GB2191345A

GB2191345A - Commutator base for an electric motor

Info

Publication number: GB2191345A
Application number: GB08613741A
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: GB8613741D0

Abstract

In order to improve heat dissipation and/or tolerance, a commutator base (10) is formed of metal and the outer surface of the base is provided with an outer electrically insulating layer of metallic compound which electrically insulates the commutator segments from one another. The insulating layer may be an oxide, e.g. aluminium oxide in association with an aluminium base and ferric oxide or zinc oxide in association with a steel base. <IMAGE>

Description

SPECIFICATION Commutator Base for an Electric Motor This invention relates to a commutator base for an electric motor, and particularly, but not exclusively to a commutator base for a fractional horsepower permanent magnet direct current motor.

It is known to provide electric motors with commutators having a commutator base which is injection moulded in thermoplastics material, and copper commutator segments secured to the base in an appropriate manner. These known commutators have the advantage that they can be made at very low cost.

However, thermoplastics material is a poor conductor of heat and in the event of, for example, excessive arcing between the commutator segments 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 a commutator base which will dissipate heat more readily and/or withstand a higher temperature rise than a base formed of thermoplastics material.

In a first aspect the invention provides a commutator base for an electric motor, formed of metal and having an electrically insulating layer of metallic compound on at least that part of its surface which in use supports commutator segments.

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

Similarly, where the base 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 commutator base is of steel, the insulating layer is zinc oxide. This is conveniently provided by depositing a layer of zinc on the base and then oxidising the outer surface of this layer of zinc.

In a second aspect the invention provides a commutator for an electric motor, comprising a commutator base according to the first aspect of the invention and a plurality of commutator segments mounted on the commutator base in direct contact with the insulating layer of the commutator base.

In a third aspect the invention provides an electric motor having a commutator according to the second 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 perspective view of an assembled commutator incorporating a commutator base embodying to the invention.

Figure 2 is a side view partially in cross section a rotor assembly incorporating the commutator of Figure 1, and, Figure 3 shows an electric motor. Referring firstly to Figure 1,the assembled commutator shown therein comprises a base 10 having a through hole 11 for mounting the base on a motor shaft and three copper commutator segments 12 mounted on the base 10.

Each segment 12 comprises a brush contacting portion 13, which bears snugly against the surface of a cylindrical support portion 14 of the base 10, and a tang 15 for connection of a wire of an armature to the segment 12. The tang 15 is integral with the brush contacting portion 13 and is located in a slot 16 provided in a collar portion 17 of the base 10.

The design of the segment 12 does not form part of the present invention and will not therefore be described in further detail. It is however more particularly described in our co-pending UK Patent Application No.8611207.

The base 10 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 outer 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, it not a good thermal insulator and consequently heat generated by, for example, arcing between the segments 12 and the brush gear of the motor will be more readily dissipated than when using commutator bases of thermoplastics material. Moreover, the aluminium base 10 can withstand higher temperatures.

In an alternative embodiment, the commutator base 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 oxidizing the outer surface of this layer of zinc.

Figure 2 shows a rotor assembly comprising a motor shaft 20 carrying the assembled commutator including base 10 and segments 12, and an armature 21. The base 10 is mounted fast on the shaft 20. The armature 21 is also mounted fast on the shaft 20 and comprises a number of mush roomshaped, laminated, arms 22 which extend radially of the shaft, each arm 22 having a coil 23 wound about its stem 24. Aware 26 is connected to tang 15 of segment 12 in a manner more particularly described in UK Patent Application No. 8611207.

The rotor assembly as illustrated in Figure 3 may be incorporated in an electric motor, such as a fractional horsepower permanent magnet direct current motor using brush gear and a stator and bearing assembly as for the applicants motor marketed under catalogue number HC213 and shown in outline in Figure 3. This motor has an outer steel casing 31 having a closed end 32 supporting a bearing assembly 33 for shaft 20, and an open end which is closed by a plastics cap 35 which supports brush gear for supplying current to the coils of the armature 21 via the commutator, and a second bearing for shaft 20. Apertures 36 are formed in the casing for cooling as is generally known in the art.

The collar portion 17, as described in the above embodiment, is an integral part of the commutator base 10. However, for some commutators the collar may be separate from and provided as an interference fit on the base. In that case the collar could be of plastics material. Moreover, the commutator base may have a cylindrical extension of reduced diameter going beyond the collar or collar portion to abut the armature. This keeps the collar or collar portion away from the armature to leave room for the winding.

The embodiment described above is given byway 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 commutator base which supports the commutator segments.

Claims

1. A commutator base for an electric motor, formed of metal and having an electrically insulating layer of metallic compound on at least that part of its surface which in use supports commutator segments.

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

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

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

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

6. A commutator base according to claim 2, formed of a metal other than aluminium, the insulating layer being zinc oxide.

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

8. A commutator base substantially as hereinbefore described with reference to the accompanying drawing.

9. A commutator for an electric motor, comprising a commutator base according to any one of the preceding claims and a plurality of commutator segments mounted on the commutator base in direct contact with the insulating layer of the commutator base.

10. An electric motor having a commutator according to claim 9.