GB2492422A - Anti-centrifugal expansion support ring for rotor bars - Google Patents

Abstract

A electric machine rotor 210 that resists expansion of the rotor components at high rotational speed includes a apertured support disk 226, 227 placed between the laminations stacks 216a, 216b, the laminations having slots to accept rotor bars 217. The support disk, which may be electrically insulated (coated or anodised) or made of electrically insulating material, slidingly receives the rotor bars and restrains the rotor bars 218 from bending outwardly at high rotational speeds. The rotor bars 218 are further restrained at the ends by end rings 214, which have apertures 114 into which ends of the rotor bars are placed, the bar ends being welded to the rings. Containment rings 204 are placed over axial extension of the end rings to prevent outward bowing at high speeds. The rotor may be directly mounted to the shaft (340 fig 7) or via a stiffener sleeve 202 providing additional resistance to expansion. Projections 222,233 bound a portion of the sleeve for receiving a tool for disassembly. In a further embodiment, the rotor end rings, support disk and bars may be injection moulded to the spaced lamination sets. Three lamination sets and two support rings may be used.

Description

ELECTRIC MOTOR ROTOR

The present invention relates to the field of electric motors and more specifically to rotors of such motors that contain magnetic field rcactivc elements suitable for high speed operations.

Particularly challenging aspects in thc design of the rotor of an electric motor that has the capability to be driven at speeds exceeding 100,000 rpm concern the prevention of centrifugal forces from radially expanding the rotor elements such that they become separated from the shaft to which they are attached. In the case of an induction motor, it is important to prevent expansion of the rotor elements to avoid them coming into contact with the stator element.

Rotor bars are typically received and supported by apertures formed in the laminations of the core. However, such laminations have been found to lack sufficient strength to retain the rotor bars at high rotational speeds, due to the large forces generated.

US2O1O/0308685 describes a set of central laminations of reduced diameter about which a retaining ring is provided to increase the radial strength of the laminations at the central position along the length of the rotor bars, to resist radial expansion. However, this solution requires the manufacture of alternate sized laminations, and the installation of the anti-expansion ring increases the complexity and cost of manufacturc. Therefore, there is a desire for an alternative for retaining expansion of the rotor bars.

It is therefore desirable to provide a rotor for an clcctric motor, and a mcthod for forming the samc, which addresses the abovc described problems and/or which offers improvements generally.

According to the present invention there is provided a method of manufacturing a rotor of an electric machine as described in the accompanying claims. In addition, there is also provided according to the present invention a rotor for an electric machine, as described in the accompanying claims.

Electric motor rotors disclosed herein are suitable for use in turbochargers and other environments where motors may be required to operate at significantly high speeds exceeding 100,000 rpm. Typically, electrically controlled turbochargers employ a high speed electrical motor to rotate the turbo sha.fl which exists between the oppositely mounted compressor and turbine. The embodiments disclosed herein provide a center supporting disk on the rotor to provide additional support to the rotor bars to minimize their outward deformation during high speed operation.

In an embodiment of the invention there is provided a method of manufacturing a rotor of an electric machine, the method comprising arranging two sets of rotor laminations on either side of a support disk; inserting rotor bars into slots defined in the rotor laminations and the support disk; and installing means for retaining the ends of the rotor bars. The support disc is configured and arranged to radially restrain the rotor bars during high speed rotation. The support disc is configured to have significantly greater radial strength than the laminations, which permits the laminations to retain a configuration which optimizes their electrical properties without concern for their radial strength with the support disc ensuring that radial distortion of the rotor bars is minimized or obviated.

The first and second plurality of laminations preferably each have a first thickness and the support disc has a second thickness greater than the first thickness of the laminations such that the radial strength of the support disc is greater than the radial strength of the laminations. A single piece support disc of greater thickness than the laminations has significantly greater radial strength than the laminations.

The means for retaining the ends of the rotor bars preferably comprises end rings at ends of the rotor bars with apertures defined in the end rings engaging with ends of the rotor bars.

The support disc preferably has a diameter equal to the diameter of the laminations, which provides the core with a constant diameter and ensures optimized balance during rotation.

The support disc is preferably a one-piece unitary component, which optimizes strength and simplifies construction and manufacture.

The support disc is preferably electrically insulated to prevent electrical interference with the laminations.

In another embodiment of the invention, there is provided a rotor of an elcctric machine, comprising a first plurality of laminations arranged axially, the first plurality of laminations defining a plurality of slots; a second plurality of laminations arranged axially, the second plurality of laminations defining a plurality of slots; a support disk defining a plurality of slots and located between the first and second plurality of lamination with slots of the first and second pluralities of lamination and slots of the support disk being aligned; and a plurality of rotor bars received within the plurality of aligned slots. The support disc is configured and arranged to radially restrain the rotor bars during high speed rotation to prevent movement of the rotor bars in the radial direction. As such the support disc simultaneously prevents distortion and damage to both the rotor bars and the laminations.

The present invention will now be described by way of example only with reference to the following illustrative figures in which: Figures IA and lB are exploded views of components included in a rotor of an electric induction motor according to an embodiment of the invention; Figure 2 is a cross-sectional plan view along the axis of an induction motor rotor assembly comprising the components shown in Figures A and 1B; Figure 3 is a plan view of a lamination taken along section line 111-111 in Figure 2; Figure 4 is an enlarged view of a lamination aperture from Figure 3, containing a rotor bar; Figure 5 is a plan view of a center support ring taken along section line V-V in Figure 2; Figure 6 is an enlarged view of a portion of Figure 5 of a center support ring aperture containing a rotor bar; Figure 7 is a cross-sectional plan view along the axis of an induction motor rotor assembly mounted directly on a shaft; and Figures 8 and 9 illustrate embodiments by which a rotor may be assembled.

In Figure IA the major components of a rotor 200 of an electric induction motor include an assembled rotor element 210, containment rings 204 and 206 and stiffener sleeve component 202 for mounting on a rotor shaft 240 (Figure 2).

In Figure lB and Figure 2, rotor element 210 is shown to include two end rings 212 and 214 (sometimes referred to as "balance" rings) having a plurality of apertures 112 and 114, a plurality of(l 9) rotor bars 218, and a plurality of (65) steel laminations in sets 21 6a and 216b arranged in axially aligned stacks. A central supporting disk (also referred to herein as an anti-expansion disk) 226 is centrally located between laminations sets 216a and 211Th. Rotor bars 218 slide through and are received and held within apertures in steel lamination sets 216a and 216b, apertures in supporting disk 226, and apertures in end rings 212 and 214. The purpose of central supporting disk 226 is to minimize the effects of centrifugal forces from distorting the rotor bars 21 8 during high speed operations as discussed in further details below.

Steel laminations 216 can be formed of high-strength electrical steel, such as Hyperco 5OTM, heat treated to provide maximum strength, and oxide coated to prevent electrical current losses between laminations. Rotor bars 218 can be madc from a high strength-to-density ratio (specific modulus) and high electrical conductivity alloy, such as 2219 Al.

During assembly, rotor lamination sets 216a and 216b are coaxially arranged in stacks on either side of support disk 226. Rotor bars 218 are inserted into (or molded in) slots 217 (21 7a -217s) and 227 (227a -227s) in the laminations 216 and support ring 226 respectively. End rings 212 and 214 are installed on each end and the ends of rotor bars are received into apertures 112 and 114 of the balance rings 212 and 214, respectively.

The assembly is then clamped togethcr axially to compress the laminations together.

Rotor bars 218 are then welded to end rings 212 and 214. Such welding may employ an electron beam process or any other process that provides effective high strength welding for such metals. Heat sinks arc attached to the rotor during this process to minimize the distortional effects of welding. After welding, rotor 210 is machined on all outside surfaces and the ID to improve concentricity of the inside diameter, ID, and outside diameter, OD, as well as balance.

Following machining, the rotor assembly 210 is slid onto the stiffener sleeve 202. The assembly is then balanced and the stiffener sleeve 202 is press fitted onto shaft 240.

While there may be some tolerance between the stiffener sleeve 202 and the ID of the laminations to prevent pre-stress in the laminations, the end rings 212 and 214 and central support disk 226 are press fitted onto the sleeve 202 to secure the rotor assembly 210 to shaft 240 under extremes in operational circumstances.

The rotor 210 can alternatively be injection molded in a high-pressure injection molding process where the rotor laminations 21 6a and 216b are placed in a mold and molten aluminum is injected into slots 217 and 227 to form the rotor bars 218. In the same process, end rings 212 and 214 and central support disk 226 are also formed.

End rings 212 and 214 are preferably fabricated from the same or similar alloy used to fabricate the rotor bars 218 and serve to minimize expansion of the rotor ends during high speed operations. Furthermore, central support disk 226 may be fabricatcd from the same or similar alloy as used for end rings 212 and 214 and rotor bars 218.

The end rings 212 and 214 radially locate and hold the rotor bars 218 in a fixed radial position. In addition, the end rings 212 and 214 act to counter the centrifugal forces experienced by the rotor bars 218 during rotation.To further mitigate the effects of centrifugal forces generated at high rotational speeds, the end rings 212 and 214 can include axial extensions 213 and 215. Extensions 213 and 215 are smaller in diameter than the main body of the end rings 212 and 214. By making end-ring extensions 213 and 215 smaller in diameter, the extensions experience much less centrifugal force and therefore retain their press fit onto the stiffener 202 and shaft 240 throughout thc broad range of operating speed.

In some embodiments, containment rings 204 and 206, formed of high strength steel, arc clamped around the end rings 212 and 214 to further ensure the integrity of the press fit between end rings, stiffener sleeve 202 and shaft 240. In Figures IA and 2, containment rings 204 and 206 are located on end ring extensions 213 and 215.

When employed in an electrically-controlled turbocharger design, motor rotors are typically elongated. While the ends of the rotor bars 218 are restrained by the end discs 212 and 214, there is a concern that the length of the rotor bars, such as is shown in Figures lB and 2, may result in the rotor bars being subjected to large centrifugal forccs at high rotational speeds that act on the central portions of the rotor bars forcing them outwardly in a radial direction sufficient to affect the motor-to-stator air gap. If distortion of the rotor bars is too great, the rotor contacts the stator.

The laminations 216 are thin disc members which are provided to minimize and reduce stray circulating currents that would result in eddy current loss. As such, the strength of the laminations 216 is compromised in favour of the electrical properties of these members. As a result, it has been found that at high rotational speeds, the laminations arc not capable of suitably restraining radial expansion of the rotor bars, In some embodiments, the individual laminations are provided with an oxide coating to prevent shorting between adjacent laminations and to prevent shorting between the surfaces of the slots formed in the laminations to the rotor bars. If large outward forces act upon the laminations the oxide coating on the surfaces of slots 217 could wear and eventually lead to shorting between laminations and the rotor bars.

Therefore, in addition to the end rings 212 and 214, the central support disk 226 is provided at an axially central location along the rotor bars 218 to restrain radial expansion of the rotor rods 218. The support disc 226 comprises a solid, one-piece disc member of unitary construction. The support disc 26 includes a plurality of apertures located radially inwards of its peripheral edge configured for receiving the rotor bars 218. As shown in Figure 2, the support disc 226 has an axial thickness greater than the axial thickness of the laminations 216. The support disc 226 preferably has a diameter equal to the diameter of the adjacent laminations 216, such that the core is of a constant diameter which is assists in stabalising the core during rotation.

The support disc 226 is preferably formed from an aluminium alloy, which may be 22 series aluminum alloy, and the end discs 212 and 214 are preferably formed of the same material. The support disc 226 is configured to radially restrain the rotor bars 218 during rotation to prevent radial expansion thereof, and has a significantly greater radial strength and resistance to radial deformation that the laminations 216.

The support disc 226 is preferably electrically insulated. In one embodiment the support disc 226 is coated in an electrically insulating material. Alternatively, the support disc 26 is anodised. In an alternative embodiment, insulating spacer members formed of an electrically-insulating material may be provided between the spacer disc 226 an the laminations 216.

In the embodiment shown in Figure 2, central support disk 226 is shown between lamination stacks 216a and 216b. In an embodiment in which it is advantageous to have a particularly long rotor, at least one additional support disk is provided between additional sets of lamination stacks. Thus, in one alternative embodiment, there are three sets of lamination stacks with a first support disk provided between first and second sets of the lamination stacks and a sccond support disk provided between second and third set of the lamination stacks.

In Figure 2, a first protuberance 222 and a second axial protuberance 223 extend outwardly in a radial direction from stiffener sleeve 202. The space between the two protuberances 222 and 223 is of a smaller diameter. This smaller diameter portion provides a shoulder for a tool to grab onto the stiffener sleeve 202 for disassembly.

Figure 3, a cross-sectional view of lamination 216a taken along section line Ill-Ill in Figure 2, shows the distribution of the 19 slots 217a -217s. In this view, the stiffener 202 is shown surrounding the rotor shaft 240. Rotor bars 218a -218s are inserted into the corresponding slots 2 17a -217s.

As call be seen in Figure 4, the enlarged view of slot 217a in lamination 216a is radially oriented. Rotor bar 2i 8a is inserted into the slot 217a. When stationary, as shown in Figure 4, slot 217a is slightly largcr than rotor bar 218a. An air gap 219 exists between slot 217a and rotor bar 21 8a. At high rotational speeds, rotor bar 218a expands more than lamination set 216a and thus more than slot 217a. Thcrefore, at high speed, air gap 219 is taken up by the expanded rotor bar 21 8a. An air gap opening 220 provides a separation so that poles are formed in the adjacent teeth (the radial portions of the laminations between the adjacent slots) In Figure 5, a cross-sectional view through central support disk 226 taken along section hue V-V in Figure 2 shows the distribution of the 19 slots 227a -227s. Central support disk 226 surrounds stiffener 202 which is press fitted to the rotor shaft 240. Rotor bars 218a -218s are inserted into the corresponding slots 217a - 21 7s. There is no air gap between slot 227a of central support disk 226 and rotor bar 218a as can be seen in detail in the embodiment shown in Figure 6 because rotor bar 218a and central support disk 226 are made of materials with similar expansion characteristics.

Further, by avoiding an air space, support disk restrains rotor bar 218a from outward movement.

Figure 7 illustrates a rotor assembly 300 that is mounted directly on rotor shaft 340 that can be used in environments where a stiffening component is not included. Rotor assembly 300 includes two end rings 312 and 314, a plurality of rotor bars 318 (only onc of which is shown in this cross section) and steel lamination sets 31 6a and 31 6b that are axially aligned stacks. A central support disk 326 is centrally located and has slots through which rotor bars 318 are inserted. Central disk 326 provides stiffening to minimize the distortion of the rotor bars at high rotational speeds. Extensions 313 and 315, extending axially from end rings 312 and 314 are smaller in diameter than the main body of the end rings 312 and 314 to reduce the mass surrounding the press fit to shaft 340.

The procedure to assemble the rotor assembly onto the shaft, according to one embodiment, is illustrated in Figure 8. In 800, laminations stacks 216a and 216b are arranged on either side of supporting disk 226. The slots are aligned so that in 802 the rotor bars 218a-s can be inserted in the slots through 216a, 216b, and 226. In 804, end rings 212 and 214 are slid onto rotor bars 218a-s with apertures in the end rings engaging with the rotor bars. In 806, the assembly is clamped together to compress the laminations axially and a heat sink is attached prior to welding the end rings 212 and 214 to the rotor bars 218. The welding process may be an electron beam process. In blocks 800 through 808, rotor assembly 210 is formed, designated as 810 in Figure 8. After rotor assembly 210 is welded, it is machined to improve its concentricity and balance. In embodiments that include containment rings 204 and 206, they are fit onto extensions 213 and 215, respectively, in 814. Rotor assembly 210 is press fit onto stiffener sleeve 202 in block 816. Tn one embodiment, only the end rings 212 and 214 and central support disk 226 are press fit on the stiffener sleeve. Lamination sets 216a and 216b are slightly oversize, with respect to the inner diameter, to avoid cracking the laminations during assembly. In block 818, stiffener sleeve 202 is press fit onto rotor shaft 240.

In one embodiment, the end rings, the central support disk, and the rotor bars are made of the same material, e.g., an aluminum alloy, and these arc produced by injection molding.

In such embodiment, the manufacture begins with stacking laminations sets 216a and 216b, as illustrated in Figure 9. starting at 900. Lamination sets 216a and 216b are placed in an injection mold and secured in place during molding, at 902. Also in 902, a die is centrally located within the laminations so that aluminum is not injected into the space reserved for rotor shaft 240 and stiffener sleeve 202. The molten aluminum alloy is injected into the mold, in 904. The end rings, central support disk, and rotor bars are one integral part, which, of course, cannot be disassembled from the laminations. The combination of parts forms the rotor assembly. The rotor assembly is cooled, 906, before being ejected from the mold, 908. In 908, the die is removed from the rotor assembly.

Group 910 designates the processes to form the rotor assembly. In 912, the rotor assembly is machined to remove artifacts from the mold process. Additionally, the machining may improve the dimensional accuracy and hence balance and fit of the rotor assembly.

In embodiments that include containment rings 204 and 206, they are fitted onto extensions of the end rings, respectively, in 914. Rotor assembly is press fit onto stiffener sleeve 202 in block 916. In one embodiment, only the end rings and central support disk 226 are press fit on the stiffener sleeve. Lamination sets 216a and 216b may be slightly oversize, with respect to the inner diameter, to avoid cracking the laminations during assembly. In block 818, stiffener sleeve 202 is press fit onto rotor shaft 240. In embodiments in which a stiffener sleeve 202 is not used, the rotor assembly is press fit directly onto rotor shaft 240.

The embodiments shown here are exemplary in nature and shall not be considered to be a restriction on the scope of the claims set forth herein.

Claims (1)

1. <claim-text>CLAIMS1. A method of manufacturing a rotor of an electric machine, the method comprising: arranging two sets of rotor laminations on either side of a support disk; inserting rotor bars into slots defined in the rotor laminations and the support disk; and installing means for retaining the ends of the rotor bars; wherein the support disc is configured and arranged to radially restrain the rotor bars during high speed rotation.</claim-text> <claim-text>2. The method of claim 1 wherein the first and second plurality of laminations each have a first thickness and the support disc has a second thickness greater than the first thickness of the laminations such that the radial strcngth of the support disc is greater than the radial strength of the laminations.</claim-text> <claim-text>3. The method of claim I or 2 where the means for retaining the ends of the rotor bars comprise end rings at ends of the rotor bars with apertures defined in the end rings engaging with ends of the rotor bars.</claim-text> <claim-text>4. The method of any preceding claim wherein the support disc has a diameter equal to the diameter of the laminations.</claim-text> <claim-text>5. The method of any preceding claim wherein the support disc is a one-piece unitary component.</claim-text> <claim-text>6. The method of any preceding claim wherein the support disc is electrically insulated.</claim-text> <claim-text>7. The method of any of claims Ito 4 wherein: the inserting rotor bars comprises injection molding the rotor bars into the slots; and the slots in the rotor laminations and the central support disk are aligned prior to the injection molding.</claim-text> <claim-text>8. The method of claim 3, further comprising: clamping the end rings axially with the rotor laminations and the support disk clamped therebetween; and welding tips of the rotor bars to the end rings.</claim-text> <claim-text>9. The method of any preceding claim wherein the rotor laminations, the rotor bars, means for retaining the ends of the rotor bars, and the support disk comprise a rotor assembly, the method further comprising: providing a heat sink in contact with the rotor assembly during the welding.</claim-text> <claim-text>10. The method of claim 9 further comprising: machining outer surfaces of thc rotor assembly to improve the balance of the rotor assembly during rotation of the rotor assembly.</claim-text> <claim-text>11. The method of claim 9 or 10 further comprising press fitting the rotor assembly onto a stiffener sleeve.</claim-text> <claim-text>12. The method of claim 11 further comprising press fitting the stiffener sleeve on a rotor shaft.</claim-text> <claim-text>13. The method of claim 9 further comprising press fitting the rotor assembly onto a rotor shaft.</claim-text> <claim-text>14. A rotor of an electric machine, comprising: a first plurality of laminations arranged axially, the first plurality of laminations defining a plurality of slots; a second plurality of laminations arranged axially, the second plurality of laminations defining a plurality of slots; and a support disk defining a plurality of slots and located between the first and second plurality of lamination with slots of the first and second pluralities of lamination and slots of the support disk being aligned; and a plurality of rotor bars received within the plurality of aligned slots; wherein the support disc is configured and arranged to radially restrain the rotor bars during high speed rotation to prevent movement of the rotor bars in the radial direction.</claim-text> <claim-text>15. The rotor of claim 14 wherein the first and second plurality of laminations each have a first thickness and the support disc has a second thickness greater than the first thickness of the laminations such that the radial strength of the support disc is greater than the radial strength of the laminations.</claim-text> <claim-text>16. The rotor of claim 14 or 15 wherein the support disc has a diameter equal to the diameter of the laminations.</claim-text> <claim-text>17. The rotor of any one of claims 14 to 16 wherein the support disc is a one-piece unitary component.</claim-text> <claim-text>18. The rotor of any one of claims 14 to 17 wherein the support disc is electrically insulated.</claim-text> <claim-text>19. The rotor of any one of claims 14 to 18, further comprising first and second end rings defining apertures with first ends of the rotor bars engaging with the apertures of the first end ring and second ends of the rotor bars engaging with the apertures of the second end ring.</claim-text> <claim-text>20. The rotor of claim 19 wherein the first ends of the rotor bars arc welded to the first end disk and the second ends of the rotor bars arc welded to the second end disk.</claim-text> <claim-text>21. The rotor of any one of claims 14 to 20, further comprising: a stiffener sleeve wherein the welded assembly is fit onto the stiffener sleeve; and a rotor shaft wherein the stiffener sleeve is fit onto the rotor shaft.</claim-text> <claim-text>22. The rotor of claim 21 wherein the stiffener sleeve comprises a protuberance extending radially outwardly from the stiffener sleeve wherein the protuberance provides a shoulder for a removal tool.</claim-text> <claim-text>23. The rotor of claim 21 or 22, further comprising: a rotor shaft wherein the welded assembly is fit onto the rotor shafi.</claim-text> <claim-text>24. A method of manufacturing a rotor of an electric machine substantially as hereinbefore described with reference to, and/or as shown in figures 1 to 9.</claim-text> <claim-text>25. A rotor of an electric machine substantially as hereinbefore described with reference to, and/or as shown in figures 1 to 9.</claim-text>