GB2062973A - A robust, stable, and cheap commutation system for compact electric motors - Google Patents

Abstract

A commutator has segments 6 supported by an insulating layer 5 on a flange 4 of a rotor disc 3, and the segments are contacted by brushes Br mounted on holders 9. Radial commutator segments (302, Figures 3, 4 not shown) may be mounted on discs (301) with the aid of insulating pads (303, 304), insulating rings (311, 316), or an annular member (314). Segments (504, Figure 5 not shown) of a radial commutator are secured to a disc (502) by lugs (505, 506). In Figure 6 (not shown), segments (605) are secured to a rotor (601) by means of a bed of insulation (603) and a flange (604) fixed to the rotor. In a brush holder 201, Figure 2, a spring 203 presses one end of a lever 205 via a sliding mass 204, and a force is applied to brush 207 by way of a further sliding mass 206. Previous applications have described the construction and advantages of "radial" type commutators. This application describes a construction which not only has the same sort of advantages for radial commutators, plus some improvements in the rigidity thereof, but is also applicable with some advantages to the construction of the more usual cylindrical commutators, albeit with use of the internal cylindrical surface rather than the external. The principles of construction are developed to the point where the commutator becomes an integral part of the rotor rather than a separate entity. An inertia-compensating brush holder without fluid, and so more suitable for use with the "internal" commutator, is also described. <IMAGE>

Description

SPECIFICATION A robust, stable, & cheap commutation system for compact electric motors BackgroundArt Two previous applications, one under the title of "A compact Electric Motor", and the other entitled "A Commutator for high-speed Electric Motors", have described some of the advantages of "radial" as opposed to the usual "cylindrical" type of commutator, especially when combined with some compact brusholder devices that can also reduce the effects of vibration and shock, upon commutation and commutator life.

This application seeks to extend such applications for further advantages of compact, improved, and more adaptable commutation of both "radial" and "cylindrical" type commutators.

General Description The first advantage in construction is that of a basically "internal" type of commutator segment structure, which is supported externally in the radial sense, as opposed to one in which the commutator segments are primarily held from their rotational centre close to the rotar axle ' iere in this invention the segments are basically supported by a rim which is external to them, so that the fixing devices which oppose the centrifugal forces of their rotation, are simply and fundamentally under compression rather than having to be so geometrically manipulated as to provide location that provides compressive forces on the insulating materials.

The previous application entitled "A commutator for High-speed Electric Motors", describes a device which employs a development of such normal fixing from inside, but where this is supplemented by a band around the periphery of the segments. Such a structure makes radially longer and heavier commutator segments than is necessary according to the present application and is also clearly unsuitable for the usual cylindrical arrangement of commutators with brushes contacting a circumferential surface of the cylinder. Neither does that construction have the axial stability of the present innovation, as its outer band provides no overall axial support apart from that of mutual alignment of the segments.

Before proceeding with explanatory embodiments of the presently proposed construction, it may also be as well to mention some of the main technical advantages to be gained in using large diameter commutators as opposed to the presently 'normal state-of-the art' smaller diameter cylindrical ones, for traction motors in particular. This should further elucidate the technical importance of the change in methods of construction proposed.

A. The greater 'sectoral' dimension of commutator segments, due to the greater mounting diameter of the same number of segments, can provide : 1. A larger contact Area for better current transfer properties and/or 2. A narrower commutatorforthe same current transfer properties, thus saving some of the limited space available for many traction motors.

3. Greater freedom of brush design with more rubust construction of "parrellel-wafer-divided" brushes with less inertia and therefore greater stability of contact and possible advantages in relation to "flashover".

4. A larger proportion of contact surface versus insulation space.

B. The higher speed of sliding, due to larger diameter commutators, can provide : 1. Quicker "make & break" commutation switching, with less arcing, and further reduction of "flashover" tendancies.

2. Better 'low-rev' performance due to "1" above. and 3. It has been shown in previous literature, that higher speed sliding of abrading surfaces may in itself reduce wear in some conditions.

Another sort of advantage is : C. in that the proposed type of construction lends itself to : 1. Use of less copper 2. Simple cheaply pressed-out commutator segments without the need for expensive taperedsection production.

3. Easily replaceable segments with saving of service costs.

Embodiments Figure 1 upper 1/2, is a sectional view of an embodiment of such a structure, in which the rotor 1 is mounted on its axle 2 and on which is mounted a rigid disc 3 with a flange 4. A hard insulating layer 5 internally lines the flange which thus supports the commutator segments 6 which are insulated from one-another. Rotor-winding connecting-lugs 7 protrude through insulated holes in the disc 3, to take rotor connections 8.

The whole structure may have as large a diameter as the rotor, and so increase commutation interruption speed as mentioned above. Accordingly the brushholders 9 are mounted on 'external access plate' 10 fixed to the motor frame 11, in which is mounted the rotor axle bearing 12. 13 is the brush-holder insulation. A duct 14 may be arranged to blow carbon dust from the commutator area, and 15 is suggested cooling and extract fan blade attached to the outer rim of the commutator assembly. 16 & 17 are the field winding and pole respectively. "Br" indicates the brush protruding from holder.

Below the centre-line is similarly represented the comparable 'state-of-the-art' cylindrical commutator and brushholder arrangement commonly used in large motors. The commutator 19 is shown on the same axle 2, and a typical brusholder 20 is shown mounted on its insulator 21 on the motor frame 22.

"Br" again indicates the protruding brushes, which must here be two for the same contact area as the one above.

It can be seen that the rotor 24 and field winding and pole 25 & 26 respectively are now further from the motorframe, and thus have less power capability within the same dimension of motor. 27 is the space required for the rotor connections.

Returning to consider the present invention, the brushholder 9 may be according to prior art whether or not of a type that includes "inertiai balancing" of brushes as described in the application entitled "A Compact Electric Motor", or alternatively the brusholder 9 may of a type probably preferred to those described in the above application, because of its operation without fluid that may find its way onto the "internal" type commutator described in Figure I. Accordingly Figure 2 shows a sectional view of an embodiment of a brush-inertia balancing brusholder as shown in outline in Figure I.

In Figure 2 the holder body 201 is fixed to the motor frame by the bolt 202, as in Figure 1, Figure 2 shows a brush pressure spring 203 pressing against the heavy balancing mass 204, which slides to operate the lever 205. Lever 205 in turn presses on the lightweight sliding block 206 which is free to push the brush 207 to the commutator. The fulcrum of the lever is the roller 208 which traverses along the top inner surface of the box 201 as the brush wears and the spring expands, so that "mechanical advantage" alters to compensate the reducing pressure of the spring as it expands, thus giving a more constant brush pressure as it shortens with wear.

The "brush-inertia balancing" is achieved by the sliding mass 204 whose inertia relative to radial accelerations is similar to that of the brush, so that its inertial forces towards or away from the lever 205 are caused to add to or subtract from, the spring pressure, by the required amount to compensate for the varying radial inertial forces on the brush, and thus give more constant pressure under conditions of external shock and vibration. A compliant membrance 209 reduces the inertial effect of this system only in relation to the small relative displacements of the commutator, but does not significantly affect the inertial balancing.

The inertial balancing is here however only approximate on account on the changing mechanical advantage for constant spring pressure of the short pressure spring. In fact this whole construction whilst comparatively cheap to manufacture, might, for traction purposes for instance, be better accomplished by using a "radial" type of commutator.

Accordingly the same basic principle of commutator construction is embodied as shown in Figure 5.

Figures 3 & 4 show two aiternative examples of how such a radial type segment might be constructed and located onto a flanged disc similar to the one shown in Figure 1. In each example the disc 301 holds a segment 302, which will here indicate the "front" of the commutator.

Figure 3 shows an insulating pad 303 which takes the centrifugal force only over the width of the lug 308, and an insulating pad 30 in the flange 305 taking centrifugal force only on the part width of the segment at shoulder 306, but not from that width taken for the connecting lug which protrudes through the flange to provide connecting slot 307.

Another connecting lug 308 at the rear helps hold the segment axially by means of insulator 309. This lug 308 also has a slot for rotor connections at 310.

Figure 4 shows the insulating ring 311 taking centrifugal force over the whole width of the segment against the flange 312. The lug at the rear has a slot 313 for rotor connections. A rim 314 fixed to the disc at 315 holds the lower ends of the segments via insulating ring 316.

Figure 5 shows another of the many possible variations in embodiment of this form of construction, here used in conjunction with brusholders described in "A Compact Electric Motor" (Refce.

application 7930430 3/Sep/79) in order to give more power per cu.ft. of total motor space, better commutation, and more economicai construction and service.

Accordingly Figure 5 depicts in schematic and sectional form, the motor frame 501 and the commutator disc 502 with its flange 503 holding segments 504 with lugs 505 & 506 and insulated as in Figures 3 & 4.

507 are cooling and dust extract fins arranged around the periphery of the flange 503. Brusholders 508 have brush-wear indicators 509.

The fact that the brusholders are electrically "live" may be overcome where necessary by for instance, moulded insulating covers, or a metal shroud.

510 indicates the space taken up by the usual "cylindrical" commutator, and the corresponding position of the rotor 511 and its connections 512, all in relation to the present rotor end at 513 and field poles 514 and axle bearing 515.

A further development of the scheme of Figure 5 is clearly to make the rotor itself act as the support disc and flange assembly, and so eliminate the commutator as a separate structural entity. This entails some re-arrangementofthe rotor-winding terminations in relation to their embedded paths to the segments, and opens up the possibility for such to be thus more robust. This results in the form of embodiment shown in Figure 6.

Figure 6 accordingly depicts a rotor 601 on its axle 602. The "commutator end" of the rotor has a bed of insulating material 603 attached to it, and retained for instance by the flange 604 located to the rotor.

The rotor connections 607 protrude from the insulating bed, and segments 605 are prepared and "location-jigged" to accept these connections to be solder-"sweated" thereto by application of heat so that both the connections and their bed form the fixing for the segments.

Further possibilities beyond the scope of this present invention would then be realiseable, with cognisance of the revised Electro-Magnetic Principles described in "IEEE Transactions in Industry Applications" Vol 1A-12 No.2, March/Apl 1976, which shows that a rotor conductor could be completely embedded, and so constructed as both parallel and series connected rods rather than windings, so that they and their commutator terminations were more conveniently and cheaply made for the commutator construction described in Figure 6.

Figure 7 is an end elevation of the commutator end of the rotor of Figure 6. It shows the segments assembled in their mounting structure, which, in this view, is not distinguishable from that of the commutator of Figure 5. In Figure 7,701 is the "flange", 702 and 703 the insulation, and 704 are the segments and 705 the rotor spindle.

Figure 8 is a diagram to illustrate in "round" numerical terms the advantages in geometric "tracing" and dynamic "tracking" ability of the brushes, that can be achieved solely by the larger diameter of commutator constructions described above. This is purely the 'first order' mechanical effect. Accordingliy the diagram shows the proposed type of commutator with radius 3R compared with a 'standard' type with radius R. The 'projection' between these two elevations represents the contact face of the standard cylindrical form.

It will be seen that in both cases the hatched area respresenting the brush contact area has been arranged to cover the same number of segments, and that the segments are of equal degree of arc in both cases. Also that the upper area of contact equals that of the lower. These "upper" and "lower" representations will henceforth be here referred-to as the "new" and "old" respectively.

For the same brush thickness "t" however, the ratio of this to the segment thickness "S", is three times greater in the new than in the old, thus giving 3X the relative geometric commutator-tracingability. Also since the width of each new brush piece is only 2/3rds that of the old, then for the same brush length, the new brush has only 2/3rds the mass of the old, thus giving 33% better dynamic tracking ability.

In all, the combined mechanical ability of the brush to follow the commutator is improved by a factor of 3 X 1.33 = 4 times, - even without the dynamic balanced brush system.

Claims (1)

1. Acommutatorforan electric motor in which the segments are held by a flange or a rim, on a disc of greater maximum diameter than that of the commutating surface, and/or held by the disc itself, which is attached in concentric manner to the rotor shaft, and where the commutation takes place either on the radially-internal faces of segments, which faces are thus parallel to the rotor shaft, or, on the faces of segments, which faces are parallel to the supporting disc, or, at any angle between these two cases mentioned.

2. A commutator as in claim 1, but where its supporting disc is integral with the rotor construction,or is the rotor itself.

3. A commutator as in claims I or 2 where the segments are held in a moulded insulating medium.

4. A brusholder for an electric motor, in which a spring pushes against a mass of similar average magnitude to that of the brush, and which mass in turn acts upon a lever to apply pressure to the brush for its contact with the commutator, so that any inertial forces on the mass, add to or subtract from the spring pressure, and thus via the lever to add to or subtract from the brush pressure on the commutator, but in opposite phase to the inertial forces experienced by the brush itself, so that the inertial forces experienced by the brush itself, so that the inertial forces from the mass tend to cancel the inertial forces of the brush on the commutator, since both are relative to the movement of their common container, brushholding or motor frame.

5. A brusholder as in claim 4, but wherein the fulcrum of the said lever changes its position relative to the spring & brush so that its mechanical advantage due to the relative distance, measured at right-angles to the line of instantaneous motion at the place of contact with brush and spring respectively, of the spring and brush from the fulcrum, is so altered as to increase this mechanical advantage as the brush wears down and the spring thus expands with its otherwise reducing pressure on the brush.

New claims or amendments to claims filed on 6 Jan 1981 Superseded claims 1,2,3 and 4 New or amended claims:

1. A brusholder for an electric motor, including one with a 'radial' or 'internal' commutator, and in which a spring pushes against a mass of similar average magnitude to that of the brush, and which mass in turn acts upon a lever to apply pressure to the brush for its contact woth the commutator, so that any inertial forces on the mass, add to or subtract from the spring pressure, and thus via the lever to add to or subtract from the brush pressure on the commutator, but in opposite phase to the inertial forces experienced by the brush itself, so that the inertial forces from the mass tend to cancel the inertial forces of the brush on the commutator, since both are relative to the movement of their common container, brusholder or motor frame.

Claim 5 re-numbered as claim 2 and appended to claim 1.