Patent Application
20140197703
This application is filed on the same day as co-pending U.S. patent applications Ser. No. ______, entitled “ELECTRIC MOTOR STATOR HOUSING INTERFERENCE GAP REDUCING METHOD AND APPARATUS,” and Ser. No. ______, entitled “ELECTRIC MOTOR ROTOR THERMAL INTERFACE WITH AXIAL HEAT SINKS.” The subject matter of these two applications is incorporated herein in its entirety.
The present invention is directed to improving the performance and thermal efficiency of electric machines and, more particularly, to methods and apparatus for improving the heat transfer process.
An electric machine is generally structured for operation as a motor and/or a generator, and may have electrical windings and/or permanent magnets, for example in a rotor and/or in a stator. Heat is produced in the windings and magnets, and by bearings or other sources of friction. Eddy currents and core losses occur within a rotor of an electric machine. Such losses result in undesirable heat within the rotor assembly. In a densely packed electric machine operating at a high performance level, excessive heat may be generated. Such heat must be removed to prevent it from reaching impermissible levels that may cause damage and/or reduction in performance or life of the motor.
Various apparatus and methods are known for removing heat. One exemplary method includes providing the electric machine with a water jacket having fluid passages through which a cooling liquid, such as water, may be circulated to remove heat. Another exemplary method may include providing an air flow, which may be assisted with a fan, through or across the electric machine to promote cooling. A further exemplary method may include spraying or otherwise directing oil or other coolant directly onto end turns of a stator winding.
There is generally an ongoing need for increasing performance and efficiency of electric machines, such by providing more power in a smaller space. Although various structures and methods have been employed for cooling an electric machine, improvement remains desirable.
It is therefore desirable to obviate the above-mentioned disadvantages by providing methods and apparatus for minimizing thermal resistance and increasing thermal efficiency.
According to an exemplary embodiment, a cooling system of an electric machine includes a hub member, a rotor core, and a thermal interfacial material interposed between respective complementary mating surfaces of the hub member and the rotor core for substantially eliminating air gaps therebetween.
According to another exemplary embodiment, a method of cooling an electric machine having a rotor core and a hub member includes placing a thermal interfacial material onto a heat transfer interface between the rotor core and the hub member, whereby the thermal interfacial material reduces contact resistance at the heat transfer interface.
According to a further exemplary embodiment, a method of cooling a stator of an electric machine includes providing a rotor core having a radially inner surface, coating at least one of a radially outer surface of a hub member and the radially inner surface of the rotor core with a thermal interfacial material, and inserting the hub member into the rotor core, whereby the thermal interfacial material is interposed between the inner surface of the rotor core and the outer surface of the hub member.
The foregoing summary does not limit the invention, which is defined by the attached claims. Similarly, neither the Title nor the Abstract is to be taken as limiting in any way the scope of the claimed invention.
The above-mentioned aspects of exemplary embodiments will become more apparent and will be better understood by reference to the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:
Corresponding reference characters indicate corresponding or similar parts throughout the several views.
The embodiments described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of these teachings.
In some embodiments, module housing 12 may include at least one coolant jacket 42, for example including passages within housing body 14 and stator 26. In various embodiments, coolant jacket 42 substantially circumscribes portions of stator assembly 26, including stator end turns 28. A suitable coolant may include transmission fluid, ethylene glycol, an ethylene glycol/water mixture, water, oil, motor oil, a gas, a mist, any combination thereof, or another substance. A cooling system may include nozzles (not shown) or the like for directing a coolant onto end turns 28. The outside surface 15 of stator 26 may be formed to snugly fit in abutment with the radially inner surface 17 of cooling jacket 42 or other housing surface, such as an interior surface of a housing formed without a cooling jacket. Housing 12 may include a plurality of coolant jacket apertures 46 so that coolant jacket 42 is in fluid communication with machine cavity 22. Coolant apertures 46 may be positioned substantially adjacent to stator end turns 28 for the directing of coolant to directly contact and thereby cool end turns 28. For example, coolant jacket apertures 46 may be positioned through portions of an inner wall 48 of body 14. After exiting coolant jacket apertures 46, the coolant flows through portions of machine cavity 22 for cooling other components. In particular, coolant may be directed or sprayed onto hub 33 for cooling of rotor assembly 24. The coolant may be pressurized when it enters the housing 12. After leaving housing 12, the coolant may flow toward a heat transfer element (not shown) outside of the housing 12, for removing the heat energy received by the coolant. The heat transfer element can be a radiator or a similar heat exchanger device capable of removing heat energy.
The interface 11 (
When rotor core 49 is a lamination stack (
In an alternative embodiment, a “hubless” rotor may be formed by providing a shaft 21 and a rotor core 49 structured for mating with one another. In such a case, rotor core inner surface 34 typically has approximately the same diameter as the outside diameter of shaft 21 and is secured thereto by an interference fit and by being torsionally interlocked, such as by the use of one or more keys (not shown) and corresponding keying slots. Since the mating of surface 34 with the outer surface of shaft 21 otherwise includes air gaps 5, TIM is applied to the interface between surface 34 and shaft 21 to fill gaps 5 and thereby reduce thermal resistance. All features such as keys, grooves, and slots of shaft 21 and surface 34 are filled with TIM, and any excess TIM is wiped off after mating insertion of shaft 21 into rotor core 49. One or more seals may then be formed at axial ends of the shaft/rotor core interface to prevent migration of uncured TIM, when appropriate.
An electric machine in various embodiments may include TIM placed onto radially outer surface(s) of shaft 21, into an interface between hub cylindrical inner bore 10 and shaft 21, and/or into the interface between hub surface 8 and rotor core surface 34. As used herein, the term “hub member” means ‘at least one of a hub and a shaft’ whereby, for example, TIM applied to a hub member is applied to a hub and/or a shaft. A hub member may have a unitary structure where only one interface exists between hub and rotor core, or a hub member may include any number of individual components that have corresponding complementary interfaces at contiguous surfaces thereof. As used herein, a hub member may include a shaft, a shaft and a hub, or a hub with no shaft. In exemplary embodiments, TIM may be applied between a rotor core and a hub where no shaft is present, TIM may be applied between a rotor core and a hub where a shaft is present but where TIM is not applied between the hub and shaft, TIM may be applied between a rotor core and a hub and also between the hub and a shaft, or TIM may be applied between a rotor core and a shaft where no hub is present.
Annular sealing members 52, 54 may be required when migration of TIM is foreseen, for example when viscosity of the TIM is low and/or when TIM at a radially outer edge may be subjected to contaminants. In some applications, such sealing may be effected by use of a temporary gasket that is only required during the manufacturing process. Seals 52, 54 may alternatively include O-rings, gaskets, resin, fiber, and/or structural barriers that block any exit paths out of TIM application region 41. When hub 53 has a rim 45, heat sink 29 may be press fit against the radially inward surface 55 of hub 53 and may be modified to abut rotor core axial end surface 51 while also providing adequate space for rim 45. Heat sink 29 may thereby be axially pressed against hub surface 55 while TIM bead 38 and/or sealant 54 are still in a liquid state, so that heat sink 29 becomes bonded to TIM bead 38 and/or sealant 54. Alternatively, heat sink 29 may be axially pressed against hub surface 55 and TIM bead 38, and an epoxy or other sealant 54 may be subsequently applied to seal any joints between rim 45, heat sink 29, hub 53, and rotor core end surface 51, thereby preventing any migration or contamination of the TIM. Similarly, heat sink 31 may be placed against rotor core end surface 56 and TIM bead 38, and an epoxy or other sealant 52 may then be used to seal any joints between hub 53 and end surface 56. In another exemplary embodiment, the TIM may be placed into interface 41 and also onto rotor core end surfaces 51, 56 so that when heat sinks 29, 31 are respectively placed onto hub end surfaces, heat sinks 29, 31 are thermally interfaced with rotor core 49 and/or hub 53. In such a case, the TIM may be continuous along surfaces 51, 34, 56, or the TIM in interface 41 may only be present for filling air gaps therewithin.
TIM may be partially or fully cured by being mixed with a hardener. Typically such curing takes approximately two hours at room temperature and approximately five minutes at an elevated temperature such as 100° C. Alternatively, TIM may remain in a liquid state when annular sealing members 52, 54 seal TIM application region 41 with separate materials such as beads of epoxy. Further, when TIM is squeezed so that one or both of annular sealing members 52, 54 includes a TIM bead, this exposed TIM may harden and effect a seal. In some applications, TIM maintains a consistency of grease and does not cure. For example, air gaps 5 that exist as a part of imperfections of surfaces 34, 44 may be isolated, and TIM displacing the air of such spaces may therefore also be isolated. Curing and an associated use of hardeners may thereby be unnecessary and/or undesirable.
When TIM has a high viscosity and no migration, the absence of thermal epoxies or other hardeners may reduce shrinkage and similar reliability issues. Depending on a particular application, TIM may contain silicone, alumina or other metal oxides, binding agents, epoxy, and/or other material. TIM has a high thermal conductivity and a high thermal stability, and may be formulated to have minimal evaporation, hardening, melting, separation, migration, or loss of adhesion. Suitable materials are available from TIMTRONICS. However, due to the small size and space of air gaps 5, the size and shapes of fillers and other ingredients of TIM, such as alumina, is typically kept below 0.03 mm.
The rate of assembly is typically as slow as is practical. Specifically, when hub 33, 53 is being inserted, a slow insertion movement helps distribute TIM into air gaps 5. The high conformability of TIM assures that nearly all air is removed. A longer cure time assures that TIM spreads and becomes uniformly distributed. For example, a nominal TIM thickness may be 0.03 mm. By slowly lowering the heated housing in an axial direction onto the TIM-coated stator, the interference fitting process removes air gaps 5 by slowly squeezing TIM. Once air gaps 5 have been filled, TIM does not readily migrate because air gaps 5 are not continuous. In other words, the tight fitment at interface 41 and the lack of channels for TIM migration prevent TIM from being displaced prior to curing. In manufacturing, TIM is metered to assure that a precise volume is being applied, whereby residue is minimized and TIM interface 41 becomes uniformly filled. In an alternative manufacture, TIM may be placed onto outer surface 44 of hub 53 prior to assembly, or both surfaces 34, 44 may be coated prior to assembly. To assure that all air gaps 5 are filled, annular rubber blade(s) or the like may be used for spreading TIM onto one or both of surfaces 34, 44 in any number of passes, prior to assembly. Since it may be desirable for TIM to have adherence properties that resist flow, the coating of surface(s) 34, 44 is typically performed by radially forcing TIM against such surface(s). For example,
Installation of heat sinks 29, 31 (
Rotor assembly 41 may be contained in a housing 12 (
Testing of exemplary embodiments has shown a significant improvement in transferring heat from a rotor core to a hub by placement of TIM at the interface therebetween. Coolant may be sprayed onto hub 33, 53 by nozzles (not shown) or by flow of the coolant through cavity 22 (
Although exemplary embodiments are described for an annular heat transfer interface between surfaces of a hub and a rotor core, the inner surface 34 of rotor core 49 may have any appropriate shape. For example rotor core 49 may be formed of individual core segments (not shown) that are connected to one another to thereby have an inner rotor core surface 34 that may include gaps, slots, protrusions, and other deviations from a relatively smooth cylinder. In such a case, large gaps and holes may be filled with a thermally conductive potting material or the like and cured, prior to TIM coating and coupling of the rotor core to a hub. By this process, the irregularities in a segmented rotor core are substantially eliminated prior to placing TIM into the heat transfer interface. Similarly, any continuous grooves, notches, or protrusions along either mating surface should be removed prior to assembly, so that TIM migration is substantially prevented by elimination of potential migration channels. In other words, by eliminating exit passageways, the TIM cannot migrate. When surfaces 34, 44 have an interference fit, TIM only spreads and fills air gaps 5. Although exemplary embodiments have been described for configurations with a rotor inside of a stator, the embodiments may also be adapted for configurations having a rotor radially outward of a stator.
The embodiments described herein may be combined, when appropriate, with aspects of the co-pending applications entitled “ELECTRIC MOTOR STATOR HOUSING INTERFERENCE GAP REDUCING METHOD AND APPARATUS” and “ELECTRIC MOTOR ROTOR THERMAL INTERFACE FOR HUB/SHAFT.”
While various embodiments incorporating the present invention have been described in detail, further modifications and adaptations of the invention may occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.
