Rotor balancing

Abstract

A rotor (330, fig 4) for an electric motor, comprising a shaft (350, fig 4), a rotor body disposed about the shaft, and first and second end pieces 420 disposed about the shaft at each end of the roto

Description

TECHNICAL FIELD The present disclosure relates to balancing of a rotor for an electric motor. Aspects of the invention relate to a rotor, to a permanent magnet synchronous motor, to a vehicle, and to a method of balancing a rotor for an electric motor. BACKGROUND It is known to provide rotors for electric motors in which elements of the rotor are retained by an end cap at each end. In particular, for a permanent magnet synchronous motor, end caps are typically provided to retain the main functional elements of the rotor, which may comprise a set of discs (“pucks”) each containing an array of permanent magnets. The end cap is constructed to retain other rotor elements, and it may also comprise keys to locate the rotor on a shaft - these keys are typically located in grooves on the shaft. Induction motors also have end pieces, typically integrally formed with the bars of a “squirrel cage” design. Such end pieces may be challenging to design so that they achieve their desired functionality while maintaining a suitable lifetime so that they do not become a point of failure for the motor. A further issue to address with such rotors is balancing - these are complicated devices containing many components, and positive action may be required to balance a manufactured rotor. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a rotor for an electric motor, a permanent magnet synchronous motor, a vehicle, and to a method of balancing a rotor for an electric motor as claimed in the appended claims. According to an aspect of the present invention there is provided a rotor for an electric motor, wherein the rotor comprises a shaft, a rotor body disposed about the shaft, and first and second end pieces disposed about the shaft at each end of the rotor body, wherein the first and second end pieces each have an inner face facing a body of the rotor, and an outer face facing outwardly from the body of the rotor. For balancing the rotor, at least one of the first and second end pieces has one or more blind holes drilled in the outer face thereof to balance the rotor, wherein each blind hole lies within predetermined size constraints. Using this approach, rotor balancing can be accomplished without introducing problematic stresses into the rotor, in use, thus achieving rotor balancing while maintaining rotor life. According to an aspect of the present invention there is provided a rotor for an electric motor, wherein the rotor comprises a shaft, a rotor body disposed about the shaft, and first and second end pieces disposed about the shaft at each end of the rotor body, wherein: the first and second end pieces each have an inner face facing a body of the rotor, and an outer face facing outwardly from the body of the rotor; and for balancing the rotor, at least one of the first and second end pieces has one or more blind holes drilled in the outer face thereof, wherein each blind hole is disposed in a load balancing region of the outer face and is shallower than a predetermined maximum depth, wherein the load balancing region is an annular region of the outer face of the end piece centred on an axis of the rotor. This strategy for rotor balancing provides constraints on load balancing holes which allows them to be introduced without a problematic increase of stresses in the rotor end piece. The number of holes provided, and their position - within the constraints provided - can be varied to achieve rotor balancing. This balancing of the rotor is in respect of the entire rotor after assembly of the pucks and end pieces. The balancing serves to ensure coincidence of the centre of mass of the rotor with the rotor axis. This may involve drilling holes in one or both the end pieces, depending on where along the rotor between the end pieces that any imbalance of the rotor is detected. That is, imbalance detected nearer one end piece than the other would mean adjusting the balance of the end piece near the imbalanced region, whereas imbalance near the centre of the rotor implies adjusting the balance of both end pieces. The centre of mass of the rotor should not only coincide with the rotor axis but also desirably, but not essentially, be located at a predetermined location along the axis in order to better suit the location and nature of bearings supporting the rotor shaft. In embodiments, the load balancing region may be an annular region disposed more than a predetermined minimum distance from the axis of the rotor. In specific embodiments, each blind hole may be disposed at substantially the same radial distance from the axis of the rotor. This may define a pitch circle around the rotor axis. This pitch circle may, for example, have a minimum diameter of 2 times the shaft diameter of the rotor -for a 50mm diameter shaft, which would be appropriate for an electric motor used in the powertrain of an electric vehicle, this pitch circle diameter would then be a minimum of 100mm, and preferably at substantially 111.5mm. In embodiments, each blind hole has a predetermined maximum diameter. This may be at a maximum diameter of 0.2 times the shaft diameter, and in particular embodiments at a maximum of 0.18 times the shaft diameter. In particular embodiments, each blind hole may have substantially the same diameter. In embodiments, the predetermined maximum depth may be 0.18 times the shaft diameter. In embodiments, each blind hole may have substantially the same depth. Using these dimensional constraints for the blind holes allows load balancing without introduction of any significant new stresses in the end piece. In certain embodiments, blind holes may be drilled in both the first and second end pieces, wherein in other embodiments, blind holes may be drilled in only one of the first and second end pieces. It is found that both strategies may be effective to achieve balancing of the rotor in particular imbalance conditions. In embodiments, the rotor may be the rotor of a permanent magnet synchronous motor, in which case the first and second end pieces are rotor end caps. Such a balancing strategy may also be applied to an induction motor, for example an induction motor of the “squirrel-cage” variety, in which case the end pieces may be or may incorporate the end rings of the rotor. In a further aspect of the invention, there is provided a permanent magnet synchronous motor comprising a rotor as described in the previous aspect. Such a motor has an effectively balanced rotor without compromise to rotor lifetime, and hence to motor lifetime. In a still further aspect of the invention, there is provided an electric vehicle having a powertrain comprising a permanent magnet synchronous motor as described in the previous aspect. In a yet further aspect of the invention, there is provided a method of balancing a rotor for an electric motor, wherein the rotor comprises a shaft, a rotor body disposed about the shaft, and first and second end pieces disposed about the shaft at each end of the rotor body, wherein the first and second end pieces each have an inner face facing a body of the rotor, and an outer face facing outwardly from the body of the rotor, the method comprising: determining imbalance for the rotor; drilling one or more blind holes in the outer face of at least one of the first and second end pieces, wherein each blind holes is drilled in an annular load balancing region of the outer face ofthe end piece centred on an axis of the rotor, and wherein each blind hole is shallowerthan a predetermined maximum depth; wherein sufficient blind holes are drilled to correct the determined imbalance. By using this approached, the rotor can be balanced effectively without introduction of stresses that could limit the lifetime ofthe part. In embodiments, the predetermined maximum depth may be 0.18 times the shaft diameter. In embodiments, each blind hole may have a predetermined maximum hole diameter. This may be at a maximum diameter of 0.2 times the shaft diameter, and in particular embodiments at a maximum of 0.18 times the shaft diameter. In particular embodiments, each blind hole may have substantially the same diameter. In particular embodiments, each blind hole may be disposed at substantially the same radial distance from the axis ofthe shaft. This may define a pitch circle around the rotor axis. This pitch circle may, for example, have a minimum diameter of 2 times the shaft diameter ofthe rotor. In embodiments, the rotor may be the rotor of a permanent magnet synchronous motor, and wherein the first and second end pieces are rotor end caps. Such a method may also be applied to the end pieces of other motor types containing a rotor, such as a “squirrel-cage” induction motor. In a yet further aspect of the invention, there is provided a rotor for an electric motor, wherein the rotor comprises a shaft having a shaft diameter and defining a rotor axis, a rotor body disposed about the shaft, and first and second end pieces disposed about the shaft at each end of the rotor body, wherein the first and second end pieces each have an inner face facing the rotor body, and an outer face facing outwardly from the rotor body; and at least one ofthe first and second end pieces has one or more blind holes in the outer face thereof and serving to balance the rotor, wherein each blind hole is disposed in a load balancing region ofthe outer face and is shallowerthan a predetermined maximum depth, wherein the load balancing region is an annular region ofthe outer face ofthe end piece centred on an axis ofthe rotor. In embodiments, the load balancing region is an annular region centred on the shaft axis and having a pitch whose diameter is at least 2 times the shaft diameter. In embodiments each blind hole is disposed at substantially the same radial distance from the axis of the rotor. In embodiments each blind hole has a predetermined maximum diameter, being not less than 0.2 times the shaft diameter. In embodiments each blind hole of two or more blind holes formed in at least one of the first and second end pieces has substantially the same diameter and the thickness of the annular region is equal to the diameter of each blind hole. In embodiments each blind hole of two or more blind holes formed in at least one of the first and second end pieces has substantially the same depth. In embodiments at least one blind hole is in both the first and second end pieces. In embodiments the maximum depth of the or each blind hole is 0.18 times the shaft diameter. In embodiments the rotor is the rotor of a permanent magnet synchronous motor, and wherein the first and second end pieces are rotor end caps. In a yet further aspect of the invention, there is provided a permanent magnet synchronous motor comprising the rotor according to embodiments of the invention. In a yet further aspect of the invention, there is provided an electric vehicle having a powertrain comprising a permanent magnet synchronous motor. In a yet further aspect of the invention, there is provided a method of balancing a rotor for an electric motor, wherein the rotor comprises a shaft, a rotor body disposed about the shaft, and first and second end pieces disposed about the shaft at each end of the rotor body, wherein the first and second end pieces each have an Inner face facing a body of the rotor, and an outer face facing outwardly from the body of the rotor, the method comprising: determining imbalance for the rotor; forming one or more blind holes in the outer face of at least one of the first and second end pieces, wherein each blind hole is drilled in an annular load balancing region of the outer face ofthe end piece centred on an axis of the rotor, and wherein each blind hole is shallowerthan a predetermined maximum depth; wherein sufficient blind holes are formed to correct the determined imbalance. In embodiments each blind hole is formed by drilling the end piece. In embodiments each blind hole is disposed at substantially the same radial distance from the axis ofthe shaft. In embodiments the rotor is the rotor of a permanent magnet synchronous motor, and wherein the first and second end pieces are rotor end caps. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in anyway and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a vehicle in which embodiments of the invention are employed; Figure 2 illustrates schematically an exemplary powertrain for an electric vehicle in which embodiments of the invention may be employed; Figure 3 shows a sectional view of a permanent magnet synchronous motor in which embodiments of the invention may be employed; Figure 4 shows an isometric view of the rotor and shaft of the permanent magnet synchronous motor of Figure 3; Figures 5A and 5B show respectively a rotor end cap according to an earlier design and a rotor end cap according to an embodiment of the disclosure; Figures 6A and 6B show respectively detailed views of a rotor cap section for the rotor caps of Figures 5A and 5B; Figures 7A and 7B show isometric view of a rotor cap section as shown in Figure 6B, with Figure 7B showing a modification according to sa further embodiment of the disclosure; Figure 8 shows a rotor end cap according to the further embodiment of the invention illustrated in Figure 7B; Figure 9 shows a rotor end cap according to an embodiment of the invention; Figure 10 shows the rotor end cap of Figure 9 with additional detail; Figure 11 shows a sectional view of the rotor end cap of Figures 9 and 10; and Figure 12 shows steps of a method of manufacturing a rotor according to an embodiment of the invention. DETAILED DESCRIPTION A permanent magnet synchronous motor in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figure 3, and in subsequent Figures. Figures 1 and 2 provide context for use of such an electric motor in the context of an electric vehicle 100. The powertrain 200 of such an electric vehicle 100 is shown in Figure 2. Power is provided by battery 210 and control from electric drive controller 220, in this case to first electric motor 230 and second electric motor 240 (other embodiments may have only one electric motor). Each electric motor 230, 240 drives vehicle wheels through a transmission 235, 245. Two main types of electric motor used for this purpose are induction motors and permanent magnet synchronous motors. An induction motor has a stator with coils, and a rotor typically with bars in a “squirrel cage” design - current in the stator generates a rotating magnetic field which in turn generates current in the squirrel cage that itself generates a magnetic field opposing the driving magnetic field of the stator. That generates torque in the rotor, causing the rotor to rotate. The permanent magnet synchronous motor uses permanent magnets in the rotor, and the result is that the stator and rotor magnetic fields are locked, and therefore synchronous, with the rotor rotating at the same speed as the magnetic field is rotated in the stator. The two types have different benefits (cost, maturity of technology, ability to freewheel for induction motors -efficiency and small size for permanent magnet synchronous motors). The electric motors used may be of the same or different types - in embodiments considered in detail here, one or both of the electric motors 230, 240 is a permanent magnet synchronous motor. A known configuration for electric vehicles is to have a first electric motor 230 which is an induction motor and a second electric motor 240 which is a permanent magnet synchronous motor, as the different properties of the two motor types can be used in a complementary manner. Figure 3 is a sectional view of a permanent magnet synchronous motor 300. The stator 310 comprises a plurality of windings 320 that establish a magnet field that interacts with the field generated by the permanent magnets 340 in a rotor 330. The rotor 330 is mounted on a shaft 350 which provides torque to a transmission (not shown). Figure 4 is an isometric view of the rotor 330 of Figure 3. The rotor 330 is mounted on the shaft 350, and comprises a series of pucks 410 and two end caps 420. Each puck 410 contains a set of permanent magnets 340 - Figure 3 essentially shows a cross section through a single puck 410. The pucks 410 are aligned on the shaft 350 and retained by the two end caps 420. As can be seen from both Figures 3 and 4, the shaft 350 has grooves 355 and the pucks 410 have puck keys 415 that locate in the grooves 355, and the end caps 420 have end cap keys 425 that also locate in the grooves 355. The rotor end caps 420 have the role of holding the elements of the rotor 330 in position, including binding the pucks 410 on to the shaft 350. Issues to address in rotor design include avoiding excessive strain, and ensuring proper rotor balance. It is found that these issues can be addressed with appropriate rotor end cap design. The design of the end cap keys 425 is addressed in embodiments of the present invention. Figure 5A illustrates an earlier end cap design, whereas Figure 5B shows an end cap design according to an embodiment of the invention. Figure 5A shows that to either side of each end cap key 425 there is an undercut 510 - this is necessary to form an effective separation between the key 425 and the shaft contact surface 520 which retains the end cap 420 on the shaft 350. The shaft contact surfaces 520 are cylindrical segments that together define an inner bore 520 of the rotor end cap 420 In this earlier design, the undercut 510 is essentially a circular section, forming a substantially semi-circular-cylindrical shape. It is found that reshaping the undercut enables problematic strains in the end cap 420 to be significantly reduced. Figure 5B shows such a reshape, with a shallower undercut 510’ that has a varying radius of curvature that is greater proximate to the shaft contact surface 520 than it is proximate to the key 425. The undercut region of each arrangement is shown in detail in Figure 6A (forthe earlier design) and Figure 6B (fora design in accordance with an embodiment of the disclosure). Figure 6A shows that for the earlier design the key 425 has a flat keyway surface 612 facing in to the axis of the rotor, and a flat land 614 parallel to an equivalent land (not shown in Figure 6A) on the other side of the key 425 - the key 425 is symmetric here, that is, the lands 614 on either side of the key 425 are parallel to a radius of the rotor passing through the centre of the key - with the flat lands 614 enabling the key 425 to slot into the groove on the shaft that also has correspondingly flat lands. Between the keyway surface 612 and the horizontal flat land 614 there is a convex curved region 616 allowing for easy engagement of the key 425 with the groove on the shaft - the horizontal flat land 614 is needed to align the end cap to the rotor shaft during assembly. The undercut 610 has an overall depth D, and a constant radius of curvature R throughout the undercut 610 until there is a transition to the shaft contact surface 620. This transition involves first a flat land 624 parallel the flat land 614 of the key, followed by a convex curved region 626 to complete the transition. The design according to an embodiment of the disclosure shown in Figure 6B preserves parallel symmetry about the key 425 but loses symmetry within the undercut 610’. The key design is essentially as before -there is a keyway surface 612 facing into the shaft, and flat lands 614 to allow the key to locate in the groove on the shaft, and a convex curved region between them. However, the undercut 610’ is no longer a circular section - it is shallower, and has a more complex shape in which the radius of curvature of the undercut varies. Extending from the flat land 614 of the key 425 there is a first undercut region 6101, and extending from the shaft contact surface 620 there is a second undercut region 6102. Both these regions are curved and form circular sections, but the radius of curvature of the first undercut region 6101 is significantly less than the radius of curvature of the second undercut region 6102. The length of the first undercut region 6101 is also significantly less than the length of the second undercut region 6102, wherein said lengths are measured tangentially to a radius of the inner bore. In the arrangement shown in Figure 6B, there is also a flat land 6103 - essentially parallel here to the keyway surface 612 - bridging between the first undercut region 6101 and the second undercut region 6102. The transition between the undercut 610’ and the shaft contact surface 620 is also now managed differently, as less accommodation is needed between them - this is a achieved through a smaller convex joining region 636’, where convex means in an opposing sense to the radius of curvature of the second curved region 6102. In the arrangement shown in Figure 6B, the overall depth D of the undercut is reduced - it is preferably less than 0.03*S in total for a shaft diameter S. The distance between the two flat lands 614 (the width of the key 425) is here designated y, which here is substantially 0.08*S. The undercut region 610’ has a length u here slightly greater than the width of the key, preferably substantially 1,2*y. Flat lands 614 are preferably at least 0.01 *S in length in this arrangement to allow effective engagement of the end cap with the rotor shaft groove. The radius of curvature r1 of the first undercut region 6101 is approximately equal to 0.2*y - this allows for ease of end cap assembly as well as avoiding excessive stress concentrations. The radius of curvature r2 for the second undercut region 6102 may in this arrangement vary, as given by the following equation: 5.6 * rl <r2 <7.2 * rl This allows for a suitably large radius of curvature while allowing tangency with smaller radius regions. The smaller convex joining region 636’ here has a radius of curvature r3 substantially equal to 0.67*r1. Using this approach, plastic strain in and around the undercut region is significantly reduced. In the arrangement of Figure 6A, plastic strain was found to be above the yield limit of the material (0.002), whereas in the arrangement of Figure 6B, the plastic strain is reduced by almost 75% and is below the yield limit. A further modification to the end cap is shown with respect to Figures 7A, 7B and 8. Figure 7A is an isometric view of the end cap of Figures 5B and 6B in the region of the undercut 610’. It is found that further reduction of strain can be achieved by adding a chamfer 720 between the flat end cap surface 700 and the curved surface 710 of the undercut facing towards the shaft, A chamfer of up to 0.007*S is found to reduce plastic strain, again taking it below the yield limit of the material, in high strain regions (particularly close to the transition between the shaft contact surface 620 and the undercut 610’) without compromising significantly engagement between the rotor end cap and the shaft. A still further modification to the end cap according to an embodiment of the invention is shown with respect to Figures 9, 10 and 11. This shows a rotor end cap 420 as shown in Figure 8 with the addition of balancing holes 910. These balancing holes 910 are blind holes drilled into the outer face of the rotor end cap - these are added to compensate for rotational imbalance inherent in the rotor. It is found that it is better in this context to subtract material ratherthan to add it, and to introduce as little stress as possible the location and dimensions of individual balancing holes 910 should be controlled. If significant balance is to be offset, this is done by adding more holes while keeping the dimensions of individual holes within such constraints. The nature of this control is shown in more detail in Figures 10 and 11. As shown in Figure 10, the balancing holes 910 are generally located in a load balancing region 920 forming an annular region of the outer face of the rotor end cap, and preferably on a pitch circle 925. Preferably the minimum diameter P of such a pitch circle 925 is 2*S. The maximum width W of each hole is preferably 0.18*S. The internal dimensions of each balancing hole 910 are shown in Figure 11 - the preferred depth H of each hole is 0.162*S. Each balancing hole has a cylindrical bore region 9101 and a conical end region 9102 - the conical end wall makes an angle A to the plane normal to the axis of the cylindrical bore. Preferably this angle A is approximately 30°, The pattern of holes made may vary from endcap to endcap, depending on the determined imbalance. There may be a different number of holes on each endcap, holes on each endcap may have the same or different positions, or holes may be present on one endcap and not on the other. However, where holes are present, they will follow the rules indicated above. Figure 12 shows a method of balancing a rotor using this approach according to an embodiment of the invention. First of all, the imbalance of the rotor is determined 1210. To address this imbalance, one or more balancing holes are drilled 1220 according to the constraints set out above, with an appropriate number of balancing holes being drilled to correct the imbalance. While particularly suitable for a rotor end cap, this approach could also be taken for rebalancing of a rotor in an induction motor also. A typical induction motor rotor is of squirrel-cage design, with a series of metal bars connecting two conductive end rings - a number of steel laminations are mounted between the end rings and between and around the metal bars. Blind balancing holes can be drilled into the outer faces of the end rings following the same principles. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A rotor for an electric motor, wherein the rotor comprises a shaft, a rotor body disposed aboutthe shaft, and first and second end pieces disposed about the shaft at each end of the rotor body, wherein:the first and second end pieces each have an inner face facing a body of the rotor, and an outer face facing outwardly from the body of the rotor; andfor balancing the rotor, at least one of the first and second end pieces has one or more blind holes drilled in the outer face thereof, wherein each blind hole is disposed in a load balancing region of the outer face and is shallower than a predetermined maximum depth, wherein the load balancing region is an annular region of the outer face of the end piece centred on an axis of the rotor.

2. The rotor of claim 1, wherein the load balancing region is an annular region disposed morethan a predetermined minimum distance from the axis of the rotor.

3. The rotor of claim 1 or claim 2, wherein each blind hole is disposed at substantially the sameradial distance from the axis of the rotor.

4. The rotor of any preceding claim, wherein each blind hole has a predetermined maximumdiameter.

5. The rotor of any preceding claim, wherein each blind hole has substantially the same diameter.

6. The rotor of any preceding claim, wherein each blind hole has substantially the same depth.

7. The rotor of any preceding claim, wherein blind holes are drilled in both the first and secondend pieces.

8. The rotor of any of claims 1 to 6, wherein blind holes are drilled in only one of the first andsecond end pieces.

9. The rotor of any preceding claim, wherein the rotor is the rotor of a permanent magnetsynchronous motor, and wherein the first and second end pieces are rotor end caps.

10. A permanent magnet synchronous motor comprising the rotor of claim 9.

11. An electric vehicle having a powertrain comprising a permanent magnet synchronous motoras claimed in claim 10.

12. A method of balancing a rotor for an electric motor, wherein the rotor comprises a shaft, a rotorbody disposed about the shaft, and first and second end pieces disposed about the shaft at each end of the rotor body, wherein the first and second end pieces each have an innerface facing a body of the rotor, and an outer face facing outwardly from the body of the rotor, the method comprising:determining imbalance for the rotor;drilling one or more blind holes in the outer face of at least one of the first and second end pieces, wherein each blind holes is drilled in an annular load balancing region of the outer face of the end piece centred on an axis of the rotor, and wherein each blind hole is shallower than a predetermined maximum depth;wherein sufficient blind holes are drilled to correct the determined imbalance.

13. The method of claim 12, wherein each blind hole has a predetermined maximum holediameter.

14. The method of claim 12 or claim 13, wherein each blind hole is disposed at substantially thesame radial distance from the axis of the shaft.

15. The method of any of claims 12 to 14, wherein the rotor is the rotor of a permanent magnetsynchronous motor, and wherein the first and second end pieces are rotor end caps.Application No: GB2410721.1Examiner: Andrew IsgroveClaims searched: 1-15Date of search: 20 January 2025Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X 1-7, 9-15 CN 209184336 U (SHENZHEN JUMA NEW ENERGY AUTOMOBILE TECH CO LTD) figures X 1-15 US 2023 / 0051422 Al (FERRARA et al.) figures; paragarghs [001], [0015], [0031] X 1-7, 12-14 CN 219145128 U (WOLONG ELECTRIC NANYANG EXPLOSION PROT GROUP INDUSTRIAL DRIVING CO LTD) figures X 1-6, 12-14 KR 20200084219 A (LG ELECTRONICS INC) figures4, 11 X 1-7, 12-14 CN 212183218 U (SUZHOU LEGO MOTORS CO LTD) figures 1, 3 X 1,2, 9-12 CN 205992811 U (ASKOLL HOLDING SRL) figures, paragraph [0087]Categories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if P Document published on or after the declared priority date but combined with one or more other documents of same category. before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:International Classification:Subclass Subgroup Valid From H02K 0007 / 04 01 / 01 / 2006 TjnOIZ rlUZix 0001 / 28 01 / 01 / 2006 H02K 0015 / 165 01 / 01 / 2025

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