GB2194687A

GB2194687A - Electric motor brushgear

Info

Publication number: GB2194687A
Application number: GB08715805A
Authority: GB
Inventors: Takaichi Mabuchi
Original assignee: Mabuchi Motor Co Ltd
Current assignee: Mabuchi Motor Co Ltd
Priority date: 1986-07-10
Filing date: 1987-07-06
Publication date: 1988-03-09
Also published as: KR880002309A; DE3722466A1; KR910002626Y1; JPH0728516B2; CN87104824A; JPS6318946A; GB8715805D0

Description

1 GB2194687A 1

SPECIFICATION

Electric motors This invention relates to electric motors, and particularly to motors in which an end cover houses the requisite brushgear for transmitting electric power to the commutator of the mo tor rotor. The invention is especially con cerned with the construction of such end 75 covers and the brushgear mounted thereon.

In many known designs of electric motors brushes for making sliding electrical contact with the commutator are mounted at the ends of resiliently flexible arms, the other ends of which are fixed relative to terminals to which external electrical connections are made. The resilience of the arms urges the brushes into contact with the commutator, and maintains such contact while the motor is running. While this assembly is acceptable for many applica tions, it can provoke problems, particularly in higher power motors. Cyclic flexure of the arms can develop, resulting in a sawing mo tion of the brush on the commutator as the arm successively bends and straightens.

To solve the above problem, it has been proposed to use rigid brush arms, and urge the brush against the commutator by means of a separate spring which also carries the electric current to the brush arm. This requires the material of the spring to be both conduc tive and sufficiently resilient, and the selection thereof can itself be difficult and/or expensive.

Using a separate electrical lead wire for each brush introduces manufacturing problems, par ticularly for mass-production.

According- to the present invention, brushgear in an end cover for an electric mo tor comprises substantially rigid brush arms each with a brush portion at its radially inner end for making sliding electrically conductive contact with the commutator, and a retainer porti on at its radially outer end, the retainer portions being supported for pivotal move ment in a plane perpendicular to the commuta tor axis about posts mounted on the cover, and being in electrically conductive engage ment therewith, and means for connecting an electrical power supply to the posts and thus to the brush portions along the brush arms.

The posts will usually comprise columns of substantially circular cross-section. Good elec trical contact is preferably established between the retainer portion of each arm and the re spective post by the creation of line or point contact therebetween. For example, the re tainer portion which is normally of arcuate cross-section, may define two substantially planar divergant sections which slidingly en gage the post to make electrically conductive contact therewith. Alternatively or additionally, the posts and/or the retainer portions may be formed with annular or part-annular projections to achieve good point or line contact therebetween. It is desirable in some embodiments to ensure that the curvature of the post of the retainer portion that makes contact with the post is greater than that of the post itself or any projections thereon.

The retainer portions in brushgear according to the invention may effectively enclose the posts, and could for example each have two lugs with aligned holes therethrough, a respective post being received in said aligned holes. In this embodiment, the boundaries of the holes will preferably be formed with divergant surfaces for engaging the respective post to achieve point or line contact therewith.

In all embodiments of the invention, springs will normally be included for urging the brushes into engagement with the motor commutator, and the retainer portions into electrically conductive contact with the posts, to prevent an increase in contact resistance when power is fed to the brush arms. These springs may also extend between spring receptacles electrically connected to the posts and the respective brush arms to serve as auxiliary electrical connections to the brush portions. Simple coil springs may be used, in tension or compression, but leaf or other spring mechanisms can also be effective.

The invention will now be described by way of example and with reference to the accompanying drawings wherein:

Figure 1 is a plan view of an end cover for an electric motor embodying this invention; Figure 2 shows in enlarged longitudinal cross-section a post of the end cover of Figure 1 with the arm retainer portion mounted thereon; Figures 3A and 3B are further enlarged lateral cross-sections of details of the post assembly shown in Figure 2; taken generally on the line A-A of Figure 2; Figures 4 to 6 show alternative embodiments of the invention; Figures 7A, 7B and 7C show various ar- rangements of brushgear disposed on the case cover; Figure 8 is a view similar to that of Figure 2 of an alternative post/retainer portion assembly; Figure 9 shows in partial cross-section, a known motor construction; and Figure 10 is a view similar to that of Figure 1 of the end cover of the motor of Figure 9, and is a section taken generally on the line X- X of Figure 9.

In Figure 1 the end cover 4 includes a bearing 7 which supports the shaft of a rotor (not shown) of the motor and from which extends a commutator 5. The commutator 5 is engaged by two brushes 10 mounted at the radially inner ends of brush arms 8. Each arm 8 is substantially rigid and has a retainer portion 15 at its radially outer end. Each arm 8 is pivotally supported on an electrically conductive column or post 12. The arm retainer 15 2 GB2194687A 2 as shown in Figure 1 has an hollow cylindrical wrapping around the post 12. As will be described later with reference to Figures 2 and 3, the post 12 makes contact with the arm retainer 15 at two points when viewed crosssectionally. That is, even when some "play" is produced between the post 12 and the arm retainer 15, a leaf spring 14 urges the arm retainer 15 onto the post 12 to keep the arm retainer 15 in good contact therewith. To increase the pushing force, the position at which the spring 14 engages the brush arm 8 can be selected at a position as close as possible to the post 12.

Although the spring 14 is supported at one end thereof by the spring receptacle 13, which is electrically connected and therefore at substantially the same potential as the post 12, and pushes the brush arm 8 at the other end thereof, it may of course be supported by 85 the brush arm 8 at one end and the other end push the spring receptacle 13.

Each post 12, which passes through the case cover 4, serves as a motor terminal, and power is fed to the brush arm 8 via the con- 90 tact point of- the respective arm retainer 15 and the post 12. The brush arm 8, which is substantially rigid, is pushed by the spring 14, and thereby the carbon brush 10 is forced onto the surface of the commutator 5. At this 95 time, the arm retainer 15 of the brush arm 8 is rotatably supported by the post 12, so that the carbon brush 10, even when worn down, is kept in close sliding contact with the sur face of the commutator 5. Although there can 100 be another electric current path running from the post 12 through the spring receptacle 13 and the spring 14 to the brush arm 8, in addition to the current path running from the post through the contact point of the post 12 105 and the arm retainer 15 to the brush arm 8, power is fed mainly through the arm retainer 15. It is possible to select the force of the spring and the postion at which the spring pushes the brush arm 8 so as not to change 110 the pushing force of the brush 10 onto the commutator surface and the pushing force of the arm retainer onto the post 12, even when the carbon brush 10-is worn down.

Figure 2 shows a post 12 in longitudinal cross-section, supporting the arm retainer portion 15, and held onto the cover 4 by a screw 17. As can be seen, the screw 17 also secures a motor terminal 9, in contact with the post 12, and as shown with the retainer 120 portion 15.

As shown in Figures 3A and 313, the arm retainer 15 is formed with tangential portions 16 at positions where the post 12 makes contact with the annular projections 12a (Figure 2). That is, when viewed cross-sectionally as shown in Figure 3A, the tangential portions 16 are provided at postions at which the post 12 makes, contact with circles formed by the annular projections 12a. The tangential por- tions 16 are adapted so as to converge at an angle of for example, 30".

With this arrangement, when the brush arm 8 is pushed by the spring 14, the tangential portions 16 are always kept in contact with the annular projections 12a on the post 12 in the direction shown by arrow P in Figure 3A. A two- point contact is effected, when viewed cross-sectionally as shown in Figure 3A. If the annular projections 12a are omitted two line contacts are actually effected with the curved peripheral surface of the post 12. In another alternative, two-point contact may also be effected by providing annular ridges 12b pro- truding toward the post 12 on the side of the arm retainer 15, as shown in Figure 8.

The tangential portions 16 need not be planar, but may be in the shape of an arc having a diameter larger than the circle formed by the annular projection 12a so that part of the arc makes contact with the annular projections 12a. Neither need the cross-sectional shape of the post 12 be circular, although it should be of such a shape that a two-point contact can be effected between the cross-section of the annular projections 12a and the tangential portions 16.

Two-point contact between the retainer 15 and the post 12 is accomplished by providing the tangential portions 16 at least when viewed crosssectionally, and urging them against the post 12 by means of the spring 14, as illustrated in the typical construction shown in Figure 3A. This minimises the risk of poor electrical contact, and helps to secure the stable rotation of the motor. In the construction shown in Figure 4A, the posts 12 are each formed into an exactly columnar shape, unlike the constructions shown in Figures 2 and 3. The arm retainer 15 has two equally spaced lugs 18, as shown in Figure 4B. The lugs 18 provided are formed with holes 19 through which the post 12 is passed so as to effect a two-point contact by the action of the spring 14, similar to the manner of contact described with reference to Figures 2 and 3. The spring 14 in this embodiment is provided in such a manner as to wrap around the post 12, and pushes the brush arm 8 onto the post 12.

The embodiment shown in Figure 5 is predominantly similar to that of Figure 1 with the difference that the spring 14 is a coil spring under tension. That of Figure 6 is the same but the spring 14 is under compression.

These constructions are not limited to the embodiments shown in the figures. The spring 14 acting on the brush arm 8 shown in Figure 4, for example, may be of the construction shown in Figure 6, consisting of the spring 14 and the spring receptacle 13.

Figures 7A, 7B and 7C 6how various possible arrangements of the brushgear disposed on the case cover, that can be adopted ac- cording to the invention.

3 GB2194687A 3 A typical motor of the prior art has the construction shown in Figures 9 and 10. A brush arm 8, made of a flexible sheet ma terial, holds a carbon brush 10 near the tip thereof, with the other end thereof fixedly fitted to a case cover 4. The brush arm 8 is formed integrally with a motor terminal 9 as shown in Figure 9 to receive electric current from the motor terminal 9. When the case cover 4 on which the brusfigear is mounted is engaged with the open end of the motor case 1, the brush arm 8 is forced open from the state shown by dotted lines to the state shown by solid lines in Figure 10 so as to permit the carbon brush 10 to make sliding 80 contact with the commutator 5.

In this type of motor, flexible brush arms 8 are employed, which simplifies the mounting of the brushgear on the case cover 4 and is favourable in terms of mass-production. How ever, the arms 8 are bent and the brushes 10 forced onto the surface of the commutator 5, as shown in Figure 10, and the rotor is caused to rotate in this state. Assuming that the commutator 5 is rotated counterclockwise in Figure 10, the carbon brush 10 is first caused to move in such a manner that the bent arm 8 extends in the direction shown by arrow a, and when the brush arm 8 is ex tended to its extreme limit, the brush 10 is then caused to move in the direction shown by arrow b so as to restore the original posi tion of the brush arm 8. That is, the carbon brush 10 performs a so-called sawing motion, on the surface of the commutator 5, as indi cated by the arrows a and b. As a result of the sawing motion, unwanted sparks tend to be generated, posing a serious problem in high-power motors.

As described above, this invention makes it possible to prevent the unwanted sawing mo tion of the brush, as experienced with the conventional type of brushgear, because rigi dity is given to the brush arms 8. In designing the material of the brush arm with this inven tion, consideration may be given mainly to electrical conductivity. The material of the spring should preferably be selected by taking into account electrical conductivity as well as its intrinsic spring force. Although the problem 115 of electrical contact between the arm guide and the brush arm is likely to occur by the fact that rigidity is given to the brush arm 8, and by the fact that the brush arm must be adapted to be rotatable with respect to the arm guide to cope with the wear of the brush, this problem can also be solved by this inven tion. With this invention, the construction of supporting the spring is relatively simple, and the spring also serves as an auxiliary path of electric current fed to the brush arm.

Claims

1. An end cover for an electric motor, which cover has mounted thereon brushgear 130 for transmitting electric current to the commutator of the rotor of the motor, the brushgear comprising substantially rigid brush arms each with a brush portion at its radially inner end for making sliding electrically conductive contact with the commutator, and a retainer portion at its radially outer end, the retainer portions being supported for pivotal movement in a plane perpendicular to the commutator axis about posts mounted on the cover, and being in electrically conductive engagement therewith, and means for connecting an electrical power supply to the posts and thus to the brush portions along the brush arms.

2. An end cover for an electric motor according to Claim 1 wherein the retainer portion of each brush arm has a section of arcuate cross-section which receives the respective post, at least part of said cross-section being in engagement with the respective post.

3. An end cover for an electric motor according to Claim 2 wherein said arcuate crosssection defines two substantially planar divergant sections which slidingly engage the post to make electrically conductive contact therewith.

4. An end cover for an electric motor according to any preceding Claim wherein the retainer portion of each arm extends around more than half the periphery of the respective Post.

5. An end cover for an electric motor according to Claim 1 wherein the retainer portion of each brush arm has a section of circu- lar cross-section which receives the respective Post.

6. An end cover for an electric motor according to any preceding Claim wherein the post s comprise columns of substantially circu- lar cross-seciton.

7. An end cover for an electric motor ac cording to Claim 6 wherein the posts are formed with axially spaced annular projections for engagement by the retainer portions.

8. An end cover for an electric motor ac cording to any preceding Claim wherein the retainer portion of each brush arm has two lugs with aligned holes therethrough, a respec tive post being received in said aligned holes.

9. An end cover for an electric motor ac cording to any preceding Claim including springs for resiliently urging the brush portions into engagement with the commutator of said motor.

10. An end cover for an electric motor ac cording to Claim 9 wherein said springs simul taneously resiliently urge the retainer portions into engagement with the posts.

11. An end cover for an electric motor ac- cording to Claim 9 wherein the springs are electrically conductive and extend between spring receptacles electrically connected to the posts and the respective brush arms.

12. An end cover for an electric motor according to any preceding Claim wherein the 4 GB2194687A 4 brush portions comprise separate brushes mounted on and electrically coupled to the brush arms.

13. An end cover for an electric motor substantially as described herein with reference to Figures 1 to 8 of the accompanying drawings.

14. An electric motor comprising a motor case with an open end; field magnets; and a rotor mounted therein for rotation between the field magnets, the rotor having a commutator at one end, and the motor having an end cover according to any preceding Claim closing the open end of the motor case with the brush portions thereof engaging the commutator on the rotor.

Published 1988 at The Patent Office, State House, 66/71 HighHolborn, London WC 1 R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.

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