GB2187600A - Electric motor brush mountings - Google Patents

Abstract

To avoid melt down of plastics end caps whilst retaining the advantages of insulated brush mounting structures, two metal posts (12) project from the inner surface of a metal end cap (11) and two insulating brush holder supports (13) are mounted on these posts (12). Each support (13) is preferably of thermoplastics material and has a tubular portion (14) and an arcuate wall portion (15) between which a respective brush leaf is supported. The tubular portion (14) is mounted as a press fit on the post (12) and the arcuate wall portion (15) is in contact with the casing (10) when the motor is assembled thus structure. Alternatively the supports providing support for the plastics are connected by a plastics annulus to simplify mounting on the end cap (see Fig. 2). <IMAGE>

Description

SPECIFICATION An electric motor This invention relates to an electric motor and particularly to a p.m.d.c. micromotor, that is to say to a permanent magnet direct current micromotor.

One of the problems facing designers of small (i.e. ffractional horsepower) p.m.d.c. motors is to find ways of dissipating the heat generated by losses in the motor. This problem is exacerbated by the continuous demand to produce more output power per volume of motor. This demand means that more heat generated by more losses has to be absorbed by less volume of material with the result that temperature rises become intolerable for some materials which have been commonly used.

For many years it has been common practice to use thermoplastic materials to form the end cap of the motor. This has been advantageous because, being an insulator, the plastics end cap has also served to support the brushgear.

However when higher power to volume ratios are required it becomes inadvisable to use thermoplastic to support the bearing in the end cover. The poor conductivity of plastics material prevents an adequate dissipation of the heat generated in the bearing journal, by friction and conduction from the commutator, and the temperature in the bearing rises rapidly, reducing the vicosity and benefit of the bearing lubrication causing more heat to be generated and even higher temperature to result. This cycle usually ends with the thermoplastic melting and the motor seizing and burning-out.

The use of metal in end caps pre-dates the use of plastics and such metallic caps are well known. Metal caps with plastics inserts or additions to carry brushgear are also known.

However in small p.m.d.c. motors the problems of designing a system of metal cap to hold the bearing combined with plastics parts to hold the brushgear is made most difficult by the geometry of such small parts.

Having overcome the problem of dissipating the heat from the bearings by mounting them in a metal end cap which conducts and radiates heat away from the bearing, the remaining problem is to find a way of conducting or radiating heat away from the brushgear and its plastic (or other electrically insulating) support which becomes the next most vulnerable part in the motor design.

The present invention seeks to provide a solution to this problem.

According to the present invention there is provided an electric motor comprising a cylindrical metal casing, an end cap formed at least essentially of metal and engaged with one end of the casing, and a rotor shaft bearing and brushgear for the motor carried by the end cap, the brushgear comprising two brush holders each in the form of a brush leaf and the end cap having two metal posts projecting from the inner surface thereof and two brush holder supports mounted on the two posts, respectively, each brush holder support having two portions between which a respective brush holder is supported, one of said portions being in contact with a respective metal post and the other portion being in contact with the casing of the motor, at least those parts of each brush holder support in contact with the brush holder being of electrically insulating material.

Preferred and/or optional features of the invention are set forth in claims 2-7.

Thus, the brush holders can be electrically insulated from the metallic end cap by thin section supports, preferably of thermoplastics material, which permit adequate conduction of heat from the brush holders to the motor casing and end cap.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a fragmentary perspective view of one embodiment of an electric motor according to the invention, prior to insertion of the end cap into the casing, qnd Figure 2 is an exploded view of an alternative end cap and brush holder supports for the motor of Figure 1.

Referring firstly to Figure 1 of the accompanying drawings, there is shown therein a p.m.d.c. micromotor having a cylindrical canlike metal casing 10 and a metal end cap 11.

The end cap 11 is engageable with the open end of the casing 10, which accommodates two arcuate diametrically opposed permanent magnets (not shown) and a wound rotor (also not shown) equipped with a commutator.

The end cap 11 supports a rotor shaft bearing 9 and has two integral metal posts 12 which project from the inner surface of the end cap 11 in a direction perpendicular thereto.

A brush holder support 13 is mounted on each post 12. Each brush holder support 13 has a radially inner tubular portion 14 which is a press fit on its respective post 12, a radially outer arcuate wall portion 15 having radius of curvature which matches that of the metal casing 10, and a web portion 16 which interconnects the wall portion 15 and the tubular portion 14 and which bears against the inner surface of the end cap 11.

A brush holder in the form of a brush leaf 17 is supported in an arcuate groove defined between the tubular portion 14 and the arcuate wall portion 15 of each brush holder support 13. The brush leaves 17 (only one is shown) are of resilient electrically conductive material, e.g. beryllium copper, and are arranged so as urge respective brushes 18 against the commutator when the motor is assembled.

The brush holder supports 13 are preferably of thermoplastics material and are made of thin sections (advantageously having a thickness of not more than 0.5mm) especially in those regions which make contact with the brush leaves 17. Plastics material has high dielectric characteristics. Consequently, very thin sections will electrically isolate the brush leaves 17 from the metal end cap 11, although they are inherently flexible and would not alone create a sufficiently rigid support to withstand the stresses set up by the spring force of the brush leaves.However, the tubular portion 14, being a press fit on the post 12, is rigidly supported by the post 12 and the arcuate wall portion 15 is so shaped and disposed that when the end cap 11 is engaged with the open end of the casing 10 this wall portion makes contact with the casing 10 and is therefore radially supported thereby.

Thus the tubular portion 14 and wall portion 15 are supported in such a manner that they will withstand the stresses set up by the spring action of the brush leaf 17 and, being of thin plastics sections, transfer heat from the leaf 17 to the post 12 and the casing 10.

Even if the plastics material is elevated to a temperature at which it softens the portions 14 and 15 will still be supported by the post 12 and the casing 10, respectively.

The web portion 16 of each brush holder support 13 has an aperture therein (not shown) which is aligned with an aperture (also not shown) in the end cap 11. A terminal portion 19 is connected to each brush leaf 1 7 and extends through the aligned apertures in the web portion 16 and the end cap 11, the support 13 supporting the brush leaf 17 such that the terminal portion 19 is spaced from the metal end cap 11.

Figure 2 shows an alternative embodiment in which the two supports 13 are integrally connected by an annulus 20 of plastics material which bears against the inner surface of the end cap. This simplifies fitting of the brush holder supports to the end cap. Moreover, in this alternative embodiment the aforesaid apertures in the end cap 11 for receiving the terminal portions 19 are replaced by arcuate recesses 21 formed in the peripheral edge of the end cap 11. These recesses 21 facilitate insulation of the terminal portions from the metal end cap 11.

In the above embodiments, the brush holder supports 13 are said to be preferably of thermoplastics material. They could, however, be of any other appropriate electrically insulating material and indeed could have a metal insert coated or lined with insulating material.

Other modifications can also be made to the above embodiments without departing from the scope of the invention.

Claims (10)

1. An electric motor comprising a cylindrical metal casing, an end cap formed at least essentially of metal and engaged with one end of the casing, and a rotor shaft bearing and brush gear for the motor carried by the end cap, the brush gear comprising two brush holders each in the form of a brush leaf and the end cap having two metal posts projecting from the inner surface thereof and two brush holder supports mounted on the two posts, respectively, each brush holder support having two portions between which a respective brush holder is supported, one of said portions being in contact with a respective metal post and the other portion being in contact with the casing of the motor, at least those parts of each brush holder support in contact with the brush holder being of electrically insulating material.

2. The motor of claim 1, wherein the brush holder supports are of plastics material.

3. The motor of claim 2, wherein the aforesaid two portions of each brush holder support are flexible.

4. The motor of claim 3, wherein the two portions of each brush holder support have a thickness substantially not more than 0.5mm.

5. The motor of anyone of the preceding claims, wherein said one portion of each brush holder support is a press fit on its respective post.

6. The motor of anyone of the preceding claims, wherein the other portion of each brush holder is of arcuate section and makes radial contact with the motor casing.

7. The motor of anyone of the preceding claims, wherein each brush holder is connected to a terminal portion which extends to the outside of the end cap, and wherein each brush holder support supports its respective brush holder such that the terminal portion is spaced from the metal end cap.

8. The motor of anyone of the preceding claims, wherein the two brush holder supports are integrally connected.

9. A permanent magnet direct current micromotor having the features set out in anyone of the preceding claims.

10. A motor substantially as hereinbefore described with reference to the accompanying drawings.