ELECTRIC MACHINE WITH IN SLOT COOLING SYSTEM

An electric machine that includes a stator core having at least one slot and a winding in the slot. An elongate cooling conduit defines at least one projecting bend that is positioned in the slot. The projecting bend has first and second axially extending legs and an intermediate section. Coolant is conveyed through a first axial end opening of the slot within the first leg and then sequentially conveyed through the intermediate section and out the first axial end opening within the second leg. The projecting bend is in thermal communication with at least one of the winding and the stator core. A method of manufacture is also disclosed and may include bending a conduit having a uniform wall thickness and cross-section to form a plurality of projecting bends. The method may also include simultaneously inserting the projecting bends into respective slots from one end of the stator core.

BACKGROUND

The present invention relates to electric machines and, more particularly, to cooling systems for electric machines.

Electric machines may take the form of a motor, a generator or a motor/generator capable of selectively operating as either a motor or a generator. When operating as a motor, electrical current is input into the electric machine to generate a mechanical torque. When operating as a generator, mechanical torque is input into the electric machine to generate electrical current. Electric machines include a stator and a rotor which rotates relative to the stator.

In some applications, electric machines require the use of a cooling system to remove excess heat from the electric machine generated during operation. Various approaches are known for removing excess heat. For example, some electric machines are air cooled with a fan directing a flow of air across the electric machine to remove heat. Another common type of cooling system for electric machines is a liquid coolant system wherein a circulating liquid coolant is used to remove heat from the electric machine.

The type and application of the electric machine will be determining factors in the location and quantity of the heat generated by the electric machine. For many electric machines, the stator windings will be responsible for generating the majority of the heat during operation of the electric machine. In such electric machines, it is generally desirable to cool the stator either by directly removing heat from the stator windings or by removing heat from the stator core.

One common method of removing heat from the stator core with a liquid coolant system is to mount the stator in an exterior housing commonly referred to as a “water jacket.” The exterior housing which forms the “water jacket” includes a plurality of liquid coolant passages and surrounds and directly engages the stator core. A liquid coolant is circulated through the housing passages to remove heat from the housing. The housing thereby removes heat from the stator core and, consequently, the stator windings.

Although the various known methods of cooling electric machines can be effective, improved cooling systems remain desirable.

SUMMARY

The present invention provides an improved liquid coolant system for an electric machine.

The invention comprises, in one form thereof, an electric machine that includes a rotor rotatable about an axis and a stator operably coupled with the rotor wherein the stator includes a stator core and at least one winding. The stator core defines at least one slot wherein the winding extends axially within the slot and wherein the slot defines a first axial end opening. The electric machine also includes an elongate cooling conduit adapted to convey a liquid coolant therein. The conduit has at least one projecting bend positioned in the at least one slot wherein the projecting bend has first and second axially extending legs and an intermediate section connecting the first and second legs. The first leg is positioned to convey coolant into the slot through the first axial end opening wherein the coolant is sequentially conveyed within the projecting bend through the first leg, the intermediate section and then the second leg. The second leg conveys the coolant out of the slot through the first axial end opening and the projecting bend is in thermal communication with at least one of the winding and the stator core.

In some embodiments, the projecting bend is advantageously configured to be insertable into the slot through the first axial end opening. In other embodiments, the at least one slot may also define a second axial end opening opposite the first axial end opening with the projecting bend extending axially within the slot to or beyond a location proximate the second axial end opening. In still other embodiments, the projecting bend may be displaced radially relative to the winding. Still other embodiments may employ other features, for example, the conduit can be secured relative to the stator core by engagement of the at least one projecting bend with the stator core. Alternatively, or additionally, the intermediate section may define a pair of axially extending conduit segments.

In yet another embodiment, the at least one slot defines a second axial end opening opposite the first axial end opening and the electric machine further comprises a second elongate cooling conduit adapted to convey a liquid coolant therein. The second conduit has at least one opposing projecting bend positioned in the at least one slot wherein the opposing bend has third and fourth axially extending legs and a second intermediate section connecting the third and fourth legs. The third leg is positioned to convey coolant into the slot through the second axial end opening with the coolant being sequentially conveyed within the opposing bend through the third leg, the intermediate section and then the fourth leg. The fourth leg conveys the coolant out of the slot through the second axial end opening and the opposing bend is in thermal communication with at least one of the winding and the stator core.

In an embodiment employing such a second conduit with an opposing bend the projecting bend may extend axially within the slot from the first axial end opening to proximate a midway point between the first and second axial end openings with the opposing projecting bend extending from the second axial end opening within the slot to proximate the midway point. Alternatively, the projecting bend and the opposing projecting bend may both extend within the slot for a distance substantially equivalent to the axial distance between the first and second axial end openings.

In still other embodiments, the at least one slot is a plurality of slots with each of the slots defining a first axial end opening and an opposite second axial end opening and wherein the at least one winding includes a plurality of windings with at least one of the plurality of windings extending axially within each of the plurality of slots. In such an embodiment, the elongate cooling conduit advantageously defines a plurality of projecting bends with each of the projecting bends having first and second axially extending legs and an intermediate section connecting the first and second legs. Each of the projecting bends is disposed in a respective one of the plurality of slots wherein the first leg is positioned to convey coolant into the one slot through the first axial end opening, the coolant being sequentially conveyed within the projecting bend through the first leg, the intermediate section and then the second leg, with the second leg conveying the coolant out of the one slot through the first axial end opening.

In such an embodiment having a plurality of slots, the elongate conduit forming the plurality of projecting bends may be a unitary length of conduit having a substantially uniform wall thickness and substantially uniform circumferential dimensions.

The invention comprises, in another form thereof, an electric machine that includes a rotor rotatable about an axis and a stator operably coupled with the rotor. The stator includes a stator core which is formed out of a stacked plurality of sheet metal laminations and defines a plurality of slots. Each of the slots has an axial length and first and second axial end openings located at opposite axial ends of the slot. The stator further includes a plurality of windings wherein at least one of the plurality of windings extends axially within each of the plurality of slots. The electric machine also includes an elongate cooling conduit adapted to convey a liquid coolant therein. The conduit defines a plurality of projecting bends wherein each of the projecting bends has first and second axially extending and substantially parallel legs and an intermediate section connecting the first and second legs. Each of the projecting bends is disposed in a respective one of the plurality of slots wherein the first leg is positioned to convey coolant into the one slot through the first axial end opening, the coolant being sequentially conveyed within the projecting bend through the first leg, the intermediate section and then the second leg, with the second leg conveying the coolant out of the one slot through the first axial end opening. The projecting bend is positioned radially adjacent the at least one winding disposed in the slot and the projecting bend is also in thermal communication with at least one of the stator core and the at least one winding disposed in the slot. The elongate conduit forming the plurality of projecting bends is a unitary length of conduit having a substantially uniform wall thickness and substantially uniform circumferential dimensions.

In some embodiments, the plurality of projecting bends are advantageously configured to be simultaneously and respectively insertable into the plurality of slots through the first axial end openings of the slots. Alternatively or additionally, the conduit can be secured relative to the stator core by engagement of the plurality of projecting bends with the stator core.

Some embodiments may also include a second elongate cooling conduit adapted to convey a liquid coolant therein. The second conduit has a plurality of opposing projecting bends each of which is positioned in a respective one of the plurality of slots with each of the opposing bends having third and fourth axially extending legs and a second intermediate section connecting the third and fourth legs. The third leg is positioned to convey coolant into the slot through the second axial end opening with the coolant being sequentially conveyed within the opposing bend through the third leg, the intermediate section and then the fourth leg. The fourth leg conveys the coolant out of the slot through the second axial end opening with the opposing bend being in thermal communication with at least one of the stator core and the least one of the winding disposed in the slot.

The invention comprises, in still another embodiment thereof, a method of manufacturing an electric machine wherein the method includes providing a stator core with a plurality of slots and providing a plurality of windings and installing at least one winding in each of the plurality of slots. The method also includes forming a plurality of projecting bends in an elongate conduit and positioning each of the projecting bends in one of the plurality of slots wherein each projecting bend has first and second axially extending and substantially parallel legs and an intermediate section connecting the first and second legs. The first leg is positioned to convey coolant into the slot proximate a first axial end of the stator core, the coolant being sequentially conveyed within the projecting bend through the first leg, the intermediate section and then the second leg, with the second leg conveying the coolant out of the slot proximate the first axial end of the stator core. The projecting bend is in thermal communication with at least one of the winding and the stator core. The method additionally includes operably coupling the stator assembly with a rotor assembly.

In some embodiments, the step of forming a plurality of projecting bends in an elongate conduit includes providing a continuous and unitary conduit having a uniform wall thickness and uniform cross-sectional shape and bending the conduit to form the plurality of projecting bends.

In some embodiments, the step of positioning each of the projecting bends in one of the plurality of slots includes simultaneously inserting each of the projecting bends into a respective one of the plurality of slots from the first axial end of the stator core.

In some embodiments, the step of positioning each of the projecting bends in one of the plurality of slots comprises securing the elongate conduit to the stator core by engaging the projecting bends with the stator core.

DETAILED DESCRIPTION

An electric machine20with an in slot cooling system is schematically depicted inFIG. 1. Electric machine20includes a rotor assembly22having a rotor core24mounted on a shaft28and rotatable about axis30. Rotor core24is formed out of a plurality of stacked sheet metal laminations and defines a plurality of radially projecting poles26.

Electric machine20also includes a stator assembly32having a stator core34which encircles rotor assembly22. Stator core34is formed out of a plurality of sheet metal laminations38and defines a plurality of radially inwardly projecting teeth36. Stator teeth36define a plurality of axially extending slots40therebetween. Windings42are disposed in the slots40. Stator assembly32is mounted within a housing44with the radially outer surface of stator core34engaging the radially inner surface of housing44. It is noted that the illustrated embodiment has concentrated windings, however, alternative embodiments could employ distributed windings. A stator with distributed windings generally has a more complex winding pattern and the area where the end turns project beyond the axial end of the stator core is more congested with wiring. The difference between distributed and concentrated windings is well-known to those having ordinary skill in the art.

In the illustrated embodiment, electric machine20is a switched reluctance motor, however, alternative embodiments of the invention may be employed with other types of electric machines. Typically, electric machines have stators which circumscribe the rotor, however, other embodiments could employ a stationary stator that is centrally disposed with the rotatable rotor encircling the stator.

Electric machine20also includes a cooling system which circulates a liquid coolant, such as a water, ethylene, glycol mixture or other suitable liquid coolant, through one or more elongate cooling conduits to remove excess heat from stator assembly32. In the embodiment ofFIGS. 1-4, electric machine20includes a single elongate conduit46. Conduit46defines a projecting bend48for each slot40in stator core32.

As can be understood with reference toFIGS. 3 and 4, projecting bends48each include first and second axially extending legs50,52which are connected by an intermediate section54.FIGS. 3 and 4, as well asFIGS. 5-7discussed below, schematically depict the stator core and cooling conduits in a linear configuration instead of their actual configuration which encircles axis30to more clearly depict the manner in which cooling conduits are inserted into stator slots40.

In the illustrated embodiment, when electric machine20is a relatively large switch reluctance motor the stator slot widths can be sufficiently large to allow conduit46to be formed out of commercially available copper tubing having a nominal outside diameter of 6 or 8 mm. A continuous and unitary length of copper tubing is bent to form a plurality of projecting bends48whereby each of the bends48can be inserted into one of the stator slots40.

Stator slots40each define a first axial end opening39on one axial end33of stator core34and a second axial end opening41on the opposite axial end35of stator core34. The projecting bends48are configured to be inserted through one of the axial end openings of slots40. In the embodiment ofFIGS. 1-4, projecting bends48are each simultaneously inserted into their respective slot40through axial end openings39, which are all on the same axial end of stator core34, when installing conduit46.

As a result of this configuration, for each of projecting bends48, first leg50will convey coolant into its respective slot40through first axial end opening39, the coolant is then sequentially conveyed within bend48through first leg50, intermediate section54and then second leg52. Second leg52conveys the liquid coolant out of slot40through the same axial end opening39that first leg50conveyed the coolant into the slot40.

Within slot40, projecting bend48is in thermal communication with at least one of stator winding42and stator core34whereby excess heat from one or both of stator winding42and stator core34can be transferred to conduit46and then to the liquid coolant within conduit46. The coolant will then be conveyed through conduit46to an external portion of the cooling system which will include a heat-exchanger, e.g., a radiator, or other suitable means for discharging the excess heat. Advantageously, projecting bend48is in direct contact with both stator core34and stator winding42within slot40to promote the efficient transfer of heat.

In this regard, it is noted that conduit46starts out as a continuous and unitary length of copper tubing having a round cross section as seen inFIG. 2. After the tubing is bent to form projecting bends48and both bends48and stator windings42are installed in slots40, the originally round tubing forming conduit46will be slightly flattened as it is compressed within slot40. This advantageously enhances the contact between conduit46and stator core34and windings42to thereby promote the transfer of thermal energy and also secures bends48within slots40. While the illustrated embodiments employ round conduits that are slightly deformed during installation, alternative embodiments could employ conduits with alternative cross sectional shapes and/or shapes that are not altered during the installation process.

In this regard, it is also noted that conduit46shown inFIGS. 1-4is a unitary length of conduit having a substantially uniform wall thickness56and a substantially uniform circumferential dimension58even though it is bent from its original configuration, e.g., coiled or straight, and the cross sectional shape is deformed during installation in electric machine20. In other words, conduit46starts out as a length of continuous and unitary tubing and is subsequently deformed during installation but this does not remove material from the tube to reduce the thickness of the wall or the circumferential extent of the tube although the interior volume of the tube may be reduced by the deformation of the tube during installation. This stands in contrast to forming a custom tube that originally has distinct sections with different wall thicknesses and/or circumferential dimensions as might occur if conduit were formed by piecing together linear lengths of tubes with elbows and other fittings. It is noted that while such piecing together of a conduit will generally be undesirable, it would be possible to form a conduit46using such a process. A unitary length of tubing, as used herein, is a length of tubing that has not been pieced together by connecting originally separate lengths of tubing with joints or connectors. In this regard, it is noted that commercially available tubing, such as copper tubing, is typically provided in unitary lengths of tubing having a substantially uniform wall thickness and a substantially uniform circumferential dimension.

The use of projecting bends48within slots also provides several additional advantages. The material used to form conduit46will need to efficiently transfer heat from the stator assembly to the liquid coolant. Many metal materials, e.g., copper, are well suited for transferring heat. Metal materials, however, are also typically good conductors of electricity and capable of having an electrical current or voltage potential induced therein by the operation of electric machine20if the metal material is positioned in slot40. If induced voltage is generated in a coolant tube positioned in slot40, this can possibly negatively impact the performance of electric machine20. The configuration of projecting bends48reduces such negative impacts by employing oppositely extending legs50,52connected by intermediate section54.

More specifically, by employing a projecting bend with a first leg50and a second leg52that extend substantially parallel to each other in the same slot40and having an intermediate section54of the tubing connect the two legs wherein the tubing forming first leg50, second leg52and intermediate section54, is a continuous electrical conductor, the induced voltage in the projecting bend minimized due to the opposing polarity of the induced voltage in the first and second legs50,52. As a result, magnetically induced losses are minimized and the performance of electric machine20is enhanced.

The use of conduit46also provides manufacturing advantages. When conduit46is formed out of a unitary length of tubing the ability to insert conduit46from one axial end into the slots eliminates the need to form multiple tube joints that would be necessary if individual tubes were routed through each slot conveying the liquid into one end and out the other end of the slots. Furthermore, the tight fit of bends48within slots40secures conduit46in place and eliminates the need to use clamps, brazing or other methods to secure the conduit.

FIG. 8shows a partial axial end view of an electric machine20and one potential arrangement of conduit46and windings42within slot40. In the arrangement ofFIG. 8, conduit46is displaced radially relative to windings42and engages stator core34but has only minimal if any direct engagement with windings42. In this embodiment, conduit46is secured relative to stator core34by engagement of projecting bends48with stator core34. It is further noted that a thin electrically insulating member60is positioned about the outer periphery of winding42except for that portion of windings42that face the radially inner opening of slot40. Members60are often called wedges and are used to protect windings42from abrasion against stator core34during the insertion of windings42and thereby prevent electrical communication between windings42and stator core34. The use of wedges60is well-known to those having ordinary skill in the art. In this regard, it is noted that such wedges60are considered to be part of windings42herein and direct contact between conduit46and windings42can be achieved by engaging conduit46with wedge60.

Returning toFIG. 4, it is noted that when a single conduit46is employed, it will generally be desirable for the projecting bends48of conduit46to extend axially within slot40from the first axial end opening39to or beyond a location proximate the second axial end opening41whereby conduit46can provide cooling for substantially the entire axial length of slot40. In the embodiment ofFIG. 4, bends48project slightly beyond axial end openings41.

Various other arrangements of projecting bends and windings can also be employed. For example, instead of positioning the projecting bend at the back of slot40as depicted inFIG. 8, i.e., displacing bend48radially outward of windings42, the projecting bend could be positioned between the windings or at the inner diameter of slot40. Several additional configurations are discussed below.

It is also noted that conduit46may be inserted into slot40either before or after windings42. Generally, the order of insertion which provides for the most efficient method of assembly will be determined by the arrangement of the conduit46and winding42in slot40. Moreover, some embodiments include multiple conduits inserted into a single slot and in such embodiments it may be desirable to insert a conduit, then the winding and then an additional conduit.

FIGS. 5 and 7illustrate embodiments which include two elongate conduits having projecting bends wherein the projecting bends of the two conduits are inserted into the slots from opposite axial ends of the stator core. InFIG. 5, a first conduit46ahas a plurality of projecting bends48awith first and second legs50a,52aand intermediate sections54athat are inserted into slots40through first axial end openings39. A second conduit46bhas a plurality of projecting bends48bwith first and second legs50b,52band intermediate sections54b.The projecting bends48a,48bfunction in the same manner as bends48described above with reference to conduit46. The difference being that bends48a,48bextend axially only halfway through slot40and meet proximate the axial midpoint61of slot40. In other words, projecting bends48aextend axially within slots40from first axial end openings39to a location proximate midway point61between first and second axial end openings39,41and opposing projecting bends48bextend from second axial end openings41with slot40to a location proximate midway point61.

FIG. 7illustrates an embodiment wherein conduits46cand48drespectively define projecting bends48c,48d.Bends48c,48denter slot40from opposite axial ends of slot40, are circumferentially offset from each other, and extend for substantially the entire axial length of slot40. In other words, both projecting bend48cand projecting bend48dextend within slot40for a distance substantially equivalent to the axial distance between first and second axial end openings39,41of slot40.

FIG. 6illustrates another embodiment wherein a single conduit46ehas projecting bends48ethat are inserted through axial end openings39of slots40. Projecting bends48ediffer from bends48in that bends48einclude a second pair of legs. As seen inFIG. 6, conduit46edefines projecting bends48ethat each include first and second legs50e,52eand an intermediate section54ewith intermediate sections54eeach defining a pair of axially extending conduit segments51e,53ewhich are connected by a turn segment54e.

FIGS. 8-12provide partial axial end views illustrating several different arrangements of the projecting bends and windings within the stator slots. As mentioned above,FIG. 8shows a projecting bend48wherein the bend48is displaced radially outwardly of windings42. Also shown inFIG. 8is a dashed line62. Line62represents where the radial outermost edge of stator core34would be if no cooling conduits were inserted in slots40and a water jacket was used to cool stator assembly32instead. As can be seen, positioning cooling conduits in slots40requires that the outer diameter of the stator core be enlarged to account for the radial enlargement of slots40. Positioning cooling conduits in slots40, however, also allows the coolant passages to be omitted from housing44. Thus, the outer diameter of the entire electric machine assembly including the housing is not necessarily enlarged by the use of cooling conduits within the stator slots. It is thought that for most applications, the enlargement of the stator core and the reduction in the size of the housing will substantially offset each other with the use of in-slot cooling conduits providing an electric machine assembly having a substantially similar outer diameter as a similar water jacket cooled electric machine with either small enlargements or small reductions in diameter being possible.

FIG. 9represents an arrangement wherein the cooling conduit could have a configuration similar to either the conduit illustrated in eitherFIG. 6or those illustrated inFIG. 7. InFIG. 9, the conduit is identified as the one illustrated inFIG. 6.

FIG. 10represents an embodiment wherein the cooling conduit is positioned radially between windings42.

FIGS. 11 and 12represent embodiments wherein two separate conduits are inserted into each slot42from opposite ends of slot40with one of the conduits being positioned radially outwardly of windings42and one of the conduits being positioned radially inwardly of windings42. InFIG. 11, projecting bends48fdisposed radially outwardly of windings42have four axially extending legs or segments such as those illustrated inFIG. 6while projecting bends48gdisposed radially inwardly of windings42have only two axially extending legs. InFIG. 12, projecting bends48hdisposed radially outwardly of windings42and projecting bends48ilocated radially inwardly of windings42both have only two axially extending legs.

FIGS. 13-16schematically represent several alternative overall cooling system flow patterns.FIGS. 13-16have been graphically simplified to omit the individual projecting bends for each slot and are intended to show the overall flow pattern. The actual flow path for these figures would include portions that extend down into a slot and then return out of the slot as can be seen inFIGS. 4-7.

InFIG. 13, a single elongate conduit47ais used to cool the electric machine. Conduit47aincludes an inlet64which receives a liquid coolant at a reduced temperature. After the coolant passes through each of the projecting bends, conduit47adischarges the coolant back to the remainder of the cooling system through an outlet66. The cooling system will then remove heat from that coolant that it picked up from the electric machine and return it to the inlet64. The elongate conduits shown inFIGS. 4 and 6, as well as other alternative configurations, could be employed with the flow pattern depicted inFIG. 13.

FIG. 14differs fromFIG. 13in that it includes two elongate conduits47a,47bwhich are located on opposite axial ends of stator core34. The elongate conduits shown inFIGS. 5 and 7, as well as other alternative configurations, could be employed with the flow pattern depicted inFIG. 14.

FIG. 15includes two elongate conduits47c,47d,however, they are both located on the same axial end of stator core34instead of being on opposite ends. In this configuration, each of the two elongate conduits47c,47dhas a plurality of projecting bends that correspond to approximately half of the total number of stator slots40. The elongate conduits shown inFIGS. 4 and 6, as well as other alternative configurations, could be employed with the flow pattern depicted inFIG. 15.

FIG. 16includes four elongate conduits47c,47d,47e,47fwith two elongate conduits being disposed on each axial end of the stator core34. Similar toFIG. 15, each of the conduits of the configuration ofFIG. 16have a plurality of projecting bends that correspond to approximately half the total number of stator slots. Similar toFIG. 14, each of the slots40in the configuration ofFIG. 16have a conduit entering from each of its opposite axial end openings. The elongate conduits shown inFIGS. 5 and 7, as well as other alternative configurations, could be employed with the flow pattern depicted inFIG. 16.

With regard toFIGS. 14 and 16, the positioning of the inlets64and outlets66on the opposite axial ends of stator core34is worth noting. When employing conduits on opposite axial ends of the stator core, it will generally be desirable to have the coolant flow in opposite directions whereby the slot40which is positioned to receive the coolant immediately downstream of inlet64(and thus receive it at its lowest temperature) from the conduit on one axial end is positioned to receive the coolant immediately upstream of the outlet66(and thus receive it at a relatively elevated temperature) from the conduit on the opposite axial end. This arrangement seeks to equalize the heat removal from the different slots40. Whether or not such equalization is sufficiently advantageous to warrant the use of multiple conduits will depend on the application and operating characteristics of the electric machine.