Commutator

Abstract

 An assembled commutator (30) has a base (31) and a plurality of segments (32) fitted thereon and secured by fixing limbs (47,48) extending into recesses (38,40) formed in the axial ends of the base. A resin (50) coats the limbs and recesses at one or both ends and optionally also covers the connection between the commutator terminals (46) and armature lead wires to bond the segments (32) to the base (31) to prevent relative movement between the segments and base and to relieve or reduce stress in the lead wires caused by vibrations, centrifugal forces and/or movement between the segments and base.

Description

[0001] This invention relates to assembled commutators, particularly of the type shown in GB 223 889A.

[0002] Commutators for fractional horsepower motors are generally of two types, moulded or assembled. Moulded commutators are superior in fixing of the segments to the commutator base as the base is moulded directly to the segments but this process is difficult and thus, slow and expensive. Assembled commutators are cheaper but have a problem of how to reliably secure the segments to the preformed base.

[0003] Assembled commutators such as those shown in GB 223889A have addressed this problem by using commutator segments with fixing means or limbs in the form of fingers and loops for mating with recesses and projections formed on the base to secure the segments. The loops are placed over the projections and the fingers are plastically deformed into the recesses to secure the segments to the base.

[0004] While this works very well when the various dies and assembly machines are brand new, with time the segments are not secured as well as they might be and after use, the segments may become loose allowing vibrations or minute movement between the segments and the base which in turn can lead to metal fatigue and failure of the fixing limbs resulting in premature failure of the commutator.

[0005] However, it has been found that by coating the fixing means at one or both ends of the commutator (depending on construction) with a bonding material such as the insulating resin commonly used to relieve stresses in the armature lead wires, the minute movement or vibrations which lead to the early failure of the commutator can be prevented. An advantage of using this resin is that simplified winding techniques can be used without fear of broken lead wires due to vibration centrifugal forces.

[0006] According, the present invention provides an assembled commutator having a preformed base; a plurality of segments fitted to the base; and bonding material wherein, each segment has a brush contact portion for making sliding contact with a brush, a terminal for making electrical contact with an armature winding, and fixing means extending from the brush contact portion and formlocked with the base to fix the segment to the base, and the bonding material at least partially covers the fixing means to bond the segments to the base.

[0007] Preferably, the base is cylindrical with first and second axial ends and fixing means include fingers plastically deformed into at least one recess formed in the first axial end of the base.

[0008] Preferably, the fixing means further includes a limb extending from the brush contact portion and engaging detent means formed on the second axial end of the base for fixing the segment to the base.

[0009] Preferably, the detent means comprises a plurality of axially extending projections and the limb forms a loop through which the projections extend.

[0010] Preferably, the base has an annular groove formed in the second axial end adjacent the root of the projections and the limbs extend into the groove, the groove forming a reservoir for further bonding material bonding the limbs to the base.

[0011] According to a second aspect, the present invention provides a wound rotor for an electric motor comprising a shaft; an armature core mounted on the shaft for rotation therewith; armature windings wound around the armature core; a commutator mounted on the shaft adjacent the armature core, the commutator having a preformed base and a plurality of segments fitted to the base, each segment having a brush contact portion for making sliding contact with a brush connected to a source of electrical power, a terminal electrically connecting the segment to the armature windings and a finger extending from the brush contact portion and plastically deformed into a recess in the base thereby fixing the segment to the base; and bonding material disposed on the fingers and the base in the region of the recess to prevent relative movement between the base and the segments.

[0012] In addition, the present invention provides an electric motor incorporating a wound rotor including a commutator as herein described.

[0013] A preferred embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a partially cut-away view of a fractional horsepower universal motor incorporating a commutator according to the present invention;

Figure 2 is an end view of a commutator with a part cut away;

Figure 3 is an opposite end view of the commutator with a part away; and

Figure 4 is a sectional view of the commutator taken along line A-A of Figure 2.

[0014] The fractional horsepower universal motor 10 of Figure 1 incorporates a commutator 30 according to the present invention. The motor has a stator assembly including a stator core 11 and a stator winding 12 (only part of the coil is visible and the connecting leads have been omitted for clarity but are well known). Mounted directly on the stator core 11 at either end are bearing brackets 13, 14. Bearing bracket 14 supports brush gear comprising two carbon brushes 15 positioned for contact with the commutator and slidably disposed in insulating brush holders 16 fitted in apertures in the bearing bracket 14. Springs located inside the brush holders 16 urge the brushes 15 into contact with the commutator surface.

[0015] The bearing brackets 13, 14 support bearings 17 in which motor shaft 18 is journalled. The shaft supports the commutator 30 and armature core 19 on which armature windings 20 are wound. The armature windings 20 are connected to the commutator 30 by lead wires 21 which are terminated on terminals or tangs of the commutator. The shaft also supports a fan 25 for cooling the motor 10.

[0016] A spacer hidden in Figure 1 by the lead wires 21 and resin 22 separates the commutator and the armature core to allow room for the armature windings. The lead wires 21 wrap around the spacer to provide support for the lead wires which are subjected to vibration and centrifugal forces. A trickling resin 22 coats the lead wires as they extend from the spacer to the commutator terminals to provide greater support.

[0017] The commutator 30 is more clearly shown in Figures 2, 3 and 4. The commutator has a base 31 of moulded insulating material on which a plurality of commutator segments 32 are fitted.

[0018] The base 31 is cylindrical in shape with a central bore 33 for fitting to the motor shaft 18. The base 31 has first and second axial ends 34, 35 and an outer cylindrical surface 36 covered by the segments. The bore 33 has a step 37 in the first axial end 34 for receiving the spacer. The first axial end which is located adjacent the commutator core is formed with a plurality of axially extending recesses 38.

[0019] The second axial end 35 which is located remote from the armature core, has a plurality of axially extending projections 39 and a recess or an annular groove 40 radially inward of the projections. A lip 41 defines the inner edge of the groove. The lip 41, groove 40 and projections 39 form a reservoir 42 for bonding material as will be described later.

[0020] Each segment 32 has an elongate brush contact portion 45 which is curved to sit on the surface 36 of the base 31 and extends axially along the base. At one end of the brush contact portion is a tang or terminal 46 for making electrical connection with an armature lead wire 21. The terminal shown is a weldable tang in the form of a U-shaped hook around which a lead wire 21 is looped and the tang is then pressed to collapse the hook around the wire while sufficient current is passed through the tang to burn off insulating varnish on the lead wire to ensure good electrical contact between the lead wire and the segment.

[0021] Each segment 32 is secured to the base 31 by fixing limbs comprising two fingers 47 extending axially from the brush contact portion 45 either side of the terminal 46 and a loop 48 extending radially from the opposite end of the brush contact portion 45. To hold the segment in place, the loop 48 is placed over a projection 39 on the second end 35 of the base 31. The fingers 47 are plastically deformed into the recesses 38 in the first end 34 of the base 31 and into engagement with the wall of recess to secure the segment in place. The segment is thus formlocked to the base. At the same time, the loop may be squeezed or pressed further into the annular groove 40 to secure the other end of the segment.

[0022] Bonding material 50, preferably the same resin used to coat the lead wires 21, is coated over the connection between the fixing limbs 47, 48 and the base 31. Depending on the construction, the resin may be applied to both ends or only one end of the commutator. Ideally, the resin may be applied to the first end of the commutator at the same time the resin is being applied to the lead wires. Alternatively, it could be applied to one or both ends after the commutator 30 has been assembled but before it is fitted to the shaft 18.

[0023] The first end of the commutator 30 as shown in Figure 2 has a plurality of recesses equal in number to the number of segments and adjacent fingers 47 of adjacent segments 32 share a common recess 38. The resin 50 as shown in the upper portion of Figure 2, covers the recesses 38 and part of the pairs of fingers 47. The resin bonds the fingers to the base to prevent any minute movement or vibration which may otherwise occur due to relaxation of the grip of insufficiency of the grip by the fingers on the base. The number of recesses 38 of course could be varied between a separate recess for each finger to multiple fingers per recess including a single recess accommodating all the fingers. The resin is omitted from the lower part of Figure 2 to show detail of the finger/recess arrangement.

[0024] The second end of the commutator as shown in Figure 3 has the loops 48 extending into the annular groove 40 and the resin 50 coats at least that part of the loops 48 within the groove to bond the loops to the base to prevent minute movement therebetween. Resin 50 fills the reservoir 42 as shown in the upper portion of Figure 3. The resin has been omitted or cut away from the lower portion to show detail of the loops, groove and projections.

[0025] The preferred bonding material is a thixotropic polyester resin such as DOLPHON CC-1133/513-D Polyester Resin by John C. Dolph Company of U.S.A.

[0026] While the resin may provide only a weak bond between the segments and the base, it is sufficient to prevent the vibrations or minute movements which can lead to premature commutator failure. At the same time, this arrangement is superior to gluing the segments directly to the commutator base as that method is difficult to do properly due to possible contamination of the commutator surface by the glue and the difficulty in achieving consistently reliable bonding due to the heat developed by the commutator in use and variations in the thickness of the glue which also affects the positioning of the segments. With the present invention, these problems are avoided.

[0027] Various modification to the described embodiments will be obvious to the reader and it is desired to include all such modifications as fall within the scope of the claims. For example, loops or fingers may be used as the fixing limbs at either or both ends of the commutator.

Claims

1. An assembled commutator comprising:

a preformed base;

a plurality of segments fitted to the base; and

bonding material wherein,

each segment has a brush contact portion for making sliding contact with a brush, a terminal for making electrical contact with an armature winding, and fixing means extending from the brush contact portion and formlocked with the base to fix the segment to the base, and

the bonding material at least partially covers the fixing means and bonds the segments to the base.

2. A commutator according to claim 1 wherein the base is cylindrical with first and second axial ends and the fixing means include fingers plastically deformed into at least one recess formed in the first axial end of the base.

3. A commutator according to claim 2 wherein each segment has two fingers extending from either side of the terminal.

4. A commutator according to claim 2 or claim 3 wherein the bonding material covers the at least one recess.

5. A commutator according to any one of claims 2 to 4 wherein the or each recess forms a reservoir retaining the bonding material.

6. A commutator according to any one of claims 2 to 5 wherein the fixing means further includes a limb extending from the brush contact portion for engaging detent means formed on the second axial end of the base for fixing the segment to the base.

7. A commutator according to claim 6 wherein the detent means comprises a plurality of axially extending projections and the limbs form loops through which the projections extend.

8. A commutator according to claim 7 wherein the base has an annular groove formed in the second axial end adjacent the root of the projections and the limbs extend into the groove, the groove forming a reservoir for further bonding material bonding the limbs to the base.

9. A commutator according to any one of the preceding claims wherein the bonding material is a resin.

10. A commutator according to claim 9 wherein the resin is a thixotropic polyester.

11. An assembled commutator substantially as hereinbefore described with reference to the accompanying drawings.

12. A wound rotor for an electric motor incorporating an assembled commutator as defined in any one of the preceding claims.

13. A wound rotor for an electric motor comprising:

a shaft;

an armature core mounted on the shaft for rotation therewith;

armature windings wound around the armature core;

a commutator mounted on the shaft adjacent the armature core, the commutator having a preformed base and a plurality of segments fitted to the base, each segment having a brush contact portion for making sliding contact with a brush connected to a source of electrical power, a terminal electrically connecting the segment to the armature windings and a finger extending from the brush contact portion and plastically deformed into a recess in the base thereby fixing the segment to the base; and

bonding material disposed on the fingers and the base in the region of the recess to prevent relative movement between the base and the segments.

14. A rotor according to claim 13 wherein there are a plurality of recesses and each finger is disposed into a selected one of the recesses.

15. A rotor according to claim 13 or claim 14 wherein lead wires extend from the armature windings and connect to the terminals of the segments and the bonding material coats the lead wires in the vicinity of the commutator.

16. A rotor according to any one of claims 13 to 15 wherein the bonding material is a polyester resin.

17. A rotor according to any one of claims 13 to 16 wherein the commutator base is cylindrical in form with the fingers and terminals being located at a first axial end of the base adjacent the armature core and fixing means are provided between the segments and the base at a second axial end of the base to secure the segments to the base.

18. A rotor according to claim 17 wherein the fixing means comprises axially extending projections formed on the second end of the base and limbs extending from the brush contact portions of the segments interlock with the projections.

19. A rotor according to claim 18 wherein the limbs form loops through which the projections extend.

20. A rotor according to claim 18 or claim 19 wherein further bonding material coats the limbs and projections to prevent relative movement therebetween.

21. A rotor according to claim 20 wherein the bonding material is disposed in a reservoir formed by an annular groove in the second end of the base adjacent the roots of the projections and into which groove the limbs extend.

22. A wound rotor for an electric motor, substantially as hereinbefore described with reference to the accompanying drawings.

23. An electric motor incorporating a wound rotor as defined in any of claims 12 to 22 or an assembled commutator as defined in any one of claims 1 to 11.

Drawing