ELECTRICAL MACHINE ROTOR

Abstract: A rotor for a rotary electrical machine is in the form of a single cylindrical two-pole permanent magnet (40) mounted between two end pieces (42, 44) and surrounded by a retaining hoop (70). The magnet, which is made of high energy product matereial, is "straight through" diametrically magnetised to provide a sinusoidal air gap flux distribution which eliminates losses due to harmonics. The cylindrical two-pole permanent magnet is built up from a plurality of smaller magnet blocks (50) which are stacked and machined to form a cylindrical configuration.

Description: The present invention relates generally to the construction and configuration of a permanent magnet rotor for electrical machines, whether motors or generators. A machine with a toothless stator is the subject of United States patent application serial no. 800,184, filed 20 November, 1985. The use of a toothless stator construction necessarily requires that the magnetic air gap is considerably larger than that generally encountered in existing machines, so it is necessary that the permanent magnet rotor provide the maximum possible magnetomotive force to make practical a machine with such a large magnetic air gap. Typical state-of-the-art rotor construction utilises a minimum of four individual precision machined magnets, a precision machined ferro-magnetic support yoke, and an aluminium cage consisting of spacers and rings and used to position and support the individual magnets. Construction of such conventional permanent magnet rotors typically includes at least twelve individual parts contained in the rotor, each of which must be precision machined to result in proper mechanical fit of the various parts. It may be appreciated that the high parts count and precision machining requirements result in relatively expensive rotor fabrication costs. DE-U-1938162 shows a rotor in the form of a cylindrical two-pole permanent magnet which is "straight through" diametrically magnetised. DE-A-3236619 discloses a rotor in which shafts are fixed to the rotor body by end pieces which are of a non-magnetisable material. EP-A-0 187 478 discloses a permanent magnet rotor in which a solid cylindrical magnet is mounted and bonded between two non-magnetic end pieces with a cylindrical retaining shell fitted around the magnet; this application lies within the terms of Art. 54(3) EPC. It is therefore an object of the present invention to reduce both the number of parts contained in the rotor and the rotor fabrication time, which is mainly due to precision machining of the various parts contained in the rotor. It may be appreciated that by accomplishing a substantial reduction in the cost of the rotor, while simultaneously utilising the substantially cheaper toothless stator construction described in the above-referenced copending patent application, the overall construction cost of a permanent magnet machine may be substantially reduced. The second objective of the present invention is to maximise machine efficiency and provide in a reduced size and weight package a machine having equivalent power and at least equivalent efficiency to a larger conventional machine. In order to reduce the size of the machine without reducing efficiency or output of the machine, it is necessary that the permanent magnet rotor provide the maximum possible magnetomotive force for a given rotor size. By so doing, the rotor of the present invention may be used with the toothless stator construction referenced above to construct a compact, efficient electrical machine. According to the present invention, there is provided a rotor for an electrical machine, comprising a cylindrical two-pole permanent magnet which is "straight through" diametrically magnetised, characterised in that the rotor includes first and second end pieces made of non-magnetisable material, and bonded to the respective ends of the cylindrical two-pole permanent magnet, and a cylindrical retaining hoop fitted around the cylindrical two-pole permanent magnet, and those portions of the first and second end pieces adjacent to the two-pole permanent magnet, the cylindrical two-pole permanent magnet being solid and being made of a high energy product material, for example, samarium cobalt or neodymium-iron-boron. By "straight through" diametrically magnetised is meant that the cylindrical magnet has its magnetic axis along a single diameter of the rotor. The cylindrical magnet is magnetised with all flux lines parallel to that magnetic axis to produce a sinusoidal air gap flux distribution which eliminates losses due to harmonics in the magnetic field. Such a rotor would consist, therefore, essentially of only four pieces, significantly reducing the amount of precision machining required by previous rotor designs. This significant reduction in rotor parts count has two advantages. First, since there are only four parts contained in the rotor, and since far less precision machining of parts is required, the cost of construction of the rotor is substantially reduced. Secondly, since the parts count of the rotor is reduced, the overall stiffness of the rotor is substantially increased, which results in a rotor operable at higher speeds, thus allowing higher output from the same size. Since the rotor of the present invention utilises a solid cylindrical magnet which forms a substantial percentage of the total volume of the rotor, the maximum possible magnetomotive force is generated for any given rotor size. Accordingly, the rotor of the present invention may be used with a large air gap, and will be functional with the toothless stator construction referenced above. The use of the rotor of the present invention with a toothless stator will result in a machine having a substantially increased power output per machine volume and weight, with little or no reduction in efficiency. The invention also relates to a method for manufacturing a rotor as defined above. The invention may be carried into practice in various ways, and certain embodiments will be described by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a schematic cross-sectional view of a toothless stator construction shown with a two-pole rotor; Figure 2 is an exploded, partially cutaway view of a rotor constructed according to the teachings of the present invention; Figure 3 is an end view of the cylindrical solid magnet used in the rotor illustrated in Figure 2; Figure 4 is an exploded view of the assembled rotor parts shown in Figure 2 and the retaining hoop; Figure 5 is a cross-sectional view of an hydraulic press used to install the assembled rotor of Figure 2 into the retaining hoop without using heating; and Figure 6 is a graph of power per weight versus magnet energy product for a conventional machine, and a machine using both a toothless stator and the rotor of the present invention, the latter being shown both with and without direct liquid cooling. A machine utilizing a toothless stator is shown in schematic fashion in Figure 1 with a two pole rotor assembly 20 comprised of a two pole permanent magnet rotor 22 shown in schematic fashion, which rotor 22 is surrounded by a retaining hoop 24 made of non-magnetizable material. The toothless stator illustrated in Figure 4 has stator windings 26 which are mounted on a winding support structure 28 made of non-magnetizable material, which winding support structure 28 has a cylindrical portion with radially outwardly extending longitudinal support fins 30 mounted thereto. The support fins 30, which extend longitudinally along the cylindrical portion of the winding support structure 28 except on the two ends thereof, provide the areas into which the stator windings 26 may be inserted. Construction of the stator is facilitated by the fact that the stator windings 26 may be laid into the outer side of the winding support structure 28, and therefore the stator windings 26 may be prefabricated resulting in reduced labor cost. The flux path is completed by a cylindrical flux collector ring 32 made of ferromagnetic material which is mounted about the outer periphery of the winding support structure 28, with the flux collector ring 32 extending around the outermost edges of the support fins 30 on the winding support structure 28. Details of the construction of the toothless stator schematically illustrated in Figure 1 are given in the above-referenced disclosure. As illustrated in Figure 1, the magnetic air gap of this machine extends from the outer periphery of the two pole permanent magnet rotor 22 to the inner periphery of the flux collector ring 32. Such a large magnetic air gap would not be feasible with low energy product magents, but recently developed high energy produce magnets make the large magnetic air gap a problem which may be overcome. It is first necessary that the magnets in the two pole rotor 22 are high energy produce magnets such as samarium cobalt or neodymium-iron-boron magnets, the former of which are presently available with magnetic energy products of 27 mega-gauss-oersted (MGO) and the latter of which are available with magnetic energy products of 35 MGO. As mentioned above, it is desireable that the two pole rotor 22 schematically illustrated in Figure 1 have the maximum possible magnetomotive force to allow the machine illustrated in Figure 1 to function with the large magnetic air gap. It is apparent that the limit on the size of the magnet contained in a rotor is 100% of the area carrying the magnet or magnets, and the attainment of this 100% absolute maximum is the accomplishment of the present invention. The present invention utilizes a "straight through" diametrically magnetized solid cylindrical two-pole magnet, which cylindrical magnet is contained between two end pieces and surrounded by a cylindrical retaining hoop. Such a rotor is illustrated in Figure 2 without the outer shell. The "straight through" diametrically magnetized two pole magnet has its magnetic axis along a single diameter of the rotor. The magnet is therefore magnetized with all flux lines therethrough parallel to that magnetic axis, to produce a sinusoidal air gap flux which eliminates harmonics related losses in the rotor and stator assemblies. A solid cylindrical two pole magnet 40 is illustrated in Figure 2 between a first end piece 42 and a second end piece 44. The first and second end pieces 42, 44 are made of non-magnetizable material, which will be attached to the two ends of the solid cylindrical two-pole magnet 40. Obviously, it is desireable that the solid cylindrical two-pole magnet 40 be made of a single piece of magnetizable medium which is then magnetized. However, with the present state-of-the-art in high magnetic energy produce permanent magnets, it is not possible to magnetize material greater than approximately 1.9 cm square cross-sectional area, with a number of manufacturers only having the capability of magnetizing material which is 0.8 cm square cross-sectional area. In the example illustrated in Figures 2 and 3, 0.8 cm by 0.8 cm by 2.54 cm magnet blocks 50 are utilized to construct the 15.2 cm long, 2.19 cm diameter solid cylindrical two-pole magnet 40. The magnet blocks 50 may be bonded together using contact cement, with each of the magnet blocks 50 aligned with the same axis of magnetic polarity to produce a diametrically magnetized construction in the solid cylindrical two pole magnet 40. Adjacent magnet blocks 50 overlap each other for greater strength. Following construction of the magnets blocks 50 into a unit, they are machined to rough size, with the ends of the solid cylindrical two-pole magnet 40 being machined flat in the preferred embodiment. It should be noted that it is possible that the magnet 40 and the end pieces 42, 44 could be machined to a non-flat surface, as, for example, conical surface, but from an economic standpoint flat surfaces are preferred. The first and second end pieces 42, 44 are then bonded to the solid cylindrical two-pole magnet 40 using contact adhesive, such as for example Eastman 910, and the rotor assembly 60 illustrated in Figure 2 is then machined to have a cylindrical outer surface. It is important that the first and second end pieces 42, 44 have a portion which is the same diameter as the solid cylindrical two-pole magnet 40 so that the rotor, when finished will have sufficient stiffness. For the example illustrated in Figure 2, the width of the portion of the first and second end pieces 42, 44 having the same diameter as the solid cylindrical two-pole magnet is approximately 1.0 cm. An outer retaining hoop 70 is then inserted over the rotor assembly 60, with the shell covering the entire solid cylindrical two-pole magnet 40 as well as the portions of the first and second end pieces 42, 44 having the same diameter as the solid cylindrical two-pole magnet 40. The typical method of installing the retaining hoop 70 on the rotor assembly 60 is by heat shrinking the outer retaining hoop 70 onto the rotor 60. Generally, the retaining hoop 70 is heated while the rotor assembly 60 is cooled, after which the hoop 70 is inserted over the rotor assembly 60. As the hoop 70 assembly cools, it shrinks and forms a high interference fit with the rotor assembly 60. In certain applications, it may also be desireable to weld the retaining hoop 70 onto the rotor assembly 60, although it has been found that the high interference fit of the hoop 70 on the rotor assembly 60 without welding is generally acceptable. High energy permanent magnets are typically very sensitive to heat, and the operation of fitting the retaining hoop 70 onto the rotor assembly 60 using a heat-shrink fit may affect the magnetic characteristics of the rotor assembly 60. While samarium cobalt magnets may be utilized in a rotor assembly 60 with a heat-shrink fitted retaining hoop 70 without substantial problem, neodymium-iron-boron magnets are three times as sensitive to heat as samarium cobalt magnets. Accordingly, a different technique must be used to insert the retaining hoop 70 over the rotor assembly 60 when the magnets are neodymium-iron-boron magnets. The alternative embodiment uses hydraulic pressure to slightly expand a retaining hoop 170 to allow the rotor assembly 60 to be inserted into the retaining hoop 170, as illustrated in Figure 5. A housing 120 made of non-ferromagnetic material open at one end is designed to receive the hoop 170, which has a diameter at the ends thereof which is slightly larger than the diameter of the portion intermediate the ends. The hoop 170 is positioned in the housing 120 with its inward movement limited by tabs 122, 124 made integrally with the housing 120. The portions of the housing 120 around the outside of the intermediate portion of the hoop 170 contain an annular recessed area 130 in which hydraulic pressure will not be present, and which may be vented. It is thus apparent that when the interior of the housing 120 is supplied with pressurized hydraulic fluid through an inlet 132, the hoop 170 will be caused to expand outwardly, slightly increasing its inner diameter. The rotor assembly 60 is placed on a hydraulic ram 140 extending through the housing 120 and having an integral plate 142 to force the rotor assembly 60 into the hoop 170. The housing 120 is sealed at its open end by a cover 150 which may be screwed onto the housing 120, and through which the hydraulic ram 140 extends. The hydraulic ram 140 is sealed by packing 160, 162 in the housing 120 and the cover 150. Hydraulic pressure is supplied to the housing 120, causing the hoop 170 to expand radially outwardly. The hydraulic ram 140 then forces the rotor assembly 60 into the hoop 170, with proper placement being limited by contact of the rotor assembly 60 with the closed end of the housing 120. During this operation, supply of hydraulic pressure to the interior of the hoop 170 is ensured by providing a hydraulic passage 164 in the housing 120 to equalize pressure on both ends of the hoop 170. By using this embodiment, it is not necessary to heat the retaining hoop 170. Accordingly, the chance of damaging the magnet 40 by heating it when the hoop 170 is installed on the rotor 60 is completely eliminated. Performance of a machine built according to the teachings of the present invention both with and without direct liquid cooling of the stator is illustrated in Figure 6 as compared with a conventional machine. It is apparent that for high energy product permanent magnets the design using the rotor of the present invention in combination with a toothless stator, particularly a toothless stator with liquid cooling, is substantially more efficient than a conventional machine. By utilizing a rotor constructed according to the teachings of the present invention in combination with a toothless stator, it is apparent that a machine smaller both in size and weight than conventional machines may be constructed to provide a given power output. For the example used above with the solid cylindrical two pole magnet 40 being constructed of 0.8 cm by 0.8 cm by 2.54 cm samarium-cobalt magnets, a machine may be produced which will operate at 45,000 rpm and produce 85 horsepower as a motor or generate 63 Kilowatts as a generator. It is therefore apparent that the rotor of the present invention may be used to produce a machine having a high power output per volume ratio. Additionally, since the rotor of the present invention is constructed of four parts rather than the typical twelve parts used in a rotor, labour cost due to machining and part cost of the rotor are reduced. The rotor of the present invention is therefore more inexpensive to manufacture than previous rotors while also possessing substantially higher performance characteristics. Accordingly, it may be perceived that the rotor of the present invention represents a significant step in the state-of-the-art and a highly advantageous construction for electrical machines.

Claims: A rotor for an electrical machine, comprising a cylindrical two-pole permanent magnet (40) which is "straight through" diametrically magnetized, characterised in that the rotor includes first and second end pieces (42, 44) made of non-magnetisable material, and bonded to the respective ends of the cylindrical two-pole permanent magnet, and a cylindrical retaining hoop (70) fitted around the cylindrical two-pole permanent magnet, and those portions of the first and second end pieces adjacent to the two-pole permanent magnet, the cylindrical two-pole permanent magnet being solid and being made of a high energy product material, for example, samarium cobalt or neodymium-iron-boron. A rotor as claimed in Claim 1, characterised in that the cylindrical two-pole permanent magnet comprises a plurality of smaller magnet blocks (50) with their magnetic axes aligned with the same polarity. A rotor as claimed in Claim 2, characterised in that adjacent magnet blocks overlie each other for greater strength. A method for manufacturing a rotor for an electrical machine comprising a two-pole permanent magnet (40) which is "straight-through" diametrically magnetized, characterized in that a cylindrical solid magnet (40) is mounted and bonded between two end pieces (42, 44) made of non-magnetisable material, and a cylindrical retaining hoop (70) is fitted around the magnet (40) and those portions of the end pieces (42, 44) adjacent the magnet (40), the magnet being made of a high energy product material such as samarium cobalt or neodymium-iron-boron. A method as claimed in Claim 4, characterised in that a number of magnetic blocks (50) are assembled to form the solid cylinder of magnetic material, with the magnetic axes of the blocks aligned with the same polarity, and the body is then machined to be of accurate cylindrical form (40) with machined ends. A method as claimed in Claim 4 or Claim 5, characterised in that the blocks (50) are assembled using an adhesive. A method as claimed in any of Claims 4 to 6, characterised in that the hoop (70) is fitted around the magnet by a heat shrink operation. A method as claimed in any of Claims 4 to 6, characterised in that the hoop (70) is installed by using hydraulic pressure to expand the interior of the hoop radially outwardly, and sliding the two-pole permanent magnet and the adjacent portions of the end pieces into the hoop. A rotary electrical machine having a rotor as claimed in any of Claims 1 to 3.

Inventors: DENK, JOSEPH, WUERTZ, KENNETH L.
Applicants: ALLIED-SIGNAL INC.
Application year: 1986
Main classification: H02K1
IPC codes: H02K1, H02K15