Two-phase liquid-cooled alternating current (AC) rotating electrical equipment

Abstract

Two-phase liquid-cooled AC rotating electrical machines include multiple cooling circuits that recirculate respective working liquids around closed-loop fluid paths, transitioning them between liquid and vapor states, to cool multiple machine components, e.g., hollow conductor rotor and stator windings, and magnetic stator cores. A portion of each fluid path is integrated with or in thermal contact with the component. Each portion of the fluid path evaporates the working liquid, maintaining operating temperatures at or near its respective different phase transition temperatures.

Description

[Technical Field]

[0001] (Reference to Related Application) This patent application claims the benefit of priority to U.S. Patent Application No. 17 / 992,728, filed November 22, 2022, which is incorporated herein by reference in its entirety.

[0002] (Technical field) The present invention relates to a high performance cooling system for AC rotating electrical machinery (generators or motors), and more particularly to providing a thermal design for operating each component of the machinery at different temperatures and heat loads by applying a two-phase liquid cooling method using different process fluids such as fluoroketones (FKs), hydrofluoroethers (HFEs), perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), perfluorohexanes (PFHs), perfluoropolyethers (PFPEs), and chlorofluorocarbons (CFCs). [Background technology]

[0003] AC rotating electrical machines are based on Faraday's law, which predicts how a magnetic field interacts with an electric coil to generate an electromotive force (emf), which then produces or absorbs torque. An AC generator converts mechanical energy (e.g., the rotation of a rotor shaft by a prime mover) into electrical energy (e.g., an AC voltage developed across a stator). An AC motor converts electrical energy (e.g., AC power applied to a stator) into mechanical energy (e.g., the rotation of a rotor shaft, which is transmitted to a load).

[0004] In most AC electrical machines, the rotor is configured to rotate around the central axis of the stator. The rotor generates a magnetic field. The rotor may be a permanent magnet type or an electromagnet type, in which rotor windings are wrapped around a ferromagnetic material and energized to generate the magnetic field. The stator includes a stator core that supports one or more stator windings. The stator core is typically a magnetic stator core that generates a stationary electromagnetic field (EM field), but in "superconducting" winding configurations, the stator core may be an "air core" made from materials such as ceramic or fiberglass. Each rotor and stator winding is typically multi-phase (e.g., three-phase), but may also be single-phase. In an equivalent configuration, a stationary electromagnetic field (e.g., stator) is positioned along the central axis, causing the core and windings (e.g., rotor) to rotate.

[0005] AC electrical equipment generates heat. In these machines, heat is generated in the rotor and excitation winding of a wound field machine, and in the magnetic stator core and one or more stator windings. Heat can degrade system performance, damage or shorten the life of critical components by causing insulation breakdown in components, and even cause a fire.

[0006] Cooling systems are implemented to remove heat so that operating temperatures remain below a predetermined temperature. The capacity of a cooling system is determined by the heat load and the desired operating temperature. Cooling system designs are often based on the worst-case heat load or operating temperature within the system. Cooling systems take up valuable space within the system, consume energy to remove heat, and add to overall costs. New equipment generates more heat in a smaller volume, requiring more efficient cooling systems.

[0007] A variety of cooling systems exist to remove heat depending on space, energy, and cost requirements. Heat sinks are passive heat exchangers that transfer heat generated by a device to a fluid medium (often air or a coolant), dissipating the heat from the device. These may take the form of "moving air" or "recirculating immersion systems." A related approach is called the "recirculating integrated system," in which cooling water or hydrogen gas is recirculated directly through components such as hollow conductor rotor or stator windings, hollow conductor excitation windings, or around the stator core (through hollow tubes wound around the core or through a volume within a vessel surrounding the core) to absorb and remove heat. In each of these configurations, heat is transferred from the device to the fluid, thereby increasing its temperature, and the fluid is removed from the component. These components are typically connected in series within a single cooling loop. The cooling water or hydrogen gas first passes through the coldest operating component (e.g., the stator core), then through each successive component, and finally through the hottest operating component (e.g., the rotor windings) before returning to the cooling water reservoir. Accurate control of the operating temperatures of critical components is difficult. Summary of the Invention

[0008] The following is a summary of the invention to provide a basic understanding of some aspects of the disclosure. This summary is not intended to identify key or critical elements of the disclosure or to delineate the scope of the disclosure. Its sole purpose is to present some concepts of the disclosure in a simplified form as a prelude to the more detailed description and claim definitions that follow.

[0009] The present disclosure provides a two-phase liquid-cooled alternating current (AC) rotating electrical machine in which components (e.g., rotor or stator windings, magnetic stator cores, or exciter armature) are cooled by circulating one or more working fluids and vaporizing the working fluids to maintain the component temperatures at or near the phase transition temperatures of the working fluids. Two-phase liquid-vapor cooling can handle a larger heat load per unit volume than single-phase fluids. Furthermore, the use of series or parallel connections within one or more fluid paths recirculating working fluids exhibiting different transition temperatures allows for optimization of the cooling system, enabling higher power densities and better control of the operating temperatures of individual components. The working fluids should be chemically inert, thermally stable, non-toxic, have high dielectric strength, and are preferably commercially available. Suitable processing fluids are selected from fluorinated ketones (FK), hydrofluoroethers (HFE), perfluorocarbons (PFC), hydrofluorocarbons (HFC), perfluorohexanes (PFH), perfluoropolyethers (PFPE), and chlorofluorocarbons (CFC), depending on the application. In consideration of environmental issues such as global warming, FK and HFE may be preferably selected.

[0010] In one embodiment, a two-phase liquid-cooled AC electric machine includes a wound-field electric machine having a rotor with a hollow conductor rotor winding wound around a ferromagnetic material and a stator including one or more hollow conductor stator windings supported by a stator core. The rotor is configured to rotate about a central axis of the stator. A pair of independent cooling circuits are configured to absorb and cool the thermal load of the hollow conductor rotor and the stator winding, respectively. Each circuit includes a closed-loop fluid path through which a working liquid recirculates and transitions between a liquid state and a vapor state within the winding. The working liquid evaporates at a phase transition temperature to cool the winding and maintain the operating temperature of the winding at or near a first phase transition temperature. The working liquids in the cooling circuits exhibit different phase transition temperatures to accommodate the different thermal loads and operating temperatures of the rotor winding and the stator winding.

[0011] In another embodiment, the stator includes multiple hollow conductor stator windings. In one configuration, each stator winding is provided with its own cooling circuit, using a working fluid selected for its particular thermal load and operating temperature. In another configuration, where the stator windings exhibit similar thermal loads, the cooling circuit includes a 1:N inlet manifold that distributes working liquid to each stator winding in parallel, whereby the working liquid evaporates within the hollow conductor stator windings and maintains their respective operating temperatures at or near their transition temperatures. In another configuration, where the stator windings exhibit different thermal loads, the stator windings are connected in series within a single cooling circuit. The working liquid evaporates in the stator winding or windings with the greatest thermal load. The remaining stator windings may be located upstream, where they are cooled by the temperature increase of the working liquid in its liquid phase, or downstream, where they are cooled by the temperature increase of the working liquid in its vapor phase.

[0012] In another embodiment, the stator core is a magnetic stator core. In one configuration, the third cooling circuit is configured to recirculate and evaporate a third working liquid around the magnetic stator core (e.g., a hollow tube wound around the stator core or a volume around the stator core) to maintain the operating temperature of the magnetic stator core at or near its transition temperature. In another configuration, one of the cooling circuits includes an inlet manifold that distributes the working liquid in parallel to the stator windings and the magnetic stator core. In another configuration, the stator core is connected in series with the stator windings within a single cooling circuit. The stator core may be cooled by increasing the temperature of the working liquid in a liquid or vapor state. Alternatively, a portion of the working liquid may evaporate within the hollow conductor stator windings and around the stator core, thereby maintaining the operating temperatures of both at or near the phase transition temperature of the fluid.

[0013] In one embodiment, each cooling circuit includes, in a closed-loop fluid path, a pump for recirculating the processing liquid, a hydraulic pressure regulator for adjusting the pressure of the processing liquid, a vapor pressure regulator for adjusting the pressure of the processing liquid in its vapor state, and a condenser coupled to the cooling liquid reservoir for condensing the processing liquid vapor into processing liquid for recirculation. The cooling circuits are suitably connected in series or parallel to a common cooling liquid (usually water) reservoir. Each cooling circuit may also include a vapor pressure sensor for sensing vapor pressure and providing feedback to the hydraulic pressure regulator to control the flow rate of the processing liquid.

[0014] In one embodiment, the processing fluid is doped with a conductive solid granular media (e.g., aluminum shot). The pump includes an electromagnetic pump (e.g., a distributed AC induction coil) that generates a moving magnetic field within a closed-loop fluid path, which interacts with the conductive solid granular media to create a force that propels both the processing fluid and the media. The media also enhances the heat transfer capabilities of the processing fluid, providing more uniform cooling in the portion of the fluid path where evaporation occurs.

[0015] In another embodiment, a two-phase liquid-cooled AC electric machine includes a rotor (permanent magnet or electromagnet) providing a magnetic field, a stator including one or more hollow conductor stator windings supported by a magnetic stator core, and a pair of independent cooling circuits configured to absorb the heat load and cool the hollow conductor stator windings and the magnetic stator core, respectively.

[0016] In another embodiment, a two-phase liquid-cooled AC electric machine includes a rotor (permanent magnet or electromagnet) that provides a magnetic field, and a stator including one or more hollow conductor stator windings supported by a magnetic stator core. A cooling circuit is configured to recirculate a working liquid around the magnetic stator core and through the one or more hollow conductor stator windings, the liquid evaporating at a phase transition temperature to cool and maintain the operating temperature of at least one hollow conductor stator winding at or near the phase transition temperature, and to cool the remaining hollow conductor stator windings and the stator core.

[0017] These and other features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, which description is to be considered in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0018] [Figure 1A] 1 illustrates an embodiment of a two-phase liquid-cooled AC rotating electrical machine. [Figure 1B] 1 illustrates an embodiment of a two-phase liquid-cooled AC rotating electrical machine. [Figure 1C] 1 illustrates an embodiment of a two-phase liquid-cooled AC rotating electrical machine. [Figure 1D] 1 illustrates an embodiment of a two-phase liquid-cooled AC rotating electrical machine. [Figure 1E] 1 illustrates an embodiment of a two-phase liquid-cooled AC rotating electrical machine. [Figure 1F] 1 illustrates an embodiment of a two-phase liquid-cooled AC rotating electrical machine. [Figure 1G] 1 illustrates an embodiment of a two-phase liquid-cooled AC rotating electrical machine. [Figure 1H] 1 illustrates an embodiment of a two-phase liquid-cooled AC rotating electrical machine. [Figure 1I] 1 illustrates an embodiment of a two-phase liquid-cooled AC rotating electrical machine. [Figure 2] FIG. 1 illustrates a portion of a closed-loop fluid path in which a processing liquid flows in the liquid phase, absorbing heat and transferring it to the gas phase. [Figure 3] FIG. 1 is a diagram showing the relationship between the saturation temperature and specific energy of a fluorinated ketone in a saturated state and the vapor pressure. [Figure 4A] FIG. 1 illustrates a portion of a closed-loop fluid path in which a processing liquid containing a conductive solid granular medium flows in response to the interaction of the medium with traveling waves generated by distributed electromagnetic pumps, absorbing heat in the liquid phase and undergoing a phase transition to a gas phase. [Figure 4B] FIG. 1 illustrates a portion of a closed-loop fluid path in which a processing liquid containing a conductive solid granular medium flows in response to the interaction of the medium with traveling waves generated by distributed electromagnetic pumps, absorbing heat in the liquid phase and undergoing a phase transition to a gas phase. [Figure 5] FIG. 1 illustrates a closed-loop fluid path in which one or more hollow core stator windings are connected in series with the stator core. [Figure 6] FIG. 1 illustrates a closed-loop fluid path in which one or more hollow core stator windings and a stator core are connected in parallel. DETAILED DESCRIPTION OF THE INVENTION

[0019] Two-phase liquid-cooled alternating current (AC) rotating electrical machinery circulates one or more process fluids within the machinery and vaporizes them to cool components (e.g., rotor or stator windings, stator core, or exciter armature) and maintain the components at or near the phase transition temperature of the process fluid. Two-phase liquid-vapor cooling can accommodate much greater heat loads than single-phase fluids. Furthermore, the use of series or parallel connections within one or more fluid paths recirculating process fluids with different transition temperatures allows for optimization of the cooling system, enabling higher power densities and better control of the operating temperatures of individual components. The process fluid should be chemically inert, thermally stable, nontoxic, have high dielectric strength, and preferably be commercially available. Depending on the application, suitable process fluids may be selected from fluorinated ketones (FKs), hydrofluoroethers (HFEs), perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), perfluorohexanes (PFHs), perfluoropolyethers (PFPEs), and chlorofluorocarbons (CFCs). 3M® Company manufactures a family of process fluids under the trademark Novec® that are examples of fluorinated ketones (FKs) that exhibit the properties required for a wide variety of applications and are used in the embodiments described herein.

[0020] "Evaporative cooling" or "two-phase liquid cooling" utilizes the phase transition of a fluid from liquid to gas to accept heat. The main advantage of this method is that the phase change from liquid to gas at a fixed transition temperature has a much greater heat capacity, or latent heat of vaporization, than heating a liquid. When properly configured, these systems can provide equal or greater cooling capacity than traditional systems, while using less space, less energy, and at lower cost.

[0021] A key feature of two-phase liquid cooling systems is that the closed-loop fluid path allows AC rotating electrical equipment to be orientation independent. The equipment can be turned upside down, placed on its side, or rocked back and forth without affecting cooling performance. This is extremely important in applications that may be mounted on moving land vehicles, ships, or aircraft.

[0022] A closed-loop fluid path also improves coverage of hot component surfaces that require cooling. Passing the process liquid through hollow conductor rotor or stator coils or around the stator core provides uniform coverage of the entire surface, resulting in more uniform cooling of the component.

[0023] Without loss of generality, this disclosure describes wound-field AC rotating electric machines in which the rotor includes a rotor winding wound on a magnetic core and the stator includes the magnetic core. The same two-phase liquid cooling can also be applied to permanent magnet AC rotating electric machines in which the rotor includes permanent magnets. A closed-loop fluid path is used to cool the stator core and one or more stator windings. In certain embodiments in which the rotor winding is superconducting, the stator core is non-magnetic and does not require cooling.

[0024] 1A-1G illustrate one embodiment of a two-phase liquid-cooled wound-field AC rotating electrical machine 10 that can be configured as a motor or a generator. As a motor, rotor shaft 12 is coupled to a load to generate mechanical energy. As a generator, rotor shaft 12 is coupled to a prime mover to receive mechanical energy and convert it to electrical power.

[0025] 1A, the machine 10 includes a main field (rotor) 14 having a rotor winding 16 wound around a ferromagnetic material 18 attached to a rotor shaft 12, and a stationary main armature (stator) 20 having one or more stator windings 22 supported by a magnetic stator core 23. Each stator winding 22 is typically a multi-pole, multi-turn electrical coil. The rotor shaft 12 is configured to rotate about a central axis 24 of the stator 20.

[0026] A wound field exciter 30 provides DC excitation to the rotor windings 16. The exciter 30 includes an exciter armature (rotor) 32 that includes an AC coil 34 wound multiple times on the rotor shaft 12, and an exciter field (stator) 36 that includes an exciter field stator winding 38. The exciter field is stationary and is energized by DC current from a regulator 33 that is powered by an external DC power source 35. Normal synchronous operation is achieved with a fixed or slowly varying DC input for the excitation power. Pulsed operation can be achieved by increasing or decreasing the DC input.

[0027] The exciter armature (rotor) 32 rotates on the rotor shaft 12 with a multi-turn AC coil 34 connected to a rotating rectifier assembly 40, which provides an adjustable level of high power DC power to the machine's main field (rotor) 14. Induction from the main field (rotor) induces current in the main armature (stator) 20. The AC electrical output 42 of the main armature is typically polyphase, but may be single phase.

[0028] To cool the components of the machine 10, a working fluid is recirculated through the magnetic stator core 23, one or more stator windings 22, the rotor winding 16, and the exciter field stator winding 38. The working fluid evaporates, changing from a liquid to a vapor state, cooling the respective components and maintaining their operating temperatures at or near the phase transition temperature of the working fluid. The magnetic stator core 23 is surrounded by a circumferential cooling jacket 44, which includes a set of non-electrically energized hollow cooling tubes 46 wound in a circumferential or longitudinal layout. These are shown as closed-loop fluid path #1 50. In this system, all three elements may have electrically energized hollow conductors within their windings. Closed-loop fluid path #2 52 flows through the stator winding 22. Closed-loop fluid path #3 54 flows through the rotor winding 16. Closed-loop fluid path No. 4 56 flows through the hollow conductors within the exciter field stator winding 38. Thus, closed-loop fluid paths No. 2, No. 3, and No. 4 each have a fluoroketone or similar process fluid with high dielectric strength operating inside an electrically conductive conductor. Alternatively, closed-loop fluid path No. 4 may reside within a cooling jacket that is electrically isolated from the coil conductors within the exciter field, while being positioned for high heat transfer to the jacket. Because all four primary electrical circuits have different current densities and power losses due to the size and shape of the coils or the configuration of the magnetic stator core, it is advantageous to allow each closed-loop fluid path to have a different coolant temperature rise and / or latent heat of vaporization.

[0029] FIG. 1B shows a typical cross section of the main stator winding 22 and main rotor winding 16 of a synchronous machine 10, shown in a cross section perpendicular to the central axis 24, with a circumferential stator core cooling jacket 44. The cooling jacket 44 and its hollow cooling tubes 46 are electrically insulated from, but thermally coupled to, the stator magnetic core 23. Each subassembly's slots 60 are surrounded by a magnetic core 23 or 18, typically formed of laminated, high-permeability electrical steel. The stator winding 22 has, for example, 18 turns, and the rotor winding 16 has, for example, 14 turns, a hollow conductor in each slot, each filled with the target process fluid. Typically, these conductors are wound in series in each slot, although parallel windings are also practical. The conductor cross-sections may be circular, square, rectangular, or hexagonal and are typically fabricated from a copper alloy.

[0030] 1C, a 3D isometric view of stator winding 22 shows the electrically energized hollow conductor structure of stator winding 22. Working liquid 61 is introduced in a liquid state at fluid inlet 62, and electrical connection 64 is made to the hollow conductor near inlet 62. Similarly, working liquid is removed in a vapor state at fluid outlet 66, and electrical connection 68 is made to the hollow conductor near outlet 66. Hollow conductor stator winding 22 is optionally wrapped with insulating tape. Hollow conductor stator winding 22 is optionally wrapped with semiconductive tape 70 and conductive tape 72 over the insulating tape to distribute the electric field throughout the high voltage stator winding.

[0031] 1D, 1E, 1F, and 1G, one embodiment of a two-phase liquid cooling system 100 includes cooling circuits 102, 104, 106, and 108 connected to a recirculating coolant (typically water) path 110 for independently cooling and maintaining the operating temperatures of the stator core 23, one or more stator windings 22, rotor winding 16, and exciter field stator winding 38 at or near predetermined levels, respectively. Each cooling circuit recirculates a working liquid selected to have a phase transition temperature at or near the desired operating temperature of the respective equipment component.

[0032] In this embodiment, each cooling circuit is coupled to a recirculating coolant path 110 and includes a condenser 112 that phase - transfers the processing liquid 114 from a vapor state to a liquid state, a liquid pump 116, a hydraulic regulator 118, a device component (such as a rotor or stator winding), or a hollow tube 119 that is in direct thermal contact with the device component (such as a jacket around the stator core) where the processing liquid 114 evaporates at its transition temperature, a vapor pressure sensor 120 configured to measure the vapor pressure and supply a control signal to the hydraulic regulator 118 to control the flow rate, and a vapor pressure regulator 122 for maintaining an appropriate vapor pressure for a specific vapor transition temperature, and a return path to the condenser 112, forming a closed - loop fluid path. The processing liquid 114 enters the hollow tube as a liquid at a relatively low inlet temperature, absorbs heat therein, its temperature rises, and it evaporates at the transition temperature. The temperature of the vapor may rise or fall when passing through the vapor pressure regulator when returning to the condenser 112.

[0033] The recirculation liquid path 110 includes a coolant reservoir 130, a pump 132, the condensers 112 of each cooling circuit, and a heat exchanger 134 that removes the heat injected from each cooling circuit and returns the liquid to the coolant reservoir 130. It can be understood that the temperatures are T1 < T2 < T3 < T4 < T5. The temperature of the cooling water supplied from the reservoir may be T1 = 10°C. Each cooling circuit may raise the water temperature by 5 - 10°C, for example, through the condenser 112. Alternatively, the flow paths can be connected in parallel to a common coolant reservoir or to dedicated coolant reservoirs respectively.

[0034] In a particular embodiment, as shown in FIGS. 1D - 1E, Table 140 of the design parameters of a specific wound - field AC rotating machine in FIG. 1H, and Table 142 of the design parameters of the 3M (registered trademark) Novec (registered trademark) processing liquid in FIG. 1I, different processing liquids 114 are recirculated through each cooling circuit to hold the operating temperatures of the stator core 23, stator winding 22, rotor winding 16, and the field - excitation stator winding 38 at independently specified values.

[0035] In the cooling circuit 102, a suitable working liquid 114 is Novec® 649 from 3M®, which has a transition temperature of 49°C and a latent heat of vaporization of 88 kJ / kg. Novec® 649 has a typical inlet temperature T6 of 19°C when it enters the jacket 44 as a liquid. As the Novec® 649 passes through the jacket 44 in the liquid state, it absorbs heat Q1 and cools the stator core 23. The Novec® 649 vaporizes at a vapor transition temperature T7 of 49°C, increasing its temperature by 30°C, maintaining the operating temperature of the stator core 23 at or near 49°C.

[0036] In the cooling circuit 104, a suitable working liquid 114 is Novec® 7200 from 3M®, which has a transition temperature of 76°C and a latent heat of vaporization of 119 kJ / kg. Novec® 7200 has a typical inlet temperature T8 = 61°C when it enters the stator windings 22 as a liquid. As Novec® 7200 passes through the stator windings 22 in its liquid state, it absorbs heat Q2, cooling the stator windings 22. Novec® 7200 vaporizes at a vapor transition temperature T9 = 76°C, increasing its temperature by 15°C, maintaining the operating temperature of the stator windings 22 at or near 76°C.

[0037] In the cooling circuit 106, a suitable working liquid 114 is Novec® 7500 from 3M®, which has a transition temperature of 128°C and a latent heat of vaporization of 89 kJ / kg. Novec® 7500 has a typical inlet temperature T10 = 103°C when it enters the rotor windings 16 as a liquid. As Novec® 7500 passes through the rotor windings 16 in its liquid state, it absorbs heat Q3, cooling the rotor windings 16. Novec® 7500 vaporizes at a vapor transition temperature T11 = 128°C with a temperature rise of 25°C, maintaining the operating temperature of the rotor windings 16 at or near 128°C.

[0038] In the cooling circuit 108, a suitable working liquid 114 is Novec® 7300 from 3M®, which has a transition temperature of 98°C and a latent heat of vaporization of 102 kJ / kg. Novec® 7300 has a typical inlet temperature T12 = 63°C when it enters the exciter armature stator windings 38 as a liquid. As Novec® 7300 passes through the exciter armature stator windings 38 in its liquid state, it absorbs heat Q4, cooling the windings. The Novec® 7300 heats up and vaporizes at its vapor transition temperature T13 = 98°C, maintaining the operating temperature of the exciter armature stator windings 38 at or near 98°C.

[0039] FIG. 2 shows an electrical hollow conductor 200 with an inlet manifold 201 and an outlet manifold 206 through which an electric current 203 is injected or excited and through which a working liquid 202 is also transported. The working liquid enters the inlet 201 and flows through the hollow conductor, which may also be an electrical metallic conductor such as those forming the stator or rotor windings of an electrical machine. At some distance along the conductor, when the accumulated heat exceeds the latent heat of vaporization (e.g., 119 kJ / kg when 3M® Novec® 7200 is used), the working liquid 202 transforms into a vapor 204 within the conductor at the vapor transition point and exits as a vapor through the outlet 206. The exact location of the vapor transition point has been found to vary depending on the actual electrical load and the resulting power dissipation losses within the conductor. From a design perspective, it is desirable for the vapor transition point, with maximum power dissipation, to be within the last half of the hollow conductor's path. A single conductor may be repeated multiple times in either the primary or secondary winding structure to allow for multiple winding turns, as is common practice in which coils are electrically connected in series or electrically connected in parallel.

[0040] Also, as shown in Figure 3, for the specific fluid Novec® No. 649, a temperature-dependent vapor pressure 220 develops within the hollow conductor, exhibiting a vapor pressure range of 1 to 4 bar at saturation temperatures 224 ranging from 49°C to 96°C. Figure 3 also shows that the latent heat of vaporization 222 decreases slightly as the vapor pressure increases, but this is within acceptable limits.

[0041] 4A-4B, one embodiment of a two-phase liquid cooling system 400 includes a working liquid 402 containing an electrically conductive solid granular medium 414. The liquid is introduced into a hollow electrical conductor 403 at an inlet 410, vaporizes within the hollow electrical conductor, and exits at an outlet 420, where it is condensed and recirculated. An electric current 413 may be injected or excited to flow through the hollow electrical conductor. The medium 414 may be used to increase overall cooling capacity, improve cooling uniformity on the hollow electrical conductor 403, and promote movement of the working liquid 402 within the flow path. The medium 414 is suitable for both electrical and thermal conductivity; suitable materials are aluminum or magnesium alloys, which are lightweight, highly thermally conductive materials that can be preferably formed into small spherical balls or "shots." The medium 414 is designed to accept the heat content from the hollow electrical conductor but not melt. Before entering system 400, a medium 414 is added to the processing liquid stream, which continues to flow throughout the system and exit through outlet 420. Medium 414 accepts (and removes) heat from the first half of the hollow electrical conductor before vaporizing, thereby improving the distribution of heat acceptance and cooling the conductor more uniformly. The conductive medium is small enough, e.g., 0.065 inches in diameter, to pass through the liquid pump, vapor pressure sensor, liquid pressure sensor, and condenser shown in Figures 1D-1E, and is constantly recirculated. Excess heat contained in conductive medium 414 can be efficiently transferred via the condenser to the external cooling liquid reservoir in Figure 1.

[0042] An electromagnetic pump 412 can be used to propel the medium 414 and working liquid 402 through the hollow electrical conductor 403. The electromagnetic pump 412, such as a coaxial electromagnetic pump, induces magnetic waves traveling through the gap 422 between the walls of the hollow electrical conductor, generating eddy currents 425 in the medium 414. The medium 414 is propelled by the cross product of the eddy currents 425 induced by the electromagnetic pump's alternating magnetic field B 427 and the strength of the B magnetic field, as shown in Figure 4B. In a preferred embodiment, the B magnetic field within the hollow conductor may have a strength of 1.5 to 1.7 Tesla. The conductive medium 414 absorbs heat from the electrical conductor, and this heat is first transferred to the fluid 402 in the first portion of the coolant path. The electromagnetic pump need not be continuous; as shown in Figure 4A, the pump can be segmented to allow for bending and curvature of the electrical hollow conductor along its actual winding.

[0043] If the capacity of the electromagnetic pump 412 is sufficient and a sufficient amount of conductive medium 414 is used as the primary transport means, the conventional type mechanical liquid pump 106 or 120 shown in Figure 1 can be replaced by only the electromagnetic pump 412. The vapor transition point within the hollow conductor preferably occurs in the latter half of the entire flow path.

[0044] To design an optimal cooling system for a particular configuration of AC rotating electrical machinery, multiple equipment components (e.g., stator core, stator winding(s), rotor winding, exciter field stator winding) may be connected in series and / or parallel within a particular cooling circuit, depending on the operating temperature and heat load requirements of the individual components. Generally, equipment components with low heat loads can be served by increasing the temperature of the process liquid in either the liquid or vapor state and may not require the heat capacity associated with a phase change. Depending on the desired operating temperature, components may be located in either the cold liquid or hot vapor portion of the flow path. Evaporation may occur with multiple equipment components in parallel or series to serve components with higher heat loads. A given cooling circuit may be configured using a combination of series and parallel connections.

[0045] As shown in FIG. 5, cooling circuit 500 recirculates working liquid 114 through jacket 502, which is in direct thermal contact with stator core 504, and through a series connection of four different stator windings 506A-506D. Each component has a predetermined operating temperature (or a temperature not exceeding the operating temperature) and a heat load that must be absorbed by cooling circuit 500 to maintain that operating temperature. In this example, working liquid 114 enters jacket 502 as a liquid at temperature T3, absorbs heat Q1, raising the liquid temperature to T4, and then enters stator winding 506A, absorbing heat Q2, further raising the liquid temperature to T5. The working liquid passes through stator winding 506A, where it absorbs heat Q3, partially evaporating the liquid. Stator winding 506C absorbs heat Q4, completing evaporation of the working liquid at a phase transition temperature T6. The vapor then passes through stator winding 506D, absorbing heat Q5, raising the vapor temperature to T7. The condenser 112 removes the accumulated absorbed heat and converts the vapor back to a lower temperature liquid state. Series connections are particularly useful for meeting the cooling requirements of equipment components with different heat loads and predetermined operating temperatures.

[0046] As shown in FIG. 6 , the cooling circuit 600 recirculates the working liquid 114 through a jacket 602 in direct thermal contact with the stator core 604 and a parallel connection of N different stator windings 606A-606N. Each component has a predetermined operating temperature (or a temperature not exceeding the operating temperature) and a heat load that must be absorbed by the cooling circuit 600 to maintain that operating temperature. Parallel connections are particularly useful for accommodating the cooling requirements of equipment components with similar heat loads and maximum operating temperatures. In this example, the working liquid 114 enters an input manifold 608, which splits the liquid at temperature T3 into N different streams (usually, but not necessarily, equal in volume) connected to the N stator windings 606A-606N and the jacket 602. Within each equipment component, sufficient heat is absorbed to at least partially vaporize the working liquid 114 at a phase transition temperature T4. An output manifold 610 combines the at least partially vaporized streams into a single stream that is delivered to the vapor pressure sensor 120. The condenser 112 removes the accumulated absorbed heat and converts the vapor back to a lower temperature liquid state.

[0047] While several illustrative embodiments of the present disclosure have been shown and described, numerous variations and alternative embodiments will occur to those skilled in the art. Such variations and alternative embodiments are contemplated and can be made without departing from the scope of the present disclosure, which is defined in the appended claims.

Claims

1. a wound field electric machine comprising: a rotor having a hollow conductor rotor winding wound around a ferromagnetic material; and a stator including one or more hollow conductor stator windings supported by a stator core, the rotor configured to rotate about a central axis of the stator; a first cooling circuit including a first closed-loop fluid path in which a first working liquid selected from fluorinated ketones (FKs), hydrofluoroethers (HFEs), perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), perfluorohexanes (PFHs), perfluoropolyethers (PFPEs), and chlorofluorocarbons (CFCs) is recirculated through the hollow conductor rotor windings, where the first working liquid evaporates at a first phase transition temperature to cool the hollow conductor rotor windings and maintain their operating temperature at or near the first phase transition temperature; a second cooling circuit comprising a second closed-loop fluid path in which a second working liquid selected from FK, HFE, PFC, HFC, PFH, PFPE, and CFC is recirculated through the one or more hollow conductor stator windings, where the second working liquid evaporates within the one or more stator windings at a second phase transition temperature to cool the one or more hollow conductor stator windings and maintain their operating temperature at or near the second phase transition temperature; the first and second processing liquids exhibit different first and second phase transition temperatures and different latent heats of vaporization; Two-phase liquid-cooled AC rotating electrical machinery.

2. 2. The two-phase liquid-cooled AC rotating electric machine of claim 1, wherein the first phase transition temperature and the operating temperature of the hollow conductor rotor winding are greater than the second phase transition temperature and the operating temperature of the one or more hollow conductor stator windings.

3. 2. The two-phase liquid-cooled AC rotating electric machine of claim 1, wherein the first and second cooling circuits each include a pump for recirculating the working liquid, a liquid pressure regulator for adjusting the pressure of the working liquid, a vapor pressure sensor for sensing vapor pressure and feeding it back to the liquid pressure regulator to control the flow rate of the working liquid, a vapor pressure regulator for adjusting the vapor pressure of the working liquid in a vapor state, and a condenser connected to a common cooling liquid reservoir for condensing the vapor of the working liquid into the working liquid for recirculation.

4. 2. The two-phase liquid-cooled AC rotating electrical machine of claim 1, wherein the first and second working fluids each include a conductive solid granular medium, and the first and second cooling circuits each include an electromagnetic pump that generates a moving magnetic field within the closed-loop fluid path, the moving magnetic field interacting with the conductive solid granular medium to generate an electrodynamic force that propels the working fluid and the conductive solid granular medium, the conductive solid granular medium enhancing heat transfer from the hollow conductor windings to evaporation of the working liquid and more uniformly cooling the hollow conductor windings.

5. 2. The two-phase liquid-cooled AC rotating electric machine of claim 1, wherein the wound field electric machine comprises a plurality, N, of hollow conductor stator windings, and the second cooling circuit further comprises: a 1:N inlet manifold that distributes the second working liquid to flow and evaporate in parallel within the N hollow conductor stator windings to maintain each of the stator windings at or near the second phase transition temperature; and an N:1 outlet manifold that collects and condenses vapor from the hollow conductor stator windings back into a liquid state for recirculation.

6. 2. The two-phase liquid-cooled AC rotating electric machine of claim 1, wherein the wound field electric machine comprises a plurality, N, of hollow conductor stator windings exhibiting different heat loads, the N hollow conductor stator windings being connected in series in the second closed-loop fluid path, and wherein the second working liquid evaporates in at least one of the stator windings exhibiting a greatest heat load, and the remainder of the stator windings are cooled by either evaporation of a remaining portion of the second working liquid or by an increase in temperature of the second working liquid in either a liquid or vapor state.

7. 2. The two-phase liquid-cooled AC rotating electric machine of claim 1, wherein the stator core is a magnetic stator core, and further comprising a third cooling circuit having a third closed-loop fluid path, in which a third working liquid selected from FK, HFE, PFC, HFC, PFH, PFPE, and CFC is recirculated around the magnetic stator, the third working liquid evaporating at a third phase transition temperature to cool the magnetic stator core and maintain its operating temperature at or near the third phase transition temperature, the third phase transition temperature being different from the first and second phase transition temperatures and the latent heat of vaporization.

8. 2. The two-phase liquid-cooled AC rotating electric machine of claim 1, wherein the stator core is a magnetic stator core, and the second cooling circuit further includes an inlet manifold that distributes the second working liquid to flow in parallel through the hollow conductor stator windings and around the stator core, evaporating within the hollow conductor stator windings and around the stator core to maintain the stator windings and stator core at or near the second phase transition temperature, and an outlet manifold that collects vapor from around the hollow conductor stator windings and stator core and condenses it to a liquid state for recirculation.

9. 2. The two-phase liquid-cooled AC rotating electric machine of claim 1, wherein the stator core is a magnetic stator core, and the stator core is connected in series with the hollow conductor stator windings in the second closed-loop fluid path, such that the second working liquid evaporates within the hollow conductor stator windings and the stator core is cooled by one of evaporating a remaining portion of the second working liquid or increasing the temperature of the second working liquid in either a liquid state or a vapor state.

10. an electric machine comprising: a rotor providing a magnetic field; and a stator including one or more hollow conductor stator windings supported by a magnetic stator core, the rotor configured to rotate about a central axis of the stator; a first cooling circuit including a first closed-loop fluid path in which a first working liquid selected from fluorinated ketones (FK), hydrofluoroethers (HFE), perfluorocarbons (PFC), hydrofluorocarbons (HFC), perfluorohexanes (PFH), perfluoropolyethers (PFPE), and chlorofluorocarbons (CFC) is recirculated around the magnetic stator core, the first working liquid evaporating at a first phase transition temperature to cool the magnetic stator core and maintain its operating temperature at or near the first phase transition temperature; a second cooling circuit comprising a second closed-loop fluid path in which a second working liquid selected from FK, HFE, PFC, HFC, PFH, PFPE, and CFC is recirculated through the one or more hollow conductor stator windings, and in which the second working liquid evaporates at a second phase transition temperature within the one or more stator windings to cool the one or more hollow conductor stator windings and maintain their operating temperature at or near the second phase transition temperature; the first and second processing liquids exhibit different first and second phase transition temperatures and different latent heats of vaporization; Two-phase liquid-cooled AC rotating electrical machinery.

11. 11. The two-phase liquid-cooled AC rotating electric machine of claim 10, wherein the first phase transition temperature and the operating temperature of the magnetic stator core are lower than the second phase transition temperature and the operating temperature of the one or more hollow conductor stator windings.

12. 11. The two-phase liquid-cooled AC rotating electric machine of claim 10, wherein the first working fluid is recirculated either in a space within a vessel disposed around the magnetic stator core or within a hollow tube wound around the magnetic stator core.

13. 11. The two-phase liquid-cooled AC rotating electric machine of claim 10, wherein the wound field electric machine comprises a plurality, N, of hollow conductor stator windings exhibiting similar heat loads, and the second cooling circuit further comprises: a 1:N inlet manifold for distributing the second working fluid in parallel through the N hollow conductor stator windings for flow and evaporation to maintain each of the stator windings at or near the second phase transition temperature; and an N:1 outlet manifold for collecting vapor from the hollow conductor stator windings, condensing it to a liquid state, and recirculating it.

14. 11. The two-phase liquid-cooled AC rotating electric machine of claim 10, wherein the wound field electric machine comprises a plurality, N, of hollow conductor stator windings exhibiting different heat loads, the N hollow conductor stator windings being connected in series in the second closed-loop fluid path, and wherein the second working liquid is evaporated in at least one of the stator windings exhibiting the highest heat load and the remainder of the stator windings are cooled by one of evaporation of a remaining portion of the second working liquid or an increase in temperature of the second working liquid in either a liquid or vapor state.

15. 11. The two-phase liquid-cooled AC rotating electrical machine of claim 10, wherein a two-phase liquid cooling system with a closed-loop fluid path allows the AC rotating electrical machine to be orientation independent.

16. 11. The two-phase liquid-cooled AC rotating electric machine of claim 10, wherein the first and second cooling circuits each include a pump for recirculating the working liquid, a liquid pressure regulator for adjusting the pressure of the liquid, a vapor pressure sensor for sensing vapor pressure and feeding it back to the liquid pressure regulator to control the flow rate of the liquid, a vapor pressure regulator for adjusting the vapor pressure of the working liquid in a vapor state, and a condenser connected to a common cooling liquid reservoir for condensing the vapor of the working liquid into the working liquid for recirculation.

17. 11. The two-phase liquid-cooled AC rotating electric machine of claim 10, wherein the first and second working fluids each include a conductive solid granular medium, and the first and second cooling circuits each include an electromagnetic pump that generates a moving magnetic field within the closed-loop fluid path, the moving magnetic field interacting with the conductive solid granular medium to generate an electrodynamic force that propels the working fluid and the conductive solid granular medium, the conductive solid granular medium enhancing heat transfer from the hollow conductor windings to evaporation of the working liquid and more uniformly cooling the hollow conductor windings.

18. an electric machine comprising: a rotor providing a magnetic field; and a stator including one or more hollow conductor stator windings supported by a magnetic stator core, the rotor configured to rotate about a central axis of the stator; a cooling circuit comprising a closed-loop fluid path in which a working liquid selected from fluorinated ketones (FKs), hydrofluoroethers (HFEs), perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), perfluorohexanes (PFHs), perfluoropolyethers (PFPEs), and chlorofluorocarbons (CFCs) is recirculated around the magnetic stator core and through the one or more hollow conductor stator windings, the working liquid evaporating at a phase transition temperature to cool at least one of the hollow conductor stator windings and maintain its operating temperature at or near the phase transition temperature, and to cool the remainder of the hollow conductor stator windings and the stator core. Two-phase liquid-cooled AC electrical equipment.

19. 20. The two-phase liquid-cooled alternating current electric machine of claim 18, wherein the cooling circuit further includes an inlet manifold that distributes the working liquid to flow in parallel through the at least one hollow conductor stator winding and around the stator core, evaporating within the at least one hollow conductor stator winding and around the stator core to maintain the hollow conductor stator winding and stator core at or near the phase transition temperature, and an outlet manifold that collects vapor from around the at least one hollow conductor stator winding and stator core and condenses it to a liquid state for recirculation.

20. 20. The two-phase liquid-cooled AC electric machine of claim 18, wherein the magnetic stator core is connected in series with the one or more hollow core stator windings in the closed-loop fluid path whereby the working liquid evaporates within the at least one hollow conductor stator winding and the stator core is cooled by one of evaporation of a remaining portion of the working liquid or a temperature increase of the working liquid in its liquid or vapor state.

Images