GB2191346A

GB2191346A - Insulating metallic brush cages for electric motors

Info

Publication number: GB2191346A
Application number: GB08613742A
Authority: GB
Inventors: Patrick Shui Chung Wang
Original assignee: Johnson Electric Industrial Manufactory Ltd
Current assignee: Johnson Electric Industrial Manufactory Ltd
Priority date: 1986-06-06
Filing date: 1986-06-06
Publication date: 1987-12-09
Also published as: GB8613742D0

Abstract

To improve heat conductance of a brush cage 25, the cage is formed of metal and has a surface layer of the cage oxidised so as to provide an outer electrically insulating layer. The cage may be aluminium or steel and aluminium oxide, ferric oxide and zinc oxide may be appropriately used as the insulator. <IMAGE>

Description

SPECIFICATION Brush Cages for Electric Motors This invention relates to brush cages or boxes (hereinafter referred to as cages) for electric motors, and particularly, but not exclusively to brush cages for a.c. or universal motors.

It is known to provide electric motors with brush cages injection moulded in thermoplastics material.

These cages slidably accommodate brushes which are urged into contact with a commutator by appropriate springs.

However, thermoplastics material is a poor conductor of heat and in the event of, for example, excessive arcing between the commutator and brush gear of the motor will tend to soften, distort and even melt as a result of the heat generated.

Accordingly, the present invention seeks to provide brush cages which will dissipate heat more readily and/or withstand a higher temperature rise than cages formed of thermoplastics material.

In a first aspect the invention provides a brush cage for an electric motor, formed of metal and having an electrically insulating layer of metallic compound on at least a part of its surface.

The insulating layer may comprise a metallic oxide formed by oxidation of the surface of a metal.

However, other metallic compounds which are electrically insulating can be used.

Where the brush cage is of aluminium, the insulating layer is conveniently formed by oxidising the surface of the cage or at least a part thereof to provide a layer of insulating aluminium oxide.

Similarly, where the cage is of steel, the insulating layer may be formed by oxidation to provide a layer of insulating ferric oxide.

In an alternative form of construction where the cage is of steel, the insulating layer is zinc oxide.

This is conveniently provided by depositing a layer of zinc on the cage and then oxidising the outer surface of this layer of zinc.

In a second aspect the invention provides an electric motor, including at least two brush cages according to the first aspect of the invention.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side elevation of a motor and gearbox unit incorporating brush cages embodying the present invention; Figures 2 and 3 are front and rear end views respectively of a brush cage similar to that shown in Figure 1; and Figure 4 is a section taken along the line IV--IV of Figure 2.

Referring to the drawings there is shown therein a universal motor and gearbox unit intended for use in a domestic food processor. The motor 10 comprises a laminated rotor and laminated stator each with respective windings. The laminated rotor is designated by reference numeral 11 and the windings of the rotor by reference numeral 12. The motor 10 has two frame parts 13 and 14, each comprising two generally U-shaped pillars 15 and 16 respectively, secured by web portions 17 to the laminated rotor 11.

An integral portion of the frame part 13 defines one bearing support 18 of the motor. The frame part 13 also supports motor brush gear designated by the general reference numeral 19.

The frame part 14 is formed integrally with a casing 20 of an epicyclic gearbox 21 and the other bearing supple rt 22 of the motor.

The two frame parts 13 and 14 are made, conveniently, from die castings such as of an aluminium orzincalloy.

A fan 23 is mounted on motor shaft 24 for drawing cooling air over the casing 20 and into the motor 10.

The brush gear 19 comprises two brush cages 25 (shown best in Figures 2 to 4) secured to the frame part 13 by fasteners (not shown) extending through holes 26 in attachment lugs 27 of the cages. Carbon brushes 28 (Figure 1) are slidably accommodated in the cages 25 and urged into contact with a commutator 29 by springs (not shown) interposed between the brushes 28 and brush gear terminals 30 attached to the rear ends of the cages 25.

Each cage 25 is a die cast aluminium or aluminium alloy part which is oxidised, such as by an anodizing process, to provide a layer of aluminium oxide over its entire surface. It could also be any other appropriate die cast which could be aluminium plated and then anodized.

Aluminium oxide, whilst possessing good electrical insulating properties, is not a good thermal insulator and consequently heat generated by, for example, arcing between the commutator 29 and the brush gear of the motor will be more readily dissipated than when using commutator cages of thermoplastics material. Moreover, the aluminium cage 25 can withstand higher temperatures.

In an alternative embodiment, each brush cage 25 could be of steel, in which case the electrically insulating layer may be formed by oxidation to provide a layer of insulating ferric oxide, or of zinc oxide provided conveniently by depositing a layer of zinc on the steel base and then oxidising the outer surface of this layer of zinc.

The embodiment described above is given by way of example only and various modifications will be apparent to those skilled in the art without departing from the scope of the invention. For example, the insulating layer need be provided on only that part of the cage 25 which is connected to frame part 13.

Moreover, the motor need not be a universal motor, but could instead be an a.c. or d.c. motor.

1. A brush cage for an electric motor, formed of metal and having an electrically insulating layer of metallic compound on at least a part of its surface.

2. A brush cage according to claim 1, in which the metallic compound is an oxide.

3. A brush cage according to claim 2, formed of aluminium or an aluminium alloy, the insulating layer being aluminium oxide.

4. A brush cage according to claim 3, in which the insulating layer of aluminium oxide has been provided by an anodizing process.

5. A brush cage according to claim 2, formed of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Brush Cages for Electric Motors This invention relates to brush cages or boxes (hereinafter referred to as cages) for electric motors, and particularly, but not exclusively to brush cages for a.c. or universal motors. It is known to provide electric motors with brush cages injection moulded in thermoplastics material. These cages slidably accommodate brushes which are urged into contact with a commutator by appropriate springs. However, thermoplastics material is a poor conductor of heat and in the event of, for example, excessive arcing between the commutator and brush gear of the motor will tend to soften, distort and even melt as a result of the heat generated. Accordingly, the present invention seeks to provide brush cages which will dissipate heat more readily and/or withstand a higher temperature rise than cages formed of thermoplastics material. In a first aspect the invention provides a brush cage for an electric motor, formed of metal and having an electrically insulating layer of metallic compound on at least a part of its surface. The insulating layer may comprise a metallic oxide formed by oxidation of the surface of a metal. However, other metallic compounds which are electrically insulating can be used. Where the brush cage is of aluminium, the insulating layer is conveniently formed by oxidising the surface of the cage or at least a part thereof to provide a layer of insulating aluminium oxide. Similarly, where the cage is of steel, the insulating layer may be formed by oxidation to provide a layer of insulating ferric oxide. In an alternative form of construction where the cage is of steel, the insulating layer is zinc oxide. This is conveniently provided by depositing a layer of zinc on the cage and then oxidising the outer surface of this layer of zinc. In a second aspect the invention provides an electric motor, including at least two brush cages according to the first aspect of the invention. The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side elevation of a motor and gearbox unit incorporating brush cages embodying the present invention; Figures 2 and 3 are front and rear end views respectively of a brush cage similar to that shown in Figure 1; and Figure 4 is a section taken along the line IV--IV of Figure 2. Referring to the drawings there is shown therein a universal motor and gearbox unit intended for use in a domestic food processor. The motor 10 comprises a laminated rotor and laminated stator each with respective windings. The laminated rotor is designated by reference numeral 11 and the windings of the rotor by reference numeral 12. The motor 10 has two frame parts 13 and 14, each comprising two generally U-shaped pillars 15 and 16 respectively, secured by web portions 17 to the laminated rotor 11. An integral portion of the frame part 13 defines one bearing support 18 of the motor. The frame part 13 also supports motor brush gear designated by the general reference numeral 19. The frame part 14 is formed integrally with a casing 20 of an epicyclic gearbox 21 and the other bearing supple rt 22 of the motor. The two frame parts 13 and 14 are made, conveniently, from die castings such as of an aluminium orzincalloy. A fan 23 is mounted on motor shaft 24 for drawing cooling air over the casing 20 and into the motor 10. The brush gear 19 comprises two brush cages 25 (shown best in Figures 2 to 4) secured to the frame part 13 by fasteners (not shown) extending through holes 26 in attachment lugs 27 of the cages. Carbon brushes 28 (Figure 1) are slidably accommodated in the cages 25 and urged into contact with a commutator 29 by springs (not shown) interposed between the brushes 28 and brush gear terminals 30 attached to the rear ends of the cages 25. Each cage 25 is a die cast aluminium or aluminium alloy part which is oxidised, such as by an anodizing process, to provide a layer of aluminium oxide over its entire surface. It could also be any other appropriate die cast which could be aluminium plated and then anodized. Aluminium oxide, whilst possessing good electrical insulating properties, is not a good thermal insulator and consequently heat generated by, for example, arcing between the commutator 29 and the brush gear of the motor will be more readily dissipated than when using commutator cages of thermoplastics material. Moreover, the aluminium cage 25 can withstand higher temperatures. In an alternative embodiment, each brush cage 25 could be of steel, in which case the electrically insulating layer may be formed by oxidation to provide a layer of insulating ferric oxide, or of zinc oxide provided conveniently by depositing a layer of zinc on the steel base and then oxidising the outer surface of this layer of zinc. The embodiment described above is given by way of example only and various modifications will be apparent to those skilled in the art without departing from the scope of the invention. For example, the insulating layer need be provided on only that part of the cage 25 which is connected to frame part 13. Moreover, the motor need not be a universal motor, but could instead be an a.c. or d.c. motor. CLAIMS

5. A brush cage according to claim 2, formed of steel, the insulating layer being ferric oxide.

6. A brush cage according to claim 2, formed of a metal otherthan aluminium, the insulating layer being zinc oxide.

7. A brush cage according to claim 6, in which the zinc oxide is formed on the outer surface of a layer of zinc applied to the metal brush cage.

8. A brush cage substantially as hereinbefore described with reference to the accompanying drawing.

9. An electric motor including at least two brush cages according to any one of the preceding claims.

10. An electric motor according to claim 9, wherein the brush cages are attached to a metal frame of the motor.

11. An electric motor according to claim 9 or claim 10, in the form of an a.c. motor.

12. An electric motor according to claim 9 or claim 10, in the form of a universal motor.