METHOD OF PROTECTING THE MAGNETS OF A PERMANENT MAGNETIC ROTOR AGAINST CORROSIVE AGENTS

Abstract: A permanent magnet rotor (1) used in a corrosive atmosphere or liquid and provided with field permanent magnets (10) inserted into slots (9) of a yoke (3) of a lamination of silicon steel plates (2), wherein the outer surfaces of the field permanent magnets (10) are covered with anticorrosive plates made of an anticorrosive material. The field permanent magnets (10) of the permanent rotor (1) can be used without being corroded, and the rotor (1) can be easily manufactured.

Description: Technical Field The present invention relates to a method of protecting the magnets of a permanent magnetic rotor against corrosive agents. Prior Art The permanent magnet rotor is well known, which generally comprises a yoke formed of a plurality of laminated silicon steel sheets and permanent magnets for field inserted into such a yoke. Referring to Fig. 8 of the attached drawings, there is shown a permanent magnet rotor 50 of prior art which includes a yoke 51. The yoke 51 is formed of, as mentioned above, a plurality of integrally laminated silicon steel sheets 52 and has along its outer periphery four magnetic poles 53 projecting radially outward. The respective magnetic poles 53 are formed with slots 54 serving to respective field permanent magnets 55. Now referring to Fig. 9, the above well known permanent magnet rotor 50 is shown in a sectional view. As seen, the permanent magnets 55 are inserted into the respective slots 54 of the yoke 51. These permanent magnets 55 are arranged so that the outer periphery of the permanent magnet rotor 50 is magnetized alternately in different polarities, as shown. Consequently, the permanent magnet rotor 50 has four magnetic poles. Magnetic flux generated from these magnetic poles interacts with electric current flowing through excitation coils (not shown) of the motor to drive the permanent magnet rotor 50. However, the conventional permanent magnet rotor has been disadvantageous in that the fluid surrounding the permanent magnet rotor tends to permeate through interstices of the laminated steel sheets and comes in contact with the permanent magnets. Particularly, in the case of the motor used in the scroll type compressor for the air-conditioner, the permanent magnets of the rotor have inevitably been corroded due to direct contact with the refrigerant, resulting in serious problems, for example, a capacity of the motor has been reduced due to demagnetization of the magnets and parts of the equipment have been clogged with magnetic particles separated from the permanent magnets. Moreover, the permanent magnets in the conventional permanent magnet rotor have usually been mechanically fragile and liable to be damaged during insertion or bonding into or onto the yoke. EP-A-228 872 discloses a rotor for an electrical machine having large permanent magnets which are located at half the pole locations. The whole rotor is surrounded by a retaining hoop which serves to retain the large permanent magnets on the yoke. There are only mechanical reasons for providing this retaining hoop. This hoop does not serve as a protection against any fluid or atmosphere. US-A-4,414,523 discloses a rotor in which annular magnets are provided on a molded core formed of plastic material. The annular magnets are covered by an encapsulating jacket which is mounted with its axial ends in the molded material. DE-U-86 03 982 discloses a rotor for a motor having a cylindrical hoop around its circumferential surface and annular covering means bonded to its end faces in order to mechanically reinforce the rotor and to protect it against dust and corrosion. This rotor is not hermetically covered. Accordingly, it is an object of the invention to provide a method of protecting the magnets of a permanent magnetic rotor against corrosive agents, like corrosive atmosphere or liquid, which method allows to easily manufacture a permanent magnetic rotor. Disclosure of the Invention: The object set forth above is achieved, in accordance with a first aspect of the invention, by a method of protecting the magnets of a permanent magnetic rotor against corrosive agents comprising the steps of forming a peripheral wall of a mold having a rectangular cross-section from an anticorrosive sheet, pouring molten metal of cast type permanent magnet and thereby molding a permanent magnet ingot, hot rolling said permanent magnet ingot by a rolling mill, simultaneously said ingot being magnetically made anisotropic, into a rolled bar of permanent magnet having a predetermined cross-sectional dimension, cutting said rolled bar of permanent magnet into an individual permanent magnet of a predetermined length, and inserting or bonding said permanent magnet into or onto the yoke. The object set forth above is achieved, in accordance with a second aspect of the invention, by a method of protecting the magnets of a permanent magnetic rotor comprising a yoke, magnets and a shaft for a brushless motor against corrosive agents comprising the steps of forming a casing having a cylindrical portion and a bottom with an opening for the shaft of a corrosion resisting material, inserting the rotor into the casing, sealing the opening of the bottom against the shaft, tightly fitting a molded member of corrosion resisting material into the opening of the cylindrical portion at the free end thereof, and sealing the opening of the molded member against the shaft and the joint between the molded member and the opening of the cylindrical Portion of the metallic casing, so that both the yoke and the permanent magnets of said rotor are hermetically sealed against corrosion. Brief Description of the Drawings: Fig. 1 is a perspective view showing an embodiment of a permanent magnet rotor constructed in accordance with the invention; Fig. 2 is a sectional view of the permanent magnet rotor shown in Fig. 1; Fig. 3 is a perspective view sketching a first manufacturing process of the permanent magnet used in the rotor of Figs. 1 and 2; Fig. 4 is a schematic diagram illustrating a second manufacturing process of the permanent magnet; Fig. 5 is an exploded perspective view showing an alternative embodiment of the permanent magnet rotor constructed in accordance with the invention; Fig. 6 is a sectional side view of the permanent magnet rotor shown in Fig. 5; Fig. 7 is a schematic diagram illustrating a spinning process for a metallic casing; Fig. 8 is a perspective view sketching a permanent magnet rotor of prior art; and Fig. 9 is a sectional view of the permanent magnet rotor shown in Fig. 8. Preferred Embodiments of the Invention : The invention will be more readily understood from the following description of preferred embodiments made with reference to the attached drawings. Referring to Fig. 1, there is shown the first embodiment of the permanent magnet rotor constructed in accordance with the invention. The permanent magnet rotor 1 has a yoke 3 comprising a plurality of integrally laminated silicon steel sheets 2. Each of these silicon steel sheets 2 has a plurality of rectangular recesses 4 formed by pressing so that these silicon steel sheets may be caulked together by forcing the recesses 4 into engagement with one another and thereby integrated into the yoke 3. The yoke 3 is centrally formed with an opening 5 axially extending therethrough to receive a rotatable shaft 6 which has, in turn, a key 7 by means of which the rotatable shaft 6 is rotated integrally with the yoke 3. The yoke 3 has along its outer periphery four magnetic poles projecting therefrom radially outward and a pair of the magnetic poles 8a, 8a which are diametrically opposed to each other are provided with slots 9, respectively, destined to receive permanent magnets. The another pair of said magnetic poles 8b, 8b are provided with no slot. The slots 9 contain therein the permanent magnets 10, respectively. The permanent magnets 10 are arranged to have their N-poles diametrically opposed to each other. The N-poles of the representative permanent magnets 10 repulse each other and, in consequence, the magnetic poles 8a, 8a are magnetized in S-polarity while the magnetic poles 8b, 8b are magnetized in N-polarity. Thus, the permanent magnet rotor 1 has along its outer periphery four magnetic poles arranged to have alternately different polarities and is rotationally driven by an interaction of these magnetic poles and electric current flowing through excitation coils (not shown). Fig. 2 shows the permanent magnetic rotor 1 in a crosssection. As shown, the permanent magnet rotor 1 includes a pair of permanent magnets 10 each comprising a cast type permanent magnet core 11 and an anticorrosion plate 12 would around the former. The anticorrosion plate 12 is made of metallic sheet having an ability to resist severe temperature and corrosive attack of refrigerant with which the plate 12 is to be in contact (referred to hereinafter as corrosion resistance) and is integrally wound around the case type permanent magnet core 11. The permanent magnet 10 is protectively covered with the anticorrosion plate 12 and therefore will be free from corrosion even if the permanent magnet rotor 1 is used directly in a severe environment where refrigerant may permeate through the interstices of the laminated silicon steel sheets 2 and comes in direct contact with the permanent magnets 10. In this manner, separation of magnetic particles from the permanent magnets 10 due to corrosion and clogging of various equipment parts with such magnetic particles are effectively avoided. Demagnetization due to corrosion of the permanent magnets is also avoided. Referring to Fig. 3, the first manufacturing process for the permanent magnets 10 is illustrated. As illustrated, the corrosion resisting metallic sheet 13 is bent and opposite free edges thereof are welded together so as to form a peripheral wall of a mold 14 having a rectangular cross-section. Bottom of the mold 14 is sealed by a suitable member. Molten metal 11a of the case type permanent magnet such as praseodymium alloy is poured into the mold 14 and thereby a permanent magnet ingot 15 is molded. Referring to Fig. 4, the second manufacturing process for the permanent magnet 10 is illustrated. The permanent magnet ingot 15 molded in the manner as has been mentioned above is then inserted into a hot working chamber 16 maintained at a predetermined temperature and therein hot rolled by a rolling mill 17 while the ingot 15 is magnetically made anisotropic under effect of an intense magnetic field. The permanent magnet ingot 15 is moved in the direction indicated by an arrow B and rolled into an elongate rolled bar 18 of permanent magnet having a predetermined cross-sectional dimension as the rolling mill 17 is rotated in a direction indicated by an arrow A. The rolled bar 18 of permanent magnet thus obtained is then cut transversely of its axis by a cutting-off wheel into the individual permanent magnets 10 each of a predetermined dimension. The permanent magnets 10 are inserted into the respective slots 9 of the yoke 3 and axially opposite and surfaces of each magnet 10 are coated with anticorrosive resinous material. The rotatable shaft 6 is inserted into the axial opening of the yoke 3 so as to complete the permanent magnet rotor 1. According to the above-mentioned manufacturing method. productivity of the permanent magnets 10 can be improved. since it is unnecessary to wrap the individual permanent magnets 10 with the individual anticorrosion plates 12 and to weld opposite free edges of the individual anticorrosion plates 12. In addition, the permanent magnets 10 can be inserted into the respective slots 9 of the yoke 3 without a risk of damage, since the permanent magnets 10 are inserted into the yoke 3 together with the anticorrosion plates 12 protectively wrapping the respective magnets 10. This provides an easy and highly productive method of making the permanent magnet rotor. While the invention has been described above in connection with the specific embodiment of permanent magnets have their magnetic poles of a same polarity being diametrically opposed to each other and mutual repulsion of these magnetic poles generates the magnetic poles the number of which is twice the number of permanent magnets along the outer periphery of the permanent magnet rotor, the invention is not limited to the permanent magnet of such construction but applicable also to the conventional permanent magnet rotor including the same number of permanent magnets as the number of magnetic poles are inserted into the yoke. While the invention has been described also above in the connection with the specific embodiment of permanent magnet rotor so constructed that the permanent magnets are inserted into the slots of the yoke comprising a plurality of integrally laminated silicon steel sheets, the invention is not limited to such a rotor but applicable also to the permanent magnet rotor so constructed that the permanent magnets are bonded onto the outer periphery of a pillar-shaped yoke. According to the first aspect of the invention, as will be apparent from the foregoing description, the permanent magnet rotor can be used in corrosive atmosphere or liquid such as refrigerant without a risk that the permanent magnets might be corroded thereby, since the outer peripheries of the respective permanent magnets are hermetically covered with the anticorrosion plates and perfectly isolated from said atmosphere or liquid. In addition to protecting the permanent magnet core against corrosion, the anticorrosion plate prevents the permanent magnet from being damaged during assembly of the permanent magnet rotor. With the method of making the permanent magnet rotor according to the invention, the manufacturing processes are substantially simplified with respect to the conventional method by which the anticorrosion plates such as metallic sheets are directly wound around the individual permanent magnets, since, according to the invention, each anticorrosion plate is used as a part of the mold into which molten metal of the cast type permanent magnet is poured to mold the permanent magnet ingot, then the permanent magnet ingot thus formed is hot rolled into the rolled bar while said ingot is magnetically made anisotropic, and finally this rolled bar is cut into a plurality of individual permanent magnets each of a predetermined length. More specifically, the processes of winding the anticorrosion plates around the individual permanent magnets and welding or bonding opposite free edges of each anticorrosion plates thus would around the permanent magnet can be eliminated, and a number of permanent magnets can be obtained at once with a high productivity. Now, a second embodiment of the invention will be described with reference to Figs. 5 through 7. According to this embodiment, the yoke and the permanent magnets of the rotor are hermetically covered with thin sheets of anticorrosive material so that the permanent magnet rotor may be used in the corrosive atmosphere or liquid. Such construction allows both the yoke and the permanent magnets to be perfectly isolated from the external atmosphere or liquid and thereby allows the permanent magnet rotor to be used in said corrosive atmosphere or liquid, for example, in refrigerant without a risk that the yoke and the permanent magnets might be thereby corroded. Referring to Fig. 5, a permanent magnet rotor 21 comprises a permanent magnet rotor proper 22, a metallic casing 23 covering the outer peripheral surface and one axial end surface of a pillar-shaped assembly of a yoke and permanent magnets constituting the permanent magnet rotor proper 22, and a molded member 24 adapted to cover the other axial end surface of the pillar-shaped assembly. The permanent magnet rotor proper 22 centrally includes the yoke 25 which is, in turn, centrally provided with an opening 26 serving to receive a rotatable shaft 27. The rotatable shaft 27 has a locking key or the like (not shown) by means of which the rotatable shaft 27 can be rotated integrally with the yoke 25. The yoke 25 has an outer periphery configured to comprise a pair of cylindrical surface sections and a pair of flat surfaces which are mutually parallel and opposed to each other. A pair of bar-shaped permanent magnets 28, 29 are bonded onto the parallel flat surfaces of the yoke 25, respectively, with their magnetic poles of a same polarity being opposed to each other. The surfaces of these permanent magnets 28, 29 not opposed to each other respectively carry auxiliary yokes 30, 31 bonded thereto, respectively, serving to form magnetic poles. Assumed that the permanent magnets 28, 29 are arranged on the mutually parallel surfaces of the yoke 25 with their N-poles being opposed to each other, mutual repulsion of these magnetic poles causes the outer peripheral sections of the outer surfaces of the auxiliary yokes 30, 31 to be magnetized in S-polarity. Consequently, the permanent magnet rotor 21 has along the outer periphery four magnetic poles and rotationally driven by interaction with excitation coils (not shown) of the motor. The metallic casing 23 comprises a cylindrical portion 32 serving to cover the outer periphery of the pillar-shaped assembly consisting of the yoke 25, the permanent magnets 28, 29 and the auxiliary yokes 30, 31 and a bottom 33 serving to cover the one axial end surface of the pillar-shaped assembly. The bottom 33 is centrally provided with an opening 34 serving to receive the rotatable shaft 27. The metallic casing 23 is formed by well known deep drawing process in which a central zone of a metal sheet is forced by a punch of a predetermined configuration into a die cavity so as to form a metallic container having a cylindrical side wall and a bottom wall which is then provided with an opening. Starting metal sheet material for the metallic casing 23 should have a resisting property (referred to hereinafter as corrosion resistance) to the severe temperature and/or corrosiveness of the particular type of refrigerant being used. Such corrosion resisting metallic material includes various kinds of metal sheets such as a stainless steel sheet and the other anticorrosive metal sheets. Referring to Fig. 6, the assembled permanent magnet rotor is shown in an axial sectional view, in which the permanent magnet rotor proper 22 has been inserted into the metallic casing 23. The outer peripheral surface 35 of said pillar-shaped assembly comprising the yoke 25, the permanent magnets 28, 29 and the auxiliary yokes 30, 31 is covered with the cylindrical portion 32 of the metallic casing 23 and the one axial end surface 26 is covered with the bottom wall 33 of the metallic casing 23. A joint between the opening 34 formed in the bottom wall 33 of the metallic casing 23 and the rotatable shaft 27 is sealed by use of resinous material or welding process. An open end of the metallic casing's cylindrical portion 32 projects slightly beyond an end surface of the pillar-shaped assembly comprising the yoke 25. the auxiliary yokes 30. 31 and the permanent magnets 28. 29. The molded member 24 comprising a corrosion resisting resinous molded member or an aluminum die case is tightly fitted into a recess defined by the end surface 37 and the cylindrical portion 32 of the metallic casing 23. Both a joint between the molded member 24 and the rotatable shaft 27 and a joint between the molded member 24 and the cylindrical portion 32 of the metallic casing 23 are sealed by use of resinous adhesive or welding process. With the construction as has been described above, the yoke 25, the permanent magnets 28. 29 and the auxiliary yokes 30. 31 constituting together the permanent magnet rotor proper 22 is perfectly isolated from the exterior and therefore the permanent magnet rotor 21 can be used even in the corrosive atmosphere or liquid without a risk that the refrigerant might come into direct contact with the yoke and the permanent magnets and corrode them. Additionally, the cylindrical portion 32 of the metallic casing 23 effectively prevents the permanent magnets 28, 29 from centrifugally flying off. The metallic casing 23 can be molded also by spinning instead of the above-mentioned deep drawing. Fig. 7 illustrates one process in the course of molding the metallic casing by means of the spinning. As seen, the metal sheet 38 is secured at its central zone by a fastener to a mold 40 and rotated by a motor 41. Adjacent the mold 40 there is provided a pallet 42 so as to be swingable around one end thereof. The pallet 42 is swung by a pallet driver 43 in the direction X as indicated by an arrow, pressing the metal sheet 38 against its outer periphery of the mold 40 to form a metallic container having a cylindrical side wall and a bottom wall. The bottom wall of this metallic container is formed with an opening and thereby the complete metallic casing 23 is obtained. Formation of the metallic casing 23 by the above-mentioned deep drawing or spinning process advantageously allows the conventionally required processes of winding the metal sheet around the permanent magnet rotor and then welding the opposite free edge of the metal sheet to be eliminated, thus improving a productivity with which the permanent magnet rotor can be manufactured. While the invention has been described above in connection with the specific embodiment of permanent magnet rotor so constructed that the permanent magnets are arranged with their magnetic poles of a same polarity being opposed to each other and mutual repulsion of these magnetic poles generates along the outer periphery of the permanent magnetic rotor the magnetic poles the number of which is twice the number of permanent magnets, the invention is not limited to such construction of permanent magnet rotor but applicable also to any other construction of permanent magnet rotor. As will be appreciated from the foregoing description, the second embodiment of permanent magnet rotor can be used in the corrosive atmosphere or liquid, for example, in refrigerant, since the outer periphery of the yoke and the permanent magnets is hermetically covered with the anticorrosive sheet material. Furthermore, by hermetically covering both the yoke and the magnets with the metallic casing formed by the deep drawing or spinning process, this embodiment advantageously allows the conventionally required process of welding to be eliminated and provides the permanent magnet rotor which can be easily manufactured and free from a risk that the refrigerant or the like might permeate through the seam of the metallic casing. Industrial Usefulness : As will be appreciated from the foregoing description, the rotor for the brushless motor constructed according to the invention is suitable for the motor such as the motor for scroll type compressor which is usually used in the corrosive atmosphere or liquid, particularly in refrigerant.

Claims: A method of protecting the magnets of a permanent magnetic rotor against corrosive agents comprising the steps of forming a peripheral wall of a mold (14) having a rectangular cross-section from an anticorrosive sheet, pouring molten metal (11a) of cast type permanent magnet and thereby molding a permanent magnet ingot, hot rolling said permanent magnet ingot by a rolling mill, simultaneously said ingot being magnetically made anisotropic, into a rolled bar (18) of permanent magnet having a predetermined cross-sectional dimension, cutting said rolled bar (18) of permanent magnet into an individual permanent magnet (10) of a predetermined length, and inserting or bonding said permanent magnet (10) into or onto the yoke (3). A method of protecting the magnets of a permanent magnetic rotor comprising a yoke (25), magnets (28, 29) and a shaft (27) for a brushless motor against corrosive agents comprising the steps of forming a casing (23) having a cylindrical portion (32) and a bottom (33) with an opening (34) for the shaft (27) of a corrosion resisting material, inserting the rotor (22) into the casing (23), sealing the opening (34) of the bottom (33) against the shaft (27), tightly fitting a molded member (24) of corrosion resisting material into the opening of the cylindrical portion (32) at the free end thereof, and sealing the opening of the molded member (24) against the shaft (27) and the joint between the molded member (24) and the opening of the cylindrical portion (32) of the metallic casing (23), so that both the yoke (25) and the permanent magnets (28, 29) of said rotor are hermetically sealed against corrosion. A method according to claim 2, wherein the casing (23) is formed as a seamless, anticorrosive metallic casing. A method according to claim 2, wherein the casing (23) is formed by deep drawing or spinning process. A method according to claim 2, wherein said molded member (24) is formed of a corrosion resisting resinous molded material or die cast material.

Inventors: ENDO KENICHI, KOIKE YOSHIKAZU, NAGATE TAKASHI, SETO TAKESHI, YAMAGISHI YOSHIHIKO, ENDO, KENICHI, KOIKE, YOSHIKAZU, NAGATE, TAKASHI, SETO, TAKESHI, YAMAGISHI, YOSHIHIKO, ENDO, KENICHI, SEIKO EPSON CORPORATION, KOIKE, YOSHIKAZU, SEIKO EPSON CORPORATION, NAGATE, TAKASHI, SEIKO EPSON CORPORATION, SETO, TAKESHI, SEIKO EPSON CORPORATION, YAMAGISHI, YOSHIHIKO, SEIKO EPSON CORPORATION
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1991
Main classification: H02K1
IPC codes: H02K1, H02K15