PERMANENT MAGNET ASSEMBLY AND METHOD OF MAKING SAME

Abstract: A rotor (85) assembly for a permanent magnet motor includes a set of laminations forming a rotor body, each lamination having a first magnet slot (97) therein. A generally rectangular permanent magnet is disposed in the magnet slot (97), the magnet slot (97) of each lamination being shaped so as to receive the rectangular permanent magnet through all the skewed laminations with a minimum air gap. A method of making an assembly such as a rotor (85) includes the steps of providing an assembly body made of ferromagnetic material defining at least one magnet slot (97) therein. The slot is at least partially filled with a mixture of magnetizable particles in a binder. The mixture is compressed and then cured to bond the magnetizable particles together in the slot (97) and the bonded part icles are then magnetized in the slot (97) to form a magnet in situ in the slot (97) without an air gap.

Description: This invention relates to permanent magnet assemblies and more particularly to self-retaining permanent magnet assemblies. Permanent magnets are used in a variety of assemblies such as electric motor rotors, electric motor stators, loudspeakers, and the like. Although the present invention is described herein with particular reference to electric motor assemblies, it should be realized that the present invention is not so limited but instead extends to permanent magnetic assemblies for a wide variety of purposes. By way of example, in certain electric motors and the like of the synchronous rotor type, the motors have a cage rotor construction with conductor bars for the rotor made of die-cast aluminum. Typically, rotor assemblies for these motors include a stack or rotor core of individual laminations constructed from suitable ferro-magnetic material. Each lamination has a central opening and a plurality of so-called satellite openings adjacent to its outer margin. The laminations are assembled in a stack and the laminations are rotated slightly with respect to one another in the stack so that their central openings are coaxial but their satellite openings are skewed relative to one another to form skewed slots. The lamination stack or core is then placed in a suitable die-casting mold and molten aluminum is injected under pressure into the molds so as to surround the core, fill the mold, and flow through the skewed slots in the core formed by the satellite opening so as to form the conductor bars of the rotor. In addition, each lamination includes one or more magnet slots in which are mounted pro-formed permanent magnets, which are commonly in the shape of rectangular bars. These bar-shaped permanent magnets must be mounted in the magnet slots and suitably secured therein. However, if these slots are skewed, it is normally necessary to use a smaller than desired magnet to fit into the skewed slot, which results in a larger than desired air gap, or to use specially designed magnets (rather than the standard bar-shaped magnets) which increases the cost of the rotor assembly substantially. Even in those cases where skewing of the rotor laminations is not needed, it would sometimes be preferable to have various non-rectangular shapes for a magnet slot to provide the desired magnetic flux. But the use of custom-made magnets of these various shapes is not economical. Even when permanent magnets of the desired shape are available, there still exists an undesirable air gap between the magnets and the laminations because of the necessary manufacturing tolerances. FR-A-2 551 931 describes a method of fabricating a rotor for an electric micro-motor. A hollow dish is filled with powder which is premagnetized under pressure. The body so obtained is agglomerated at elevated temperature and/or under pressure. The agglomerated body is mechanically finished before exposing it to a magnetic field for final magnetization. WO-A-85/05507 discloses a stepping motor with tooth stator poles and tooth rotor poles having magnetized inserts place therein. These inserts are prepared by wiping magnetic material into slots in the pole surfaces of the stator and rotor and thereafter drying and magnetizing the magnetic material. A primary object of the present invention is the provision of a magnetic assembly which permits the use of, in particular, rectangular permanent magnets even when the magnet slots thereof are skewed with respect to each other, but in which permanent magnets may also be of arbitrary shape and are still obtained at an economical cost. Secondary objects of the present invention are the provision of a rotor assembly which is economical in construction, the provision of a permanent magnet assembly with a minimum air gap, and/or the provision of a method of making a permanent magnet assembly which allows the designer increased choice in the placement and shape of the permanent magnets in the assembly. According to one aspect of the invention, the above object is achieved by a method of making a magnetic assembly comprising the steps of: providing an assembly body of ferromagnetic material or the like, filling at least a portion of the body at least partially with a mixture of magnetizable particles and a binder, compressing the mixture in the body, curing the compressed mixture, and after the curing step magnetizing the bonded particles to form a magnet, characterized by the steps of: providing an assembly body which defines a plurality of skewed magnet slots therein, each extending generally longitudinally of the assembly body, wherein each slot is at least partially filled with said mixture and each said slot being configured to retain a magnet in place therein; said compressed mixture in the slot being cured without significantly shrinking the material in the slots to bond the magnetizable particles together in the slots; said cured mixture of magnetizable particles in the binder being completely unmagnetized; and magnetizing the bonded particles in the slots to form a magnet in situ in each slot. According to another aspect of the invention, the above object is achieved by a permanent magnet assembly including an assembly body of ferromagnetic material or the like and a permanent magnet formed in situ in the assembly body, characterized by: said assembly body defining a plurality of enclosed magnet slots therein, said slots being configured to retain each a magnet in place therein said assembly body being composed of a plurality of laminations which are skewed with respect to each other, said permanent magnet slots being thereby skewed as well, said permanent magnets being formed in situ in each of said enclosed slots without an air gap between the magnet and the slot according to the foregoing method. Embodiments of the invention are claimed in the dependent claims. Brief Description of the Drawings Fig. 1 is a right side elevation of a dynamoelectric machine utilizing a rotor assembly of the present invention; Fig. 2 is a sectional view illustrating the construction of the rotor assembly of the present invention; Fig. 3 is a top plan view illustrating one embodiment of the laminations used with the rotor assembly of Fig. 2 illustrating the position of a rectangular bar magnet in a magnet slot of such a lamination; Fig. 4 is a view similar to Fig. 3 illustrating the positions of a magnet slot in a skewed stack of laminations; Fig. 5 is a top plan view of a second embodiment of the laminations used in the present invention; Fig. 6 is a top plan view of a third embodiment of the laminations of the present invention; Fig. 7 is a top plan view of a fourth embodiment of the laminations of the present invention; Fig. 8 is a top plan of a stator assembly of the present invention; Fig. 9 is an elevation of the stator assembly of Fig. 8; Fig. 10 is a top plan illustrating a fixture used with the rotors of Figs. 5 through 7; Fig. 11 is a cross-sectional view taken generally along line 11-11 of Fig. 10; and Fig. 12 is a top plan view of a fifth embodiment of the laminations of the present invention. Similar reference characters indicate similar parts throughout the several views of the drawings. A permanent magnet motor 11 (Fig. 1) includes a rotor assembly 13 of the first embodiment of the present invention suitably journalled for rotation with respect to a stator 15. Rotor 13 includes a rotor shaft 17 to which are suitably secured a stack of laminations 19 having a series of magnet slots therein for receiving a plurality (four in this example) of rectangular permanent magnets 21. Permanent magnets 21 are of conventional rectangular bar magnet construction to minimize the cost of permanent magnet motor 11. Each lamination is, for example, approximately (0.022 inches) 0.056 cm in width. Laminations 19 each have a series of openings therein (see Figs. 2 and 3). For example, all of the laminations have a central bore 23 therein as well as a plurality (e.g. twenty-eight) of rotor cage slots 25 around the periphery of each lamination. Between the cage slots 25 and central bore 23 are permanent magnet slots labelled 27 in Fig. 2 for receiving permanent magnets 21. A similar set of magnet slots 33 particularly useful when the laminations are skewed, are shown in Fig. 3. To construct the rotor assembly of Fig. 2, laminations 19 are stacked together in a core approximately (1.875 inches) 4.76 cm in height, for example, in the proper orientation with respect to each other. A rotor cage of aluminum is then die-cast in place to hold the laminations in place with respect to each other. A pair of end rings 29 approximately (one-half inch) 1.27 cm in height are die-cast in place at each end of the stack at the same time. The rotor shaft may be secured to the rotor assembly in this die-casting step or otherwise suitably secured to the assembly. Turning to Fig. 3, a single lamination 19 of the rotor assembly 31 of the present invention is shown in combination with one permanent magnet 21. The four permanent magnets of Fig. 1 fit in the pair of generally V-shaped slots 33 disposed in each of the laminations. The legs of the V-shaped slots are not straight but instead have a "bow-tie" configuration. That is, the end and the base of each leg define the widest portion of the corresponding magnet slot and these slots taper from those points to the midpoint of the slot. The angle of the taper is generally equal to the angle through which the laminations are rotated so that even when skewed, the laminations still accept the rectangular bar magnets 21. This is illustrated in more detail in Fig. 4 which shows by hatched lines the outline of the various slots 33 of the laminations in a skewed stack. Although the actual outlines of these slots differ because of the skewing or rotation of the laminations with respect to each other, it can be seen that the resulting passage 35 through all the laminations is generally rectangular in shape. This allows magnet 21 to be inserted in passage 35 without binding upon any of the laminations. It should be appreciated that skewing of the laminations with respect to each other results in four rectangular passageways through the set of laminations 19 for receipt of all four permanent bar magnets 21 even though only two of these passageways are shown in Fig. 4 for purposes of clarity. Moreover, this particular slot configuration minimizes the air gap between the magnets and the laminations while maximizing the size of rectangular bar magnet which can be used in the slots. Also shown in Figs. 3 and 4 are a pair of slots 37 which provide reluctance barriers between magnets 21. These barriers may be filled with aluminum during the die-casting process. In the embodiment of Figs. 1-4, the designer is limited by several factors, including the limited number of sizes and shapes of magnets 21 available and the air gap between these magnets and their slots which, although minimized, are of necessity present when the stack of laminations is skewed. These problems are completely overcome in the embodiments of Figs. 5-12, which free the designer to concentrate on the necessary flux without substantial restriction as to magnet size or shape. An alternative embodiment of lamination 19, labeled 41, is shown in Fig. 5. Magnet slots 43 in this embodiment are generally V-shaped but have straight sides instead of the "bow-tie" configuration of slots 33. These laminations have cage slots 25 and reluctance barrier slots 37, although neither forms a necessary part of the present invention. The magnets for slots 43 are molded in situ using a low shrinkage permanent magnet material such as that sold by Delco Remy under the trade designation Magnequench MQ-1. This particular material is 98% boron-neodymium-iron with a 2% epoxy binder, and is believed to be made in accordance with U.S. patent 4,496,395 to Croat. This particular material is placed in powder form into slots 43 and then compressed with a force of approximately (66 tons per square) 91,000·10⁵ Pa inch. It is then cured at (300 degrees F) 150°C to form the magnetic material into a solid piece in the slots 43 in the stack of laminations. This not only allows a magnet to be formed in situ for any particular desired shape of magnet slot but also creates a low cost way of holding the magnets in place. By way of contrast, the permanent magnets 21 of the first embodiment are held in place by die-cast aluminum, or shims, or adhesive. The material which is cured in magnet slots 43 is in an unmagnetized state during the curing process. During curing there is hardly any shrinkage of the formed magnet in the slot due to the nature of the magnetic material itself. Thus, there results a magnet in slots 43 which is without an air gap between the magnet itself and the laminations 41. Once the magnet is formed in place it is then magnetized in situ to form a two-pole rotor, for example. Of course, various magnetizations could be used depending upon the particular magnet construction one wanted the rotor to have. Note that with the proper fixtures, the magnetizable material may be compressed to the proper compression whether or not the lmainations 41 are skewed. A third embodiment of the lamination of the present invention is shown in Fig. 6 and labeled 51. This particular lamination includes four reluctance barrier slots 37 disposed as shown with a pair of rotor cage slots 25 disposed between each pair of reluctance barrier slots 37. In addition, the magnet slots labeled 53 of this lamination are generally arc-shaped. The in situ magnetic material discussed in connection with Fig. 5 may be inserted in the arc-shaped slots of lamination 51 and formed into a suitable pair of magnets therein using the same process as described above in connection with Fig. 5. The magnets so formed may have any desired polarity. For example, the north pole of the rightmost magnet in the configuration shown in Fig. 6 could be on the outer side of that arc while the north pole on the left most magnet as shown in Fig. 6 could be on the inner side of the corresponding magnet arc. Of course, since the magnet is formed in situ, the laminations 51 may be skewed or not as desired for the particular application. Note that using the method of the present invention, the designer is freed from using permanent magnets of only certain shapes. If the designer wants a certain flux and number of poles which are not available with one configuration a different arbitrary magnet configuration can be chosen, independently of the permanent magnet shapes which are commercially available. The designer is freed to make the best permanent magnet assembly, not just the best assembly which can be made from commercially available components. In yet another embodiment of the present invention, a lamination 61 (Fig. 7), similar to that shown in Fig. 6 includes the pair of arc-shaped magnet slots 53 and rotor cage slots 25 around its periphery. This particular lamination 61 includes a pair of reluctance barrier slots 63 having a generally anvil-shape. These reluctance barrier slots may be filled with aluminum or other suitable material during the die-casting operation. Thereafter, the magnetizable material is inserted into slots 53 and the permanent magnets are formed therein in situ as described above. Although the invention has been described to this point with respect to a rotor assembly, it is not so limited. As shown in Figs. 8 and 9, the present invention is equally applicable to stator assemblies for dynamoelectric machines and is in general applicable to any permanent magnet assembly. A stator assembly 71 of the present invention (Fig. 9) includes a stack or body of laminations 73 of suitable ferromagnetic material or the like. Each lamination has formed therein a plurality of magnet slots 75 in which are formed in situ a plurality of permanent magnets 77 as described above in connection with a rotor assembly. In Fig. 8, eight magnet slots 75 and eight permanent magnets 77 are shown. These can be magnetized to create at least an eight-pole stator. Of course this number is illustrative. Any desired number of poles could be formed in this manner. The slots 75 of stator 71 differ from the magnet slots previously described in that they are open to the interior central bore 79 of the stator assembly. It should be noted that the magnet slots 75 are smaller at their mouths than at the rear portion of the magnet so that the shape of the slot and the magnet itself holds the magnet in its respective slot. Of course, when forming the magnets 77 it is necessary to place some fixture in central bore 79 to prevent the powder-like material from flowing out of magnet slot 75 until it is compressed and/or cured. Similar shapes could be used for the magnet slots of the rotor assemblies described above so that those magnet slots also extend to the periphery of their laminations. All that is required is that the laminations be one continuous piece and that the shape of the slot be configured so that it holds its associated permanent magnet in place, once the magnet is formed therein. A fixture 81 suitable for providing the necessary compression to the magnetizable material used with the laminations of Figs. 5 through 7 is shown in Figs. 10 and 11. This fixture includes a cylindrical collar 83 which supports a rotor 85 made up of laminations such as that shown in Fig. 7 against lateral forces. Due to the large nature of the forces involved in compressing the magnetizable material, collar 83 and the load-bearing members of fixture 81 are preferably made of tool-steel or the like. Collar 83 and rotor 85 rest upon a base 87 of tool-steel. Base 87 includes a pedestal 89 suitable for supporting rotor 85 from below. Rotor 85 is suitably secured to base 87 by means of a bolt 91 secured at its bottom to base 87. Bolt 91 extends upwardly through the central bore of rotor 81 and is secured to a threaded washer 93 at the top of the assembly, which washer holds rotor 85 in place. Of course, this connection need not be particularly strong since it is acting in the same direction as the compressive forces which are applied to rotor 85. If necessary, a bushing 95 may be disposed around bolt 91 to fill the central bore of the rotor. Rotor 85 includes a pair of magnet slots 97 in which are disposed the magnetizable particles described above. Fixture 81 also includes a plunger 99 having a pair of downwardly extending ears 101 which are shaped to precisely fit in the slots 97 filled with the magnetizable material. Application of force in the direction shown by the arrows in Fig. 11 thereby results in compression of the magnetizable material in slots 97. As should be appreciated, if the laminations of rotor 85 are skewed, it would be desirable to use plungers 99 of suitable shape at both the top and the bottom of slots 97 to ensure complete compression of all the magnetizable particles. Although the rotors previously described are all of cage construction, the present invention is not so limited. In Fig. 12 there is shown a rotor lamination 105 of suitable ferromagnetic material having a central opening 107 for the rotor shaft and a pair of arc-shaped magnet openings 109, but no cage slots. In this embodiment, the laminations of the rotor stack are suitably secured together by an adhesive or the like before the magnetizable powder is inserted in magnet slots 109 and compressed.

Claims: A method of making a magnetic assembly comprising the steps of: providing an assembly body of ferromagnetic material or the like, filling at least a portion of the body at least partially with a mixture of magnetizable particles and a binder, compressing the mixture in the body, curing the compressed mixture, and after the curing step magnetizing the bonded particles to form a magnet, characterized by the steps of: providing an assembly body which defines a plurality of skewed magnet slots therein, each extending generally longitudinally of the assembly body, wherein each slot is at least partially filled with said mixture and each said slot being configured to retain a magnet in place therein; said compressed mixture in the slot being cured without significantly shrinking the material in the slots to bond the magnetizable particles together in the slots; said cured mixture of magnetizable particles in the binder being completely unmagnetized; and magnetizing the bonded particles in the slots to form a magnet in situ in each slot. The method as set forth in claim 1 wherein the assembly body is composed of a set of laminations, further including the step of at least temporarily securing the laminations together before the mixture is placed in the slot. The method as set forth in claim 1 wherein the binder is an epoxy binder. The method as set forth in claim 1 wherein the curing step occurs at a temperature of approximately 150°C (300°F). The method as set forth in claim 1 wherein the providing step includes selecting a shape for the magnet slot independently of what permanent magnet sizes and shapes are available. A permanent magnet assembly including an assembly body of ferromagnetic material or the like and a permanent magnet formed in situ in the assembly body, characterized by: said assembly body defining a plurality of enclosed magnet slots (27; 33; 43; 53; 75) therein, said slots being configured to retain each a magnet in place therein said assembly body being composed of a plurality of laminations which are skewed with respect to each other, said permanent magnet slots being thereby skewed as well, said permanent magnets (21; 77) being formed in situ in each of said enclosed slots without an air gap between the magnet and the slot according to claim 1. The permanent magnet assembly as set forth in claim 6 wherein said permanent magnet has a density substantially greater than the density of the loose magnetizable particles of which it is composed, said particles in the process of making the permanent magnet being subjected to a compressive force. The permanent magnet asembly as set forth in claim 7 wherein the compressive force is approximately 91.000·10⁵ Pa (66 tons per square inch). The permanent magnet assembly as set forth in claim 6 wherein the permanent magnet is formed in situ in its slot without significant shrinkage of the material of which the magnet is composed to preclude the formation of an air gap. The permanent magnet assembly as set forth in claim 6 wherein the magnetizable particles of the slot are compressed together in the process of forming the magnet in situ, said compressed particles being thereafter cured in situ without significant shrinkage, thereby precluding formation of an air gap. The permanent magnet assembly as set forth in claim 6 wherein said assembly is a rotor (19) for a dynamoelectric machine, each lamination having a plurality of magnet slots formed therein, the laminations being aligned so as to form magnet slots (27; 33; 43; 53) extending through the stack; a plurality of permanent magnets (21) disposed one in each magnet slot without an air gap between the magnet and the slot, said permanent magnets comprising a cured mixture of magnetizable particles in a binder, said permanent magnets having been formed in situ in the magnet slot. The permanent magnet assembly as set forth in claim 11 wherein the assembly is a stator assembly (71) for a dynamoelectric machine.

Inventors: LLOYD, JERRY DEAN
Applicants: EMERSON ELECTRIC CO.
Application year: 1987
Main classification: H02K15
IPC codes: H02K15, H02K21, H02K1, H01F41, H02K23, H01F7