Abstract:
Peak performance of an electric motor can be enhanced by effective cooling of windings of the stator to avoid overheating. A liquid coolant is effective at cooling the stator; but in high-speed motors, it is advisable to avoid allowing coolant on the rotor to avoid high frictional losses. A shield provided in the air gap between the rotor and the stator guides the coolant back to a sump without gaining access to the rotor. Furthermore, if the electric machine is proximate a high temperature component, the shield may further prevent radiative and conductive heat transfer to the electric machine.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority benefit from U.S. provisional patent application 61/692,727 filed 24 Aug. 2012. 
     
    
     FIELD 
       [0002]    The present disclosure relates to cooling electric motors, particularly high-speed motors coupled to turbomachines. 
       BACKGROUND 
       [0003]    The peak performance of an electric machine can be enhanced by effective cooling of windings of the stator. Heat is generated predominantly in the windings of the stator. Eddy currents in the rotor are high, but contribute very little to thermal losses, thus reducing the need for forced cooling. Often a liquid coolant is employed to extract heat from the stator. In conventional electric motors, the coolant may contact the rotor with little consequence. However, in very high speed motors, such as an electric motor coupled to a turbomachine, in which the speeds can approach 350,000 rpm, it is desirable to avoid oil contacting the rotor to avoid high losses due to a high shear rate of the coolant. A system and method to provide liquid coolant onto the windings of the stator, while avoiding coolant contact with the rotor, is sought. 
         [0004]    In systems in which the coolant is a lubricant that is also provided to bearings associated with the electric motor or turbomachine, lubrication of the bearings should be maintained at all times during operation to maintain the system&#39;s integrity. 
         [0005]    If the turbomachine associated with the electric motor operates at high temperature, another contributor to high temperatures in the electric machine is due to heat transfer, primarily radiation, from hot components of the turbomachine to the electric motor. 
       SUMMARY 
       [0006]    To overcome at least one problem in the prior art, an electronically-controlled turbocharger (ECT) is disclosed that includes: a turbocharger shaft onto which are mounted a turbine wheel and a compressor wheel and an electric machine disposed between the turbine wheel and the compressor wheel. The electric machine has a rotor coupled to the shaft, a stator concentrically arranged around the rotor, and a plurality of cores made up of a plurality of laminations with a plurality of coils wrapped around each plurality of laminations. An air gap is provided between the rotor and the stator. The electric machine includes a shield having at least a hollow cylindrical portion disposed in the air gap between the rotor and the stator. The cylindrical portion of the shield is comprised of a material having low electromagnet permeability. The shield may further include a first end cap affixed to a first end of the cylindrical portion and a second end cap affixed to a second end of the cylindrical portion. The cylindrical portion is disposed proximate the stator with the remaining air gap disposed between the cylindrical portion and the rotor. The cylindrical portion is thinner than the end caps. The end caps are affixed to the cylindrical portion via one of: friction welding, welding, soldering, gluing, threads, and snapping. At least a portion of surfaces of the end caps that face away from the cylindrical portion may be coated with an insulating material such as ceramic. 
         [0007]    The ECT further includes a housing into which the electric machine is mounted with a pressurized coolant supply passage defined in the housing and an opening in an outer surface of the housing, a coolant manifold defined in the housing and fluidly coupled to the pressurized coolant supply passage, and a first coolant passage fluidly coupled to the coolant manifold and direct flow of coolant to the coils. The shield substantially prevents coolant from contacting the rotor. A drain is further defined in the housing. The shield substantially directing coolant toward the drain. In one embodiment, the end caps substantially form bell mouths. 
         [0008]    A method to assemble an electric machine is disclosed that includes: assembling a stator, affixing a first end cap to a first end of a cylindrical sleeve, inserting the cylindrical sleeve with the stator, affixing a second end cap to a second end of the cylindrical sleeve, assembling a rotor, and inserting the rotor into the cylindrical sleeve. The method may further include coating a convex surface at least one of the end caps. 
         [0009]    The end caps are affixed to the cylindrical sleeve by one of: welding, friction welding, gluing, threading, and snapping. 
         [0010]    Also disclosed is a high-speed electric motor that includes a motor housing, a stator mounted in the motor housing, a rotor disposed within the stator with an air gap separating the stator and the rotor, first and second bearings disposed on first and second ends of the rotor (with the first and second bearing supported in the motor housing), and a liquid cooling apparatus. The liquid cooling apparatus includes: a pressurized coolant supply orifice defined in the motor housing and a plurality of coolant passages defined in the motor housing to direct flow to the stator; and a shield provided to substantially prevent coolant from contacting the rotor. 
         [0011]    The shield is made up of a hollow cylindrical portion that is disposed in the air gap separating the stator and the rotor, a first end cap affixed to a first end of the cylindrical portion, and a second end cap affixed to a second end of the cylindrical portion. The end caps may be in the shape of bell mouths. The cylindrical portion is substantially thinner than the first and second end caps. The cylindrical portion of the shield is comprised of a material having low electromagnet permeability. The cylindrical portion is disposed proximate the stator with the remaining air gap disposed between the cylindrical portion and the rotor. The end caps are affixed to the cylindrical portion via one of: friction welding, welding, soldering, gluing, threading, and snapping. In some embodiments, at least a portion of surfaces of the end caps that face away from the cylindrical portion are coated with an insulating material. Throughout the disclosure the commonly-used term, electric motor, may be used to mean electric machine, i.e., a device that can be operated both as a motor and as a generator. 
         [0012]    Advantages, according to embodiments of the disclosure, are that liquid cooling is provided to the stator, but the shield prevents coolant from getting to the rotor and the end caps of the shield introduce a heat transfer barrier from hotter components, such as an exhaust turbine, to the electric machine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic representation of an engine system having an electronically-controlled turbocharger (ECT); 
           [0014]      FIG. 2  is a cross-sectional illustration of an ECT; 
           [0015]      FIG. 3  is a cross sectional illustration of an electric motor associated with an ECT with the cross section taken perpendicular to the axis of the motor; 
           [0016]      FIG. 4  is a cross sectional illustration of the electric motor taken along the axis of the motor; 
           [0017]      FIG. 5  is a cross-sectional illustration of the shield in an expanded view; 
           [0018]      FIG. 6  is a cross-sectional illustration of the shield in an assembled view; 
           [0019]      FIG. 7A  is an isometric view of the stator of the electric machine and the shield in an expanded view; 
           [0020]      FIG. 7B  is an isometric view of the stator of the electric machine and the shield as assembled; 
           [0021]      FIG. 8  is a flowchart depicting one embodiment of assembling the shield within the electric machine; and 
           [0022]      FIG. 9  illustrates a strategy to control current to the electric machine. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce alternative embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desired for particular applications or implementations. Those of ordinary skill in the art may recognize similar applications or implementations whether or not explicitly described or illustrated. 
         [0024]    An internal combustion engine  10  having an electronically controlled turbocharger (ECT)  12 , a type of turbomachine, is represented schematically in  FIG. 1 . ECT  12  includes: a compressor  14  that compresses intake gases supplied to engine  10 ; a turbine  16  that extracts energy from exhaust gases from engine  10 ; a shaft  18  that couples compressor  14  with turbine  16 ; and an electric machine (or motor)  20  that drives, or may be driven by, shaft  18 . 
         [0025]    Engine  10  has an oil pump  30  to lubricate and cool the engine as well as supplying oil to: electric motor  20  and bearings associated with ECT  12  and turbine shaft  16 . Oil returning to engine  10  drains to sump  28  wherein it is picked up by oil pump  30  to be pressurized and provided to oil passages in engine  10  and ECT  12 . 
         [0026]    An electronic control unit  32  receives signals from various sensors  36  and receives signals from and provides signals to various actuators  34 . ECU  32  also provides signals to actuators on engine  10  and a power electronics module  38  that provides current to electric motor  20  of ECT  20  and receives signals from sensors on engine  10  and ECT  20  and others. A single ECU  32  is shown; alternatively, distributed computing using a plurality of ECUs is used. For example, sensors  36  may include an oil pressure sensor within engine  10  and/or located at the inlet to ECT  20 , a temperature sensor located proximate coils of the electric motor or at an outlet of ECT  20 , as examples. Furthermore, based on models of the system, temperatures, pressures, and other parameters can be estimated based on a minimum set of sensor signals and actuator signals. For example, if temperature within the coils of the stator is sought, the flow of oil to the stator for cooling, the temperature of the oil to and from the stator, the current command to the electric machine, and a heat transfer model of the system can be employed to determine the temperature. The present description is one non-limiting example of how a particular temperature, pressure, or other condition can be determined based on a combination of sensor information, actuator information, and a model (or, alternatively, a lookup table). 
         [0027]    A cross section of an ECT  40  is shown in  FIG. 2 . The ECT includes a compressor section  50 , an electric machine section  52 , and a turbine section  54 . Coupled to a common shaft  60  are: a compressor wheel  62  fixed axially by nut  64 , a rotor  66  of the electric machine, and a turbine wheel  68  (welded). Alternatively, turbine wheel  68  may be threaded onto shaft  60 . Additional detail concerning the components that make up rotor  66  is provided in the description related to  FIG. 4 . The embodiment in  FIG. 2  includes four housing sections that are coupled together: a compressor housing section  70 , two electric machine housing sections  72  and  73 , and a turbine housing section  74 . (In an embodiment without a turbomachine, i.e., just a high-speed electric machine, the housing for the motor may include fewer sections.) Rotating shaft  60  is supported in the housings by bearings  76  and  78 . A thrust bearing  58  is provided between the compressor and the housing. An electrical connector  56 , which couples with high power electronics (not shown), exits ECT  40 . 
         [0028]    In the embodiment in  FIG. 2 , lubricant is used as the coolant for the electric motor. Thus, the lubrication system and the cooling system are integrated. Alternatively, the two systems are separated, which allows different fluids to be used in the systems. 
         [0029]    Pressurized lubricant, which is engine oil in one embodiment, is provided to ECT  40  through inlet  80 . Oil from inlet  80  fills manifold  82 . Manifold  82  is fluidly coupled to oil passages  84  and  86  with passage  84  providing lubricant to bearings  76  and  78  and passage  86  providing lubricant to bearing  78 . A plug  85  is provided at the outside end of passage  84  to seal off the drilling to form passage  84 . 
         [0030]    Manifold  82  is also fluidly coupled to check valves  92 ,  94 , and  96 . When pressure in manifold  82  exceeds the opening pressure of the check valve, the check valve opens to allow flow through the check valve. The outlet side of valve  92  directs oil onto a first end  98  of windings of the electric machine; the outlet side of valve  94  directs oil to an oil gallery  100 , and the outlet side of valve  96  directs oil onto a second end  102  of the windings. Gallery  100  is shown as a groove in a back iron  108  of the stator. Gallery  100  is contained between housing  72  and a groove in the back iron  108 . Alternatively, a groove is provided in housing  72  with the outer surface of back iron  108  being without a groove. 
         [0031]    Check valves  92 ,  94 , and  96  ensure that when oil pressure provided to ECT  40  is lower than the opening pressure, that oil is not directed away from bearings  58 ,  76 , and  78 . That is, bearings  58 ,  76 , and  78  receive priority lubrication. When pressure in manifold  82  is higher than the opening pressure, there is sufficient pressure in the system to provide cooling to the electric machine without negatively impacting the bearings. In the above discussion, the implication is that the opening pressure in each of check valves  92 ,  94 , and  96  is the same. The opening pressures may be purposely set slightly different so that oil to the bearings is affected in a stepwise fashion. In another situation, the check valve opening pressures may be different due to manufacturing tolerances and effects that come into play during operation, such as deposits forming in the check valve or spring tension in the valves changing over time. 
         [0032]    Oil provided to the various components travel to a collector  104  within the housing and drains through a drain hole  106 . A shield  110  substantially prevents oil from accessing rotor  66 . Shield  110  is provided circumferentially between rotor  66  and the stator (described in more detail below). In the view in  FIG. 2 , a cross section through a diameter of shield  110 , shows an upper and lower portion of the shield; but, the shield extends circumferentially around rotor  66 . 
         [0033]    In  FIG. 3 , a cross section of the electric motor  200  is shown as taken in a perpendicular direction with respect to the view in  FIG. 2 . At the center would be the shaft (not shown) surrounded by a stiffener  120 . A plurality of magnets  122  (four in the present embodiment) are provided around stiffener  120  with keystone wedges  124  between adjacent pairs of magnets  122 . An even number of magnets are arranged radially. A rotor sleeve  126  located exterior to magnets  122  and wedges  124  is provided to contain them. The rotor includes stiffener  120 , magnets  122 , wedges  124 , sleeve  126 , and rotor ends caps  128  (only a portion of one rotor end cap is visible in  FIG. 3 . An air gap  148  separates the rotor and the stator. The stator includes: cores  130  (six in the present embodiment), that are formed out of a plurality of laminations, with bobbins  134  onto which a conductor is wound forming coils  136 . The bobbins  134  are provided to simplify assembly of, and to electrically insulate stator coils from motor cores of motor  200 ; alternatively, the coils are wound directly onto cores or laminations  130 . The illustration in  FIG. 3 , taken as a cross section, does not show the separate laminations that form cores  130 . However, this is known to one skilled in the art. The laminations continue through a stator back iron  138 . That is, back iron  138  is also formed of laminations; back iron  138  is circumferentially arranged around cores  130 . The cores and back iron are comprised of the same laminations and are contiguous with the two separate numerals used to indicate the two sections. A groove in the periphery forms the gallery  140  for oil. Recall that gallery  140  is formed between back iron  138  and the motor housing, the latter of which is not shown in  FIG. 3 . Orifices  142  are provided in the back iron to allow oil from gallery  140  into voids  144  inside the stator. It may appear from  FIG. 3  that oil builds up inside voids  144 , but it will become apparent how the oil drains away out of voids  144  in viewing  FIG. 4 . Shield  110  is provided in air gap  148  to prevent oil within the stator from accessing the rotor. In  FIG. 3 , orifices  142  appear substantially equal in diameter. Alternatively, orifices  142  are sized to provide a desired quantity of coolant through the various orifices. 
         [0034]      FIG. 4  shows a cross section of motor  200  as indicated in  FIG. 3 . Like elements in  FIG. 4  use the same numeral as that used in  FIG. 3 . The cross-sectional view is not taken through a diameter so that it shows a cross section through windings  136  and an orifice  142 . On the lower side, the cross section is taken through cores  130 . In the embodiment portrayed in  FIG. 4 , there are three permanent magnets  122  axially. From  FIG. 3 , there are four permanent magnets  122 , as considered radially. Thus, in the embodiment of  FIGS. 3 and 4  have twelve permanent magnets. Magnets that are segmented in an axial direction reduce magnet eddy current losses. 
         [0035]    In  FIG. 5 , shield  110  is shown to include three pieces: a cylindrical sleeve  201  and first and second end caps  202  and  204 . In the embodiment in  FIG. 5 , end caps  202  and  204  are substantially in the shape of bell mouths. However, this is but one non-limiting example. Bell mouths  202  and  204  have a cut back section  206  to present a shoulder to cylinder  201 . In all embodiments, shield  110  is made of two parts to allow assembly. In one embodiment, one of the bell mouths is coupled to cylinder  201  prior to insertion into the air gap of the motor or the bell mouth is integrally formed with the cylindrical sleeve  201 . The bell mouths  204  each couple to an end of cylinder  201  via any suitable technique, including, but not limited to: gluing, snapping, threading, friction welding, and welding. In an embodiment which uses threads, there are threads in cutback section  206  which engage with threads at the ends of cylindrical sleeve  201  (threads not shown in  FIG. 5 ). An assembled version of shield  110  is shown in  FIG. 6 . 
         [0036]    Turbine section  54  of ECT  40  is provided exhaust gases from an engine, thus consequently runs hot. Energy is dissipated in electric machine  200  ( FIG. 4 ) both when operating as a motor or as a generator. To avoid damaging electric machine  200 , the dissipated energy is managed. It is desirable to avoid any radiative or conductive heat transfer to the electric machine from turbine section  54 . Bell mouths  202  and  204  serve a dual purpose of preventing oil from dripping onto the rotor and preventing radiative heat transfer from the turbomachine to the electric motor. To improve the insulating characteristics of shield  110 , surfaces  208  of bell mouths  202  and  204  are coated with an insulating ceramic or other suitable insulator or reflector. The coating insulates thermally, electrically, or both. 
         [0037]    The thickness of cylindrical sleeve  200  is selected to take up as little of the air gap as possible while having sufficient structural integrity. It can be seen in  FIGS. 5 and 6  that cylindrical sleeve  200  is much thinner than bell mouths  202  and  204 . 
         [0038]    Referring to  FIG. 3 , shield  110  allows oil, under gravitational pull, to move downwardly toward the drain  106 , but without contacting the rotor. 
         [0039]    In  FIG. 7A , an isometric view of stator  210  and the shield (expanded as cylindrical sleeve  200  and end caps  202  and  204 ) is shown. The shield and stator are shown in an assembled state in  FIG. 7B . 
         [0040]    Assembly of shield is shown in a flowchart in  FIG. 8 . The stator is assembled in  230 . One of the end caps is attached to an end of the cylindrical sleeve in block  232 . The cylindrical sleeve is inserted though the stator in block  234 . The other end cap is attached to the other end of cylindrical sleeve in block  236 . The rotor is inserted into the stator in block  238 . The operations in  FIG. 7  are shown in the preferred order. Blocks  232  and  234  may be performed in the opposite order. In another alternative, blocks  230  and  232  may be performed in the opposite order. 
         [0041]    The coolant can be any suitable fluid. In the case of an ECT that is coupled to an internal combustion, engine lubricant is a fluid that is available under pressure to provide to the ECT for both cooling and lubricating purposes. In the embodiment in which lubricant serves as the coolant for the electric machine  20  ( FIG. 1 ), drain  106  ( FIG. 2 ) can be fluidly coupled to sump  28  of engine  10  ( FIG. 1 ). 
         [0042]    As described above, lubrication of the bearings is prioritized over cooling the electric machine. For example, at startup, the oil pressure is likely less than that needed to provide oil both for cooling and lubrication and the check valves providing oil to the electric machine are closed. This may, in some situations, coincide with the desire to provide a high current to the electric machine to compress air in the turbomachine. The electric machine can tolerate a high burst of current for a short duration without overheating. However, without additional cooling measures being provided, the duration of such a burst is limited. A strategy to avoid overheating during such a situation in which the check valves are closed starts in block  300  in  FIG. 9 . In  302  the pressure in the oil system is determined (Poil) and compared to the opening pressure of the check valves (Popen). When the pressure in the oil system is greater, then the check valves are open and control passes to block  306  in which normal control of the current provided to or extracted from the electric machine proceeds. If, however, the pressure in the system is not high enough to open the check valves, control passes to block  204  in which it is determined whether the temperature of the coils (Tcoil) of the electric motor exceeds a threshold temperature (Tthresh). Based on a measurement of temperature in the coils or by a model, the temperature in the coils can be estimated or determined. As long as the temperature in the coils is lower than the threshold, control passed to block  306  for normal control of current. However, if the temperature exceeds the threshold, control passes to block  308 , which is an alternative strategy for controlling the current to (or from) the electric machine to protect the electric machine from overheating. In the vast majority of normal operating conditions, the occurrence of insufficient oil pressure to both cool the coils of the electric machine and lubricate the bearings is brief, most likely confined to startup. Nevertheless, it is useful to provide an operating strategy that limits current, such as called for in block  308 , to avoid damage of the electric machine during those unusual occurrences. 
         [0043]    The ability of the electric motor to provide torque is often limited by the current flux capacity as a result of the temperature that is generated in the coils or windings. Providing cooling to the windings effectively leads to a higher output motor. To that end, liquid cooling is known to be provided onto the windings. For high speed motors, however, the liquid cooling should be kept away from the rotor. The energy dissipated in the rotor is much lower than in the stator; thus, no liquid cooling is needed. In high speed motors, e.g., approaching 350,000 rpm in some ECTs, shearing of the coolant at such high speeds leads to a high frictional load as well as losses as the coolant is atomized into a mist. To keep the coolant from obtaining access to the rotor, a sleeve portion of a shield is placed between the rotor and the stator occupying a portion of the air gap. The shield has a cylindrical section and two bell mouth sections, one on each end. The cylindrical section, which is separated from the permanent magnets by a small air gap, is formed out of a material having low permeability so as to avoid undue interference with the flux lines set up in the motor. The permeability referred to herein relates to electromagnetic permeability. The material may be a polymer, composite, non-ferrous, or any other material with relatively low permeability. As the bell mouths are not within the air gap between the rotor and the stator, the bell mouths may be made of a material substantially without regard to the permeability. 
         [0044]    A notch  146 , as shown in  FIG. 4 , is provided in the outer surface of the stator so that a set screw can be engaged with notch  146 . The set screw  103  is shown in  FIG. 2  (notch is shown in  FIG. 2  but not separately called out with a numeral and a lead line) engaged with the notch. See in  FIG. 8  that notches  146  are evenly spaced around the periphery of stator  210 . Only one of notches  146  engages with set screw  103 . However, for proper operation of the electric machine, it is desirable to evenly distribute notches  146  on the outer surface of stator  210  and coordinated with the coils. The notch  146  and set screw  103  serve to counteract the torque generated by the motor. Alternatively, a plurality of set screws can be provided to protect for backing out of any one set screw. 
         [0045]    While the best mode has been described in detail with respect to particular embodiments, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. While various embodiments may have been described as providing advantages or being preferred over other embodiments with respect to one or more desired characteristics, as one skilled in the art is aware, one or more characteristics may be compromised to achieve desired system attributes, which depend on the specific application and implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. The embodiments described herein that are characterized as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.