GB2178245A - Commutators - Google Patents

Abstract

A method of construction of a commutator in which a tubular support member (1) is formed from a stack of laminations, the support member (1) having a number of axial grooves (3) running along its external periphery. Respective pairs of conductive segments (5, 7) are placed in each of the number of folds of a flexible insulating material (9), one fold then being placed in each groove (3). Respective flexible wedge members (11) are then inserted between each pair of segments (5, 7) to secure the fold and segments (5, 7) within each groove (3). The whole assembly so formed is then subjected to a vacuum pressure impregnation process and the completed commutator finish machined in the normal way. <IMAGE>

Description

SPECIFICATION Commutators This invention relates to methods of constructing commutators for electrical machines.

In such machines the armature conductors are normally externally connected in sequence by means of brushes sliding over the segments of a commutator, each'segment being connected to a conductor on the armature. The basic requirement for such a commutator is that each segment is electrically insulated from the other segments and also from the structure supporting the segments. Furthermore the pitching of the segments should be uniform, and there should be no relative movement between the segments as a result of mechanical or thermal effects when the machine is in operation.

Known commutators for electrical machines, especially large D.C. machines employ anchor bars running the length of the commutator, a respective anchor bar being provided for each segment or pair of segments. To construct the commutator a section of each segment is slid into a re-entrant portion of each anchor bar having a complementary profile to the section, an insulating layer being interposed between each anchor bar and its mating segment, the layer extending round each segment. The anchor bars are then fastened together by welding to form a ring from which the segments radially project.

Such a method of construction has the disadvantage, however, that it is difficult to assemble the anchor bars already supporting the segments to form the ring to achieve a uniformly pitched, stable commutator. Furthermore during the assembly of the ring it is necessary to employ a special jig to compress the insulating layer round each neighbouring pair of segments.

Other known commutators for large D.C.

machines employ so-called "V-rings". These are two V-sectioned rings which mate with two complementary notches at either end of each segments so as to connect the segments in a radial configuration. Again, however, it is difficult to construct such a commutator to achieve equal pitching of the segments.

It is an object of the present invention to provide a method of construction of a commutator wherein these difficulties are at least alleviated.

According to the present invention a method of construction of a commutator comprises: forming a tubular support member having a plurality of structures round its external periphery for holding conductive segments in the commutator; placing an anchor portion of each of a plurality of conductive segments into a respective fold of a flexible insulating material, each of the anchor portions having a complementary profile to that of each of the structures; securing each anchor portion within one of the folds to the support member by a structure, and subjecting the assembly so formed to a bonding process.

Preferably the bonding process is a vacuum pressure impregnation process.

Preferably a stack of laminations are used to form the support member.

Preferably the structures are in the form of grooves running axially along the support member the segments being in the form of one or more metal bars. In such a preferred method, preferably wedge members are provided to fasten the fold and bar or bars within each groove. Preferably a plurality of bars are placed within each fold, a respective wedge member being provided between each adjacent pair of bars within a fold. Preferably each wedge member and the surfaces which it contacts within each fold are shaped so as to prevent removal of the wedge from the fold.

One method of construction of a commutator in accordance with the invention will now be described, by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic diagram showing a step in the method; Figure 2 is a schematic diagram showing a side view of part of the assembled commutator; Figure 3 and 4 are schematic diagrams illustrating alternative segment and wedge shapes; and Figure 5 and 6 are schematic diagrams illustrating alternative segment and groove shapes.

Referring firstly to Fig. 1 the method includes forming a tubular support member 1 from a stack of steel laminations, the laminations being held together by any suitable means such as bolts, rivets, welds or glue (not shown). The laminations are shaped and are secured such that the stack has a number of parallel, wide bottomed grooves 3 running axially along the stack, a central aperture (not shown) running through the stack in which the shaft of a machine employing the commutator will be located. The laminations may take the form of complete circles or alternatively segmental laminations may be interleaved so as to form the tubular support member.

In respect of each groove there are provided a pair of electrically conductive segments in the form of copper bars 5, 7, an end portion of each being of complementary profile to an edge of the grooves, the profiled ends being placed in a fold of one or more layers of flexible electrically insulating material 9, for example glass cloth, interspersed with plastic film. The profiled end of one of the bars 5 is then pushed, together with the layer of material 9 into the groove 3, the profiled end of the second bar 7 then also being pushed into the groove as shown in Fig. 1, the dimensions of the bars and groove being chosen to make this possible. Further pairs of similar bars, within further folds of insulating material are then inserted into all the other grooves 3 round the periphery of the tube 1.

Referring now to Fig. 2, respective wedges 11 of a rigid insulating material are then driven between each pair of bars 5, 7 in each groove 3, so as to lock each pair of bars 5, 7 into their respective groove. The whole structure is then subjected to a Vacuum Pressure Impregnation process using a suitable resin which when cured bonds the commutator into a solid rigid structure. Generally a channel (not shown) will be formed along an upstanding edge of each groove which, when the bars 5, 7 and loop of material 9 are inserted in the groove, will form a gallery to assist in the complete filling of the groove during the Vacuum Pressure Impregnation process. After the Vacuum Pressure Impregnation process the commutator is then turned and finished in the normal way. Thus by this method of construction, the need for completion construction jigs is eliminated.Furthermore the accuracy of the pitching of the segments will be approximately the accuracy of the spacing of the grooves, which can be made very precise.

It will be appreciated that the shape of the wedges 11 and bars 5, 7 may be chosen to prevent them being thrown out of their groove in the event that the bond formed during the Vacuum Pressure Impregnation process fail.

Two examples of suitable shapes are shown in Figs. 3, and 4 in each case part of the wedge fitting into a complementary part of the bars. In such cases, generally the wedge may be positioned between the bars within the loop of insulating material by being axially slid into the groove from one end of the groove, rather than being pushed radially inward. The bars and wedge within each loop could also be slid axially into the groove rather than pushed in as shown in Fig. 1.

It will also be appreciated that the grooves and profiled end of the bars may have many different forms, examples of two such forms being shown in Figs. 5 and 6.

It will be appreciated that the use of laminations to form the support tube avoids the problem of having to machine grooves or other support structures in a solid tube. In some circumstances it may be preferable to form the tube from a number of spaced, discrete rings rather than a continuous tube. Additionally, it may in some circumstances be preferable to form the outermost laminations from an insulating material in order to improve the resistance to tracking.

Whilst in the commutators shown in the figures, each groove holds a pair of bars which are electrically insulated from each other, it will be appreciated that in some circumstances the pair of bars may be electrically connected so that they act as a single segment of the commutator.

It will also be appreciated that in some circumstances only one bar, or more than two bars may be inserted in each groove.

Claims (8)

1. A method of construction of a commutator comprising: forming a tubular support member having a plurality of structures round its external periphery for holding conductive segments of the commutator; placing an anchor portion of each of a plurality of conductive segments into a respective fold of a flexible insulating material, each of the anchor portions having a complementary profile to that of each of the structures; securing each anchor portion within one of the folds to the support member by a structure; and subjecting the assembly so formed to a bonding process.

2. A method according to Claim 1 in which the bonding process is a vacuum pressure impregnation process.

3. A method according to either of the preceding Claims in which a stack of laminations are used to form the support member.

4. A method according to any one of the preceding Claims in which the structures are in the form of grooves running axially along the support member the segments being in the form of one or more metal bars.

5. A method according to Claim 4 in which wedge members are provided to fasten the fold and bar or bars within each groove.

6. A method according to Claim 5 in which a plurality of bars are placed within each fold, a respective wedge member being provided between each adjacent pair of bars within a fold.

7. A method according to Claim 6 or Claim 7 in which each wedge member and the surfaces which it contacts within each fold are shaped so as to prevent the removal of the wedge from the fold.

8. A method of construction of a commutator substantially as hereinbefore described with reference to the accompanying figures.