elements have higher motor-inverter efficiency than IGBT elements, and that SiC elements achieve maximum motor-inverter efficiency at a carrier frequency of 2-3 kHz.

[0029] In Figure 7(B), considering only the motor × inverter efficiency, it appears that increasing the carrier frequency is unnecessary. However, as shown in Figure 7(A), when considering motor cooling efficiency, it is clear that increasing the carrier frequency is beneficial.

[0030] Furthermore, comparing Figure 6(B) and Figure 7(B), the maximum efficiency of the SiC element at rated load occurs at a carrier frequency of 2 kHz, while the maximum efficiency of the SiC element at 25% of rated load occurs at 6 kHz. In other words, the carrier frequency at which maximum efficiency occurs at 25% load is three times higher than the carrier frequency at which maximum efficiency occurs at rated load. By analogy with the relationship between the carrier frequencies at which maximum efficiency appears due to the above load changes, the carrier frequency at which maximum efficiency occurs at 50% load is 1.5 times higher than the carrier frequency at which maximum efficiency occurs at rated load. Therefore, the control unit 200 of the control panel 12 (see Figure 2) may set the carrier frequency to 1.5 times or more the rated load frequency when the load is light (50% or less of the rated load).

[0031] To improve the overall efficiency of a chiller system, it is preferable to use SiC elements and set the carrier frequency in the range of 3 to 10 kHz or higher. Furthermore, comparing the efficiency results at 25% load and 100% load, the maximum motor × inverter efficiency at 25% load is observed at higher carrier frequencies. In particular, at 25% load, it can be seen that setting the carrier frequency to 10 kHz or higher significantly improves the motor × inverter efficiency of SiC elements compared to IGBT elements.

[0032] Figure 8 shows the relationship between the heat transfer area of ​​a heat exchanger and the overall compression efficiency. The heat exchanger is a multi-tube heat exchanger. A multi-tube heat exchanger consists of a shell and tubes. The performance of the heat exchanger is determined by the number of tubes, the shell size, the approach temperature, etc. The approach temperature is the difference between the inlet temperature and the outlet temperature of the heat exchanger. When the approach temperature is small, the inlet temperature can be lowered relative to the outlet temperature, so the overall efficiency improves. However, when the approach temperature is small, the temperature difference is small and the heat transfer rate from the tubes decreases, so a heat transfer area is required to maintain the amount of heat exchanged. In other words, the overall efficiency depends on the heat transfer area of ​​the heat exchanger and the overall compression efficiency, and as the overall compression efficiency increases, the heat transfer area of ​​the heat exchanger can be reduced. In particular, just a 1.2% improvement in the overall compression efficiency from 74.7% can reduce the heat transfer area by nearly half. The reason why a 1.2% improvement in overall compression efficiency can reduce the heat transfer area by nearly half is that it is used in areas where the change in the heat transfer area of ​​the heat exchanger relative to the overall compressor efficiency is large, as customization is required according to the customer's needs for water-cooled heat exchangers, such as multi-tube heat exchangers. Note that an overall compression efficiency of 74.7% is achieved when using IGBT elements and operating at a carrier frequency of 5kHz, while a 1.2% improvement in overall compression efficiency is achieved when using SiC elements and operating at a carrier frequency of 10kHz. Overall compressor efficiency is the product of motor efficiency, inverter efficiency, and compression efficiency. Compression efficiency is the product of adiabatic efficiency and compression chamber sealing efficiency, and depends on fluid loss and leakage loss outside the compression chamber. By adopting SiC elements, the overall compression efficiency is increased, which raises the cost of the inverter, but the cost of the heat exchanger is significantly reduced, thus lowering the overall cost of the chiller system.

[0033] Figure 9 shows the relationship between the ratio of the gap length to the rotor outer diameter and the power factor of the motor 11. As mentioned above, a large gap ratio results in motor 11 with excessive copper loss, while a small gap ratio increases iron loss. By reducing the gap ratio of motor 11, the power factor of motor 11 can be improved. Improving the power factor allows for a reduction in the output current requirement of the inverter. Although a small gap ratio results in motor 11 with high iron loss, as will be discussed later, there is a correlation between iron loss and carrier frequency, so increasing the carrier frequency can reduce iron loss.

[0034] When increasing the carrier frequency using SiC elements, motors with higher iron losses show a greater improvement in efficiency. In Figure 9, the ratio of iron loss to copper loss is 1:1 when the gap ratio is 0.035. When the gap ratio is 0.035 or less, the iron loss becomes greater than the copper loss, so it is desirable for the gap ratio to be 0.035 or less.

[0035] The power factor of the motor 11 can be improved by reducing the air gap 114. In other words, reducing the air gap 114 increases the proportion of the electrical energy input to the motor that is actually used effectively for the motor's rotation. Improving the motor's power factor reduces the output current requirement of the inverter, so it is desirable to reduce the gap, but this results in a motor with high iron loss. For this reason, in the chiller device 1 of this embodiment, the carrier frequency is increased to significantly improve motor efficiency.

[0036] Figure 10(A) is a characteristic diagram showing the relationship between the carrier frequency at rated load and the magnet iron loss of the motor. The motor is a PMSM (Permanent Magnet Synchronous Motor) and has either an SPM (Surface Permanent Magnet) structure or an IPM (Interior Permanent Magnet) structure. Figure 10 shows that increasing the carrier frequency can reduce the magnet iron loss of the motor 11. When the carrier frequency is increased from 5kHz to 10kHz, the magnet iron loss decreases from approximately 2.9kW to 1.3kW, a significant reduction of approximately 68%. Reducing the magnet iron loss can suppress the temperature rise of the magnet. Figure 10(B) is a diagram showing the relationship between the carrier frequency at rated load and the temperature rise of the magnet. There is a tendency for the temperature rise of the magnet to decrease as the carrier frequency increases. In particular, the effect of reducing the temperature rise is large when the carrier frequency is between 2kHz and 10kHz.

[0037] Figure 11(A) shows the change in magnet iron loss when the carrier frequency at rated load is changed from 10 kHz to 20 kHz. By changing the carrier frequency from 10 kHz to 20 kHz, the magnet iron loss is reduced from approximately 1.3 kW to 0.72 kW, a reduction of approximately 45%. Figure 11(B) shows the change in magnet iron loss when the carrier frequency at rated load is changed from 20 kHz to 30 kHz. By changing the carrier frequency from 20 kHz to 30 kHz, the magnet iron loss is reduced from approximately 0.72 kW to 0.55 kW, a reduction of approximately 24%.

[0038] Figure 12 shows the relationship between carrier frequency and ripple current ratio. This relationship is based on a theoretical calculation formula that assumes the point where the duty cycle is maximized, taking into account the motor's electrical time constant, PWM frequency, resistance, inductance, etc. Note that the small motor 1 in the legend was calculated with a winding resistance of 10 mΩ, inductance of 8.7 mH, and motor time constant of 0.87. The small motor 2 was calculated with a winding resistance of 5 mΩ, inductance of 2.1 mH, and motor time constant of 0.42. The large motor 1 was calculated with a winding resistance of 1 mΩ, inductance of 0.15 mH, and motor time constant of 0.14. The large motor 2 was calculated with a winding resistance of 1 mΩ, inductance of 0.11 mH, and motor time constant of 0.10. The motor with numerically limited specifications was calculated with a winding resistance of 10 mΩ, inductance of 0.5 mH, and motor time constant of 0.05. Here, the large motors 1 and 2 and the motor with numerically limited specifications are within the range of winding resistance R = 0.01 Ω or less and inductance value L = 0.5 mH or less, while the small motors 1 and 2 are outside the above range. In the above, motors were described using winding resistance and inductance values, but they can also be described using rated rotational speed N [rpm] and rated power P [kW]. Small motors 1 and 2 and motors with numerically limited specifications have an N√P value of less than 180,000, while large motors 1 and 2 have an N√P value of 180,000 or more. Rated rotational speed refers to the rotational speed of the motor shaft when the motor is operated at the guaranteed usable output (power) value or the maximum output (power) value with the rated voltage and rated frequency applied to the motor. Rated power refers to the mechanical output value that can be drawn from the motor shaft when the motor is operating at the rated rotational speed.

[0039] Figure 12 shows that the ratio of current ripple to average current decreases as the carrier frequency increases. Generally, motor iron loss depends on the amount of carrier ripple current, and the carrier ripple current depends on the inverter's carrier frequency and the motor's design value. As shown in Figure 12, for small motors, the carrier ripple current is relatively small even without increasing the carrier frequency. Therefore, iron loss is small and there is little need to increase the carrier frequency. On the other hand, for large motors, the carrier ripple current ratio to the average current is large, resulting in large iron loss. To improve this, it is necessary to lower the carrier ripple current ratio by increasing the carrier frequency. This is because lowering the carrier ripple current ratio contributes to reducing iron loss. There is a close correlation between carrier ripple current and the motor's magnet iron loss. Therefore, the effect of reducing magnet iron loss by increasing the carrier frequency in Figures 10 and 10, and the reduction in the ripple current ratio when the carrier frequency is increased as shown in Figure 12, can be inferred from the similarity in the shape of the reduction curves in the graphs.

[0040] Large motors that rotate at high speeds and handle high currents, where the product of the square root of the rated rotational speed [rpm] and rated power [kW] is large, have fewer winding turns, resulting in smaller winding resistance and inductance values. Therefore, the motor's heat generation can be significantly reduced by increasing the carrier frequency, improving the motor's cooling efficiency. Furthermore, the greatest reduction in motor loss due to the reduction in carrier ripple current is in the magnet iron loss. In particular, the chiller device in this embodiment uses a magnet bearing direct drive without variable speed gears, allowing for the use of a motor capable of high-speed rotation. Since the carrier ripple current as a percentage of the motor current depends on the carrier frequency, the improvement in motor efficiency at partial load is significant. Figure 12 shows that for large motors with numerically limited specifications, the ripple current ratio falls below 100% when the carrier frequency exceeds 10 kHz. In other words, for high-capacity, high-speed motors where the product of the square root of the rated rotational speed [rpm] and rated power [kW] is large, increasing the carrier frequency to 10 kHz or higher significantly reduces the amount of heat generated by the motor and improves cooling efficiency.

[0041] Figure 13 is a table comparing various efficiencies of SiC elements and IGBT elements. Here, motor 11 is assumed to be a BPM motor. Figure 13 shows load factor, mechanical efficiency, motor efficiency, inverter efficiency, overall efficiency, cooling loss, and overall efficiency. Overall efficiency is the efficiency considering mechanical efficiency, motor efficiency, inverter efficiency, and cooling loss. Cooling loss is the cooling performance of motor 11. Overall efficiency is the efficiency of the entire chiller system 1.

[0042] Compared to IGBT elements, SiC elements offer improved motor efficiency, VFD efficiency, overall efficiency, cooling performance, and overall efficiency of the chiller system 1.

[0043] The chiller device in the above embodiment is water-cooled, but an air-cooled chiller device may also be used.

[0044] <Effects> The chiller device 1 in the first aspect includes a refrigerant circuit 70 including a compressor 10 with a built-in motor 11, a condenser 20, a throttle device 40, and an evaporator 50, a bypass circuit 72 branched from the refrigerant circuit 70, an inverter 14 for driving the motor 11, and a control unit 200. The motor 11 that drives the compressor 10 is cooled by the refrigerant flowing through the bypass circuit 72, the control unit 200 that controls the motor 11 operates the inverter 14, which includes a wide-bandgap semiconductor element, at a carrier frequency of 10 kHz or higher, and the motor 11 has a product of the square root of its rated rotational speed [rpm] and rated power [kW] of 180,000 or higher. In this case, a highly efficient chiller device can be provided that includes a motor with reduced heat generation and a heat exchanger with reduced heat transfer area. The chiller device 1 in the second aspect is the chiller device in the first aspect, wherein the motor 11 has a permanent magnet. In this case, a significant reduction in magnet iron loss due to an increase in carrier frequency can be achieved. The chiller device 1 in the third view is a chiller device in the first or second view, wherein the control unit 200 monitors or estimates the temperature of the stator 110, rotor 112 of the motor 11, or the magnetic bearing 120 of the compressor 10. In this case, the flow rate of the refrigerant or cooling water and the carrier frequency can be controlled based on the motor temperature. The chiller device 1 in the fourth aspect is the chiller device in the first to third aspects, wherein when the motor 11 is hot, the control unit 200 increases the flow rate of the refrigerant or cooling water, or increases the carrier frequency. In this case, efficient control can be achieved by performing feedback control to increase the flow rate of the refrigerant or cooling water, or to increase the carrier frequency, when the motor temperature increases. The chiller device 1 in the fifth aspect is the chiller device in the first to fourth aspects, wherein the motor 11 has a ratio of the air gap 114 between the rotor 112 and the stator 110 to the outer diameter 116 of the rotor 112 of the rotor 112 of 0.035 or less. In this case, the power factor is improved and heat transfer is improved. The chiller device 1 according to the sixth aspect is the chiller device according to the first to fifth aspects, wherein the control unit 200 sets the carrier frequency to 1.5 times or more the frequency at the rated load when the load is light (50% or less of the rated load). In this case, the degree of improvement in motor efficiency at light load can be increased. The chiller device 1 according to the seventh aspect is the chiller device according to the first to sixth aspects, wherein the carrier frequency is 20 kHz or higher. In this case, the iron loss of the motor 11 can be reduced by approximately 45%. The chiller device 1 according to the eighth aspect is the chiller device according to the first to sixth aspects, wherein the carrier frequency is 30 kHz or higher. In this case, the iron loss of the motor 11 can be reduced by approximately 24%. The chiller device 1 in the ninth aspect is the chiller device in the first to eighth aspects, wherein the wide bandgap semiconductor element is a SiC element. In this case, even if surges become severe due to high current and high frequency, high voltage resistance can be achieved with the SiC element. The chiller device 1 according to the tenth aspect is the chiller device according to the first to ninth aspects, wherein the device structure of the wide-bandgap semiconductor element is a MOSFET structure. In this case, the short-circuit capacity can be improved. The chiller device according to the eleventh aspect is a chiller device according to the first to tenth aspects, wherein the condenser is cooled with cooling water. In this case, a chiller device with higher efficiency than an air-cooled chiller device can be provided.

[0045] Although embodiments have been described above, the technical scope of this disclosure is not limited to the embodiments described above. It is clear from the claims that various modifications or improvements to the embodiments described above are also included in the technical scope of this disclosure. Furthermore, the order in which each operation in the control unit is executed is not limited to the order described in each of the embodiments described above, but may be changed individually. [Explanation of symbols]

[0046] 1...Chiller unit, 5...Compressor drive system, 10...Compressor, 11...Motor, 12...Control panel, 13...Power supply, 14...Inverter, 15...Internal power supply, 16...Magnet bearing controller, 20...Condenser, 30...Cooling tower, 32...Cooling water pump, 34...Cooling water pump drive motor, 36...First cooling circuit, 40...Throttling device (expansion valve), 50...Evaporator, 60...Load, 62...Chilled water pump, 64...Chilled water pump drive motor, 66...Second cooling circuit 70...Refrigerant circuit, 72...Bypass circuit, 110...Stator, 111...Rotor core, 112...Rotor, 114...Air gap, 115...Permanent magnet, 116...Rotor outer diameter, 117...Teeth, 118...Coil, 119...Protective sleeve, 120...Magnet bearing, 125...Shaft, 130...Impeller, 200...Control unit, f1~f3...Refrigerant flow in the refrigerant circuit, f4~f7...Cooling water flow, f8, f9...Refrigerant flow in the bypass circuit

Claims

1. A chiller device comprising a refrigerant circuit including a compressor, condenser, throttle device, and evaporator, each containing a motor; a bypass circuit branched from the refrigerant circuit; an inverter for driving the motor; and a control unit, wherein the motor for driving the compressor is cooled by the refrigerant flowing through the bypass circuit; the control unit for controlling the motor operates the inverter, which includes a wide-bandgap semiconductor element, at a carrier frequency of 10 kHz or higher; and the motor has a product of the square root of its rated rotational speed [rpm] and rated power [kW] of 180,000 or higher.

2. The chiller device according to claim 1, wherein the motor has a permanent magnet.

3. The chiller device according to claim 1 or 2, wherein the control unit monitors or estimates the temperature of the stator, rotor, or magnetic bearing of the motor.

4. When the motor is hot, the control unit increases the flow rate of the refrigerant or cooling water, or increases the carrier frequency. The chiller device according to claim 1 or 2.

5. The chiller device according to claim 1 or 2, wherein the motor has a ratio of the air gap between the rotor and the stator to the outer diameter of the rotor of 0.035 or less.

6. The chiller device according to claim 1 or 2, wherein the control unit sets the carrier frequency to 1.5 times or more the frequency at the rated load when the load is light (50% or less of the rated load).

7. The chiller device according to claim 1 or 2, wherein the carrier frequency is 20 kHz or higher.

8. The chiller device according to claim 1 or 2, wherein the carrier frequency is 30 kHz or higher.

9. The chiller apparatus according to claim 1 or 2, wherein the wide-bandgap semiconductor element is SiC.

10. The chiller apparatus according to claim 1 or 2, wherein the device structure of the wide-bandgap semiconductor element is a MOSFET structure.

11. The chiller apparatus according to claim 1 or 2, wherein the cooling circuit cools the condenser with cooling water.

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