ELECTRIC MOTORS AND STATORS THEREFOR

Abstract: A spirally wound electric motor yoke of generally polygonal cross-section has part-cylindrical surfaces (58) machined for receiving mating bosses (60) of an end member (59) to provide improved concentricity. The invention also provides improved methods of forming the spirally wound yoke.

Description: "Electric motors and stators therefor" This invention relates to electric motor stator structures having circumferentially-laminated magnetic paths, and to electric motors incorporating such structures. My British Patent No. 1,561,032 describes stator yokes produced from metal strip by coiling the strip spirally on a mandrel and bonding the turns of the spiral together. The present application concerns developments of this technique, and the application of a similar approach to other types of motor. One aspect of the invention also provides for particular methods of mounting poles and end frames to a spirally wound laminated stator, to achieve greater accuracy of pole location. Another aspect of the invention resides in operating on the strip prior to winding by punching, shearing or forming. This enables various features to be incorporated in the yoke in a simple and economical manner. For example, it allows bolt holes, access openings and cooling fins to be formed. It also provides a simple manner of forming a yoke with a varying thickness of flux path, which makes it possible to apply the laminated foil stator technique to the production of permanent magnet motors. The invention also provides a permanent magnet motor in which the magnets are secured to a cylindrical liner on which a wound laminated structure is relatively rotatable, thus providing speed control. Further, the invention provides improved methods and apparatus for applying heat and pressure to the wound yoke. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in whicht- Fig. 1 illustrates a method of forming a stator yoke in accordance with the invention; Fig. 2 illustrates a similar method in which a different yoke is being formed; Figs. 3 and 4 are respectively a longitudinal and a transverse cross-section of another motor having a stator yoke formed by the above method; Fig. 5A is a partial cross-section of one stator having a yoke which may be formed by the above method; Fig. 5B is a perspective exploded view of the stator of Fig. 5A and an associated end frame; Fig. 6 is a perspective view of a permanent magnet motor which may be formed by the above method? Fig. 7 is a half-section of another permanent magnet motor; Figs. 8 and 9 are quarter-sections of modified permanent magnet motors; Fig. 10 is a half-section of another permanent magnet motor; Fig. 11 is a side view, half in section, of a further type of motor; Fig. 12 is a perspective view of a modification of the motor of Fig. 11; Fig. 13 is a diagrammatic side view illustrating one method of curing a wound stator yoke or other laminated articles; Fig. 14 is a cross-section illustrating a method similar to that of Fig. 13; and Fig. 15 is a cross-section showing apparatus for effecting another method of curing. Refering to Figs. 1 and 2, the present invention provides a method in which a stator yoke is formed by winding a continuous metal strip 10 in a spiral fashion on a suitably-shaped mandrel 12, a material (not shown) having bonding and insulating properties being interposed between the turns of the spiral. Such a method is described in greater detail in by British Patent No. 1,561,032 referred to above. In the present method, however, the strip 10 is subjected to punching and/or forming operations before being wound on the mandrel 12. (It should also be noted that the spirally wound strip could be secured by means other than inter-turn bonding material, e.g. by welding). As shown in Fig. 1, the edge of the strip 10 may be punched to form laminae 14 for subsequent use in forming poles, or other components, and the surrounding material then sheared off as indicated at 16 to give a clean edge for winding. Holes 18 are punched to form bolt holes in the wound yoke for securing the poles. Portions of the edge may be left not sheared away, to provide particular end formations for the yoke assembly; in Fig. 1 such a portion is shown punched to form a hole 20 which subsequently forms part of a cable entry. This pre-winding punching is suitably effected by one or more machines (not shown) operating to a pre-arranged program so that the spacing between items such as the bolt holes 18 along the strip is varied to bring these items into juxtaposition in the successive turns of the stator yoke. This can suitably be achieved by means such as a shaft encoder 21 measuring the angular position of the mandrel and means such as a shaft encoder 23 driven by a contact wheel 25 measuring the feed of the strip 10. This data is fed to a micropro¬ cessor 27 which uses said pre-arranged program to control the operation of the press or the like such that features such as the bolt .holes 18 are in register in the finished wound yoke. Fig. 2 shows a similar process in which the yoke is formed with a cable entry 20 and with a brushgear access port 2. In this embodiment, the strip is also pressed to form a flanged aperture 24 which cooperates with differently-sized flanged apertures such as 26 to form a flux break and cooling fins, as will be described in greater detail below. Figs. 3 and 4 illustrate a complete motor formed by this method. Poles 30 are bolted to the yoke 32 and have associated field coils 34. The rotor 36 is carried in bearings 38 mounted in end members 40 secured to the yoke. The brushgear apertures 22 are aligned with the commutator 42 and may suitably be provided with louvered covers such as 44. Ribs 46 for locating the poles may be provided by preforming of the strip; the preforming may also provide a cutaway at 47 to give greater internal space at the commutator end. The accurate location of poles on the yoke can cause difficulties, particularly with flat- sided (square or octagonal section) stators. The ribs described above are one approach. Others are illustrated in Fig. 5, which deals with an octagonal yoke for carrying four main poles and four commutating poles. Each main pole such as 48 may be more accurately located by seating it in a recess 50 formed by aperturing the inner turns of the yoke, which can be done as described with reference to Figs. 1 and 2. Shims 51 may be provided to give an accurate pole projection. The same could be done for the commutating pole 52, but this could be more difficult in the more restricted space available. I prefer to mount the commutating pole 52 on a face 54 of the yoke which is a cylindrical surface, either by suitably shaping the mandrel or by machining. This gives two line contacts which prevent skewing. It should be remarked that it is highly desirable for the surface 56 adjacent the main pole 48 to be flat, since this allows the use of a flat field coil. However, this consider¬ ation does not apply to the commutating poles. Figs. 5A and 5B illustrate another aspect of the present invention. There is also a problem in locating the end members asccurately on the yoke, which is of significance in achieving accurate alignment of the main bearings and thus uniformity of pole-to-rotor gaps and of interpole-to-rotor gaps. As seen in Fig. 5A, the outer surfaces of the yoke at the short sides of the octagon are machined as cylindrical surfaces, as indicated at 58. The radius of this surface is chosen such that the surface 58 lies principally within one layer of the yoke spiral (preferably the outermost but one) to minimise magnetic losses. The end member 59 (Fig. 5B) is formed with axial bosses 60 having mating cylindrical surfaces. It is preferred, as shown in Fig. 5A, that the boss 60 does not extend across any area such as 61 where the machined surface intersects a line of bonding since this could trap moisture and lead to delamination. The significance of this feature is that cylindrical surfaces can be readily machined to a high degree of accuracy and concentricity, whereas attachment of end frames by, for example, axial bolts and bores can give rise to problems in accurate centering of the rotor within the stator. It also allows the accuracy of the yoke itself to be less critical, leading to simpler jigging when bonding the wound stator yoke. It would also be possible to .provide the part- cylindrical surfaces by machining the interior face of the stator yoke. Turning to Figs. 6 to 10, the application of the invention to permanent magnet motors will now be described. Fig. 6 shows the general principal, ceramic magnets 62 being secured (e.g.~ by adhesive bonding) to the interior of a wound stator 64. In permanent magnet motors, the section thickness of the magnet material can be reduced by some 25% for the same demagnetization performance if the yoke section is slotted on the centreline of the pole. With the present invention, the strip is punched before winding to provide areas in which only thin ribbons 66 are left at the end of the yoke, and shaped slots 68 are provided. If desired, the ribbons 66 could be cut off after the yoke has been bonded. Fig. 7 illustrates a similar construction for a four-pole motor. Fig. 8 illustrates a modified four-pole motor in whicht he strip adjacent the slot edges is bent up to form cooling fins 70 (cf. Fig. 2). In a further modification shown in Fig. 9, the cooling effect is enhanced by the attachment of heat sink members 72 and 74 which may suitably be of extruded aluminium. Fig. 10 illustrates another form of permanent magnet motor stator, which is punched and wound in the same manner as before. In this case however the magnets 62 are provided with flux concentrating pole shoes 76, preferably of sintered or bonded ferrous particles. It is possible for the laminated spiral yoke to be wound on a thin cylindrical shell. This is advantageous in providing in a simple manner a smooth, accurate surface to which the magnets can be bonded. Fig. 11 illustrates such an arrangement, in which the shell is formed by a "can" 78 formed by deep drawing to give a centre section in which a rotor bearing 80 is press fitted. This stator again uses permanent magnets 62 and wound laminations 80 formed with gaps 82. As shown, the laminations are also graded in axial length in their final form, this can be achieved with a winding process by scoring or slitting the strip before winding and subsequently peeling away the edge portions after the yoke has been formed. The use of a wound laminated structure on the exterior of a cylindrical or can give rise to a further development. If the laminated structure is controllably rotatable with respect to the magnets, by up to one-half pole pitch, the amount of flux in the yoke is controlled and hence the motor speed. An example of this is illustrated in Fig. 12. The laminated structure 80 is rotatable on the can 78, a low-friction separator 84 such as p.t.f.e. being inserted between them, and is provided by pre-punching with notches 86. The notches 86 are engagable by a spur wheel 88 carried by shaft 89 rotatably mounted in a bearing 90 secured to the can 78. The speed of the motor can thus be controlled by rotating the shaft 89. The embodiments of Figs. 6 to 12 are described above in terms of permanent magnet motors. It would be possible to apply the same principles to wound-pole motors. It is also possible to achieve variable speed control using the principle of varying the amount of flux in the yoke by relative axial movement rather than relative rotational movement. Again, the poles would be mounted on an inner canister or shell, a laminated yoke structure being axially slidable thereon. In this case, the motor can be of square or polygonal cross-section. The present application is also concerned with the manner in which the laminated stator yoke is cured or bonded. The turns of the yoke are suitably insulated and structurally bonded by " a synthetic resin applied to the strip immediately before winding, e.g. an epoxy resin. It is then preferable for the resin to be cured under heat, and during this step it is necessary to ensure that the turns do not separate and no foreign material such as gas pockets intervene between the turns. In one process used hitherto, the spirally wound structure was clamped by means of circumferentially arranged platens driven by hydraulic rams. However, mechanical clamping of this nature has several dis¬ advantages. It is slow, cumbersome, and not well suited to volume production. It is also wasteful of energy, since it is necessary to heat the mass of the platens. The present invention overcomes these problems by the use of a pressurised medium (gas, liquid, or particulate) to apply pressure and/or heat. One simple approach is illustrated in principle in Fig. 13. The laminated structure 100 is enveloped in a plastics membrane 102 sealed by adhesive tape 104. Within the membrane 102 are positioned apertured p.t.f.e. tubes 106. The open ends of the tubes 106 are positioned within a porous medium 108 such as glass fibre or Dacron braid. The other ends of tubes 106 are connected to a vacuum source, thus removing air from within the membrane, and the structure 100 can then be heated and pressed by, for example, positioning in an autoclave fed with steam or a mixture of steam and compressed air. Fig. 14 shows a more developed form of this approach. A stator yoke 110 is wound on a mandrel 112. After these items are removed from the winding machine, a plastics sleeve 114 is applied and is held in position by rings 116 seating in grooves 118 in the ends of the mandrels. The mandrel 112 is provided with bores 120 communicating with connectors 122 to which vacuum lines may be applied, to remove air as before. Where holes through the yoke ar present, as indicated at 124, additional bores such as 120a may be provided to assist in removing air. Suitably, any such hole will be packed with porous material (e.g. plastics foam) before the sleeve 114 is applied, to resist excessive deformation of the sleeve which might lead to a puncture. Tape 126 may likewise be applied to sharp edges. The assembly may then be processed in an autoclave or in an bath of pressurised hot liquid. The mandrel 112 is hollow to assist in heat transfer. A rib 128 is shown which provides mechanical support, and also aids heat transfer. It would be possible to further improve heat transfer by providing further ribs, or fins, on the interior surface of the mandrel. The vacuum connection may be maintained during curing, to allow the vacuum to be monitored and thus any leaks in the sleeve to be detected. After curing has been completed, the sleeve 114 may be removed and the yoke 110 retained on the mandrel 112 for further processing, such as the machining of the end faces and/or faces for mounting end members. For this purpose, the mandrel is provided with accurate locating surfaces 130 to allow it to be mounted in a lathe or other machine tool. Fig. 15 shows an apparatus in which particulate material (e.g. sand or metal shot) is used to apply pressure while heating. A cylindrical vessel 137 has a movable floor formed by piston 140, these two being sealed together by a flexible liner 142 to provide a container for particulate material 143. A movable lid assembly 138 carries an apertured cylindrical support 146 on which a flexible sleeve 147 is mounted and clamped by rings 151. A laminated yoke structure 153 can be secured to the lid assembly by positioning it over the sleeve 147 and then expanding the latter by a pressurizing gas admitted via conduit 149; if necessary, a spacer may be interposed as indicated at 125. The particulate material 143 is fluidised by hot gas admitted via conduit- 145 and apertured base plate 144, and the lid assembly 138 is then lowered to close the chamber, sealing it by means of seal 139, and immerse.the yoke 153 in the particulate material. The piston 140 may be moved to adjust the volume of the chamber and/or apply additional pressure. Movable wall elements 141 may be provided which can be driven (by means not shown) to apply radial force to the particulate medium. Outlet valves 150 allow the gas to be vented. The inner pressurising gas supplied via 149 is preferably also hot gas. Flexible tape 152 is preferably applied to the ends of the yoke 153 to prevent gas penetrating between the layers; alternatively the yoke 153 may be completely wrapped in a plastics cover.

Claims: CLAIMS 1. A method of manufacturing a stator for an electrical machine, comprising winding a metal strip in spiral fashion, securing the turns of the spiral to form a monolithic laminated yoke structure, machining parts of said structure to form part-cylindrical mounting faces adjacent at least one end of the structure, and mounting on the or each said end an end member provided with complementary part-cylindrical surf ces. 2. The method of claim 1, in which , the machining is such that each said mounting face has a major portion lying within one lamina of the yoke structure. 3. The method of claim 2, in which said complementary surfaces are dimensioned not to overlie any exposed line of bonding. 4. A stator yoke for an electrical machine, comprising a polygonal yoke structure of circumferential laminations, portions of the yoke structure adjacent at least one end being formed as arcuate surfaces for receiving and locating complementary surfaces of an end frame. 5. A method for accurately ' locating commutating poles on a yoke wherein the surface of the yoke in the region where the pole is to be mounted is shaped or machined to form a cylindrical surface thereby providing two line contacts for the pole to prevent skewing. 6. A method of manufacturing a wound, laminated stator yoke comprising winding a metal strip in a spiral fashion with a material having insulating and bonding properties interposed between the turns of the spiral, and wherein the metal strip is operated on, by punching, shearing or forming. prior to winding, thus enabling various features such as bolt holes, access openings and cooling fins to be incorporated in the yoke. 7. A method according to claim 6 wherein the edge of the metal strip is punched to form laminae for subsequent use in forming other components and the surrounding material sheared off to provide a clean edge for winding. 8. A method according to claim 6 or claim 7 wherein holes and apertures are punched in the metal strip, the spacing between said holes and apertures along the length of the strip being such that, upon winding of the strip, said holes and apertures are brought into juxtaposition to form features such as bolt-holes, access openings and cable ports. 9. A method according to any of claims 6 to 8, wherein the metal strip is punched to form a series of progressively larger apertures along its length, said apertures cooperating, upon winding of the strip, to form a shaped slot to act as a flux break. 10. A method according to claim 9 wherein the edges of said apertures are bent upwards to form cooling fins. 11. A method for accurately locating poles on a yoke manufactured in accordance with any of claims 6 to 10, wherein the inner turns of the yoke . are apertured to form recesses wherein the poles may be seated. 12. A motor including a stator yoke manufactured in accordance with claim 9 or 10, the yoke being mounted on a cylindrical shell, said shell having a plurality of poles mounted on the inside surface thereof, the yoke being controllably rotatable with respect to the shell and hence the position of the flux break slots being controllable with respect to the poles, thereby controlling the amount of flux in the yoke and hence the motor speed. 13. A motor comprising an inner shell to which poles are secured, and an outer laminated flux path yoke, the outer yoke being axially slidable on the inner member to controllably vary the amount of flux in the yoke and hence the motor speed. 14. The motor of claim 12 or claim 13, in which the poles are permanent magnets. 15. The motor of claim 12, claim 13, or claim 14 in which the yoke is provided with equispaced formations engageable by a mechanical element (pinion, rack, or the like) for effecting said relative movement. 16. The motor of any of claims 12 to 15, in which the inner shell and outer yoke are separated by friction-reducing means such as fluorocarbon film. 17. Apparatus for manufacturing wound laminated stators, comprising a mandrel, means for feeding strip metal to the mandrel, means for rotating the mandrel to wind the strip therearound, and means for shaping the strip by punching, cutting or forming immediately prior to the strip approaching the mandrel. 18. Apparatus according to claim 17, including means for sensing the angular position of the mandrel, means for measuring feed movement of the strip, and control means responsive thereto for operating said shaping means to correctly position features in register in successive layers of the stator. 19. A method of curing a wound laminated yoke in which the insulating and bonding material is a synthetic resin, wherein pressure or heat or both are applied to the yoke by means of a pressurised medium such as gas, liquid or particulate solid. 20. A method according to claim 19 wherein the laminated yoke is enveloped in a plastics membrane and the membrane evacuated prior to the application of heat and pressure to the yoke. 21. A method according to claim 19 or 20 wherein heat and pressure are applied to the yoke by position¬ ing said yoke in an autoclave fed with steam or a mixture of steam and compressed air. 22. A method according to claim 20 or 21 wherein the yoke, while still in position on the mandrel upon which it was wound, is enveloped in a plastics sleeve, said mandrel being provided with a plurality of bores to assist the evacuation of said plastics sleeve. 23. A method according to claim 22 wherein said mandrel is hollow and is provided with a plurality of ribs or fins or both on its inner surface to assist heat transfer. 24. A method according to claim 19 wherein the yoke is positioned within a vessel and pressure applied to said yoke by a particulate material contained in said vessel. 25. A method according to claim 24 wherein said vessel is cylindrical, and is provided with a movable floor formed by a piston and a flexible liner to provide a container for said particulate material, said vessel also being provided with a movable lid assembly- to which a laminated yoke may be secured. 26. A method according to claim 25 wherein said particulate material is fluidised by hot gas, thereby allowing the lid assembly to " be lowered, immersing the yoke in the particulate material, and sealed, and wherein sasid piston may be moved to adjust the volume of the vessel, or to apply additional pressure, or both. 27. A method according to claim 25 or 26 wherein said lid assembly is provided with an apertured cylindrical support with a flexible sleeve mounted thereon, the yoke being secured to the lid assembly by positioning said yoke over said sleeve and expanding said sleeve by means of a pressurizing gas. 28. A method according to claim 27 wherein said pressurizing gas is hot gas. 29. A method according to claim 27 or 28 wherein flexible tape is applied to the ends of the yoke to prevent gas penetrating between the layers. 30. A method according to claim 27 or 28 wherein the yoke is wrapped in a plastics cover. 31. A method according to any of claims 25 to 30 wherein said vessel is provided with a plurality of movable wall elements which may be driven by suitable driving means to apply radial force to the particulate material.

Inventors: WORKMAN JOHN, WORKMAN, JOHN
Applicants: WORKMAN JOHN, WORKMAN, JOHN
Application year: 1985
Main classification: H02K15
IPC codes: H02K1, H02K5, H02K9, H02K15, H02K23