GB2367193A

GB2367193A - Cooling brushes in a miniature electric motor

Info

Publication number: GB2367193A
Application number: GB0023023A
Authority: GB
Inventors: Cyrus Cuthbertson
Original assignee: Johnson Electric SA
Current assignee: Johnson Electric SA
Priority date: 2000-09-20
Filing date: 2000-09-20
Publication date: 2002-03-27
Also published as: GB0023023D0

Abstract

An insulating brush holder 10 for a miniature electric motor has a through hole for receiving a brush. The walls 14 of the through hole have a plurality of integral ridges 16 or a corrugated brass insert 24 forming a contact surface for the brush and defining cooling air pathways 22 for direct convection cooling of the brush. Alternatively the brush holder may be formed of a conductor (brass) and formed as shown in figs 4 or 5. The metal holder/insert additionally act as heat sinks.

Description

Brush Cage Assembly For A Miniature Motor This invention relates to a brush holder for a miniature electric motor. Brush holders are tubes, usually of square or rectangular cross-section, in which a carbon brush slides for making electrical contact with a commutator or slip ring of an electric motor or generator.

In miniature electric motors, especially subfractional horsepower motors, the brush holders are commonly molded from heat resistant resinous material such as phenolic which also insulates the brush electrically from the surrounding motor parts.

Sometimes the brush holder is formed from brass or similar material and sometimes a brass liner is provided in a resin molded brush holder.

The molded holders also thermally insulate the brush which, in use, can get very hot.

For low current motors, this is not a problem as the motor cooling system can maintain the brush and commutator at acceptable temperature levels. However, for high power motors drawing high currents at or close to the maximum current rating of the brush, having the brush fully enclosed in a molded brush holder does present a major cooling barrier. Brass liners and brass cages do aid cooling but the temperature of the brush does limit the continuous current rating of the motor, even though the maximum current rating of the brush has not been ached.

Thus, it is desired to have a brush holder, either of the molded type or brass type, which allows effective cooling of the brush thereby effectively increasing the power of the motor.

Accordingly, in one aspect thereof, the present invention provides a brush holder for an electric motor as defined in Claim 1.

Preferred embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a perspective view of a molded brush cage; Figure 2 is a diagrammatic cross-section of the brush holder of Figure 1;

Figure 3 is a diagrammatic cross-sectional drawing similar to Figure 2 of an alternative embodiment ; Figure 4 is a perspective view of a brass brush cage; and Figure 5 is a perspective view of an alternate brass brush cage.

Referring to Figure 1, it can be seen that the brush holder 10 has a molded body 12 of resin material, preferably a heat resistance material such as phenolic or BMC. The brush holder may be a separate part, as shown, or may be an integral part of a brush card assembly, motor end cap or similar arrangement.

The brush holder 10, as its primary function, guides a brush into sliding electrical contact with the commutator. This is achieved by providing the body with a tube through which the brush can slide to guide the brush to the commutator. In the present embodiment, the inner surface or walls 12 of the tube has a plurality of ridges 16 which form a contact surface along which the brush slides. The contact surface supports the brush against lateral movement. The dimensions of the opening defined by the contact surfaces depend on the dimensions of the brush it is designed to receive and the desired clearance to provide a sliding fit for the brush. Lateral movement is undesirable as lateral movement would allow the brush to drift about the surface of the commutator which would alter the commutation angle of the motor and thus, affect the efficiency of the motor.

Figure 2 is a cross-sectional diagrammatic view of the body or brush cage 12 of the holder of Figure 1 on an enlarged scale. Here, the contact surface is easier to see.

The inner walls 14 of the cage or tube have a plurality of axially extending ribs or ridges 16. The ridges 16 are formed with flat tops 18 but could be rounded or any other suitable shape. The tops 18 of the ridges contact the brush (shown in phantom) to support the brush against lateral movement.

Between the ridges 16 are recesses or pathways 22. During operation of the motor, cooling air circulates or flows through these pathways 22 in direct contact with the brush, providing direct cooling of the brush. During high current operation of the motor, significant heat is generated, not only at the brush/commutator interface, but also by the I2R losses in the brush itself. By direct cooling of the brushes, the motor runs cooler, reducing the heat build-up in the brushes. If the brushes get too hot, the

resistance of the brushes increases which creates more heat which can lead to catastrophic failure of the motor. Figure 3 shows a brush holder 10 with a brass brush cage liner 24. Brass is the preferred material although other metallic or conducting materials may be used. The brass liner 24 forms the actual cage in which the brush slides. The cage liner 24 has a plurality of ridges 16 and pathways 22,26 formed by corrugations in the walls of the cage. In this arrangement, the brush liner 24 also acts as a heat sink and transfers the heat from the brush more effectively by increasing the cooling surface of the brush by convection and conduction whereas the embodiment of Figure 2 relies on convection cooling only. Thus pathways 22 directly cool the brush while pathways 26 cool the brush cage liner 24 which in turn, cools the brush.

The ridges and pathways could be formed in the brush cage by slots in the walls of the cage. Here it is necessary for the ends of the slots to be open to the surrounding air to ensure good air flow.

A brass brush holder 28 is shown in Figure 4. This type of brush holder is commonly referred to as a brush cage and requires electrical insulation from the surrounding motor parts. Hence it is a cooler design than the molded brush holder with or without a brass liner. The embodiment of Figure 4 is an improvement over the standard brass brush cage due to the brush cage ridges 16 and pathways 22,26 formed by corrugations in the walls of the brush cage, similar to the liner 24 of the brush holder of Figure 3. Thus, the brush is cooled by both convection and conduction cooling.

The embodiment of Figure 5 is a brass brush cage 30 similar to the brush cage of Figure 4 where the ribs and pathways are formed by slots 32 in the walls of the cage allowing convection and conduction cooling of the brush.

While the preferred embodiments have been described, various modifications and variations will be readily apparent to those skilled in the art and it is intended to cover all such variations as fall within the scope of the appended claims.

Claims

1. A brush holder for an electric motor comprising : a body defining a tubular hole having four walls for receiving a brush, wherein at least two walls have a plurality of ribs defining a contact surface for making sliding contact with the brush and supporting the brush against lateral movement, the ribs defining therebetween a plurality of pathways for cooling air to make direct cooling contact with the brush.

2. A brush holder according to claim 1 wherein the body is a molded resin body.

3. A brush holder according to claim 2 wherein the ridges and pathways are integrally molded with the resin body and extend axially along the tubular hole.

4. A brush holder according to any one of the preceding claims wherein the brush holder is an integral part of a brush holder assembly.

5. A brush holder according to claim 2 wherein the brush holder has a brass liner lining the tubular hole and the liner has a plurality of slots in each side forming the plurality of ribs and cooling air pathways.

6. A brush holder according to claim 2 wherein the brush holder has a brass liner lining the tubular hole and the brass liner forms the ridges and pathways on the walls of the tubular hole.

7. A brush holder according to claim 6 wherein the ridges and pathways are formed by axially extending corrugations.

8. A brush holder according to claim 1 wherein the brush holder is a metallic material cage and the ribs and pathways are formed by axially extending slots.

9 A brush holder according to claim 1 wherein the body is a metallic material cage and the ribs and pathways are formed by corrugations formed in the wall of the cage.

10. A brush holder according to claim 8 or claim 9 wherein the metallic material is brass.

11. A brush holder according to any one of the preceding claims wherein each wall has said plurality of ribs.

12. A brush holder for an electric rotating machine substantially as hereinbefore described with reference to the accompanying drawings.