ELECTRIC BLOWER HAVING IMPROVED RETURN PASSAGE FOR DISCHARGED AIR FLOW

Abstract: An electric blower has an electric motor (2) driving a rotating centrifugal impeller (32), an end bracket (21) for separating the electric motor (2) from the centrifugal impeller (32), and a diffuser (31). A flat plate (31b) extends between the impeller (32) and the end bracket (21) for supporting diffuser vanes (31a) and has a plurality of guide vanes (31c) formed opposite to the diffuser vanes (31a). A passage is defined by the end bracket (21), the flat plate (31b) and the guide vanes (31c) so as to return the air flow discharged from the centrifugal impeller (32). The end bracket (21) is formed in a substantially convex shape so as to come away from the flat plate portion (31b) of the diffuser (31) according as the end bracket (21) extends from its central portion toward its periphery, thereby securing an opening area of the discharge air flow return passage in which a sound absorbing material (11; 111; 311) is arranged to absorb sounds from the discharge air flow.

Description: BACKGROUND OF THE INVENTION The present invention relates to an electric blower which is incorporated in, for instance, in an electric vaccum cleaner to be used therewith, and in particular, to a structure of an electric blower for reducing noise. Generally, an electric blower of this type has a structure wherein an impeller is directly connected to a rotary shaft of an electric motor for miniaturization and a discharge air flow from the impeller is returned inwardly to cool the electric motor. The devices in which such electric blowers are incorporated are mainly for home use, and it is desirable that noise emitted from these devices is as low as possible. Therefore, structures in which sound absorbing materials are provided in respective electric blowers so as to reduce noise have been disclosed, for instance, in Japanese Utility Model Unexamined Publication No. 61-188000, Japanese Utility Model Unexamined Publication No. 62-16797, and Japanese Patent Unexamined Publication No. 55-107100. The invention disclosed in the last one of the above publications has been assigned to the present assignee. The electric blowers described in Japanese Utility Model Unexamined Publication No. 61-188000 and No. 62-16797 respectively have sound absorbing materials which are mounted on a motor frame or an air guide within a return passage for a discharge air flow. The electric blower descried in Japanese Patent Unexamined Publication No. 55-107100 has a sound absorbing material mounted on passage walls of return guide vanes to absorb noises from a discharge air flow passing through the passage. Further, in the electric blower shown in Japanese Patent Unexamined Publication No. 60-33000, plural openings are formed in portions of a fan casing which face a passage for a discharge air flow in the fan casing, and those openings are covered with a sound absorbing material. When a discharge air flow passes through this passage, parts of the discharge air flows through the openings out of the casing so that the sound absorbing material can absorb noises from the air flowing out of the casing to thereby reduce noise. Japanese Patent Unexamined Publication No. 62-16798 also discloses an electric blower having a sound absorbing material, wherein a space for absorption of noise is defined between an outer wall of an electric motor and a motor frame and the sound absorbing material is arranged in the space. U.S. Patent No. 4,120,616 issued to Dwyer, et al on October 17, 1978 shows an electric blower in which a sound absorbing material is installed in an area, to which a cooling air for a motor collides, so as to reduce noise. On the other hand, two of the inventors of the present application have proposed a structure of an electric blower, which can be decreased in entire length, in U.S. Patent No. 4,767,285 issued on August 30, 1988 and entitled "ELECTRIC BLOWER". Considering the use of this type electric blowers as described above, it is desirous for them to have a structure which is compact and low in noise. SUMMARY OF THE INVENTION It is an object of the present invention to provide an electric blower which is capable of reducing noise without increasing in size. Another object of the invention is to provide an electric blower which can produce a sufficient air flow for cooling an electric motor to secure the life span thereof while having a higher effect of sound absorption as compared with those of the conventional blowers. The present invention, as will be described later in detail, has been accomplished on the basis of the results of analyses which were made on the conventional structures by the present inventors, and is intended to attain the above-mentioned objects by properly forming a return passage for a discharge air flow and disposing a sound absorbing material in this passage. According to the invention, there is provided an electric blower comprising an electric motor, a centrifugal impeller connected to a rotary shaft of the electric motor for rotation therewith, an end bracket for separating the electric motor from the centrifugal impeller, a diffuser including a flat plate portion extending between the centrifugal impeller and the end bracket and a plurality of diffuser vanes formed on the flat plate portion close to the outer periphery of the centrifugal impeller, the flat plate portion having a plurality of guide vanes formed on an opposite side surface of the flat plate portion from the diffuser vanes, a return passage for a discharged air flow which is defined by the end bracket, the flat plate portion and the guide vanes to return the discharge air flow from the centrifugal impeller toward the interior of the blower, and a sound absorber arranged in the return passage. The end bracket is formed in a substantially convex shape so as to come away from the flat plate portion of the diffuser according as the end bracket extends from its central portion to its periphery, thereby securing the opening area of the discharge air flow return passage. In the above structure, the end bracket is substantially convex with respect to the flat plate portion of the diffuser. Hence, the discharged air flow return passage which is defined between them increases in width as the passage extends from the central portion of the blower toward its periphery. Accordingly, the return passage can have a larger opening area to secure sufficient air flow without increasing in its structure of body, as compared with a conventional structure in which an end bracket and the flat plate portion of a diffuser are substantially parallel to each other. Further, the sound absorber is arranged in the discharge air flow return passage so as to achieve a good effect of sound absorption. BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned and other objects, features and advantages of the present invention, will be apparent from the detailed description which will be made hereinafter with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view showing the electric blower according to an embodiment of the invention; Fig. 2 is a exploded perspective view of a frame, a sound absorber and an end bracket provided in the embodiment of Fig. 1; Fig. 3 is a cross-sectional view showing the relation between a centrifugal impeller, a diffuser and the end bracket in the embodiment of Fig. 1; Fig. 4 is a cross-sectional view showing the relation between a centrifugal impeller, a diffuser and an end bracket in a conventional electric blower; Fig. 5 is a diagram showing a characteristic curve of the noise frequency of the electric blower according to the embodiment of the invention in comparison with that of a conventional electric blower; Fig. 6 is a perspective view showing a diffuser and a sound absorber of the electric blower according to another embodiment of the invention; Fig. 7 is a cross-sectional view showing the diffuser and the sound absorber shown in Fig. 6; Fig. 8 is a partially broken away perspective view showing a frame and a sound absorber of the electric blower according to still another embodiment of the invention; Fig. 9 is a cross-sectional view showing the mounting relation of a centrifugal impeller, a diffuser and a sound absorber of the electric blower according to a still further embodiment of the invention; and Fig. 10 is a cross-sectional view showing the relation of a centrifugal impeller, a diffuser and a sound absorber in a conventional electric blower for comparing the same with the relation shown in Fig. 9. DESCRIPTION OF THE PREFERRED EMBODIMENTS At the outset problems of the conventional electric blowers will be described. These problems have been clarified through the analyses made by the present inventors, which analyses constitute the foundation of the invention. In the conventional electric blowers described above, no consideration can be found to be given for realizing both miniaturization of the structure and reduction of noise without deteriorating performance. More particularly, in the structure wherein the sound absorbing material is arranged in the discharge air flow return passage, a good effect of sound absorption can be expected, because the sound absorbing material directly contacts with the discharge air flow. However, the aerodynamic performance of the blower is affected due to the passages being narrowed by the sound absorbing material and so on. As a solution for this problem, it may be thought of taking such a measure to enlarge the opening area of the passage by increasing the distance between the members which define the passage. If the members are simply separated in an axial direction from each other, however, the size of the blower as a whole is increased. On the other hand, there is a problem also in the structure wherein the sound absorbing material is arranged adjacent to the return passage for discharge air flow, for example, in the electric blower as described in the above-mentioned Japanese Patent Unexamined Publication No. 60-33000. That is, to cover the outer periphery of the side wall of the fan casing with the sound absorbing material results in laterally enlarging the electric blower by the thickness of the sound absorbing material. In addition, since parts of the discharge air flow are released outside the fan casing from the opening in the side wall thereof as the discharge air passes through the return passage, the amount of air flowing into the electric motor decreases. There is a possibility, therefore, that the motor can not be cooled sufficiently and the working life of the electric blower may be shortened. Further, since only parts of the discharge air flow contact with the sound absorbing material on the side wall of the fan casing, a good effect of sound absorption can not be expected. Therefore, it can be found that the sound absorbing material had better to be arranged in the return passage for discharge air flow, in order to obtain a good effect of sound absorption. Further, for decreasing the influence on the aerodynamic performance due to the sound absorbing material arranged in the return passage, it will be evident that the return passage needs to secure at least a sufficient opening area. However, because such securing is accompanied by the enlargement of the electric blower as described above, it is essential for achieving both the reduction of noise and the miniaturization of structure to find a solution for the enlargement of the electric blower. For this end, various examinations have been made as to the structure of the electric blower. As a result, the present inventors, direct their attention to the end bracket of the electric motor, and have come to a conclusion that the opening area of the passage can be secured by forming the end bracket in a convex shape. Such formation, as a conventional end bracket is substantially flat, is unexcpected in view of the common sense of design which usually aims at the simplification of structure. However the convex shape is remarkably effective in compatibly achieving the reduction of noise and the miniaturization of structure. More particularly, by virtue of the above formation, the return passage for discharge air flow has a large opening area, and the sound absorbing material can be arranged within the return passage. Therefore, it is possible to prevent such lateral enlargement of the blower as is in the case of the electric blower described in Japanese Patent Unexamined Publication No. 60-33000. The end bracket thus formed increases also in height in its axial direction due to the shape. However, it is possible to contain or receive the outer peripheral portion of the convex-shaped by utilizing a space within the blower to prevent the thus shaped bracket from affecting the total height of the electric blower. By virture of the arrangement of the sound absorbing material in the return passage for discharge air flow, it is unnecessary to provide an opening for the sound absorbing material which is disposed at the outside of the passage as in the case of the electric blower described in Japanese Patent Unexamined Publication No. 60-33000. In addition, the return passage has a sufficient opening area even after the sound absorbing material has been arranged therein. Therefore, it is possible to secure a sufficient air flow so as to prevent insufficient cooling of the electric motor due to reduction of an inflow of air, thereby preventing the working life of the electric blower from shortening. Further the sound absorbing material arranged in the discharge air flow return passage can sufficiently contact with the discharge air flow from the diffuser to exhibit a good effect of sound absorption. The sound absorbing material is arranged closely near the noise sources of the electric blower, that is, sound generated from vanes of the centrifugal impeller and the diffuser, sound of whirls generating at the time when the discharge air flow is changed in its direction, and so on, and therefore, noises can be absorbed before diffusion significantly. Hereinafter, description will be made on the preferred embodiments of the invention with reference to the accompanying drawings. Referring now to Fig. 1, the construction of the electric blower, generally designated by the reference numeral 1, is roughly divided into an electric motor section 2 and a blower section 3. On one side of the motor section 2 an end bracket 21 is mounted. The end bracket 21 is composed of a bearing holder portion 21a which is positioned at the central portion to hold a bearing 23 for a rotor 22, an annular flat portion 21b at the periphery, and a support arm portion 21c which interconnects the holder portion 21a and the flat portion 21b. As shown in Fig. 2, in the support arm portion 21c, there is formed openings 21d for allowing a discharge air flow from the blower section 3 to flow into the motor section 2. And the support arm portion 21c is formed in a convex shape such that the support arm portion 21c come further away from a return passage for the discharge air flow which will be described later, or from the upper of Fig. 1, according as the support arm portion 21c extends from the central portion of the end bracket 21 toward the periphery thereof. A diffuser 31 is arranged on the end bracket 21. Above the diffuser 31, a centrifugal impeller 32 is fixed to a rotary shaft 24 of the rotor 22 for rotation therewith by means of a nut 25. Reference numeral 33 denotes a fan casing. The fan casing 33 is press-fitted onto the periphery of the end bracket 21 to be fixed thereto in such a state that the fan casing convers the centrifugal impeller 32 and the diffuser 31. The diffuser 31 has a flat plate 31b which is positioned on the peripheral side of the centrifugal impeller 32. Diffuser vanes 31a are formed on the upper surface of the flat plate 31b. Further guide vanes 31c for returning a flow of discharged air are formed on the lower surface of the flat plate 31b. The guide vanes 31c cooperate with the end bracket 21 to form a return passage for the discharge air flow which passage leads the discharge air flow to the openings 21d shown in Fig. 2. On the annular flat portion 21b of the end bracket 21, disposed is a frame 12 in which a sound absorbing material 11 is contained. As shown in Fig. 2, the frame 12 is integrally secured to the end bracket 21 with screws 13, and has cutouts 12a formed in a face which is opposite to and abuts against the guide vanes 31c shown Fig. 1. Thus, the sound absorbing material 11 within the frame 12 is exposed at the portions of the frame facing the discharge air flow return passage. Incidentally the sound absorbing material 11 is usually made up of such a porous material as a sponge, MOLTPREN (Trade name, Nishiyama Rubber Co.) and so on, and has flexibility. Such material is hard to handle. In particular, it is difficult to automate the assembly of such material and, inevitably, the assembly has to be carried out manually. When the sound absorbing material 11 is housed in the frame 12, however, it becomes easy to handle the material, and it becomes possible to automate the assembly. In the construction described above, air is sucked through a central opening of he fan casing 33 by the centrifugal impeller 32, flows from the inner periphery of the im peller to the outer periphery thereof, and is discharged to the peripheral side. Succeedingly, the air current recovers its static pressure at the diffuser vanes 31a. The air current is changed in direction of flow at the outer periphery of the diffuser, and flows back through the return passage from the outer periphery toward the central portion of the diffuser 31. At this time, the air current contains the sound emitted by the centrifugal impeller 32 and the diffuser vanes 31a, the sound of whirls generated at the time when the direction of the discharge air flow is charged, and so on . These noises, when the air flows through the return passage, are absorbed by the sound absorbing material 11 which is arranged closely near the sound sources, that is, the sound absorbing material 11 which faces to the discharge air flow return passage, and therefore, a good effect of noise reduction can be obtained. Fig. 5 is a comparative diagram of the frequency characteristic of the noise made in the electric blower which has been manufactured by way of trial according to the embodiment of the invention, and that in a conventional electric blower. In this figure, reference character A , or a broken line, shows the analytic result of the frequency of the noise emitted from the conventional electric blower, while reference character B , or a solid line, shows that of the electric blower manufactured by way of trial according to the embodiment of the invention. According to the invention, the effect of noise reduction is remarkable, particularly, in the high frequency region over 4KHz, though the effect varies depending on the sound absorption characteristics of the sound absorbing material, in which region noise can be reduced by 4 to 5 dB at overall values. Besides, usually, it is difficult to reduce noise in the low frequency region under 3KHz only by the provision of a sound absorbing material, because the noise is appreciably affected by components of rotational vibration, etc. However, according to the embodiment, with the help of the rigidity of the frame structure 12 containing the sound absorbing material 11, it is possible to reduce the noise by improving the rigidity of the electric blower 1 so as to shift the natural frequency thereof into a higher frequency region or by utilizing the resonance effect and so on. The present invention is of the constitution described above. As is also evident from the description of the embodiment, according to the invention, the sound absorbing material 11 is arranged in the discharge air flow return passage which is defined by the discharge air flow guide vanes 31c formed on the lower surface of the diffuser 31 and the end bracket 21 of the electric motor. Accordingly, it is possible to eliminate the disadvantage which is caused by covering the periphery of the side wall of the fan casing with sound absorbing material as in the case of the electric blower described in the above-mentioned Japanese Patent Unexamined Publication No. 60-33000, that is to say, the disadvantage that the electric blower is made laterally large-sized by the thickness of the sound absorbing material. Moreover, according to the invention, it is not required to form openings for passage of a discharge air flow in the side wall of the fan casing as in the case of the electric blower described in the Japanese Patent Unexamined Publication No. 60-33000. Differently to the electric blower described in the above publication, therefore, no discharge air flow is released in the middle of the passage, because of no opening which is formed in the surface of the side wall of the fan casing facing the discharge air flow passage. Consequently, it is also possible to solve the problem that the air flowing in the electric motor decreases in amount so that the motor may be insufficietnly cooled to result in shortening the working life of the electric blower. In addition, according to the invention, since the sound absorbing material 11 arranged in the discharge air flow return passage contacts sufficiently with the discharge air flow, the effect of sound absorption can be remarkably improved as compared with the electric blower described in the above-mentioned Japanese Patent Unexamined Publication No. 60-33000 wherein the discharge air flow contacts with the sound absorbing material only outside the discharge air flow passage, or only outside the openings formed in the side wall of the fan casing. In the invention, the sound absorbing material 11 is arranged in the discharge air flow return passage which is positioned closely near the sources of noise of the electric blower, that is, the sounds emitted by the centrifugal impeller 32 and the diffuser vanes 31a, the sound of whirls generated at the time when the direction of the discharge air flow is changed, and so on. Therefore, it is possible to absorb the sounds before the noise is significantly diffused in the circumference. Thus, the effect is remarkable, which is achieved by arranging the sound absorbing material 11 in the return passage for discharged air flow. Fig. 3 shows the centrifugal impeller 32, the diffuser 31 and the end bracket 21 shown in Fig. 1 by omitting the other elements. On the other hand, Fig. 4 shows a centrifugal impeller 1032, a diffuser 1031 and an end bracket 1021 of a conventional electric blower, by omitting the other elements, for comparison with Fig. 3. As shown in Figs. 1 to 3, the support arm portion 21c, which connects the bearing holder portion 21a and the annular flat portion 21b of the electric motor end bracket 21 with each other, is formed in a convex shape so as to come away or become remote from the discharge air flow passage, or the upper part of Fig. 1, according as the arm portion 21c extends from the central portion toward the periphery of the end bracket. Therefore, the cross-sectional area of the discharge air flow return passage, which is defined by the guide vanes 31c of the diffuser 31 and the end bracket 21, can be made large as compared with the conventinal electric blower shown in Fig. 4. Thus, it is possible to furthermore enhance the aerodynamic performance of the electric blower while making the effect of noise reduction more effective. Incidentally, when the support arm portion 21c of the end bracket 21 of the electric motor is formed in the convex shape as described above, the axial height of the end bracket 21 itself becomes larger than that of the conventional end bracket shown in Fig. 4. However, this increment of the height does not increase, to any degree, the size of the electric blower in the longitudinal direction thereof as a whole, because the increment of the height can be regarded as being eliminated by effectively utilizing the space within the fan casing 33. Fig. 6 shows the diffuser 31 and a sound absorbing material 111 of the electric blower according to the second embodiment of the invention, and Fig. 7 is a longitudinal section of the shown in Fig. 6. In the following description, the constituent elements similar to those of the first embodiment will be designated by the same reference numerals, and description thereof will be omitted. In the first embodiment, the sound absorbing material 11 has been arranged on the annular flat portion 21b of he end bracket 21 of the electric motor. However, in the second embodiment shown in Figs. 6 and 7, the sound absorbing material 111 is arranged on the rear side of the flat plate 31b which constitutes a part of the diffuser 31. Also in this embodiemtn, the sound absorbing material 111 is arranged in the discharge air flow return passage of the electric blower. Fig. 8 shows a frame 212 and the sound absorbing material 11 of the electric blower according to the third embodiment of the invention. In this embodiment, a large number of small holes 212b are perforated in a surface of the frame 212 on the side thereof facing the discharge air flow return passage, in which frame the sound absorbing material 11 is contained, as a modification of the first embodiment shown in Fig. 1 and 2. According to this embodiment, in addition to the effect which is common to the first and second embodiments, it is possible to furthermore enhance rigidity of the frame 212 by forming therein a large number of small holes 212b instead of the cutouts 12a in the first embodiment. Fig. 9 shows the assembled construction of the centrifugal impeller 32, the diffuser 31 and a sound absorbing material 311 in the electric blower according to the fourth embodiment of the invention. Fig. 10 is a longitudinal section of the centrifugal impeller 1032 and the diffuser 1031 of the conventional electric blower by omitting the other elements for comparison with Fig. 9. In the second embodiment shown in Figs. 6 and 7, the sound absorbing material 111 is arranged on the rear side of the flat plate 31b which constitutes a part of the difuser 31. In the fourth embodiment shown in Fig. 9, however, the sound absorbing material 311 is arranged on the surfaces of the discharge air flow guide vanes 31 which constitute another part of the diffuser 31, and is inclined toward the directions of the openings, see the openings 21d shown in Fig. 2, of the end bracket (see the end bracket 21 shown in Fig. 1). According to this embodiemtn it is possible to allow the air current discharged from the diffuser vanes 31a to change the direction of flow smoothly into the return passage, as shown by an arrow in the figure, as compared to the conventional electric blower shown in Fig. 10. Accordingly, an aerodynamic loss of the air current can be decreased, thereby reducing noise to the upmost which noise is accompanying with the aerodynamic loss. Although the invention has been described that the sound absorbing material is arranged in the discharge air flow return passage, it should be noted that an effect of sound absorption can be expected to a certain extent only by sufficiently enlarging the opening area of the discharge air flow return passage by means of the end bracket of the convex shape. As described above, according to the invention, it is possible to, without enlargening the size of the electric blower, eliminate the short-coming or disadvantage that the amount of air flowing into the electric motor is decreased and thereby the cooling of the motor is insufficient to shorten the working life of the electric blower. Further, the invention can provide the improved electric blower which is capable to enhance the effect of sound absorption thereof as compared with the conventional electric blower. Thus, the meritorious results achieved by the invention are remarkable.

Claims: An electric blower having: an electric motor; a centrifugal impeller connected to a rotary shaft of the electric motor for rotation therewith; end bracket means for separating the electric motor from the centrifugal impeller; a diffuser including a flat plate portion extending between the centrifugal impeller and the end bracket means and a plurality of diffuser vanes formed on the flat plate portion close to an outer periphery of the centrifugal impeller, the flat plate portion having a plurality of guide vanes formed on a opposite surface of the flat plate portion from the diffuser vanes; a discharge air flow return passage defined by the end bracket means, the flat plate and the guide vanes for returning a discharge air flow from the centrifugal impeller inwardly of the blower; and an sound absorbing material arranged in the return passage, characterized in that said end bracket means (21) is formed in a substantially convex shape so as to come away from said flat plate portion (31b) of said diffuser (31) according as said end bracket means extends from a central portion thereof toward an outer periphery thereof, thereby securing an opening area of said discharge air flow return passage. The electric blower according to claim 1, characterized in that said electric motor (2) includes a case and a rotor (22) housed in said case, and said end bracke means (21) includes a central portion (21a) for holding a bearing of said rotor (22), an annular flat portion (21b) of an outer periphery, support portion (21c) for interconnecting said central portion (21a) and said annular flat portion (21b) with each other, and at least one through-hole (21d) formed for allowing the discharge air flow to flow through said case of said electric motor (2). The electric blower according to claim 1, characterized in that said sound absorbing material (11) is arranged on said annular flat portion (21b) of said end bracket means (21). The electric blower according to claim 1, characterized in that said sound absorbing material (111; 311) is attached onto a surface of said flat plate portion (31b) of said diffuser (31) on the guide vanes (31c) side. The electric blower according to claim 4, characterized in that said end bracket means (21) has at least one through-hole (21d) for allowing the discharge air-flow to pass to cool said electric motor (2), and said sound absorbing material (311) includes a surface inclined toward said through-hole (21d). The electric blower according to any one of claims 1 to 5, characterized in that said sound absorbing material (11) is received in at least one frame (12; 212) and is attached through said frame (12; 212) inside the discharge air-flow return passage. The electric blower according to claim 6, characterized in that said frame (12; 212) is formed in the shape of an annulus. The electric blower according to claim 6 or 7, characterized in that said frame (12) is formed on at least a side thereof facing said return passage with an opening (12a) which extends along a longitudinal direction of said frame (12). The electric blower according to claim 6 or 7, characterized in that said frame (212) is formed with a large number of small holes (212b) in a portion of said frame facing said discharge air flow return passage.

Inventors: ISHII, YOSHITARO,  3-7, IWASE, YUKIJI,  1071-22, JYORAKU, FUMIO, SATO, SHIGENORI,  2625-3-2-504, TOYOSHIMA, HISANORI, SAKURAGAWARYO, Ishii, Yoshitaro, 3-7, Iwase, Yukiji, 1071-22, Sato, Shigenori, 2625-3-2-504
Applicants: HITACHI, LTD.
Application year: 1990
Main classification: F04D29
IPC codes: F04D25, A47L9, F04D29, A47L5