Rotor assembly for an electric machine including a vibration damping member and method of manufacturing same

A rotor for an electric machine includes a shaft that is rotatable about an axis and defines a first diameter normal to the axis. A first core portion defines a first aperture having a first aperture diameter that is larger than the first diameter. The first core portion is positioned adjacent the shaft to define a first space. A second core portion defines a second aperture having a second aperture diameter that is larger than the first diameter. The second core portion is positioned adjacent the shaft to define a second space. A damping member is positioned in the first space and the second space. The damping member at least partially interconnects the shaft, the first core portion, and the second core portion.

BACKGROUND

The invention relates to a rotor assembly for an electric machine and a method of manufacturing the same. More specifically, the invention relates to a rotor including a vibration damping member.

SUMMARY

In one embodiment, the invention provides a rotor for an electric machine. The rotor includes a shaft that is rotatable about an axis and defines a first diameter normal to the axis. A first core portion defines a first aperture having a first aperture diameter that is larger than the first diameter. The first core portion is positioned adjacent the shaft to define a first space. A second core portion defines a second aperture having a second aperture diameter that is larger than the first diameter. The second core portion is positioned adjacent the shaft to define a second space. A damping member is positioned in the first space and the second space. The damping member at least partially interconnects the shaft, the first core portion, and the second core portion.

In another embodiment, the invention provides a rotor for an electric machine. The rotor includes a shaft that is rotatable about an axis and has an outer surface that defines a first cross-sectional area normal to the axis. A first rotor core portion is formed from a plurality of stacked first laminations. Each first lamination defines a first lamination surface that is substantially the same as the outer surface. The first rotor core portion is positioned adjacent the shaft. A second core portion is formed from a plurality of stacked second laminations. Each second lamination has an aperture that defines a second lamination area that is larger than the first cross-sectional area. The second core portion is positioned adjacent the shaft to define a first space. A third core portion is formed from a plurality of stacked second laminations. The third core portion is positioned adjacent the shaft to define a second space. A damping member is positioned in the first space and the second space. The damping member at least partially interconnects the shaft, the first core portion, the second core portion, and the third core portion.

The invention also provides a rotor for an electric machine. The rotor includes a shaft that has an outer surface that defines a first cross-sectional area. A first rotor core portion defines a first tooth portion and a first aperture having a second cross-sectional area. The first cross-sectional area and the second cross-sectional area cooperate to define a first space. A second rotor core portion defines a first recessed portion, and a second aperture having a third cross-sectional area that is larger than the first cross-sectional area. The third cross-sectional area and the second cross-sectional area cooperate to define a second space. The first tooth engages with the first recessed portion to couple the first rotor core portion and the second rotor core portion for rotation. A resilient member is positioned within the first space and the second space to couple the shaft, the first rotor core portion, and the second rotor core portion for rotation.

Other aspects and embodiments of the invention will become apparent by consideration of the detailed description and accompanying drawings.

DETAILED DESCRIPTION

As schematically illustrated inFIG. 1, a motor10generally includes a rotor15disposed within a stator20. The rotor15includes a rotor core25and a shaft30that extends from one or both ends of the rotor core25to provide support points and to provide a convenient shaft power take off point. Generally, two or more bearings35engage the rotor shaft30and support the rotor15such that it rotates about a rotational axis40. The stator20generally includes a housing45that supports a stator core50. The stator core50defines a substantially cylindrical aperture55that is centered on the rotational axis40. When the rotor15is in its operating position relative to the stator20, the rotor core25is generally centered within the aperture55such that a small air gap is established between the rotor core25and the stator core50. The air gap allows for relatively free rotation of the rotor15within the stator20.

The motor10illustrated inFIG. 1is a permanent magnet brushless motor. As such, the rotor15includes permanent magnets (not shown) that define two or more magnetic poles. The stator20includes windings that can be selectively energized to produce a varying magnetic field. The permanent magnets of the rotor15interact with the magnetic field of the stator20to produce rotor rotation. As one of ordinary skill will realize, the present invention is well suited to many types of motors (e.g. induction motors), in addition to the permanent magnet brushless motors10illustrated herein. As such, the invention should not be limited to only these types of motors. Furthermore, one of ordinary skill will realize that the present invention can also be applied to many types of generators. In addition, figures and description presented herein are directed to a rotor15and/or a motor10. However, some of the features described and illustrated could be applied to stators. Thus, while the figures and description refer to a brushless motor10and/or a rotor15, other applications are possible.

In many constructions, the rotor core25is formed by stacking a plurality of laminations and attaching permanent magnets to the stacked laminations. The magnets (shown inFIGS. 8 and 9) can be, for example, mounted on the rotor surface facing the air-gap or inserted in the interior of the rotor core. Generally, the laminations are punched or cut from electrical grade steel as is known in the art. The laminations, once stacked, are positioned over the shaft30to complete the rotor15. A rotor core and shaft subassembly15a, illustrated inFIG. 2, includes a plurality of first laminations60and a plurality of second laminations65stacked on top of one another. The first lamination60, shown inFIG. 4, includes a generally circular outer surface70and a central aperture75that cooperates with adjacent laminations60to define an inner surface80. Three teeth or tangs85extend radially inward from the inner surface80to a tooth diameter90that is large enough to receive the shaft30and define a space95therebetween. In other words, the tooth diameter90is larger than a shaft diameter100in the area where the laminations60will eventually be positioned.

In the illustrated construction, the three tangs85are evenly positioned approximately 120 degrees from one another. Of course, other constructions may include unevenly spaced tangs85, or more than three tangs85that are evenly or unevenly spaced. For example, another construction may include five tangs85that are spaced approximately 72 degrees apart. As one of ordinary skill in the art will realize, many different shapes, quantities and combinations of tangs85are possible.

Still other constructions may employ first laminations60that do not include tangs85but rather include a non-circular aperture. For example,FIG. 6illustrates a lamination105that includes an elliptical central aperture110. Because the aperture110is not axisymetric, laminations105can be rotated relative to one another and stacked to position a portion of one lamination105over a portion of the aperture110of another lamination105.FIG. 7illustrates yet another arrangement in which a lamination115includes a square central aperture120. Again, because the square aperture120is not axisymetric, one lamination115can be rotated with respect to another lamination115to position a portion of the lamination115over a portion of the aperture120of the adjacent lamination115.

Each of the second laminations65, shown inFIG. 5, includes an outer surface125that defines a substantially circular profile. In preferred constructions, the outer surface70of the first laminations60and the outer surface125of the second laminations65are similarly sized. The second laminations65also define a central aperture130that has a diameter135that is substantially the same as the shaft diameter100. As such, the second laminations65fit snugly against the shaft35when the rotor core and shaft subassembly15ais assembled. Several recesses140extend radially outward from the central aperture130to provide clearance space between the shaft35and the laminations65. In the illustrated construction, four elliptical recesses140are equally spaced (i.e., 90 degrees apart) from one another. As one of ordinary skill will realize, other shaped recesses140or a different number of recesses140may be employed if desired. In addition, the recesses140may be unevenly spaced if desired.

Each of the second laminations65may include apertures145positioned outward of the recesses140. The construction illustrated inFIG. 5includes four rectangular apertures145that are spaced apart from one another by about 90 degrees. The apertures145are also rotated with respect to the elliptical recesses140by about 45 degrees such that the rectangular apertures145are positioned between the elliptical recesses140. In other constructions, other shaped or other numbers of apertures145may be employed. In some constructions, the apertures145may be differently positioned or omitted.

Before proceeding, it should be noted that laminations of the type described herein often include alignment members such as indentations, lances, or apertures that facilitate the axial alignment of the various laminations. In some constructions, the alignment members are formed during the punching process that forms the lamination. The alignment members generally define an indentation on one side of the lamination and a protrusion on the opposite side of the lamination. The protrusions of one alignment member fit within the indentations of an adjacent lamination to align and fasten the laminations as desired.

The rotor core and shaft subassembly15aofFIGS. 2,3, and3aincludes several main core portions150each formed by stacking several of the first laminations60on top of one another and an alignment core portion155formed by stacking a plurality of second laminations65. The laminations60,65may be bonded to one another or may be stacked without bonding. In the construction illustrated inFIG. 3, eight main core portions150are formed in substantially the same way and are attached to one another to at least partially define the rotor core and shaft subassembly15a. The alignment core portion155is positioned with four main core portions150on either side to complete the rotor core and shaft subassembly15a. In the construction ofFIG. 3, only a single alignment core portion155is employed, with other constructions using two or more alignment core portions155.

A first main core portion150ais positioned adjacent the alignment core portion155on a first side of the rotor core and shaft subassembly15aand a second main core portion150bis positioned adjacent the alignment core portion155on a second side of the rotor core and shaft subassembly15a. In the construction illustrated inFIG. 3, the first and second main core portions150a,150bare positioned to have the same radial alignment with respect to one another. In other words, when viewed from the end, as inFIG. 2, the tangs85of the first and second main core portions150a,150balign with one another.

A third main core portion150cis positioned adjacent the first main core portion150aand a fourth main core portion150dis positioned adjacent the second core portion150b. In preferred constructions, the third and fourth main core portions150c,150dalign with one another, but are rotated with respect to the first and second main core portions150a,150b. As illustrated inFIG. 2 and 3a, the third and fourth main core portions150c,150dare rotated about 60 degrees with respect to the first and second core portions150a,150b.

The described process continues with a fifth main core portion150epositioned adjacent the third main core portion150cand aligned with the first main core portion150a. Similarly, a sixth main core portion150fis positioned adjacent the fourth main core portion150dand aligned with the second main core portion150b. A seventh main core portion150gis positioned adjacent the fifth main core portion150eand aligned with the third main core portion150c. Similarly, an eighth main core portion150his positioned adjacent the sixth main core portion150fand aligned with the fourth main core portion150d. In the preferred constructions, the rotor core is manufactured by aligning and bonding the main and alignment core portions and then the core is fitted to the shaft. In preferred constructions, a very close fit, such as interference or shrink fit exists between the alignment core portion155and the shaft30. The procedure described ensures that all the core sections are concentric with the shaft. Furthermore, this procedure produces a castellated (staggered) structure of the core15aand the space95around the shaft. Before proceeding, it should be noted that other arrangements are possible and are contemplated by the present invention. For example, other arrangements may vary the alignment of each core portion150rather than aligning every other core portion150. In addition, other rotors may include additional, or fewer, main core portions150or may include additional alignment core portions155.

The shaft30, eight main core portions150a-150h, and one alignment core portion155are then positioned within a mold such that a resilient material160such as plastic can be injection molded. The plastic160fills the spaces95between the main core portions150a-150hand the shaft30and also fills the space95between the shaft30and the alignment core portion155defined by the recesses140. In constructions that employ apertures145in the second laminations65, plastic also fills these apertures145. The plastic160serves to connect the various core portions150,155to the shaft30for rotation in unison, while simultaneously providing a damping member. The plastic160also locks the axial position of the core portions150,155on the shaft30. The castellated structure of the space95enhances the coupling between the core15a, the shaft30and the plastic160. Furthermore, undercuts161made into the shaft (seeFIG. 3) and/or knurling of the shaft surface enhances the coupling between the shaft30and the plastic160. During motor operation, some torque variations that would be transmitted through a more solid connection are dampened by the plastic connection. In other constructions, other materials are employed rather than plastic. For example, synthetic rubber or another injectable material may be used in place of plastic. The recesses140and the apertures145, when present, allow the plastic to flow axially during the injection molding from one end to another enhancing the manufacturability of the rotor.

It is important to note that the main core portions150a-150hinclude a back iron portion165that extends only part way to the shaft30, as also illustrated inFIG. 9. As such, a portion of the back iron165, that in more traditional rotor constructions would be part of the magnetic circuit (seeFIG. 8), is eliminated in the present construction and replaced with resilient material160.FIG. 8illustrates the magnetic flux lines in a motor170that includes a rotor back iron portion171that extends to the shaft30or nearly to the shaft30. As can be seen, very little magnetic flux crosses a surface of a radius175.FIG. 9illustrates the magnetic flux in a rotor portion (e.g. the main core portions150a-150h) that extends only to the aforementioned radius175. As can be seen, the magnetic flux in the back iron portion165is compressed slightly. However, this effect is minor and has a very small effect on the motor's overall performance. The minimum back iron radial thickness, defined as the difference between the radius at the base of the magnet (RBM)176and the radius175is calculated from the following equation and has been verified using the finite element method (as shown inFIGS. 8 and 9).
Minimum Back Iron Radial Thickness=RBM*PI/(# Poles)
In practice, a preferred range equal to 75 percent to 125 percent of the above value can be employed, with more preferred ranges being less than or equal to 100 percent of the calculated value.

The aforementioned equation can be used to design a rotor core having an optimal rotor yoke (back iron) radial thickness, dependent of the number of magnetic poles (# Poles). In a rotor construction with the magnets mounted on the rotor surface RBM176is defined as shown inFIGS. 8-9. In a rotor construction with the magnets inserted in the rotor and radially magnetized, commonly referred as an interior permanent magnet (IPM) rotor, RBM is defined as the minimum radius measured from the motor center to the face of a magnet. In a squirrel cage rotor, RBM is defined as the minimum radius measured from the motor center to a rotor bar. Throughout the text, twice the value of RBM is also referred as the “outside diameter”.

FIGS. 10 and 11illustrate another construction of a rotor156that includes a shaft180, and a rotor core182including several first laminations185, and at least two second laminations190. The shaft180is similar to the shaft30ofFIGS. 2-7and includes a core support portion that defines a radius195. The first laminations185, better illustrated inFIG. 11, define a central aperture200that has a radius that closely matches the shaft radius195and a plurality of outer apertures205arranged around the central aperture200and positioned radially outward. The outer apertures205reduce the weight of the rotor156, thereby reducing mechanical losses during operation. A plurality of first laminations185are stacked to define a large portion of the rotor core182. In some constructions, the outer apertures205align with one another to define cylindrical spaces210that extend the length of the stacked laminations. In the preferred constructions, the outer apertures205are placed closer to the shaft within the calculated diameter, which is equal to twice the radius175, in order to ensure a minimum back iron radial thickness that is substantially equal to the value calculated with the aforementioned equation.

Thus, the rotor core portion182illustrated inFIG. 11includes a first portion206that has a first (volumetric mass) density and a second portion207that has a second density. Each lamination185includes an outer portion and an inner portion that cooperate to define the first portion206and the second portion207respectively. In preferred constructions, the first portion206includes a ferromagnetic material that has a density that is substantially equal to the density of the first portion206. In other words, the first portion206includes solid ferromagnetic material with few, if any, apertures passing therethrough. The second portion207also includes ferromagnetic material. However, the outer apertures205that pass through the second portion207significantly reduce the density of the second portion207when compared to the density of the ferromagnetic material. In preferred constructions, the second density is at least 20 percent less than the density of the ferromagnetic material. When arranged as illustrated inFIG. 11, the effect of the outer apertures205on the rotor magnetic field and motor performance is greatly reduced.

Each of the second laminations190is positioned on one end of the stack to cover the outer apertures205. The second lamination190covers the open ends of the cylindrical spaces210and reduces windage losses that would typically occur if the cylindrical spaces210had remained uncovered. In other constructions, in which the use of the end laminations190is optional, the apertures205are filled with a light-weight material such as plastic to reduce the windage losses without significantly increasing the weight of the rotor core and shaft subassembly15b.

FIGS. 12-15illustrate another construction of a rotor core and shaft subassembly15cthat includes two rotor core portions215formed using laminations. The rotor core and shaft subassembly15cincludes a shaft220having a diameter225, a plurality of first laminations230, and a plurality of second laminations235. The first laminations230, several of which are illustrated inFIG. 13, are substantially annular rings that define an inside diameter240and an outside diameter245. Each first lamination230includes several lances250or indentations that define a pocket on one side of the lamination230and a protrusion on the other side of the lamination230. The protrusions of one lamination230fit within the depressions of the adjacent lamination230to align the laminations230as desired. Lances250of this type or other similar types could be employed with any laminations discussed herein.

The second laminations235, several of which are illustrated inFIG. 14, define an outside diameter255that substantially matches the outside diameter245of the first laminations230and an inside diameter260that substantially matches the shaft diameter225. Each of the second laminations235also includes lances250that correspond with, and are engageable with, the lances250of the first laminations230. Thus, the second laminations235can abut and align with the first laminations230.

Turning toFIG. 15, one of the rotor core portions215is illustrated. The core portion215includes several first laminations230positioned adjacent one another to at least partially define an internal space265. Several second laminations235are then positioned adjacent the first laminations230. The second core portion215is similar to the first core portion215and is positioned adjacent the first core portion215to completely define the internal space265. The second laminations235are positioned on either end of the internal space265and closely engage the shaft220to attach the core portions215to the shaft220, as shown inFIG. 12. In preferred constructions, the laminations230,235interlock to maintain their position and alignment. In some constructions, the internal space265is filled with a lightweight material such as plastic. The rotor ofFIG. 12is lightweight, thus reducing the motor's mechanical losses, and yet provides enough material (i.e., back iron) to conduct the magnetic flux as desired. In addition, the positioning of the second laminations235on the outer ends of the rotor core, rather than near the center, increases the stability and rigidity of the rotor core and shaft subassembly15cduring operation and reduces windage losses.

Before proceeding, it should be noted that all of the constructions described herein may include fasteners or other attachment systems (e.g., adhesive, welding, etc.) to hold the various laminations together. These systems can be permanent (e.g., adhesive, welding, etc.), or can be temporary. For example, one construction uses bolts that extend the length of the rotor core and hold the various laminations together. The bolt may be a permanent part of the motor or may be removed after magnets are attached to the rotor core. In other constructions, two or more laminated rotor sections, can be produced using a multiple stage punching (stamping) and interlocking (fastening) tool. For example, in the construction shown inFIG. 15, lances250are used to align and fasten several laminations230,235as well as the two core sections produced with laminations230and235, respectively, resulting a solid and rigid core portion215. As such, the invention should not be limited to rotors that include only the features illustrated herein.

FIGS. 16-28illustrate various constructions of rotors15that are manufactured from solid components rather then stacked laminations. The solid portions could be manufactured using, among other things, cast metallic elements, machined components, and/or powdered metal components. Powdered metal components, if employed, are formed by compressing a ferromagnetic powder or a soft magnetic composite in a mold that is shaped to define the final component. After the part is compressed, it may require a sintering step to complete the part. In still other constructions, final machining of the part may be required to add features and/or meet the required tolerances of the final part. The use of powdered metal to form rotor components has several advantageous over other manufacturing techniques. For example, intricate shapes can be formed in a single process without the need for expensive machining. In addition, the use of powdered metal allows for various compounds to be combined that otherwise could not be combined as an alloy. This property allows for greater control over the material properties of the finished parts. Also, the amount of scrap material for rotor fabrication is greatly reduced.

FIGS. 16-19illustrate a rotor core and shaft subassembly15dthat includes a shaft270and a rotor core275attached to the shaft270and including a first solid portion280, and a second solid portion285. The shaft270is a substantially cylindrical component that defines a shaft diameter290. While the illustrated shaft270includes a substantially uniform diameter portion in the region where the rotor core275attaches to the shaft270, other constructions may include a shaft270that includes portions with larger or smaller diameters in the region adjacent the rotor core275. In fact, any construction discussed herein may include a shaft that includes portions with larger or smaller diameter portions in the region adjacent the rotor core.

Each of the solid portions280,285defines an outside surface295and an inside aperture300. The inside aperture300defines an inner surface305having a diameter310that is larger than the shaft diameter290such that when positioned adjacent one another, the shaft270and each of the solid portions280,285cooperate to define a space315therebetween. From an electromagnetic point of view, the diameter310is selected such that the rotor back iron is equal to, or larger than the value calculated with the aforementioned equation. Furthermore, in the preferred construction, the minimum rotor core back iron in any rotor cross-section substantially equals the value calculated with the aforementioned equation. With reference toFIG. 17, three fingers320extend from the inner surface305toward the shaft270in a substantially radial direction.

The fingers320include a rounded inner most end325that when assembled abuts the shaft270. The rounded end325reduces the amount of material in contact with the shaft270after assembly and aids in centrally locating the shaft270. Because very little surface area contacts the shaft270, it is easier for that material to yield and move to accommodate and center the shaft270. Other constructions may employ a different number of fingers320or different shaped fingers320as desired. However, an odd number of fingers320is preferred as this reduces the likelihood of parasitic coupling with the magnetic field harmonics.

As illustrated inFIG. 18, each solid portion280,285also includes a plurality of teeth330positioned adjacent the outside surface295and extending axially to define a portion of the outside surface295. In the illustrated construction, three teeth330are spaced apart from one another by about 120 degrees and are sized to define spaces335between the adjacent teeth330that are about the same size as the teeth330. The resulting pattern, sometimes referred to as a castellated pattern, allows the two solid portions280,285to interconnect with one another such that they rotate with the shaft270in unison. It should be noted that because the first solid portion280and the second solid portion285are substantially the same (i.e., are interchangeable), the fingers320of the second solid portion285are rotated with respect to the fingers320of the first solid portion280by about 60 degrees. Other constructions may employ more or fewer teeth330as desired. In addition, different shaped teeth330(e.g., triangular semicircular, elliptical, etc.) could be employed if desired. In constructions that employ more or fewer teeth330as compared to the quantity of fingers320, it is possible to arrange the first solid portion280and the second solid portion285such that the fingers320align with one another or are rotated relative to one another at angles other than those discussed herein. In the preferred constructions, the teeth330are dimensioned and shaped in order to ensure, when the two solid portions280and285are mated together, very small or no air-gaps in the rotor core at least over the minimum back iron radial thickness, previously defined and calculated with the aforementioned formula. To enhance the coupling of core portions280and285an interference or shrink fit is employed for teeth330.

As shown inFIG. 18, each solid portion280,285includes a cylindrical alignment surface340that receives an annular ring345. The annular ring345includes an outer surface350that closely fits within the alignment surface340and an inner surface355that closely fits the shaft270.

To assemble the rotor ofFIGS. 16-19, the annular rings345are positioned adjacent the alignment surfaces340of the solid portions280,285. In some constructions an adhesive or other attachment system is employed to hold the annular rings345in place. In still other constructions, a press fit or interference fit between the annular rings345and the solid portions280,285holds the annular rings345in place. The solid portions280,285slide onto the shaft270and are positioned as desired. As shown inFIG. 19, the two solid portions280,285cooperate to define a hollow inner space360between the two solid portions280,285, with the annular rings345substantially sealing this space360. Resilient material362such as plastic or another material is injection molded into the spaces335to attach the solid portions280,285to the shaft270. In some constructions, plastic362is also injected into the hollow space360between the first solid portion280and the second solid portion285. After the plastic362(or other resilient material) has cured, magnets are attached to the outer surface295of the solid portions280,285, or inserted in the interior of the core to complete the rotor core and shaft subassembly15d. Electric motors, such as for example electrically commutated brushless PM machines often produce an uneven torque that may cause unwanted vibrations at the device being driven by the motor. Because the fingers320have only minimal surface contact with the shaft270, the torque is transmitted through the body of resilient material362, which reduces the transmission of torque ripple and vibrations between the core275and the shaft270.

FIGS. 20-24illustrate another construction of a rotor core and shaft subassembly15ethat includes a shaft365and a rotor core370made-up of a first core portion375and a second core portion380. As with prior constructions, the shaft365is substantially cylindrical and defines a shaft diameter385. As with other constructions, the shaft365may include different diameter portions (i.e., larger and/or smaller) as may be required by the particular application.

Each of the core portions375,380defines an outer surface390having an outer diameter and an inner surface395having an inner diameter. As shown inFIG. 22, three fingers400extend radially inward from the inner surface395such that each finger400contacts the shaft365when the core portions375,380are positioned on the shaft365. As with prior constructions, more or fewer fingers400or differently shaped fingers400could be employed if desired. Each core portion375,380also includes a contoured inner surface405that extends from the inner surface395in a first axial direction and three teeth410that extend axially in the opposite direction along the outer surface390. The contoured surface405reduces the weight of the rotor core portions375,380and enhances the torque transmission from the surface to the inner part of the rotor core370and the shaft365.

As illustrated inFIG. 22, the three teeth410align with the fingers400such that the fingers400extend the length of the teeth410. As with the construction ofFIGS. 16-19, the teeth410are spaced approximately 120 degrees apart and are sized to define a gap415between adjacent teeth410that is sized to receive a tooth410, of a mating core portion. Thus, the teeth410of the first core portion375fit within the gaps415of the second core portion380and the teeth410of the second core portion380fit within the gaps415of the first core portion375to couple the first and second core portions375,380for rotation. In preferred constructions, the first core portion375and the second core portion380are similar to one another such that they are interchangeable. Thus, as shown inFIG. 21, when the first core portion375and the second core portion380are interlocked, the fingers400of the second core portion380are rotated about 60 degrees with respect to the fingers400of the first core portion375. In constructions that employ a different number of fingers400or a different spacing for the fingers400, the relative angle between the fingers400of the first core portion375and the second core portion380may be greater then or less then 60 degrees. The core portions375and380, and in particular the fingers410together with the surface405are designed such that when the two core portions375and380are mated together, there are very small or no air-gaps in the rotor core at least over the minimum back iron radial thickness, previously defined and calculated with the aforementioned formula. To enhance the coupling of core portions375and380an interference or shrink fit is employed for teeth330.

A resilient material417, such as plastic, is positioned in the space defined between the shaft365and the inner surface of the first core portion375and the second core portion380. The resilient material417, shown inFIG. 24, extends between the teeth410such that the resilient material417couples the shaft365, the first core portion375, and the second core portion380for rotation. In some constructions, resilient material417is also positioned in the space defined between the contoured inner surface405and the shaft365. Preferably, an injection-molded plastic is employed as the resilient material417. However, other constructions may employ other materials or other methods to position the material.

The construction ofFIGS. 16-19differs from the construction ofFIGS. 20-24in that a device or means, e.g. the annular ring345, is required in the construction ofFIGS. 16-19to contain the resilient material between the fingers320as it is injected. The construction ofFIGS. 20-24does not require this device as the fingers400are positioned near the center of the core370rather than at the ends. However, the construction ofFIGS. 16-19is advantageous over the construction ofFIGS. 20-24for other reasons. For example, the solid portions280,285of the construction ofFIGS. 16-19are such that the attachment between the solid portions280,285and the shaft270is located near the ends of the core275, thus enhancing the mechanical properties of the rotor core275. In addition, the solid portions280,285ofFIGS. 16-19include a substantially large flat or planar surface420, which can be used to press against during the powder compression process and as a support during the sintering process. To some extent such a flat surface is represented in the construction ofFIGS. 20-24by the flat faces of the teeth410.

FIGS. 25-28illustrate another construction of a rotor core and shaft subassembly15fthat is similar to the construction ofFIGS. 20-24. As shown inFIG. 25, the rotor core and shaft subassembly15fincludes a shaft425and a rotor core430that includes a first core portion435and a second core portion440. As with prior constructions, the shaft425is a generally cylindrical component that defines a shaft diameter445. In some constructions, the shaft425may include larger or smaller diameter portions as desired.

Each of the core portions435,440include an outer surface450that defines an outer diameter and an inner surface455that defines an inside diameter. The inside diameter closely matches the shaft diameter445to align the core portions435,440on the shaft425. A contoured inner surface460extends from the inner surface455in a first direction and cooperates with the shaft425to define a space465.

Three teeth470extend axially from each of the core portions435,440in substantially the opposite direction as the contoured inner surface460. Each tooth470has a substantially trapezoidal axial cross-section with a cylindrical inner surface475and a cylindrical outer surface480that is generally coincident with the outer surface450. The cylindrical inner surface475defines a diameter that is larger than the shaft diameter445. Thus, the cylindrical inner surface475and the shaft425cooperate to define an interior space485, as shown inFIG. 28. Each tooth470is spaced approximately 120 degrees from the adjacent teeth470and cooperates with the adjacent teeth470to define a gap490sized to receive a tooth470. As such, the teeth470of the first core portion435fit within the gaps490of the second core portion440and the teeth470of the second core portion440fit within the gaps490of the first core portion435to interlock the core portions435,440. In preferred constructions, the first core portion435and the second core portion440are substantially the same such that they are interchangeable. However, other constructions may vary the first core portion435with respect to the second core portion440.

In some constructions, a resilient material495, such as plastic, may be positioned within the interior space485to attach the first core portion435and the second core portion440to the shaft425for rotation. In addition, the resilient material495may be positioned in the space between the contoured inner surfaces460and the shaft425. Preferably, an injection-molded plastic is employed as the resilient material495. However, other constructions may employ other materials or other methods to position the material. It should be noted that the resilient material495as used in the construction ofFIGS. 25-28does not provide significant damping. Thus, reduced cogging, torque ripple, noise and vibration for this construction must be achieved using other methods, such as skewed magnets.

The constructions previously described are especially suited for motors with relatively thin back iron, such as high pole count motors. The constructions are also suitable for motors for which the performance is less influenced by the value of the rotor magnetic permeance and that have a relatively low specific torque output per unit length, such as, for example, brushless permanent magnet machines with ferrite magnets mounted on the outer surfaces of the rotor.

As with all of the constructions discussed herein, permanent magnets can be attached to the outer surface of the rotor cores or inserted in the rotor cores to complete the rotor assembly. It should be noted that the present invention could be employed with other types of motors or generators. For example, the present invention could be applied to interior permanent magnet motors as well as squirrel cage motors. In addition, the present invention could be applied to inside-out motors if desired.

The rotor constructions of the invention reduce the torque ripple, noise, and force vibrations, that prior art rotors transmit. Specifically, the use of resilient material between the rotor core and rotor shaft at least partially isolates the two components such that noise, torque ripple, or force vibrations applied to the core are at least partially damped by the resilient material, rather than being transmitted to the rotor shaft.

In addition, the shape of the laminations or solid core portions greatly increase the concentricity of the shaft to rotor core over that of the prior art. The improved concentricity reduces the need for balancing and reduces the vibrations caused by rotor mechanical imbalance and unbalanced magnetic forces.

Furthermore, many of the constructions illustrated herein include a reduced back iron portion. The reduction in back iron reduces the weight of the rotor and reduces the amount of material required to produce the rotor. The reduction in weight improves the efficiency of the motor and reduces the rotational stress applied to the motor components, while also reducing the material used and the cost of the motor. For example, the constructions ofFIGS. 16-28include large spaces that may or may not be filled with a resilient material. The large spaces reduce the quantity of back iron in the rotor core but do not greatly affect the flow of magnetic flux within the core, as illustrated inFIGS. 8 and 9. The constructions ofFIGS. 2-7and10-15similarly include a reduced back iron portion that does not greatly affect the flow of magnetic flux within the core.

Thus, the invention provides, among other things, a new and useful rotor for an electric machine. The constructions of the rotor and the methods of manufacturing the rotor described herein and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the invention. Various features and advantages of the invention are set forth in the following claims.