Electric rotary machine

Abstract: A front frame (5) and a rear frame (6 in Figures 3 and 2) of an alternator house a rotor (3 in Fig.3) and a stator (4 in Fig.3) therein and are secured by through bolts (22 in Fig.3) and nuts (23 in Fig.3) to each other in an axial direction at supports (50,60) which are formed on the circumferential portion of the frames (5,6 in Fig.3). Cooling air intake windows (24,25) are formed on the end surfaces of the frames and cooling air discharging windows (26,27) are formed on the circumferential surface of the frames (5,6). Window ribs (26a,26b,27a,27b) formed between the discharging windows (26,27) are arranged so that the ribs (26a,27a) adjacent to the supporting portion (50,60) have a width which is greater than the width of other ribs (26b,27b), thereby providing sufficient rigidity to suppress resonant vibrations and the generation of magnetic noise which occurs when the armature reaction of the alternator is transmitted from the stator (4 in Fig.3) to the entirety of the frames (5,6 in Fig.3).

Description: The present application is based on and claims priority from Japanese Patent Application No. Hei 6-125139 filed on June 7, 1994. The present invention relates to an electric rotary machine according to the preamble of claim 1, in particular to an alternator for a vehicle. In an alternator for a vehicle, magnetic noise is generated by a stator or the like under magnetic vibration. The magnetic noise is amplified due to resonance of the alternator frame, particularly in a relatively low rotational speed range (e. g., 1500 - 4000 RPM) and may feel harsh to the driver and passengers of the vehicle. In order to prevent such magnetic noise, the structure shown in Figs. 10 and 11 has been proposed in which the rigidity of front and rear frames 100 and 200 of an alternator is increased by increasing the thickness of the peripheral walls 110 and 210 of the frames 100 and 200. However, if the thickness of the peripheral walls 110 and 210 of the frames 100 and 200 is increased in order to provide higher rigidity, the weight and size of the alternator are also increased. E.g. from the US-A-5 028 826 or the US-A-5 194 770 an AC-generator is known comprising an electric rotary machine including a rotor, a stator disposed around the rotor, front and rear frames for sandwiching the stator and fastening members for axially fastening the frames. At least one of said frames comprises a plurality of supports which are rigidly formed on a circumferential portion thereof and a plurality of cooling windows, a plurality of ribs which are formed along the circumference of the frames and define the cooling windows. These documents concern to the reduction of wind noise caused on the one hand by air ventilation due to a relationship of the overlapping degree of vane of a fan and inlet openings and, on the other hand caused by the air ventilation between a clearance where an inner edge face of one bracket axially faces vanes of a fan. The present invention is made in view of the foregoing problems, and has a main object of providing an electric rotary machine which generates reduced magnetic noise without increasing the weight and size of the rotary machine. This object is achieved by a rotary machine comprising the features of claim 1. Further improvements of the present invention are the subject-matters of the appended dependent claims. Another object of the present invention is to provide reinforced ribs adjacent to the supports (hereinafter referred to as adjacent ribs) which have greater circumferential width than the rest of the ribs. Another object of the present invention is to provide adjacent ribs which have a greater axial thickness than the rest of the ribs. A further object of the present invention is to provide adjacent ribs which have a greater radial thickness than the rest of said ribs; and A further object of the present invention is to provide adjacent ribs which have thicker bent portions extending from the radial direction to the axial direction than do the rest of the ribs. One or both of the frames may preferably incorporate these features. In an electric rotary machine according to the present invention, magnetic vibrations generated in a stator during rotation of a rotor are transmitted to fastening members such as bolts and nuts used as supports. In this way, the magnetic vibrations are absorbed by adjacent window ribs. Since the magnetic vibrations are absorbed by the adjacent ribs, resonant vibration of the frames is suppressed, thereby reducing magnetic noise. In the present invention, only a part of the window ribs are reinforced to enhance rigidity. Therefore, effective reduction of the magnetic noise is attained without significantly increasing the weight and size of the rotary machine. BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: Fig. 1 is a front view of a front frame of an alternator according to a first embodiment of the present invention; Fig. 2 is a front view of a rear frame of the embodiment shown in Fig. 1; Fig. 3 is a partially cross-sectional side view illustrating an alternator according to the first embodiment; Fig. 4 is a graph showing a relationship between rotational speed and noise level in a rotary machine; Fig. 5 is a cross-sectional view illustrating a reinforced window rib adjacent to a support according to a second embodiment of the present invention; Fig. 6 is a cross-sectional view illustrating a reinforced window rib adjacent to a support according to a third embodiment of the present invention. Fig. 7 is a cross-sectional view illustrating a reinforced window rib adjacent to a support according to a fourth embodiment of the present invention. Fig. 8 is a cross-sectional view illustrating a reinforced window rib adjacent to a support according to a fifth embodiment of the present invention; Fig. 9 is a cross-sectional view illustrating a window rib which is not reinforced; Fig. 10 is a front view illustrating a prior art front frame; and Fig. 11 is a front view illustrating a prior art rear frame. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments according to a first embodiment of the present invention will now be described with reference to Figs. 1 through 4. As shown in Fig. 3, an alternator 1 according to a first embodiment of the present invention consists of a rotor 3 having a rotary shaft 2, a stator 4 disposed around the rotor 3, a front frame 5 and a rear frame 6, both of which accommodate the rotor 3 and the stator 4, and a rear cover 7. The rotary shaft 2 is rotatably supported by bearings 8 and 9 on the front frame 5 and the rear frame 6 and extends from those frames. One end of the rotary shaft 2 extending from the front frame 5 has a pulley 10. An engine driving force is transmitted to the pulley 10 through a belt (not shown) to rotate the rotor 3. The other end of the rotary shaft 2 extending from the rear frame 6 has a pair of slip rings 11 to be rotatable together. In addition to the rotary shaft, the rotor 3 is composed of a pole core 12 having a plurality of claw poles 12a on its peripheral portion, a cylindrical yoke (not shown) formed in the center of the pole core and a field coil (not shown). The pole 12 and the yoke may be formed as a unit by forging. Cooling fans 15 are fixed to both sides of the pole core 12 and rotate along with the rotor 3 to take cooling air in through the front and rear frames 5 and 6. The stator 4 is composed of a stator core 16 and a stator coil 17 wound on the stator core 16. The stator core 16 is made of a plurality of laminated thin annular iron plates and is press-fitted into the inner periphery of the front frame 5 around the pole core 12. The stator core 16 is seated on a step portion 5a which is formed on the inner periphery of the front frame 5. The stator coil 17 is wound in a Y or ▵ connection and inserted in a plurality of slots (not shown) formed in the inner periphery of the stator core 16. A rear cover 7 is secured to the rear frame 6 and protects brushes 18, a rectifying unit 19, a regulator and the like. which are disposed outside the rear frame 6. The brushes 18 are held in a brush holder 21 and biased against the outer periphery of the slip rings 11. The front frame 5 and the rear frame 6 have respective supports 50 and 60 formed on four corresponding locations on the outer circumference of the frames to receive through bolts 22. The front frame supports 50 have threaded holes 51 as shown in Fig. 1 and the rear frame supports 60 have recesses 61 as shown in Fig. 3 to receive washer-based nuts 22a, and through holes 62 for receiving the rear portions of the through bolts 22. The front frame 5 and the rear frame 6 are fastened by a plurality of through bolts 22 and nuts 23. The washer based nuts 22a of the through bolts 22 press on one end of the stator core 16, the other end of which is press-fitted into the inner periphery of the front frame 5 as the through bolts 22 are screwed into the threaded hole 51 and seat the stator core 16 on a step portion 5a of the front frame 5. Cooling air intake windows 24 and 25 (shown in Figs. 1 and 2) are formed on the end surfaces of the front frame 5 and the rear frame 6 to open in an axial direction thereof so that cooling air flow is generated by rotation of the cooling fans 15. A plurality of cooling air discharge windows 26 and 27 are formed on the circumferential surface of the respective frames 5 and 6 to open in a radial direction thereof. The cooling air discharge windows 26 and 27 are shaped into ellipses with their long axes oriented in the axial direction of the alternator. Reinforced window ribs 26a and 27a, and other window ribs 26b and 27b formed between the cooling windows 26 and 27. The reinforced window ribs 26a and 27a are formed adjacent to the supports 50 and 60 and have the width 'a' which is greater than the width 'b' of the rest of the window ribs, as illustrated in Fig. 1 and Fig. 2. The width 'a' is preferably about 1.2 to 3.0 times as wide as the width 'b'. Thus, the rigidity of the window ribs (adjacent ribs) 26a and 27a adjacent to the supporting portion 50 and 60 provides sufficient rigidity to prevent transmission of magnetic vibrations generated in the stator 4 to the entirety of the frames 5 and 6. Next, operation of this embodiment will be explained. When an engine runs, driving force is transmitted through a V belt (not shown) to the pulley 10 to rotate the rotary shaft 2. An electric current is supplied from outside through the brushes and the slip rings to energize the field coil 14. As a result, the claw poles 12a of the pole core 12 become polarized so that an alternating output current is generated in the stator coil 17 while the rotor 3 rotates. Magnetic vibrations generated due to armature reaction of the stator 4 are transmitted through the through bolts 22 and the supports 50 and 60 to the entirety of the frame. Since there are reinforced adjacent ribs 26a and 27a provided in the path of the magnetic vibration, the magnetic vibrations transmitted through the supports 50 and 60 are absorbed by the adjacent ribs 26a and 27a. Since the magnetic vibrations transmitted from the supporting portion to the rest of the frames is reduced, magnetic noise is reduced. Particularly, since the magnetic vibration transmitted through the supporting 50 and 60 to the entirety of the frames is reduced and resonant vibrations of the frames are suppressed, the peak level of the magnetic noise generated in the low rotational range may be effectively reduced as indicated by a solid line shown in Fig. 4. The graph in Fig. 4 shows noise level relative to rotational speed, and the solid line indicates the noise level generated by the present embodiment when the cross-sectional area of the window ribs 26a and 27a is about 1.3 times that of other ribs, and the broken line indicates the noise level of a prior art device in which the cross-sectional areas of the window ribs adjacent to the supports are equal to the cross-sectional areas of other window ribs. In the embodiment, since only the adjacent ribs 26a and 27a are reinforced in the above mentioned manner, excessive increase in weight and size is eliminated and :greater ease of installation and reduced fuel consumption are maintained when compared to a conventional device in which all ribs are reinforced and thickened. A second embodiment will be described next. Fig. 5 is a cross-sectional view of an adjacent rib 26a or 27a. While in the first embodiment, the width 'a' of the adjacent ribs 26a and 27a is greater than other window ribs 26b and 27b, the thickness in the radial direction (the vertical direction in Fig. 5) of the adjacent ribs 26a and 27a is greater than other window ribs 26b and 27b one of which is shown in Fig. 9. A third embodiment of the present invention will be explained next. Fig. 6 is a cross-sectional view of an adjacent rib 26a or 27a. As shown in Fig. 6, the adjacent ribs 26a and 27a are thicker in the axial direction (horizontal direction in Fig. 6) than other window ribs 26b or 27b one of which is shown in Fig. 9. A fourth embodiment of the present invention will be explained next. Fig. 7 is a cross-sectional view of an adjacent rib 26a or 27a. As shown in Fig. 7, the adjacent ribs 26a and 27a are thicker at a bend 28 bending from the radial direction to the axial direction than the bends of other ribs 26b and 27b one of which is shown in Fig. 9. In this embodiment, in order to have an appropriate clearance between the coil 17 and the frames 5 and 6 around the bends 28, the radius the bend is decreased to provide the thicker bend 28. A fifth embodiment of the present invention will be explained next. Fig. 8 is a cross-sectional view of an adjacent rib 26a or 27a. As shown in Fig. 8, the adjacent ribs 26a and 27a are thicker over the whole rib body than other ribs. In the embodiments 2 through 5 described above, the width 'a' of the adjacent ribs 26a and 27a may be made equal to the width 'b' of other ribs 26b and 27b, however it may be made greater than the width 'b' of others 26b and 27b to enhance the rigidity of the window ribs 26a and 27a further. [Variation] In the above embodiments, only the adjacent ribs 26a and 27a are reinforced to have more rigidity than other window ribs 26b and 27b, however, the window ribs 26b and 27b which are closer to the supports 50 and 60 may be made to have more rigidity than the rest of the ribs. In this case, the adjacent ribs 26a and 27a which are closest to the supports 50 and 60 are most rigid. Although in the first embodiment all the adjacent ribs 26a and 27a are reinforced to increase the rigidity, either one of the adjacent rib 26a or 27a of either front or rear frame 5 or 6 may be reinforced to increase the rigidity.

Claims: An electric rotary machine including a rotor (3), a stator (4) disposed around said rotor (3); front and rear frames (5, 6) for sandwiching said stator (4); and fastening members (22, 23) for axially fastening said frames (5, 6), wherein at least one of said frames (5, 6) comprises: a plurality of supports (50, 60) rigidly formed on a circumferential portion thereof; a plurality of cooling windows (26, 27); a plurality of ribs (26b, 27b) formed along the circumference of said frames (5, 6) and defining said cooling windows (26, 27); characterized by reinforced ribs (26a, 27a) formed adjacent to said supports (50, 60) and comprising at least a portion having greater rigidity than the rigidity of any of said other ribs (26b, 27b). An electric rotary machine according to claim 1, wherein each of said reinforced ribs (26a, 27a) having a cross-section which is greater than the cross-section of any of said other ribs (26b, 27b). An electric rotary machine according to claim 2, wherein said reinforced ribs (26a, 27a) comprise ribs having a cross-sectional area which is about 1.2 to 3 times that of said other ribs (26b, 27b). An electric rotary machine according to claim 2 or claim 3, wherein said reinforced ribs (26a, 27a) comprise greater circumferential width than said other ribs (26b, 27b). An electric rotary machine according to claim 2 or claim 3, wherein said reinforced ribs (26a, 27a) have greater axial thickness than the said other ribs (26b, 27b). An electric rotary machine according to claim 2 or claim 3, wherein said reinforced ribs (26a, 27a) comprise greater radial thickness than said other ribs (26b, 27b). An electric rotary machine according to claim 2, wherein said reinforced ribs (26a, 27a) comprise bend portions bending from the radial direction to the axial direction which are thicker than those of said other ribs (26b, 27b). An electric rotary machine according to claim 2, wherein said reinforced ribs (26a, 27a) having portions to prevent resonance of said frame (5, 6) with magnetic vibration generated by said stator (4). An electric rotary machine according to any of the preceding claims, wherein said reinforced ribs (26a, 27a) having a greater rigidity are disposed on regions of said circumferential portion other than a region having a stay portion, said regions are defined by said supports (50, 60). An electric rotary machine according to any of the preceding claims, wherein said reinforced ribs (26a, 27a) having a greater rigidity define a cooling air discharge window cooperatively with said support (50, 60).

Inventors: FUJIMOTO HIROFUMI, KONAYA KAZUYOSHI, SAKAKIBARA HIROSHI, FUJIMOTO, HIROFUMI, KONAYA, KAZUYOSHI, SAKAKIBARA, HIROSHI
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1995
Main classification: H02K5
IPC codes: H02K5